Search

The thermocline depth in the tropical Indian Ocean has experienced dramatic decadal variations in recent decades. Using analysis and reanalysis datasets, we find that the decadal thermocline depth anomalies show large seasonal differences. The seasonal differences are modulated by two major modes. The first mode shows a zonal dipole pattern, with opposite thermocline depth anomalies in the equatorial eastern Indian Ocean and western Indian Ocean, and is prominent in summer and winter. The second mode is characterized by marked thermocline depth anomalies in the southern Indian Ocean and is significant in spring and fall. The amplitude of the seasonal oscillation in these two modes has increased substantially in the twenty-first century. Their phase change is in good agreement with the observed thermocline depth anomalies in each season. The results also show that the seasonality of the decadal thermocline depth anomalies arises directly from surface wind variations within the Indian Ocean. The first mode is mainly caused by equatorial zonal wind anomalies. The second mode is dominated by local wind stress curl anomalies. These wind anomalies are both significantly correlated with the ENSO-like SST anomalies in the Pacific Ocean. The findings improve our understanding of the decadal thermocline anomalies, and will help to better evaluate their impact on seasonal phase-locked oceanic and atmospheric processes.

The thermocline-sea surface temperature (SST) feedback is the most important component of the Bjerknes feedback, which plays an important role in the development of the air-sea coupling modes of the Indian Ocean. The thermocline-SST feedback in the Indian Ocean has experienced significant decadal variations over the last 40 a. The feedback intensified in the late twentieth century and then weakened during the hiatus in global warming at the early twenty-first century. The thermoclidal variations of feedback are similar in these two regions, there are still differences in the underlying mechanisms. The decadal variations of feedback in the southeastern Indian Ocean are dominated by variations in the depth of the thermocline, which are modulated by equatorial zonal wind anomalies. Whereas the decadal variation of feedback in the southwestern Indian Ocean is mainly controlled by the intensityne-SST feedback is most prominent in the southeastern and southwestern Indian Ocean. Although the deca of upwelling and thermocline depth in winter and spring, respectively. The upwelling and thermocline depth are both affected by wind stress curl anomalies over the southeastern Indian Ocean, which excite anomalous Ekman pumping and influence the southwestern Indian Ocean through westward propagating Rossby waves.

The Indian Ocean Dipole (IOD) is a prominent mode of ocean-atmosphere interannual variability with great climate and socioeconomic impacts. Early positive IOD (pIOD), a newly discovered type of pIOD, induces pronounced rainfall anomalies in boreal summer more than canonical pIOD. It also contributes to more frequent consecutive pIODs, causing devastating droughts and floods. How early pIOD responds to global warming remains unknown. Here, we show that early pIOD has increased substantially in the past decades, reaching the same frequency as canonical pIOD. The increase is caused by intensified Bjerknes feedback and an early summer monsoon onset, which is the major trigger for early pIOD. Model simulations suggest that the increased frequency of early pIOD is likely to continue under greenhouse warming by the same mechanisms as in the observations, increasing boreal summer climate variability and leading to more climate extremes in affected regions.

The seasonal characteristics of the mesoscale coupling between sea surface temperature (SST) and wind speed in the South China Sea (SCS) are investigated using satellite observations. The correlation between mesoscale SST and wind speed is highest in winter. The region of high correlation is located in the central SCS in the early stage of the winter monsoon. It then gradually shifts northward in the following months and is located in the northern SCS in the late stage of the winter monsoon. In summer, the region of high correlation is located to the east of the Vietnam coast. Two controlling factors are crucial in mesoscale SST–wind speed coupling: the mesoscale SST gradient and the wind speed steadiness. The mesoscale SST gradient is fundamental in mesoscale coupling, but a steady wind speed also plays an important role. The development of significant coupling depends on the relative contribution of these two factors. For regions where the mesoscale SST gradient is relatively weak, a very steady wind field is required for detectable mesoscale coupling to occur, whereas in regions where the wind speed is less steady, a stronger mesoscale SST gradient must exist for coupling to develop. Variations in wind speed steadiness can well explain the inconsistency between the spatial patterns of the mesoscale SST gradient and the intensity of coupling. The wind speed steadiness is a good factor with which to evaluate the constraining effect of the background wind field variability on the development of mesoscale coupling in the SCS.

