Using 20 models of the Coupled Model Intercomparison Project Phase 5 （CMIP5）, the simulation of the Southwest Indian Ocean （SWIO） thermocline dome is evaluated and its role in shaping the Indian Ocean Basin （IOB...Using 20 models of the Coupled Model Intercomparison Project Phase 5 （CMIP5）, the simulation of the Southwest Indian Ocean （SWIO） thermocline dome is evaluated and its role in shaping the Indian Ocean Basin （IOB） mode following E1 Nifio investigated. In most of the CMIP5 models, due to an easterly wind bias along the equator, the simulated SWIO thermocline is too deep, which could further influence the amplitude of the interannual IOB mode. A model with a shallow （deep） thermocline dome tends to simulate a strong （weak） IOB mode, including key attributes such as the SWIO SST warming, antisymmetric pattern during boreal spring, and second North Indian Ocean warming during boreal summer. Under global warming, the thermocline dome deepens with the easterly wind trend along the equator in most of the models. However, the IOB amplitude does not follow such a change of the SWIO thermocline among the models; rather, it follows future changes in both ENSO forcing and local convection feedback, suggesting a decreasing effect of the deepening SWIO thermocline dome on the change in the IOB mode in the future.展开更多
This study investigates the interannual relationship between El Nino-Southern Oscillation(ENSO)and tropical cyclone(TC)genesis potential(measured by the Dynamic Genesis Potential Index or DGPI)over the Southwest India...This study investigates the interannual relationship between El Nino-Southern Oscillation(ENSO)and tropical cyclone(TC)genesis potential(measured by the Dynamic Genesis Potential Index or DGPI)over the Southwest Indian Ocean(SWIO)and its modulation by the Interdecadal Pacific Oscillation(IPO)during 1959-2020 based on observation and reanalysis.Results show that DGPI can well capture the spatial distribution and seasonal cycle of TC genesis over the SWIO.A positive correlation is found between ENSO and the SWIODGPI on interannual timescales,with higher(lower)DGPI and increased(decreased)TC occurrence frequency during El Nino(La Nina)years,indicating an important role of ENSO in modulating TC activity in the SWIO.Further analyses show that ENSO favors the SWIO TC genesis primarily through modulating vertical wind shear and upward motion.Furthermore,the interannual relationship between ENSO and the SWIO DGPI also exhibits remarkable interdecadal variations that are likely modulated by the IPO with strong(weak)correlation in positive(negative)IPO phases.This is because the ENSO-driven anomalies of vertical wind shear,upward motion,and low-level vorticity are larger under positive IPO phases than its negative phases,which favor the sensitivity of the SWIO TC genesis to ENSO and thus promote a higher correlation between the two.These findings highlight the importance of considering interdecadal variability when assessing the ENSO-TC relationship over the SWIO and contribute to improving seasonal prediction of TCs in the region.展开更多
Regardless of the slowdown in global warming during the hiatus period, sea surface temperatures(SSTs) in the southwestern Indian Ocean(SWIO) have experienced sustained decadal warming for more than two decades since t...Regardless of the slowdown in global warming during the hiatus period, sea surface temperatures(SSTs) in the southwestern Indian Ocean(SWIO) have experienced sustained decadal warming for more than two decades since the mid-1990 s. The SWIO SSTs warmed steadily during 1996–2016, causing a warming hot spot of 0.4 K decade-1 in a large region east of Madagascar. An upper-layer heat budget analysis indicated that heat advection by ocean currents was the greatest contributor to the warming of the SWIO SSTs. The existence of an anticyclonic geostrophic current along the western boundary of the SWIO tended to maintain such warming by transporting warmer water from the west into the SWIO region. In addition, net positive heat transport by ocean currents also occurred at the southern boundary of the SWIO