Severe hypoxia was observed in the submarine canyon to the east of the Changjiang estuary in July 14, 2015, two days after typhoon Chan-hom. The oxygen concentration reached as low as 2.0 mg/L and occupied a water col...Severe hypoxia was observed in the submarine canyon to the east of the Changjiang estuary in July 14, 2015, two days after typhoon Chan-hom. The oxygen concentration reached as low as 2.0 mg/L and occupied a water column of about 25 m. A ROMS model was con?gured to explore the underlying physical processes causing the formation of hypoxia. Chan-hom passed through the Changjiang estuary during the neap tide. The strati?cation was completely destroyed in the shallow nearshore region when typhoon passing. However, it was maintained in the deep canyon, though the surface mixed layer was largely deepened. The residual water in the deep canyon is considered to be the possible source of the later hypoxia. After Chan-hom departure, not only the low salinity plume water spread further of fshore, but also the sea surface temperature(SST) rewarmed quickly. Both changes helped strengthen the strati?cation and facilitate the formation of hypoxia. It was found that the surface heat ?ux, especially the solar short wave radiation dominated the surface re-warming, the of fshore advection of the warmer Changjiang Diluted Water(CDW) also played a role. In addition to the residual water in the deep canyon, the Taiwan Warm Current(TWC) was found to ?ow into the deep canyon pre-and soon post-Chan-hom, which was considered to be the original source of the hypoxia water.展开更多
Spatial and seasonal variabilities of submesoscale currents in the northeastern South China Sea are investigated by employing a numerical simulation with a horizontal resolution of 1km. The results suggest that submes...Spatial and seasonal variabilities of submesoscale currents in the northeastern South China Sea are investigated by employing a numerical simulation with a horizontal resolution of 1km. The results suggest that submesoscale currents are widespread in the surface mixed layer mainly due to the mixed layer instabilities and frontogenesis. In horizontal, submesoscale currents are generally more active in the north than those in the south, since that active eddies, especially cyclonic eddies, mainly occur in the northern area. Specifically, submesoscale currents are highly intensified in the east of Dongsha Island and south of Taiwan Island. In temporal sense, submesoscale currents are more active in winter than those in summer, since the mixed layer is thicker and more unstable in the winter. The parameterization developed by Fox-Kemper et al. is examined in terms of vertical velocity, and the results suggest that it could reproduce the vertical velocity if mixed layer instability dominates there. This study improves our understanding of the submesoscale dynamics in the South China Sea.展开更多
This paper is focused on the seasonality change of Arctic sea ice extent （SIE） from 1979 to 2100 using newly available simulations from the Coupled Model Intercomparison Project Phase 5 （CMIP5）. A new approach to ...This paper is focused on the seasonality change of Arctic sea ice extent （SIE） from 1979 to 2100 using newly available simulations from the Coupled Model Intercomparison Project Phase 5 （CMIP5）. A new approach to compare the simulation metric of Arctic SIE between observation and 31 CMIP5 models was established. The approach is based on four factors including the climatological average, linear trend of SIE, span of melting season and annual range of SIE. It is more objective and can be popularized to other comparison of models. Six good models （GFDL-CM3, CESM1-BGC, MPI-ESM-LR, ACCESS-1.0, HadGEM2-CC, and HadGEM2-AO in turn） are found which meet the criterion closely based on above approach. Based on ensemble mean of the six models, we found that the Arctic sea ice will continue declining in each season and firstly drop below 1 million km2 （defined as the ice-free state） in September 2065 under RCP4.5 scenario and in September 2053 under RCP8.5 scenario. We also study the seasonal cycle of the Arctic SIE and find out the duration of Arctic summer （melting season） will increase by about I00 days under RCP4.5 scenario and about 200 days under RCPS.5 scenario relative to current circumstance by the end of the 21st century. Asymmetry of the Arctic SIE seasonal cycle with later freezing in fall and early melting in spring, would be more apparent in the future when the Arctic climate approaches to ＂tipping point＂, or when the ice-free Arctic Ocean appears. Annual range of SIE （seasonal melting ice extent） will increase almost linearly in the near future 30-40 years before the Arctic appears ice-free ocean, indicating the more ice melting in summer, the more ice freezing in winter, which may cause more extreme weather events in both winter and summer in the future years.展开更多
The Arctic near-surface air temperatures are increasing more than twice as fast as the global average-a feature known as Arctic amplification （AA）. A modified AA index is constructed in this paper to emphasize the c...The Arctic near-surface air temperatures are increasing more than twice as fast as the global average-a feature known as Arctic amplification （AA）. A modified AA index is constructed in this paper to emphasize the contrast of warming rate between polar and mid-latitude regions, as well as the spatial and temporal characteristics of AA and their influence on atmospheric circulation over the Northern Hemisphere. Results show that AA has a pronounced annual cycle. The positive or negative phase activities are the strongest in autumn and winter, the weakest in summer. After experiencing a remarkable decadal shift from negative to positive phase in the early global warming hiatus period, the AA has entered into a state of being enlarged continuously, and the decadal regime shift of AA in about 2002 is affected mainly by decadal shift in autumn. In terms of spatial distribution, AA has maximum warming near the surface in almost all seasons except in summer. Poleward of 20~N, AA in autumn has a significant influence on the atmospheric circulation in the following winter. The reason may be that the autumn AA increases the amplitude of planetary waves, slows the wave speeds and weakens upper-level zonal winds through the thermal wind relation, thus influencing surface air temperature in the following winter. The AA correlates to negative phase of the Arctic oscillation （AO） and leads AO by 0-3 months within the period 1979-2002. However, weaker relationship between them is indistinctive after the decadal shift of AA.展开更多
Gaofen-3(GF-3) is the first Chinese space-borne satellite to carry the C-band multi-polarization synthetic aperture radar(SAR). Marine applications, i.e., winds and waves retrieved from GF-3 SAR images, have been oper...Gaofen-3(GF-3) is the first Chinese space-borne satellite to carry the C-band multi-polarization synthetic aperture radar(SAR). Marine applications, i.e., winds and waves retrieved from GF-3 SAR images, have been operational since January 2017. In this study, we have collected more than 1000 quad-polarization(vertical-vertical(VV); horizontal-horizontal(HH); vertical-horizontal(VH); horizontal-vertical(HV)) GF-3 SAR images, which were acquired around the China Seas from September 2016 to September 2017. Wind streaks were visible in these images in co-polarization(VV and HH) channel. Geophysical model functions(GMFs), including the CMOD5N together with polarization ratio(PR) model and C-SARMOD, were used to retrieve winds from the collected co-polarization GF-3 SAR images. Wind directions were directly obtained from GF-3 SAR images. Then, the SAR-derived wind speeds were compared with the measurements at a 0.25? grid from the Advanced Scatterometer on board the Metop-A/B and microwave radiometer WindSAT. Based on the analysis, empirical corrections are proposed to improve the performance of the two GMFs. Results of this study show that the standard deviation of wind speed is 1.63 m s^(-1) with a 0.19 m s^(-1) bias and 1.71 m s^(-1) with a 0.26 m s^(-1) bias for VV-and HH-polarization GF-3 SAR, respectively. Our work not only systematically evaluates wind retrieval by using the two advanced GMFs and PR models but also proposes empirical corrections to improve the accuracy of wind retrievals from GF-3 SAR images around the China Seas and thus enhance the accuracy of near real-time operational SAR-derived wind products.展开更多
