Interannual variability of the Antarctic Circumpolar Current (ACC) strength is studied in stream-coordinate with twenty-year Absolute Dynamic Topography data from satellite altimetry. The stream-coordinate projection ...Interannual variability of the Antarctic Circumpolar Current (ACC) strength is studied in stream-coordinate with twenty-year Absolute Dynamic Topography data from satellite altimetry. The stream-coordinate projection method separates the ACC from adjacent subtropical and subpolar gyres, enabling consideration of the zonal asymmetry of the ACC rather than assuming that the ACC is a purely zonal flow. It is shown that the ACC strength has large interannual variations with two recent peaks around 2000 and 2009. The interannual variability appears mainly in the Indo-Pacific sector of the Southern Ocean and the strongest signal is located south of Australia. The intensification of the westerly wind in 1998 and 2008 appears to cause the strengthening of the ACC via baroclinic processes.展开更多
The Kuroshio inflow northeast of Taiwan Island plays an important role in the heat and nutrient balances over the East China Sea(ECS). Based on merged satellite altimeter data and the PCM-1 mooring observation at the ...The Kuroshio inflow northeast of Taiwan Island plays an important role in the heat and nutrient balances over the East China Sea(ECS). Based on merged satellite altimeter data and the PCM-1 mooring observation at the East Taiwan Channel(ETC), the study employs a correlation iteration scheme to find the optimal transport index for the Kuroshio inflow. The sea level difference with the highest correlation to the ETC transport is across the ECS shelf break rather than along the PCM-1 line. The counter-intuitive result is caused by large signal noise and poor track coverage of altimeters near the Taiwan coast. The optimal altimetric index is highly correlated with the two-year in-situ measurements as well as the ten-year output of the global assimilation model. It serves as a better estimator of Kuroshio inflow than those using tidal gauge data, and helps pinpoint a 5 cm mismatch of mean sea level in the Keelung tidal record. The mean transport of Kuroshio inflow based on the twenty-year altimetric index is 20.55 Sv with a standard deviation of 3.05 Sv. Wavelet spectrum of the index reveals that semi-annual period dominates the Kuroshio variation northeast of Taiwan Island.展开更多
Recent discovery of low-dimensional coherent structure in oceanic currents along with a new merged altimeter product called Absolute Dynamic Topography (ADT) makes it possible to derive subsurface ocean information fr...Recent discovery of low-dimensional coherent structure in oceanic currents along with a new merged altimeter product called Absolute Dynamic Topography (ADT) makes it possible to derive subsurface ocean information from satellite remote sensing data.An altimetric geostrophic empirical mode (η-GEM) is developed in this study by projecting hydrographic transects onto the ADT sea surface height coordinate,based on which the four-dimensional thermohaline and velocity structures of oceanic currents are reconstructed from satellite surface observations.In the WOCE/SR3 area,the η-GEM fields capture more than 95% of the total thermal variance.The GEM-derived flow has equivalent-barotropic structure and represents the velocity profile better than traditional dynamic modes.Comparison with mooring observations also demonstrates that the η-GEM provides good estimates of the deep thermohaline fields.展开更多
A streamfunction EOF method is applied to a time series of hydrographic sections in the Southern Ocean south of Australia to study water mass variations. Results show that there are large thermohaline variations north...A streamfunction EOF method is applied to a time series of hydrographic sections in the Southern Ocean south of Australia to study water mass variations. Results show that there are large thermohaline variations north of the Subantarctic Front (SAF) at 300–1500 dbar level, indicating upwelling and downwelling of the Antarctic Intermediate Water (AAIW) along isopycnal surfaces. Based on the latest altimeter product, Absolute Dynamic Topography, a mechanism due to frontal wave propagation is proposed to explain this phenomenon, and an index for frontal waves is defined. When the frontal wave is in positive (negative) phase, the SAF flows northeastward (southeastward) with the fresh AAIW downwelling (upwelling). Such mesoscale processes greatly enhance cross-frontal exchanges of water masses. Spectral analysis shows that frontal waves in the Southern Ocean south of Australia are dominated by a period of about 130 days with a phase speed of 4 cm/s and a wavelength of 450 km.展开更多