as the climatological northward transport of cold water from the Southern Ocean weakened. This reduction in northward ocean currents at the surface was caused by local wind stress changes, leading to a southward Ekman current. Below the surface, an anticyclonic geostrophic current pattern existed around the warming center near the southeastern SWIO, which reduced the transport of cold waters from the Southern Ocean and warmed the SWIO. These processes near the two boundaries formed a self-sustaining positive feedback mechanism and favored the maintenance of sustained warming in the SWIO. More attention is needed to analyze the sustained long-lasting warming in the SWIO, as it is a unique phenomenon occurring under the background of the ongoing global warming.展开更多
During February–March 2023,the record-breaking tropical cyclone(TC)Freddy caused widespreadflooding and damages across southeastern Africa.While<5%of TCs make landfall into southern Africa,TC Freddy made landfall t...During February–March 2023,the record-breaking tropical cyclone(TC)Freddy caused widespreadflooding and damages across southeastern Africa.While<5%of TCs make landfall into southern Africa,TC Freddy made landfall twice and is the only TC in the past two decades that has tracked over 8000 km across the entire southern Indian Ocean.To understand why TC Freddy was so unique,this study investigated the evolution,track and atmospheric-oceanic mechanisms driving TC Freddy using the ERA5,CFSv2,OSTIA,NCEP-NCAR datasets and track data from various sources.It was found that SSTs were>27◦C during TC Freddy’s lifetime,while TC Dingani and a split Mascarene High played a role in steering TC Freddy across the southern Indian Ocean.Leading up to the development of TC Freddy,conditions were favourable for TC genesis,as indicated by the levels of the Genesis Potential Parameter(GPP)and its modified version(GPPI),the tropical cyclone heat potential levels,and elevated SSTs.Ridging subtropical anticyclones and the Mascarene High alongside favourable steeringflow and GPP(and GPPI)conditions resulted in Freddy’s double landfall in Mozambique.In assessing the tracks,it was found that there are discrepancies in the track of the commonly used IBTrACS when compared to ERA5 and RSMC tracks,which has implications for impact studies due to the underestimation of landfall considerations.This study reveals the unique characteristics and atmospheric-oceanic mechanisms driving TC Freddy,emphasising the impor-tance of accurate representation of favourable conditions and track data for enhancing TC forecasting and impact assessments.展开更多
基金supported by the National Basic Research Program of China (Grant Nos.2012CB955600 and 2015CB954300)the National Natural Science Foundation of China (Grant Nos. 41106010 and 41476003)+1 种基金the State Key Laboratory of Tropical Oceanography, Chinese Academy of Sciences (Grant Nos. LTO1206 and LTOZZ1202)a China Meteorological Public Welfare Science Research Project (Grant No. GYHY201306027)
文摘Using 20 models of the Coupled Model Intercomparison Project Phase 5 （CMIP5）, the simulation of the Southwest Indian Ocean （SWIO） thermocline dome is evaluated and its role in shaping the Indian Ocean Basin （IOB） mode following E1 Nifio investigated. In most of the CMIP5 models, due to an easterly wind bias along the equator, the simulated SWIO thermocline is too deep, which could further influence the amplitude of the interannual IOB mode. A model with a shallow （deep） thermocline dome tends to simulate a strong （weak） IOB mode, including key attributes such as the SWIO SST warming, antisymmetric pattern during boreal spring, and second North Indian Ocean warming during boreal summer. Under global warming, the thermocline dome deepens with the easterly wind trend along the equator in most of the models. However, the IOB amplitude does not follow such a change of the SWIO thermocline among the models; rather, it follows future changes in both ENSO forcing and local convection feedback, suggesting a decreasing effect of the deepening SWIO thermocline dome on the change in the IOB mode in the future.