Chinese Gaofen-3（GF-3） is the first civilian satellite to carry C-band（5.3 GHz） synthetic aperture radar（SAR）.During the period of August 2016 to December 2017, 1 523 GF-3 SAR images acquired in quad-polarizatio...Chinese Gaofen-3（GF-3） is the first civilian satellite to carry C-band（5.3 GHz） synthetic aperture radar（SAR）.During the period of August 2016 to December 2017, 1 523 GF-3 SAR images acquired in quad-polarization（vertical-vertical（VV）, horizontal-horizontal（HH）, vertical-horizontal（VH）, and horizontal-vertical（HV）） mode were recorded, mostly around China＇s seas. In our previous study, the root mean square error（RMSE） of significant wave height（SWH） was found to be around 0.58 m when compared with retrieval results from a few GF-3 SAR images in co-polarization（VV and HH） with moored measurements by using an empirical algorithm CSARWAVE. We collected a number of sub-scenes from these 1 523 images in the co-polarization channel,which were collocated with wind and SWH data from the European Centre for Medium-Range Weather Forecasts（ECMWF） reanalysis field at a 0.125° grid. Through the collected dataset, an improved empirical wave retrieval algorithm for GF-3 SAR in co-polarization was tuned, herein denoted as CSARWAVE2. An additional 92 GF-3 SAR images were implemented in order to validate CSARWAVE2 against SWH from altimeter Jason-2, showing an about 0.52 m RMSE of SWH for co-polarization GF-3 SAR. Therefore, we conclude that the proposed empirical algorithm has a good performance for wave retrieval from GF-3 SAR images in co-polarization.展开更多
Version 4(v4) of the Extended Reconstructed Sea Surface Temperature(ERSST) dataset is compared with its precedent, the widely used version 3b(v3b). The essential upgrades applied to v4 lead to remarkable differences i...Version 4(v4) of the Extended Reconstructed Sea Surface Temperature(ERSST) dataset is compared with its precedent, the widely used version 3b(v3b). The essential upgrades applied to v4 lead to remarkable differences in the characteristics of the sea surface temperature(SST) anomaly(SSTa) in both the temporal and spatial domains. First, the largest discrepancy of the global mean SSTa values around the 1940 s is due to ship-observation corrections made to reconcile observations from buckets and engine intake thermometers. Second, differences in global and regional mean SSTa values between v4 and v3b exhibit a downward trend(around-0.032℃ per decade) before the 1940s, an upward trend(around 0.014℃ per decade) during the period of 1950–2015, interdecadal oscillation with one peak around the 1980s, and two troughs during the 1960s and 2000s, respectively. This does not derive from treatments of the polar or the other data-void regions, since the difference of the SSTa does not share the common features. Third, the spatial pattern of the ENSO-related variability of v4 exhibits a wider but weaker cold tongue in the tropical region of the Pacific Ocean compared with that of v3b, which could be attributed to differences in gap-filling assumptions since the latter features satellite observations whereas the former features in situ ones. This intercomparison confirms that the structural uncertainty arising from underlying assumptions on the treatment of diverse SST observations even in the same SST product family is the main source of significant SST differences in the temporal domain. Why this uncertainty introduces artificial decadal oscillations remains unknown.展开更多
In this study, we examine the results obtained by the Finite-volume Coastal Ocean Circulation Model(FVCOM) regarding the effects of eddy viscosity and bathymetry on the three-dimensional(3 D) Lagrangian residual veloc...In this study, we examine the results obtained by the Finite-volume Coastal Ocean Circulation Model(FVCOM) regarding the effects of eddy viscosity and bathymetry on the three-dimensional(3 D) Lagrangian residual velocity(LRV) in a narrow bay. The results are cast in terms of two nondimensional numbers: the ratio of friction to local acceleration(δ) and the ratio of the minimum depth over shoals to the maximum depth in the channel(ε). The ratio δ depends on the eddy viscosity and mean depth. For a given eddy viscosity, when ε > 0.5, the along-estuary LRV tends to be vertically sheared and when ε < 0.5, the exchange is laterally sheared. When ε << 1, the structure of the 3 D, depth-integrated, and breadth-averaged LRV changes only slightly as δ increases. For ε values between 0.33 and 0.5, the structure of the 3 D LRV is mainly laterally sheared. In the same ε range, the 3 D and depth-integrated LRV exhibit reversed structures from high to low δ values. In addition, the breadth-averaged LRV weakens the typical twolayered circulation when δ decreases. When ε is 1, the two-layered vertical structure reverses direction, and a three-layered vertical structure develops in the outer bay as δ decreases.展开更多
Several bottom-mounted Acoustic Doppler Current Profiler (ADCP) moorings were deployed in the northern Yellow Sea (NYS) during the four seasons of 2006–2007 and also the summertime of 2009. A synthesis analysis on th...Several bottom-mounted Acoustic Doppler Current Profiler (ADCP) moorings were deployed in the northern Yellow Sea (NYS) during the four seasons of 2006–2007 and also the summertime of 2009. A synthesis analysis on the time-continuous records was performed to examine the characteristics and variations of tidal currents and mean flow over the observation period at these stations. Tidal currents accounted for ~75% of the total kinetic energy, with the absolute dominance of M2 constituent. Visible vertical variations of tidal flow were found on all sites, featured by the decrease of amplitude, increase of rotation rate as well as a decreasing trend of the phase for M2 component with depth. A notable exception was in the central NYS, where the maximum tidal currents occurred in the upper or middle layers (~20–40 m) instead of near the surface (<10 m). The observed mean flow was relatively weak, smaller than 15 cm/s. Velocity on the northern end of Yellow Sea Trough (YST) was characterized by low magnitude and an obvious layered structure vertically. In the Bohai Strait (BS) and the northern slope area, the currents weakened and the flow direction presented a major trend to deflect counterclockwise with depth in most observations. Summertime cyclonic circulation around the Yellow Sea Cold Water Mass (YSCWM), its intensification on the frontal zone and the Yellow Sea Warm Current (YSWC) for the winter season were all evident by our direct current measurements. However, the details of water exchange through the BS appeared partly diff erent from the traditionally-accepted pattern. The vertical diff erences of tidal and mean flow were larger in summer than that in winter, implying the influence of thermal structure to the local currents. Aff ected by the water stratification, mean flow usually reached its maximum near the thermocline in spring and summer, while showing a nearly uniform vertical distribution during winter.展开更多
Based on 22 of the climate models from phase 3 of the Coupled Model Intercomparison Project, we investigate the ability of the models to reproduce the spatiotemporal features of the wintertime North Pacific Oscillatio...Based on 22 of the climate models from phase 3 of the Coupled Model Intercomparison Project, we investigate the ability of the models to reproduce the spatiotemporal features of the wintertime North Pacific Oscillation(NPO), which is the second most important factor determining the wintertime sea level pressure field in simulations of the pre-industrial control climate, and evaluate the NPO response to the future most reasonable global warming scenario(the A1B scenario). We reveal that while most models simulate the geographic distribution and amplitude of the NPO pattern satisfactorily, only 13 models capture both features well. However, the temporal variability of the simulated NPO could not be significantly correlated with the observations. Further analysis indicates the weakened NPO intensity for a scenario of strong global warming is attributable to the reduced lower-tropospheric baroclinicity at mid-latitudes, which is anticipated to disrupt large-scale and low-frequency atmospheric variability, resulting in the diminished transfer of energy to the NPO, together with its northward shift.展开更多