A streamfunction projection method called gravest empirical mode(GEM) is applied to the hydrographic section at 137°E to filter out eddy noises in the western North Pacific and derive quantitative ensemble-averag...A streamfunction projection method called gravest empirical mode(GEM) is applied to the hydrographic section at 137°E to filter out eddy noises in the western North Pacific and derive quantitative ensemble-average water mass properties in the North Equatorial Current region. The GEM fields capture more than 80% of total property variances in the thermocline layer. The core layer structures of key water masses, including the North Pacific Tropical Water(NPTW) and the North Pacific Intermediate Water(NPIW), are examined with a definition of water mass boundary based on property gradient. It shows that a tongue of maximal root-mean-square(RMS) residual exists in the upper half of NPIW for all water properties. These subsurface RMS tongues appear to be close to sharp property gradients. It is the first time a GEM diagnosis is applied to nutrient data, which reveals a drastic difference of N/P reaction rate ratio above and below the maximal-nutrient core at 1250 m. Additionally, a GEM velocity reconstruction successfully produces the North Equatorial Undercurrent(NEUC), demonstrating the stable thermal-wind nature of this newly-discovered current.展开更多
The vertical thermohaline structure in the western equatorial Pacific is examined with a Gravest Empirical Mode(GEM)diagnosis of in-situ mooring measurements. The poor GEM performance in estimating deep thermohaline v...The vertical thermohaline structure in the western equatorial Pacific is examined with a Gravest Empirical Mode(GEM)diagnosis of in-situ mooring measurements. The poor GEM performance in estimating deep thermohaline variability from satellite altimetry confirms a lack of vertical coherence in the equatorial ocean. Mooring observation reveals layered equatorial water with phase difference up to 6 months between thermocline and sub-thermocline variations. The disjointed layers reflect weak geostrophy and resemble pancake structures in non-rotating stratified turbulence. A coherency theorem is then proved, stating that traditional stationary GEM represents in-phase coherent structure and can not describe vertically out-of-phase variability. The fact that stationary GEM holds both spatial and temporal coherence makes it a unique tool to diagnose vertical coherent structure in geophysical flows. The study also develops a non-stationary GEM projection that captures more than 40% of the thermohaline variance in the equatorial deep water.展开更多
基金supported by the Knowledge Innovation Program of Chinese Academy of Sciences (KZCX2-YW-Q11-02)the National Natural Science Foundation of China(41006114 and 40890151)
文摘Interannual variability of the Antarctic Circumpolar Current (ACC) strength is studied in stream-coordinate with twenty-year Absolute Dynamic Topography data from satellite altimetry. The stream-coordinate projection method separates the ACC from adjacent subtropical and subpolar gyres, enabling consideration of the zonal asymmetry of the ACC rather than assuming that the ACC is a purely zonal flow. It is shown that the ACC strength has large interannual variations with two recent peaks around 2000 and 2009. The interannual variability appears mainly in the Indo-Pacific sector of the Southern Ocean and the strongest signal is located south of Australia. The intensification of the westerly wind in 1998 and 2008 appears to cause the strengthening of the ACC via baroclinic processes.
基金supported by the National Basic Research Program of China(Grant No.2012CB417400)the National Natural Science Foundation of China(Grant Nos.41421005,U1406401)
文摘The Kuroshio inflow northeast of Taiwan Island plays an important role in the heat and nutrient balances over the East China Sea(ECS). Based on merged satellite altimeter data and the PCM-1 mooring observation at the East Taiwan Channel(ETC), the study employs a correlation iteration scheme to find the optimal transport index for the Kuroshio inflow. The sea level difference with the highest correlation to the ETC transport is across the ECS shelf break rather than along the PCM-1 line. The counter-intuitive result is caused by large signal noise and poor track coverage of altimeters near the Taiwan coast. The optimal altimetric index is highly correlated with the two-year in-situ measurements as well as the ten-year output of the global assimilation model. It serves as a better estimator of Kuroshio inflow than those using tidal gauge data, and helps pinpoint a 5 cm mismatch of mean sea level in the Keelung tidal record. The mean transport of Kuroshio inflow based on the twenty-year altimetric index is 20.55 Sv with a standard deviation of 3.05 Sv. Wavelet spectrum of the index reveals that semi-annual period dominates the Kuroshio variation northeast of Taiwan Island.