文摘This study investigates the interannual relationship between El Nino-Southern Oscillation(ENSO)and tropical cyclone(TC)genesis potential(measured by the Dynamic Genesis Potential Index or DGPI)over the Southwest Indian Ocean(SWIO)and its modulation by the Interdecadal Pacific Oscillation(IPO)during 1959-2020 based on observation and reanalysis.Results show that DGPI can well capture the spatial distribution and seasonal cycle of TC genesis over the SWIO.A positive correlation is found between ENSO and the SWIODGPI on interannual timescales,with higher(lower)DGPI and increased(decreased)TC occurrence frequency during El Nino(La Nina)years,indicating an important role of ENSO in modulating TC activity in the SWIO.Further analyses show that ENSO favors the SWIO TC genesis primarily through modulating vertical wind shear and upward motion.Furthermore,the interannual relationship between ENSO and the SWIO DGPI also exhibits remarkable interdecadal variations that are likely modulated by the IPO with strong(weak)correlation in positive(negative)IPO phases.This is because the ENSO-driven anomalies of vertical wind shear,upward motion,and low-level vorticity are larger under positive IPO phases than its negative phases,which favor the sensitivity of the SWIO TC genesis to ENSO and thus promote a higher correlation between the two.These findings highlight the importance of considering interdecadal variability when assessing the ENSO-TC relationship over the SWIO and contribute to improving seasonal prediction of TCs in the region.
基金the National Key Research and Development Program of China (2016YFA0600602)National Natural Science Foundation of China (41776039)。
文摘Regardless of the slowdown in global warming during the hiatus period, sea surface temperatures(SSTs) in the southwestern Indian Ocean(SWIO) have experienced sustained decadal warming for more than two decades since the mid-1990 s. The SWIO SSTs warmed steadily during 1996–2016, causing a warming hot spot of 0.4 K decade-1 in a large region east of Madagascar. An upper-layer heat budget analysis indicated that heat advection by ocean currents was the greatest contributor to the warming of the SWIO SSTs. The existence of an anticyclonic geostrophic current along the western boundary of the SWIO tended to maintain such warming by transporting warmer water from the west into the SWIO region. In addition, net positive heat transport by ocean currents also occurred at the southern boundary of the SWIO as the climatological northward transport of cold water from the Southern Ocean weakened. This reduction in northward ocean currents at the surface was caused by local wind stress changes, leading to a southward Ekman current. Below the surface, an anticyclonic geostrophic current pattern existed around the warming center near the southeastern SWIO, which reduced the transport of cold waters from the Southern Ocean and warmed the SWIO. These processes near the two boundaries formed a self-sustaining positive feedback mechanism and favored the maintenance of sustained warming in the SWIO. More attention is needed to analyze the sustained long-lasting warming in the SWIO, as it is a unique phenomenon occurring under the background of the ongoing global warming.
基金the funding obtained by the first author,from the South African National Research Foundation(NRFGrant number:PMDS22092860092).
文摘During February–March 2023,the record-breaking tropical cyclone(TC)Freddy caused widespreadflooding and damages across southeastern Africa.While<5%of TCs make landfall into southern Africa,TC Freddy made landfall twice and is the only TC in the past two decades that has tracked over 8000 km across the entire southern Indian Ocean.To understand why TC Freddy was so unique,this study investigated the evolution,track and atmospheric-oceanic mechanisms driving TC Freddy using the ERA5,CFSv2,OSTIA,NCEP-NCAR datasets and track data from various sources.It was found that SSTs were>27◦C during TC Freddy’s lifetime,while TC Dingani and a split Mascarene High played a role in steering TC Freddy across the southern Indian Ocean.Leading up to the development of TC Freddy,conditions were favourable for TC genesis,as indicated by the levels of the Genesis Potential Parameter(GPP)and its modified version(GPPI),the tropical cyclone heat potential levels,and elevated SSTs.Ridging subtropical anticyclones and the Mascarene High alongside favourable steeringflow and GPP(and GPPI)conditions resulted in Freddy’s double landfall in Mozambique.In assessing the tracks,it was found that there are discrepancies in the track of the commonly used IBTrACS when compared to ERA5 and RSMC tracks,which has implications for impact studies due to the underestimation of landfall considerations.This study reveals the unique characteristics and atmospheric-oceanic mechanisms driving TC Freddy,emphasising the impor-tance of accurate representation of favourable conditions and track data for enhancing TC forecasting and impact assessments.