The sea level variabilities, especially the atmosphere-driven sea level variabilities, which are diff erent in studies on diverse areas and timescales, need to be further documented in the Bohai Bay. Coastal sea level...The sea level variabilities, especially the atmosphere-driven sea level variabilities, which are diff erent in studies on diverse areas and timescales, need to be further documented in the Bohai Bay. Coastal sea level data and coincident meteorological data collected hourly at two observation stations (E1 and E2) in the Bohai Bay, which is a typical semi-enclosed coastal sea in China, are analyzed for the period from 19 August 2014 to 18 November 2014. The sub-sampled low-pass (<0.8 cpd) sea levels (SLSLs) at E1 and E2 are almost the same as each other, while the winds are not. On the whole, SLSLs at E1 and E2 are dominantly influenced by the across-shore wind;in detail, the dominant wind orientation at E1 is 65° measured clockwise from north, and SLSL at E2 is significantly influenced by the sub-sampled wind (SW) at 55°. Regression of SLSL onto the corresponding SW in dominant orientation and the atmospheric pressure is used to predict SLSL, which make the frequency of occurrences when the predicted total sea level is within 0.15 m from the observed values increase to 66.03% and 58.08% at E1 and E2 from original 36.71% and 34.80% without using it, respectively. The results indicate that for the prediction of the total sea level variability in the coastal shallow waters, the SLSL influenced by the atmospheric forcing, including local wind and atmospheric pressure, can be predicted using the multivariable linear regression model.展开更多
The‘Two Oceans and One Sea’area(West Pacific,Indian Ocean,and South China Sea;15°S–60°N,39°–178°E)is a core strategic area for the‘21st Century Maritime Silk Road’project,as well as national ...The‘Two Oceans and One Sea’area(West Pacific,Indian Ocean,and South China Sea;15°S–60°N,39°–178°E)is a core strategic area for the‘21st Century Maritime Silk Road’project,as well as national defense.With the increasing demand for disaster prevention and mitigation,the importance of 10–30-day extended range prediction,between the conventional short-term(around seven days)and the climate scale(longer than one month),is apparent.However,marine extended range prediction is still a‘blank point’in China,making the early warning of marine disasters almost impossible.Here,the authors introduce a recently launched Chinese national project on a numerical forecasting system for extended range prediction in the‘Two Oceans and One Sea’area based on a regional ultra-high resolution multi-layer coupled model,including the scientific aims,technical scheme,innovation,and expected achievements.The completion of this prediction system is of considerable significance for the economic development and national security of China.展开更多
The physical cause of amplified deep Arctic tropospheric warming in winter in the Barents-Kara Seas （BKS） is examined. The authors propose that changes in the atmospheric circulation patterns are important for deep ...The physical cause of amplified deep Arctic tropospheric warming in winter in the Barents-Kara Seas （BKS） is examined. The authors propose that changes in the atmospheric circulation patterns are important for deep Arctic tropospheric warming in winter. It is found that the retrograde Urals blocking （UB） event concurrent with a negative North Atlantic Oscillation （NAO-） that arises from a prior negative Arctic Oscillation （AO-） is not favorable for tropospheric warming because of less water vapor over the BKS. Such UB events are related to more winter BKS sea ice associated with the negative sea surface temperature （SST） anomaly in the BKS. In contrast, a UB occurring together with a positive North Atlantic Oscillation （NAO~） shows less movement and can significantly enhance tropospheric warming over the BKS through increasing tropospheric sensible heat energy due to a persistent BKS water vapor increase. This type of quasi-stationary UB event is related to prior less BKS sea ice associated with a positive BKS SST anomaly that coexists with the North Atlantic SST tripole structure. In summary, because warm, wet and low sea-ice winters in the BKS are related to UB events with an NAO~, and depend on the winter prior sea-ice condition, the tropospheric warming is to some extent a manifestation of the sea-ice-blocking-moisture feedback in the BKS.展开更多
Near-inertial waves(NIWs), which can be generated by wind or the parametric subharmonic instability(PSI) of internal tides, are common in the South China Sea(SCS). Moored current observations from the northern SCS hav...Near-inertial waves(NIWs), which can be generated by wind or the parametric subharmonic instability(PSI) of internal tides, are common in the South China Sea(SCS). Moored current observations from the northern SCS have revealed that the PSI of semidiurnal(D_2) internal tides is another source of NIWs. The objective of this study was to examine the energy variance in the PSI of D_2 tides. The PSI of D_2 internal tides generated NIWs and waves with frequencies around the difference frequency of D_2 and f. The observed NIWs induced by PSI could be distinguished clearly from those elicited by typhoon Krosa. Shortly after Krosa entered the SCS, NIWs began to intensify on the surface and they propagated downward over subsequent days. The near-inertial currents were damped quickly and they became relatively weak before the waves were reinforced beneath the mixed layer when wind stress was relatively weak. Rotation spectra indicated an energy peak at exactly the difference frequency D_2–f of the NIWs and D_2, indicating nonlinear wave-wave interaction among D_2, f, and D_2–f. Depth-time maps of band-pass fi ltered velocities of D_2 –f showed the waves amplifi ed when the NIWs were reinforced, and they intensifi ed at depths with strong D_2 tides. The energies of the NIWs and D_2 –f had high correlation with the D_2 tides. The PSI transferred energy of low-mode D_2 internal tides to high-mode NIWs and D_2–f waves. For the entire observational period, PSI reinforcement was observed only when mesoscale eddies emerged and when D_2 was in spring tide, revealing a close connection between mesoscale eddies and NIWs. Mesoscale eddies could increase the energy in the f-band by enhancing the PSI of D_2 internal tides. Thus, this represents another mechanism linking the energy of mesoscale eddies to that of NIWs.展开更多
A nine-month mooring record was used to investigate seasonal variation and modal content of internal tides(ITs) on the continental slope in the northern South China Sea(SCS). Diurnal tides at this site show clear seas...A nine-month mooring record was used to investigate seasonal variation and modal content of internal tides(ITs) on the continental slope in the northern South China Sea(SCS). Diurnal tides at this site show clear seasonal change with higher energy in winter than in spring and autumn, whereas semidiurnal tides show the opposite seasonal pattern. The consistency of ITs with barotropic tides within the Luzon Strait, which is the generation region of the ITs, implies that the seasonal variation of ITs depends on their astronomical forcing, even after extended propagation across the SCS basin. Diurnal tides also differ from semidiurnal tides in relation to modal content; they display signals of high modes while semidiurnal tides are dominated by low modes. Reflection of the diurnal tides on the continental slope serves as a reasonable explanation for their high modes. Both diurnal and semidiurnal tides are composed of a larger proportion of coherent components that have a regular 14-day spring-neap cycle. The coherent components are dominated by low modes and they show obvious seasonal variation, while the incoherent components are composed mainly of higher modes and they display intermittent characteristics.展开更多
The vertical mixing parameterization scheme,by providing the eff ects of some explicitly missed physical processes and more importantly closing the energy budgets,is a critical model component and therefore imposes si...The vertical mixing parameterization scheme,by providing the eff ects of some explicitly missed physical processes and more importantly closing the energy budgets,is a critical model component and therefore imposes signifi cant impacts on model performance.The Yellow Sea Cold Water Mass(YSCWM),as the most striking and unique phenomenon in the Yellow Sea during summer,is dramatically aff ected by vertical mixing process during its each stage and therefore seriously sensitive to the proper choice of parameterization scheme.In this paper,a hindcast of YSCWM in winter of 2006 was implemented by using the Regional Ocean Modeling System(ROMS).Three popular parameterization schemes,including the level 2.5 Mellor-Yamada closure(M-Y 2.5),Generic Length Scale closure(GLS)and K-Profi le Parameterization(KPP),were tested and compared with each other by conducting a series of sensitivity model experiments.The infl uence of diff erent parameterization schemes on modeling the YSCWM was then carefully examined and assessed based on these model experiments.Although reasonable thermal structure and its seasonal variation were well reproduced by all schemes,considerable diff erences could still be found among all experiments.A warmer and spatially smaller simulation of YSCWM,with very strong thermocline,appeared in M-Y 2.5 experiment,while a spatially larger YSCWM with shallow mixed layer was found in GLS and KPP schemes.Among all the experiments,the discrepancy,indicated by core temperature,appeared since spring,and grew gradually by the end of November.Additional experiments also confi rmed that the increase of background diff usivity could eff ectively weaken the YSCWM,in either strength or coverage.Surface wave,another contributor in upper layer,was found responsible for the shrinkage of YSCWM coverage.The treatment of wave eff ect as an additional turbulence production term in prognostic equation was shown to be more superior to the strategy of directly increasing diff usivity for a coastal region.展开更多