基金supported by National Basic Research Program of China(Grant Nos. 2007CB411804 and 2012CB417401)Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KZCX2-YW-Q11-02)National Natural Science Foundation of China (Grant No.41006114)
文摘Recent discovery of low-dimensional coherent structure in oceanic currents along with a new merged altimeter product called Absolute Dynamic Topography (ADT) makes it possible to derive subsurface ocean information from satellite remote sensing data.An altimetric geostrophic empirical mode (η-GEM) is developed in this study by projecting hydrographic transects onto the ADT sea surface height coordinate,based on which the four-dimensional thermohaline and velocity structures of oceanic currents are reconstructed from satellite surface observations.In the WOCE/SR3 area,the η-GEM fields capture more than 95% of the total thermal variance.The GEM-derived flow has equivalent-barotropic structure and represents the velocity profile better than traditional dynamic modes.Comparison with mooring observations also demonstrates that the η-GEM provides good estimates of the deep thermohaline fields.
基金supported by the National Natural Science Foundation of China (Grant No. 41006114)the National Science and Technology Major Project (Grant No. 2012YQ12003907)+2 种基金the National Key Basic Research Program of China (Grant Nos. 2012CB417401 and 2013CB956202)the CAS Strategic Priority Research Program (Grant No. XDA11010101)the NSFC-Shandong Joint Fund for Marine Science Research Center (Grant No. U1406401)
文摘A streamfunction EOF method is applied to a time series of hydrographic sections in the Southern Ocean south of Australia to study water mass variations. Results show that there are large thermohaline variations north of the Subantarctic Front (SAF) at 300–1500 dbar level, indicating upwelling and downwelling of the Antarctic Intermediate Water (AAIW) along isopycnal surfaces. Based on the latest altimeter product, Absolute Dynamic Topography, a mechanism due to frontal wave propagation is proposed to explain this phenomenon, and an index for frontal waves is defined. When the frontal wave is in positive (negative) phase, the SAF flows northeastward (southeastward) with the fresh AAIW downwelling (upwelling). Such mesoscale processes greatly enhance cross-frontal exchanges of water masses. Spectral analysis shows that frontal waves in the Southern Ocean south of Australia are dominated by a period of about 130 days with a phase speed of 4 cm/s and a wavelength of 450 km.
基金supported by the National Basic Research Program of China(Grant No.2012CB417400)the National Natural Science Foundation of China(Grant Nos.41421005,U1406401)
文摘A streamfunction projection method called gravest empirical mode(GEM) is applied to the hydrographic section at 137°E to filter out eddy noises in the western North Pacific and derive quantitative ensemble-average water mass properties in the North Equatorial Current region. The GEM fields capture more than 80% of total property variances in the thermocline layer. The core layer structures of key water masses, including the North Pacific Tropical Water(NPTW) and the North Pacific Intermediate Water(NPIW), are examined with a definition of water mass boundary based on property gradient. It shows that a tongue of maximal root-mean-square(RMS) residual exists in the upper half of NPIW for all water properties. These subsurface RMS tongues appear to be close to sharp property gradients. It is the first time a GEM diagnosis is applied to nutrient data, which reveals a drastic difference of N/P reaction rate ratio above and below the maximal-nutrient core at 1250 m. Additionally, a GEM velocity reconstruction successfully produces the North Equatorial Undercurrent(NEUC), demonstrating the stable thermal-wind nature of this newly-discovered current.
基金supported by the National Basic Research Program of China (Grant No.2012CB417400)the National Natural Science Foundation of China (Grant Nos. 41576017 & U1406401)
文摘The vertical thermohaline structure in the western equatorial Pacific is examined with a Gravest Empirical Mode(GEM)diagnosis of in-situ mooring measurements. The poor GEM performance in estimating deep thermohaline variability from satellite altimetry confirms a lack of vertical coherence in the equatorial ocean. Mooring observation reveals layered equatorial water with phase difference up to 6 months between thermocline and sub-thermocline variations. The disjointed layers reflect weak geostrophy and resemble pancake structures in non-rotating stratified turbulence. A coherency theorem is then proved, stating that traditional stationary GEM represents in-phase coherent structure and can not describe vertically out-of-phase variability. The fact that stationary GEM holds both spatial and temporal coherence makes it a unique tool to diagnose vertical coherent structure in geophysical flows. The study also develops a non-stationary GEM projection that captures more than 40% of the thermohaline variance in the equatorial deep water.