The seasonal characteristics and formation mechanism of the thermohaline structure of mesoscale eddy in the South China Sea are investigated using the latest eddy dataset and ARMOR3D data. Eddy-centric composites reveal that the horizontal distribution of temperature anomaly associated with eddy in winter is more of a dipole pattern in upper 50 m and tends to be centrosymmetric below 50 m, while in summer the distribution pattern is centrosymmetric in the entire water column. The horizontal distribution of eddy-induced salinity anomaly exhibits similar seasonal characteristics, except that the asymmetry of the salinity anomaly is weaker. The vertical distribution of temperature anomaly associated with eddy shows a monolayer structure, while the salinity anomaly demonstrates a triple-layer structure. Further analysis indicates that the vertical distribution of the anomalies is related to the vertical structure of background temperature and salinity fields, and the asymmetry of the anomalies in upper 50 m is mainly caused by the horizontal advection of background temperature and salinity.

The ear-shaped thermal front (ESTF), formed by the convergence of the Yellow Sea Warm Current (YSWC) and the Shandong Coastal Current (SCC), is a very important oceanic phenomenon in the Yellow Sea (YS) in winter. In situ measurements and reanalysis datasets all demonstrate that the ESTF has been weakening during 1950s–1990s, and a similar weakening trend is also found in winter monsoon over the YS. Numerical experiments show that the weakening of winter monsoon can induce an anomalous circulation in the YS on multi-decadal timescale with northward anomalous currents along China’s coast and southward anomalous currents in the central YS—generally opposite to seasonal mean circulation. The anomalous circulation causes slowdown of the YSWC and the SCC, and thus weakens the ESTF. Since the ESTF plays important roles in regional ocean dynamics and air-sea interactions, its weakening has important implications for regional climate in the YS in winter.

The decadal variability in the subsurface temperature of the tropical Indian Ocean shows a strong seasonality and features a prominent east–west dipole structure. The seasonal characteristics of the decadal variability shows a semiannual cycle in the rapid warming period of the twentieth century and then shift to an annual cycle during the recent hiatus period. The semiannual cycle is in phase with the climatological seasonal cycle, and therefore strengthens the climatological seasonal variations. Whereas during the recent hiatus period, the annual cycle shows a negative (positive) Indian Ocean Dipole pattern in the first (second) half of the year. Such regime shift in the decadal variability of subsurface temperature is strongly correlated with that of the surface wind in the tropical Indian Ocean, especially the equatorial zonal wind. Our result suggests that an abrupt weakening in both summer and winter Indian monsoon at the beginning of this century is the major cause of the regime shift in decadal variability of surface winds.

The coupling between sea surface temperature (SST) and sea surface wind speed over mesoscale eddies in the South China Sea (SCS) was studied using satellite measurements. Positive correlations between SST anomalies (SSTA) and wind speed anomalies were found over both cyclonic and anticyclonic eddies. In contrast to the open oceans, the spatial patterns of the coupling over mesoscale eddies in the SCS depend largely on the seasonal variations of the background SST gradient, wind speed, and wind directional steadiness. In summer, the maximum SSTA location coincides with the center of eddy-induced sea surface height anomalies. In winter, the eddy-induced SSTA show a clear dipole pattern. The spatial patterns of wind speed anomalies over eddies are similar to those of the SSTA in both seasons. Wind speed anomalies are linearly correlated with SSTA over anticyclonic and cyclonic eddies. The coupling coefficients between SSTA and wind speed anomalies in the SCS are comparable to those in the open oceans.

Although the Indian Ocean dipole (IOD) and ENSO are significantly correlated, there are indeed some IODs independent of ENSO. In this research, the characteristics of independent IOD are investigated and a new triggering mechanism is proposed based on case study and statistical analysis. Results show that the independent IODs peak in an earlier season and have a weaker intensity compared with the IODs associated with ENSO. The wind anomaly associated with the independent IOD is very unique and shows a monsoonlike pattern, in addition to the equatorial easterly wind anomaly (EEWA) common to all IODs. The evolution of the EEWA associated with the independent IOD is well captured by the second EOF mode of the equatorial zonal wind interannual variability, suggesting that the independent IOD is an important climate mode inherent to the tropical Indian Ocean. The EEWA associated with the independent IOD is tightly linked to Indian summer monsoon activities in spring, and the convection anomalies associated with early summer monsoon onset in the Bay of Bengal plays a key role in inducing the EEWA. The EEWA can persist through spring and summer and causes a series of processes similar to those related to the IODs associated with ENSO. The correlation between the independent IOD and Indian summer monsoon activities increases dramatically after the 1980s, which is probably due to the mean state change in the tropical Indian Ocean climate system.