Tropical Atlantic climate change is relevant to the variation of Atlantic meridional overturning circulation(AMOC) through different physical processes. Previous coupled climate model simulation suggested a dipole-lik...Tropical Atlantic climate change is relevant to the variation of Atlantic meridional overturning circulation(AMOC) through different physical processes. Previous coupled climate model simulation suggested a dipole-like SST structure cooling over the North Atlantic and warming over the South Tropical Atlantic in response to the slowdown of the AMOC. Using an ocean-only global ocean model here, an attempt was made to separate the total influence of various AMOC change scenarios into an oceanicinduced component and an atmospheric-induced component. In contrast with previous freshwater-hosing experiments with coupled climate models, the ocean-only modeling presented here shows a surface warming in the whole tropical Atlantic region and the oceanic-induced processes may play an important role in the SST change in the equatorial south Atlantic. Our result shows that the warming is partly governed by oceanic process through the mechanism of oceanic gateway change, which operates in the regime where freshwater forcing is strong, exceeding 0.3 Sv. Strong AMOC change is required for the gateway mechanism to work in our model because only when the AMOC is sufficiently weak, the North Brazil Undercurrent can flow equatorward, carrying warm and salty north Atlantic subtropical gyre water into the equatorial zone. This threshold is likely to be model-dependent. An improved understanding of these issues may have help with abrupt climate change prediction later.展开更多
Melt ponds on Arctic sea ice are of great significance in the study of the heat balance in the ocean mixed layer, mass and salt balances of Arctic sea ice, and other aspects of the earth-atmosphere system. During the ...Melt ponds on Arctic sea ice are of great significance in the study of the heat balance in the ocean mixed layer, mass and salt balances of Arctic sea ice, and other aspects of the earth-atmosphere system. During the 7th Chinese National Arctic Research Expedition, aerial photographs were taken from an Unmanned Aerial Vehicle over an ice floe in the Canada Basin. Using threshold discrimination and three-dimensional modeling, we estimated a melt pond fraction of 1.63% and a regionally averaged surface roughness of 0.12 for the study area. In view- of the particularly foggy environment of the Arctic, aerial images were defogged using an improved dark channel prior based image defog algorithm, especially adapted for the special conditions of sea ice images. An aerial photo mosaic was generated, melt ponds were identified from the mosaic image and melt pond fractions were calculated. Three-dimensional modeling techniques were used to generate a digital elevation model allowing relative elevation and roughness of the sea ice surface to be estimated. Analysis of the relationship between the distributions of melt ponds and sea ice surface roughness show-s that melt ponds are smaller on sea ice with higher surface roughness, while broader melt ponds usually occur in areas where sea ice surface roughness is lower.展开更多
The co-variation of surface wind speed and sea surface temperature (SST) over the Gulf Stream frontal region is investigated using high-resolution satellite measurements and atmospheric reanalysis data. Results show t...The co-variation of surface wind speed and sea surface temperature (SST) over the Gulf Stream frontal region is investigated using high-resolution satellite measurements and atmospheric reanalysis data. Results show that the pattern of positive SST-surface wind speed correlations is anchored by strong SST gradient and marine atmospheric boundary layer (MABL) height front, with active warm and cold-ocean eddies around. The MABL has an obvious transitional structure along the strong SST front, with greater (lesser) heights over the north (south) side. The significant positive SST-surface wind-speed perturbation correlations are mostly found over both strong warm and cold eddies. The surface wind speed increases (decreases) about 0.32 (0.41) m/s and the MABL elevates (drops) approximate 55 (54) m per 1℃ of SST perturbation induced by warm (cold) eddies. The response of the surface wind speed to SST perturbations over the mesoscale eddies is mainly attributed to the momentum vertical mixing in the MABL, which is confirmed by the linear relationships between the downwind (crosswind) SST gradient and wind divergence (curl).展开更多
Paleo reconstructions and model simulations have suggested the Bering Strait plays a pivotal role in climate change. However, the contribution of the Bering Strait throughflow to oceanic meridional heat transport (OMH...Paleo reconstructions and model simulations have suggested the Bering Strait plays a pivotal role in climate change. However, the contribution of the Bering Strait throughflow to oceanic meridional heat transport (OMHT) is about 100 times smaller than the OMHT at low latitudes in the modern climate and it is generally ignored. Based on model simulations under modern and Last Glacial Maximum (LGM,~21 ka;ka=thousand years ago) climate conditions, this study highlights the importance of the Bering Strait throughflow to OMHT. The interbasin OMHT induced by the Bering Strait throughflow is estimated by interbasin-intrabasin decomposition. Similar to barotropic-baroclinic-horizontal decomposition, we assume the nonzero net mass transport induced by interbasin throughflows is uniform across the entire section, and the interbasin term is separated to force zero net mass transport for the intrabasin term. Based on interbasinintrabasin decomposition, the contribution of the Bering Strait throughflow is determined as ~0.02 PW (1 PW=10 15 W) under the modern climate, and zero under the LGM climate because the closed Bering Strait blocked interbasin throughflows. The contribution of the Bering Strait throughflow to OMHT is rather small, consistent with previous studies. However, comparisons of OMHT under modern and LGM climate conditions indicate the mean absolute changes are typically 0.05 and 0.20 PWin the North Atlantic and North Pacific, respectively. Thus, the contribution of the Bering Strait throughflow should not be ignored when comparing OMHT under diff erent climate conditions.展开更多
基金Supported by the National Key Research and Development Program of China(No.2016YFC1402000)the National Natural Science Foundation of China(Nos.41406008,41706022)+2 种基金the Fundamental Research Funds for the Central Universities(No.201762032)the Natural Science Foundation of Shandong Province(No.ZR2014DQ023)the SRF for ROCS,SEM[2014] No.1685
文摘Severe hypoxia was observed in the submarine canyon to the east of the Changjiang estuary in July 14, 2015, two days after typhoon Chan-hom. The oxygen concentration reached as low as 2.0 mg/L and occupied a water column of about 25 m. A ROMS model was con?gured to explore the underlying physical processes causing the formation of hypoxia. Chan-hom passed through the Changjiang estuary during the neap tide. The strati?cation was completely destroyed in the shallow nearshore region when typhoon passing. However, it was maintained in the deep canyon, though the surface mixed layer was largely deepened. The residual water in the deep canyon is considered to be the possible source of the later hypoxia. After Chan-hom departure, not only the low salinity plume water spread further of fshore, but also the sea surface temperature(SST) rewarmed quickly. Both changes helped strengthen the strati?cation and facilitate the formation of hypoxia. It was found that the surface heat ?ux, especially the solar short wave radiation dominated the surface re-warming, the of fshore advection of the warmer Changjiang Diluted Water(CDW) also played a role. In addition to the residual water in the deep canyon, the Taiwan Warm Current(TWC) was found to ?ow into the deep canyon pre-and soon post-Chan-hom, which was considered to be the original source of the hypoxia water.
基金Supported by the Natural Science Foundation of China(No.41576009)the National Key Research and Development Program(No.2016YFC1401403)the State Key Laboratory of Tropical Oceanography,and the Global Change and Air-Sea Interaction Project(Nos.GASIIPOVAI-01-03,GASI-IPOVAI-01-02)
文摘Spatial and seasonal variabilities of submesoscale currents in the northeastern South China Sea are investigated by employing a numerical simulation with a horizontal resolution of 1km. The results suggest that submesoscale currents are widespread in the surface mixed layer mainly due to the mixed layer instabilities and frontogenesis. In horizontal, submesoscale currents are generally more active in the north than those in the south, since that active eddies, especially cyclonic eddies, mainly occur in the northern area. Specifically, submesoscale currents are highly intensified in the east of Dongsha Island and south of Taiwan Island. In temporal sense, submesoscale currents are more active in winter than those in summer, since the mixed layer is thicker and more unstable in the winter. The parameterization developed by Fox-Kemper et al. is examined in terms of vertical velocity, and the results suggest that it could reproduce the vertical velocity if mixed layer instability dominates there. This study improves our understanding of the submesoscale dynamics in the South China Sea.
基金The National Basic Research Program of China(973 Program)under contract No.2015CB953904the National Natural Science Foundation of China under contract No.41575067
文摘This paper is focused on the seasonality change of Arctic sea ice extent （SIE） from 1979 to 2100 using newly available simulations from the Coupled Model Intercomparison Project Phase 5 （CMIP5）. A new approach to compare the simulation metric of Arctic SIE between observation and 31 CMIP5 models was established. The approach is based on four factors including the climatological average, linear trend of SIE, span of melting season and annual range of SIE. It is more objective and can be popularized to other comparison of models. Six good models （GFDL-CM3, CESM1-BGC, MPI-ESM-LR, ACCESS-1.0, HadGEM2-CC, and HadGEM2-AO in turn） are found which meet the criterion closely based on above approach. Based on ensemble mean of the six models, we found that the Arctic sea ice will continue declining in each season and firstly drop below 1 million km2 （defined as the ice-free state） in September 2065 under RCP4.5 scenario and in September 2053 under RCP8.5 scenario. We also study the seasonal cycle of the Arctic SIE and find out the duration of Arctic summer （melting season） will increase by about I00 days under RCP4.5 scenario and about 200 days under RCPS.5 scenario relative to current circumstance by the end of the 21st century. Asymmetry of the Arctic SIE seasonal cycle with later freezing in fall and early melting in spring, would be more apparent in the future when the Arctic climate approaches to ＂tipping point＂, or when the ice-free Arctic Ocean appears. Annual range of SIE （seasonal melting ice extent） will increase almost linearly in the near future 30-40 years before the Arctic appears ice-free ocean, indicating the more ice melting in summer, the more ice freezing in winter, which may cause more extreme weather events in both winter and summer in the future years.
基金The Global Change Research Program of China under contract No.2015CB953904the National Natural Science Foundation of China under contract Nos 41575067 and 41376008
文摘The Arctic near-surface air temperatures are increasing more than twice as fast as the global average-a feature known as Arctic amplification （AA）. A modified AA index is constructed in this paper to emphasize the contrast of warming rate between polar and mid-latitude regions, as well as the spatial and temporal characteristics of AA and their influence on atmospheric circulation over the Northern Hemisphere. Results show that AA has a pronounced annual cycle. The positive or negative phase activities are the strongest in autumn and winter, the weakest in summer. After experiencing a remarkable decadal shift from negative to positive phase in the early global warming hiatus period, the AA has entered into a state of being enlarged continuously, and the decadal regime shift of AA in about 2002 is affected mainly by decadal shift in autumn. In terms of spatial distribution, AA has maximum warming near the surface in almost all seasons except in summer. Poleward of 20~N, AA in autumn has a significant influence on the atmospheric circulation in the following winter. The reason may be that the autumn AA increases the amplitude of planetary waves, slows the wave speeds and weakens upper-level zonal winds through the thermal wind relation, thus influencing surface air temperature in the following winter. The AA correlates to negative phase of the Arctic oscillation （AO） and leads AO by 0-3 months within the period 1979-2002. However, weaker relationship between them is indistinctive after the decadal shift of AA.
基金partly supported by the National Key Research and Development Program of China (Nos. 2016YFC1401605, 2016YFC1401905, and 2017YFA0604 901)the National Natural Science Foundation of China (Nos. 41806005 and 41806004)the National Social Science Foundation of China (No. 15ZDB170)
文摘Gaofen-3(GF-3) is the first Chinese space-borne satellite to carry the C-band multi-polarization synthetic aperture radar(SAR). Marine applications, i.e., winds and waves retrieved from GF-3 SAR images, have been operational since January 2017. In this study, we have collected more than 1000 quad-polarization(vertical-vertical(VV); horizontal-horizontal(HH); vertical-horizontal(VH); horizontal-vertical(HV)) GF-3 SAR images, which were acquired around the China Seas from September 2016 to September 2017. Wind streaks were visible in these images in co-polarization(VV and HH) channel. Geophysical model functions(GMFs), including the CMOD5N together with polarization ratio(PR) model and C-SARMOD, were used to retrieve winds from the collected co-polarization GF-3 SAR images. Wind directions were directly obtained from GF-3 SAR images. Then, the SAR-derived wind speeds were compared with the measurements at a 0.25? grid from the Advanced Scatterometer on board the Metop-A/B and microwave radiometer WindSAT. Based on the analysis, empirical corrections are proposed to improve the performance of the two GMFs. Results of this study show that the standard deviation of wind speed is 1.63 m s^(-1) with a 0.19 m s^(-1) bias and 1.71 m s^(-1) with a 0.26 m s^(-1) bias for VV-and HH-polarization GF-3 SAR, respectively. Our work not only systematically evaluates wind retrieval by using the two advanced GMFs and PR models but also proposes empirical corrections to improve the accuracy of wind retrievals from GF-3 SAR images around the China Seas and thus enhance the accuracy of near real-time operational SAR-derived wind products.