Influence of the Indian Ocean Dipole (IOD) on the Indian Ocean Meridional Heat Transport (MHT) is studied using 60-yr SODA data. Results show that there is anomalous heat transport from the equatorial region to higher latitudes in both hemispheres during IOD events, and the anomalous poleward MHT occurs mainly in the western Indian Ocean. IOD has strong influence on the MHT during late summer and fall, which is consistent with the seasonal phase-locking characteristics of IOD itself. The MHT anomaly caused by anomalous meridional velocity, which is induced by wind, is the major contributor of the total MHT anomalies. EOF analysis shows that the IOD is a key factor for interannual variability of the MHT in the Indian Ocean. These findings are very useful for understanding the Indian Ocean heat balance and climatic implications of IOD.

Indian Ocean Dipole (IOD) can be divided into two types according to their SST anomaly (SSTA) patterns: the symmetric IODs and the asymmetric IODs. Dynamical mechanisms for the two types of IODs are investigated using an ocean general circulation model, and the numerical experiments indicate that the setup time of the equatorial easterly wind anomalies (EEWA) associated with IODs is crucially important for the symmetrical characteristics of the SSTA patterns. Early setup of the EEWA in spring can induce intense cooling in the east pole but weak warming in the west pole, making the two poles asymmetric. The intense cooling in the east pole is linked to the seasonal variations of thermocline depth, horizontal SST gradient, and unusually long duration of the EEWA, and the weak warming in the west pole is due to unusually cold zonal advection resulted from zonal SST gradient change. The numerical experiments demonstrate that the strength of the EEWA also plays an important role in the symmetrical characteristics of IODs. The SSTA in the two poles both intensify as the EEWA strength increases only when the EEWA speed is below a critical value. Above the critical value, the warming in the west pole starts to decrease while the cooling in the east pole still keeps increasing, leading to an asymmetric SSTA pattern. The mechanism behind this phenomenon is similar to the one in the situation when EEWA sets up earlier.

Performances of 5 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) in simulating the chlorophyll concentration over the tropical Indian Ocean are evaluated. Results show that these models are able to capture the dominant spatial distribution of observed chlorophyll concentration and reproduce the maximum chlorophyll concentration over the western part of the Arabian Sea, around the tip of the Indian subcontinent, and in the southeast tropical Indian Ocean. The seasonal evolution of chlorophyll concentration over these regions is also reproduced with significant amplitude diversity among models. All of 5 models is able to simulate the interannual variability of chlorophyll concentration. The maximum interannual variation occurs at the same regions where the maximum climatological chlorophyll concentration is located. Further analysis also reveals that the Indian Ocean Dipole events have great impact on chlorophyll concentration in the tropical Indian Ocean. In the general successful simulation of chlorophyll concentration, most of the CMIP5 models present higher than normal chlorophyll concentration in the eastern equatorial Indian Ocean.

Interannual variations in the surface and subsurface tropical Indian Ocean were studied using HadISST and SODA datasets. Wind and heat flux datasets were used to discuss the mechanisms for these variations. Our results indicate that the surface and subsurface variations of the tropical Indian Ocean during Indian Ocean Dipole (IOD) events are significantly different. A prominent characteristic of the eastern pole is the SSTA rebound after a cooling process, which does not take place at the subsurface layer. In the western pole, the surface anomalies last longer than the subsurface anomalies. The subsurface anomalies are strongly correlated with ENSO, while the relationship between the surface anomalies and ENSO is much weaker. And the subsurface anomalies of the two poles are negatively correlated while they are positively correlated at the surface layer. The wind and surface heat flux analysis suggests that the thermocline depth variations are mainly determined by wind stress fields, while the heat flux effect is important on SST.