基金The National Key Research and Development Program of China under contract Nos 2016YFC1401905 and2017YFA0604901the National Natural Science Foundation of China under contract Nos 41776183,41676014,41606024 and 41506033the National Social Science Foundation of China under contract No.15ZDB170
文摘Chinese Gaofen-3（GF-3） is the first civilian satellite to carry C-band（5.3 GHz） synthetic aperture radar（SAR）.During the period of August 2016 to December 2017, 1 523 GF-3 SAR images acquired in quad-polarization（vertical-vertical（VV）, horizontal-horizontal（HH）, vertical-horizontal（VH）, and horizontal-vertical（HV）） mode were recorded, mostly around China＇s seas. In our previous study, the root mean square error（RMSE） of significant wave height（SWH） was found to be around 0.58 m when compared with retrieval results from a few GF-3 SAR images in co-polarization（VV and HH） with moored measurements by using an empirical algorithm CSARWAVE. We collected a number of sub-scenes from these 1 523 images in the co-polarization channel,which were collocated with wind and SWH data from the European Centre for Medium-Range Weather Forecasts（ECMWF） reanalysis field at a 0.125° grid. Through the collected dataset, an improved empirical wave retrieval algorithm for GF-3 SAR in co-polarization was tuned, herein denoted as CSARWAVE2. An additional 92 GF-3 SAR images were implemented in order to validate CSARWAVE2 against SWH from altimeter Jason-2, showing an about 0.52 m RMSE of SWH for co-polarization GF-3 SAR. Therefore, we conclude that the proposed empirical algorithm has a good performance for wave retrieval from GF-3 SAR images in co-polarization.
基金supported by the National Key Basic Research and Development Plan (No.2015CB953900)the Natural Science Foundation of China (Nos.41330960 and 41776032)
文摘Version 4(v4) of the Extended Reconstructed Sea Surface Temperature(ERSST) dataset is compared with its precedent, the widely used version 3b(v3b). The essential upgrades applied to v4 lead to remarkable differences in the characteristics of the sea surface temperature(SST) anomaly(SSTa) in both the temporal and spatial domains. First, the largest discrepancy of the global mean SSTa values around the 1940 s is due to ship-observation corrections made to reconcile observations from buckets and engine intake thermometers. Second, differences in global and regional mean SSTa values between v4 and v3b exhibit a downward trend(around-0.032℃ per decade) before the 1940s, an upward trend(around 0.014℃ per decade) during the period of 1950–2015, interdecadal oscillation with one peak around the 1980s, and two troughs during the 1960s and 2000s, respectively. This does not derive from treatments of the polar or the other data-void regions, since the difference of the SSTa does not share the common features. Third, the spatial pattern of the ENSO-related variability of v4 exhibits a wider but weaker cold tongue in the tropical region of the Pacific Ocean compared with that of v3b, which could be attributed to differences in gap-filling assumptions since the latter features satellite observations whereas the former features in situ ones. This intercomparison confirms that the structural uncertainty arising from underlying assumptions on the treatment of diverse SST observations even in the same SST product family is the main source of significant SST differences in the temporal domain. Why this uncertainty introduces artificial decadal oscillations remains unknown.
基金supported by the National Natural Science Foundation of China (No. 41676003)the National Natural Science Foundation of China–Shandong Joint Fund for Marine Science Research Centers (No.U1606402)
文摘In this study, we examine the results obtained by the Finite-volume Coastal Ocean Circulation Model(FVCOM) regarding the effects of eddy viscosity and bathymetry on the three-dimensional(3 D) Lagrangian residual velocity(LRV) in a narrow bay. The results are cast in terms of two nondimensional numbers: the ratio of friction to local acceleration(δ) and the ratio of the minimum depth over shoals to the maximum depth in the channel(ε). The ratio δ depends on the eddy viscosity and mean depth. For a given eddy viscosity, when ε > 0.5, the along-estuary LRV tends to be vertically sheared and when ε < 0.5, the exchange is laterally sheared. When ε << 1, the structure of the 3 D, depth-integrated, and breadth-averaged LRV changes only slightly as δ increases. For ε values between 0.33 and 0.5, the structure of the 3 D LRV is mainly laterally sheared. In the same ε range, the 3 D and depth-integrated LRV exhibit reversed structures from high to low δ values. In addition, the breadth-averaged LRV weakens the typical twolayered circulation when δ decreases. When ε is 1, the two-layered vertical structure reverses direction, and a three-layered vertical structure develops in the outer bay as δ decreases.
基金Supported by the State Key Program of National Natural Science of China(Nos.41430963,U1706215)the National Science Foundation for Young Scientists of China(No.41506012)
文摘Several bottom-mounted Acoustic Doppler Current Profiler (ADCP) moorings were deployed in the northern Yellow Sea (NYS) during the four seasons of 2006–2007 and also the summertime of 2009. A synthesis analysis on the time-continuous records was performed to examine the characteristics and variations of tidal currents and mean flow over the observation period at these stations. Tidal currents accounted for ~75% of the total kinetic energy, with the absolute dominance of M2 constituent. Visible vertical variations of tidal flow were found on all sites, featured by the decrease of amplitude, increase of rotation rate as well as a decreasing trend of the phase for M2 component with depth. A notable exception was in the central NYS, where the maximum tidal currents occurred in the upper or middle layers (~20–40 m) instead of near the surface (<10 m). The observed mean flow was relatively weak, smaller than 15 cm/s. Velocity on the northern end of Yellow Sea Trough (YST) was characterized by low magnitude and an obvious layered structure vertically. In the Bohai Strait (BS) and the northern slope area, the currents weakened and the flow direction presented a major trend to deflect counterclockwise with depth in most observations. Summertime cyclonic circulation around the Yellow Sea Cold Water Mass (YSCWM), its intensification on the frontal zone and the Yellow Sea Warm Current (YSWC) for the winter season were all evident by our direct current measurements. However, the details of water exchange through the BS appeared partly diff erent from the traditionally-accepted pattern. The vertical diff erences of tidal and mean flow were larger in summer than that in winter, implying the influence of thermal structure to the local currents. Aff ected by the water stratification, mean flow usually reached its maximum near the thermocline in spring and summer, while showing a nearly uniform vertical distribution during winter.
基金Supported by the China National Global Change Major Research Project(No.2013CB956201)the National Science Foundation of China(NSFC)Key Project(No.41130859)+1 种基金the NSFC(Nos.41506009,41521091)the NSFC Major Project(No.41490643)
文摘Based on 22 of the climate models from phase 3 of the Coupled Model Intercomparison Project, we investigate the ability of the models to reproduce the spatiotemporal features of the wintertime North Pacific Oscillation(NPO), which is the second most important factor determining the wintertime sea level pressure field in simulations of the pre-industrial control climate, and evaluate the NPO response to the future most reasonable global warming scenario(the A1B scenario). We reveal that while most models simulate the geographic distribution and amplitude of the NPO pattern satisfactorily, only 13 models capture both features well. However, the temporal variability of the simulated NPO could not be significantly correlated with the observations. Further analysis indicates the weakened NPO intensity for a scenario of strong global warming is attributable to the reduced lower-tropospheric baroclinicity at mid-latitudes, which is anticipated to disrupt large-scale and low-frequency atmospheric variability, resulting in the diminished transfer of energy to the NPO, together with its northward shift.
基金Supported by the Fundamental Research Funds for the Central Public Welfare Research Institutes(No.TKS160224)the Key Research and Development Program of Shandong Province(No.2016ZDJS09A02)+2 种基金the National Key Research and Development Program of China(No.2016YFC1402304)the National Natural Science Foundation of China(No.41606006)the Open Fund of Key Laboratory of Engineering Sediment of the Ministry of Transport,Tianjin Research Institute for Water Transport Engineering,M.O.T
文摘The sea level variabilities, especially the atmosphere-driven sea level variabilities, which are diff erent in studies on diverse areas and timescales, need to be further documented in the Bohai Bay. Coastal sea level data and coincident meteorological data collected hourly at two observation stations (E1 and E2) in the Bohai Bay, which is a typical semi-enclosed coastal sea in China, are analyzed for the period from 19 August 2014 to 18 November 2014. The sub-sampled low-pass (<0.8 cpd) sea levels (SLSLs) at E1 and E2 are almost the same as each other, while the winds are not. On the whole, SLSLs at E1 and E2 are dominantly influenced by the across-shore wind;in detail, the dominant wind orientation at E1 is 65° measured clockwise from north, and SLSL at E2 is significantly influenced by the sub-sampled wind (SW) at 55°. Regression of SLSL onto the corresponding SW in dominant orientation and the atmospheric pressure is used to predict SLSL, which make the frequency of occurrences when the predicted total sea level is within 0.15 m from the observed values increase to 66.03% and 58.08% at E1 and E2 from original 36.71% and 34.80% without using it, respectively. The results indicate that for the prediction of the total sea level variability in the coastal shallow waters, the SLSL influenced by the atmospheric forcing, including local wind and atmospheric pressure, can be predicted using the multivariable linear regression model.
基金supported by the National Key Research and Development Program of China(Grant Nos.2017YFC1404105,2017YFC1404100,2017YFC1404101,2017YFC1404102,2017YFC1404103 and 2017YFC1404104)
文摘The‘Two Oceans and One Sea’area(West Pacific,Indian Ocean,and South China Sea;15°S–60°N,39°–178°E)is a core strategic area for the‘21st Century Maritime Silk Road’project,as well as national defense.With the increasing demand for disaster prevention and mitigation,the importance of 10–30-day extended range prediction,between the conventional short-term(around seven days)and the climate scale(longer than one month),is apparent.However,marine extended range prediction is still a‘blank point’in China,making the early warning of marine disasters almost impossible.Here,the authors introduce a recently launched Chinese national project on a numerical forecasting system for extended range prediction in the‘Two Oceans and One Sea’area based on a regional ultra-high resolution multi-layer coupled model,including the scientific aims,technical scheme,innovation,and expected achievements.The completion of this prediction system is of considerable significance for the economic development and national security of China.
基金supported by the National Natural Science Foundation of China[grant number 41430533],[grant number41375067]
文摘The physical cause of amplified deep Arctic tropospheric warming in winter in the Barents-Kara Seas （BKS） is examined. The authors propose that changes in the atmospheric circulation patterns are important for deep Arctic tropospheric warming in winter. It is found that the retrograde Urals blocking （UB） event concurrent with a negative North Atlantic Oscillation （NAO-） that arises from a prior negative Arctic Oscillation （AO-） is not favorable for tropospheric warming because of less water vapor over the BKS. Such UB events are related to more winter BKS sea ice associated with the negative sea surface temperature （SST） anomaly in the BKS. In contrast, a UB occurring together with a positive North Atlantic Oscillation （NAO~） shows less movement and can significantly enhance tropospheric warming over the BKS through increasing tropospheric sensible heat energy due to a persistent BKS water vapor increase. This type of quasi-stationary UB event is related to prior less BKS sea ice associated with a positive BKS SST anomaly that coexists with the North Atlantic SST tripole structure. In summary, because warm, wet and low sea-ice winters in the BKS are related to UB events with an NAO~, and depend on the winter prior sea-ice condition, the tropospheric warming is to some extent a manifestation of the sea-ice-blocking-moisture feedback in the BKS.
基金Supported by the Natural Science Foundation of Shandong Province of China(No.ZR2014DM017)the Natural Science Foundation of Zhejiang Province(No.LY15D060001)+4 种基金the National High Technology Research and Development Program of China(863 Program)(No.2013AA09A502)the National Natural Science Foundation of China(Nos.41606006,41371496)the National Key Technology Research and Development Program(No.2013BAK05B04)the 111 Project of Ministry of Education of China(No.B07036)the China Postdoctoral Science Foundation(No.2017M611979)
文摘Near-inertial waves(NIWs), which can be generated by wind or the parametric subharmonic instability(PSI) of internal tides, are common in the South China Sea(SCS). Moored current observations from the northern SCS have revealed that the PSI of semidiurnal(D_2) internal tides is another source of NIWs. The objective of this study was to examine the energy variance in the PSI of D_2 tides. The PSI of D_2 internal tides generated NIWs and waves with frequencies around the difference frequency of D_2 and f. The observed NIWs induced by PSI could be distinguished clearly from those elicited by typhoon Krosa. Shortly after Krosa entered the SCS, NIWs began to intensify on the surface and they propagated downward over subsequent days. The near-inertial currents were damped quickly and they became relatively weak before the waves were reinforced beneath the mixed layer when wind stress was relatively weak. Rotation spectra indicated an energy peak at exactly the difference frequency D_2–f of the NIWs and D_2, indicating nonlinear wave-wave interaction among D_2, f, and D_2–f. Depth-time maps of band-pass fi ltered velocities of D_2 –f showed the waves amplifi ed when the NIWs were reinforced, and they intensifi ed at depths with strong D_2 tides. The energies of the NIWs and D_2 –f had high correlation with the D_2 tides. The PSI transferred energy of low-mode D_2 internal tides to high-mode NIWs and D_2–f waves. For the entire observational period, PSI reinforcement was observed only when mesoscale eddies emerged and when D_2 was in spring tide, revealing a close connection between mesoscale eddies and NIWs. Mesoscale eddies could increase the energy in the f-band by enhancing the PSI of D_2 internal tides. Thus, this represents another mechanism linking the energy of mesoscale eddies to that of NIWs.
基金Supported by the China Postdoctoral Science Foundation(No.2017M611979)the National Natural Science Foundation of China(Nos.41606006,41371496)+3 种基金the Natural Science Foundation of Zhejiang Province(No.LY15D060001)the National High Technology Research and Development Program of China(863 Program)(No.2013AA09A502)the National Key Technology Research and Development Program(No.2013BAK05B04)the 111 Project of Ministry of Education of China(No.B07036)
文摘A nine-month mooring record was used to investigate seasonal variation and modal content of internal tides(ITs) on the continental slope in the northern South China Sea(SCS). Diurnal tides at this site show clear seasonal change with higher energy in winter than in spring and autumn, whereas semidiurnal tides show the opposite seasonal pattern. The consistency of ITs with barotropic tides within the Luzon Strait, which is the generation region of the ITs, implies that the seasonal variation of ITs depends on their astronomical forcing, even after extended propagation across the SCS basin. Diurnal tides also differ from semidiurnal tides in relation to modal content; they display signals of high modes while semidiurnal tides are dominated by low modes. Reflection of the diurnal tides on the continental slope serves as a reasonable explanation for their high modes. Both diurnal and semidiurnal tides are composed of a larger proportion of coherent components that have a regular 14-day spring-neap cycle. The coherent components are dominated by low modes and they show obvious seasonal variation, while the incoherent components are composed mainly of higher modes and they display intermittent characteristics.
基金Supported by the National Natural Science Foundation of China(Nos.41606005,41430963,41676004)the National Program on Global Change and Air-Sea Interaction(No.GASI-GEOGE-03)+1 种基金the Liaoning Revitalization Talents Program(No.XLYC1807161)the Dalian Highlevel Talents Innovation Support Plan(No.2017RQ063)。
文摘The vertical mixing parameterization scheme,by providing the eff ects of some explicitly missed physical processes and more importantly closing the energy budgets,is a critical model component and therefore imposes signifi cant impacts on model performance.The Yellow Sea Cold Water Mass(YSCWM),as the most striking and unique phenomenon in the Yellow Sea during summer,is dramatically aff ected by vertical mixing process during its each stage and therefore seriously sensitive to the proper choice of parameterization scheme.In this paper,a hindcast of YSCWM in winter of 2006 was implemented by using the Regional Ocean Modeling System(ROMS).Three popular parameterization schemes,including the level 2.5 Mellor-Yamada closure(M-Y 2.5),Generic Length Scale closure(GLS)and K-Profi le Parameterization(KPP),were tested and compared with each other by conducting a series of sensitivity model experiments.The infl uence of diff erent parameterization schemes on modeling the YSCWM was then carefully examined and assessed based on these model experiments.Although reasonable thermal structure and its seasonal variation were well reproduced by all schemes,considerable diff erences could still be found among all experiments.A warmer and spatially smaller simulation of YSCWM,with very strong thermocline,appeared in M-Y 2.5 experiment,while a spatially larger YSCWM with shallow mixed layer was found in GLS and KPP schemes.Among all the experiments,the discrepancy,indicated by core temperature,appeared since spring,and grew gradually by the end of November.Additional experiments also confi rmed that the increase of background diff usivity could eff ectively weaken the YSCWM,in either strength or coverage.Surface wave,another contributor in upper layer,was found responsible for the shrinkage of YSCWM coverage.The treatment of wave eff ect as an additional turbulence production term in prognostic equation was shown to be more superior to the strategy of directly increasing diff usivity for a coastal region.
基金supported by the National Natural Science Foundation of China under Grant Nos. 41276013 41576004 and U1406401funded by the China Scholarship Council to visit Texas A&M Uni versity for one year under file No. 201406335002
文摘Tropical Atlantic climate change is relevant to the variation of Atlantic meridional overturning circulation(AMOC) through different physical processes. Previous coupled climate model simulation suggested a dipole-like SST structure cooling over the North Atlantic and warming over the South Tropical Atlantic in response to the slowdown of the AMOC. Using an ocean-only global ocean model here, an attempt was made to separate the total influence of various AMOC change scenarios into an oceanicinduced component and an atmospheric-induced component. In contrast with previous freshwater-hosing experiments with coupled climate models, the ocean-only modeling presented here shows a surface warming in the whole tropical Atlantic region and the oceanic-induced processes may play an important role in the SST change in the equatorial south Atlantic. Our result shows that the warming is partly governed by oceanic process through the mechanism of oceanic gateway change, which operates in the regime where freshwater forcing is strong, exceeding 0.3 Sv. Strong AMOC change is required for the gateway mechanism to work in our model because only when the AMOC is sufficiently weak, the North Brazil Undercurrent can flow equatorward, carrying warm and salty north Atlantic subtropical gyre water into the equatorial zone. This threshold is likely to be model-dependent. An improved understanding of these issues may have help with abrupt climate change prediction later.
基金funded by the National Natural Science Foundation of China (Grant no.41276193)the Global Change Research Program of China (Grant no.2015CB953901)the National Key Research and Development Program of China (Grant no.2016YFC1402704)
文摘Melt ponds on Arctic sea ice are of great significance in the study of the heat balance in the ocean mixed layer, mass and salt balances of Arctic sea ice, and other aspects of the earth-atmosphere system. During the 7th Chinese National Arctic Research Expedition, aerial photographs were taken from an Unmanned Aerial Vehicle over an ice floe in the Canada Basin. Using threshold discrimination and three-dimensional modeling, we estimated a melt pond fraction of 1.63% and a regionally averaged surface roughness of 0.12 for the study area. In view- of the particularly foggy environment of the Arctic, aerial images were defogged using an improved dark channel prior based image defog algorithm, especially adapted for the special conditions of sea ice images. An aerial photo mosaic was generated, melt ponds were identified from the mosaic image and melt pond fractions were calculated. Three-dimensional modeling techniques were used to generate a digital elevation model allowing relative elevation and roughness of the sea ice surface to be estimated. Analysis of the relationship between the distributions of melt ponds and sea ice surface roughness show-s that melt ponds are smaller on sea ice with higher surface roughness, while broader melt ponds usually occur in areas where sea ice surface roughness is lower.
基金Supported by the China’s National Key Research and Development Projects(No.2016YFA0601803)the National Natural Science Foundation of China(Nos.41490641,41521091,U1606402)the Qingdao National Laboratory for Marine Science and Technology(No.2017ASKJ01)
文摘The co-variation of surface wind speed and sea surface temperature (SST) over the Gulf Stream frontal region is investigated using high-resolution satellite measurements and atmospheric reanalysis data. Results show that the pattern of positive SST-surface wind speed correlations is anchored by strong SST gradient and marine atmospheric boundary layer (MABL) height front, with active warm and cold-ocean eddies around. The MABL has an obvious transitional structure along the strong SST front, with greater (lesser) heights over the north (south) side. The significant positive SST-surface wind-speed perturbation correlations are mostly found over both strong warm and cold eddies. The surface wind speed increases (decreases) about 0.32 (0.41) m/s and the MABL elevates (drops) approximate 55 (54) m per 1℃ of SST perturbation induced by warm (cold) eddies. The response of the surface wind speed to SST perturbations over the mesoscale eddies is mainly attributed to the momentum vertical mixing in the MABL, which is confirmed by the linear relationships between the downwind (crosswind) SST gradient and wind divergence (curl).
基金Supported by the China’s National Key Research and Development Project(No.2016YFA0601803)the National Natural Science Foundation of China(Nos.41490641,41521091,U1606402)the Qingdao National Laboratory for Marine Science and Technology(No.2017ASKJ01)
文摘Paleo reconstructions and model simulations have suggested the Bering Strait plays a pivotal role in climate change. However, the contribution of the Bering Strait throughflow to oceanic meridional heat transport (OMHT) is about 100 times smaller than the OMHT at low latitudes in the modern climate and it is generally ignored. Based on model simulations under modern and Last Glacial Maximum (LGM,~21 ka;ka=thousand years ago) climate conditions, this study highlights the importance of the Bering Strait throughflow to OMHT. The interbasin OMHT induced by the Bering Strait throughflow is estimated by interbasin-intrabasin decomposition. Similar to barotropic-baroclinic-horizontal decomposition, we assume the nonzero net mass transport induced by interbasin throughflows is uniform across the entire section, and the interbasin term is separated to force zero net mass transport for the intrabasin term. Based on interbasinintrabasin decomposition, the contribution of the Bering Strait throughflow is determined as ~0.02 PW (1 PW=10 15 W) under the modern climate, and zero under the LGM climate because the closed Bering Strait blocked interbasin throughflows. The contribution of the Bering Strait throughflow to OMHT is rather small, consistent with previous studies. However, comparisons of OMHT under modern and LGM climate conditions indicate the mean absolute changes are typically 0.05 and 0.20 PWin the North Atlantic and North Pacific, respectively. Thus, the contribution of the Bering Strait throughflow should not be ignored when comparing OMHT under diff erent climate conditions.
