An idealized study of the impact of extratropical climate change on El Niño–Southern Oscillation

Extratropical impacts on the tropical El Niño–Southern Oscillation (ENSO) are studied in a coupled climate model. Idealized experiments show that the remote impact of the extratropics on the equatorial thermocline through oceanic tunnel can substantially modulate the ENSO in both magnitude and frequency. First of all, an extratropical warming can be conveyed to the equator by the mean subduction current, resulting in a warming of the equatorial thermocline. Second, the extratropical warming can weaken the Hadley cells, which in turn slow down the mean shallow meridional overturning circulations in the upper Pacific, reducing the equatorward cold water supply and the equatorial upwelling. These oceanic dynamic processes would weaken the stratification of the equatorial thermocline and retard a buildup (purge) of excess heat content along the equator, and finally result in a weaker and longer ENSO cycle. This study highlights a nonlocal mechanism in which ENSO behavior is related to the extratropical climate conditions.     

Surface currents and equatorial thermocline in a coupled upper ocean-atmosphere GCM

The Oregon State University coupled upper ocean-atmosphere GCM is evaluated in terms of the simulated winds, ocean currents and thermocline depth variations. Although the zonal wind velocities in the model are underestimated by a factor of about three and the zonal current velocities are underestimated by a factor of about five, the model is seen to qualitatively simulate the major features of the gyral scale currents, and the phases of the seasonal variation of the principal equatorial currents are in reasonable agreement with observations. The simulated tropical currents are dominated by Ekman transport and the eastern boundary currents do not penetrate far enough equatorward, while the western boundary currents do not penetrate far enough poleward. The subtropical trade wind belt and the mid-latitude westerlies are displaced equatorward of observations; hence, the mid-latitude eastward currents, principally the Kuroshio-North Pacific Drift and the Gulf Stream-North Atlantic Current are displaced equatorward. In spite of these shortcomings the surface current simulation of this two-layer upper ocean model is comparable with that of other ocean GCMs of coarse resolution. The coupled model successfully simulates the deepening of the thermocline westward across Pacific as a consequence of the prevailing Walker circulation. The region of most intense simulated surface forcing is located in the western Pacific due to a southwestward displacement of the northeast trade winds relative to observations; hence the equatorial Pacific is dominated by eastward propagation of thermocline depth variations. The excessively strong Ekman divergence and upwelling in the western Pacific cools the local warm pool, while incorrectly simulated westerlies in the eastern Pacific suppress upwelling and inhibit cooling from below. These features reduce the simulated trans-Pacific sea-surface temperature gradient, weakening the Walker circulation and the anomalies associated with the simulated Southern Oscillation. Offprint requests to: KR Sperber     

Blunt ocean dynamical thermostat in response of tropical eastern Pacific SST to global warming

Using an intermediate ocean–atmosphere coupled model (ICM) for the tropical Pacific, we investigated the role of the ocean dynamical thermostat (ODT) in regulating the tropical eastern Pacific sea surface temperature (SST) under global warming conditions. The external, uniformly distributed surface heating results in the cooling of the tropical eastern Pacific “cold tongue,” and the amplitude of the cooling increases as more heat is added but not simply linearly. Furthermore, an upper bound for the influence of the equatorially symmetric surface heating on the cold tongue cooling exists. The additional heating beyond the upper bound does not cool the cold tongue in a systematic manner. The heat budget analysis suggests that the zonal advection is the primary factor that contributes to such nonlinear SST response. The radiative heating due to the greenhouse effect (hereafter, RHG) that is obtained from the multi-model ensemble of the Climate Model Intercomparison Project Phase III (CMIP3) was externally given to ICM. The RHG obtained from the twentieth century simulation intensified the cold tongue cooling and the subtropical warming, which were further intensified by the RHG from the doubled CO₂ concentration simulation. However, the cold tongue cooling was significantly reduced and the negative SST response region was shrunken toward the equator by the RHG from the quadrupled CO₂ concentration simulation, while the subtropical warming increased further. A systematic RHG forced experiment having the same spatial pattern of RHG from doubled CO₂ concentration simulation with different amplitude of forcing revealed that the ocean dynamical response to global warming tended to enhance the cooling in the tropical eastern Pacific by virtue of meridional advection and upwelling; however, these cooling effects could not fully compensate a given RHG warming as the external forcing becomes larger. Moreover, the feedback by the zonal thermal advection actually exerted the warming over the equatorial region. Therefore, as the global warming is intensified, the cooling over the eastern tropical Pacific by ODT and the negative SST response area are reduced.     

An analysis on the physical process of the influence of AO on ENSO

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Niño-like warming in the tropical eastern Pacific in winter.     

近赤道海温对西太平洋副高强度的影响机理——模糊映射诊断

针对近赤道海温对西太平洋副高影响的复杂性和不确定性问题,引入自适应模糊推理系统方法,系统地分析了西太平洋副高强度对赤道东太平洋、赤道西太平洋和赤道印度洋等海区海温状况和冷暖变化的响应态势和响应程度。研究表明:副高对赤道东太平洋海温的变冷(增暖)过程将出现正距平的增强(减弱)响应,响应幅度随时间减弱(增强);副高对赤道印度洋海温增暖(变冷)的响应态势与对赤道东太平洋海温的响应态势相反,响应幅度远小于赤道东太平洋海温变化的响应幅度;副高对赤道西太平洋海温的增暖急缓表现出不同的响应态势,迅速增暖利于副高衰减,缓慢增暖则利于副高增强,而对于海温的变冷过程则有滞后的增强响应趋势。此外,上述海区的海温冷暖变化幅度和速度是导致副高变异的主要因素,较之海温基本热力状况对副高强度的影响更为显著。     

Anatomizing the Ocean’s Role in Maintaining the Pacific Decadal Variability

ABSTRACT The role of ocean dynamics in maintaining the Pacific Decadal Variability （PDV） was investigated based on simulation results from the Parallel Ocean Program （POP） ocean general circulation model developed at the Los Alamos National Laboratory （LANL）. A long-term control simulation of the LANL-POP model forced by a reconstructed coupled wind stress field over the period 1949-2001 showed that the ocean model not only simulates a reasonable climatology, but also produces a climate variability pattem very similar to observed PDV. In the Equatorial Pacific （EP） region, the decadal warming is confined in the thin surface layer. Beneath the surface, a strong compensating cooling, accompanied by a basin-wide-scale overturning circulation in opposition to the mean flow, occurs in the thermocline layer. In the North Pacific （NP） region, the decadal variability nonetheless exhibits a relatively monotonous pattern, characterized by the dominance of anomalous cooling and eastward flows. A term balance analysis of the perturbation heat budget equation was conducted to highlight the ocean＇s role in main- taining the PDV-like variability over the EP and NP regions. The analyses showed that strong oceanic adjustment must occur in the equatorial thermocline in association with the anomalous overturning circulation in order to maintain the PDV-like variability, including a flattening of the equatorial thermocline slpoe and an enhancement of the upper ocean＇s stratification （stability）, as the climate shifts from a colder regime toward a warmer one. On the other hand, the oceanic response in the extratropical region seems to be confined to the surface layer, without much participation from the subsurface oceanic dynamics.     

Relative roles of the equatorial upper ocean zonal current and thermocline in determining the timescale of the tropical climate system

Summary In this study, it is demonstrated that the amplitude of the equatorial upper-ocean zonal current anomaly induced by the fast-varying wind forcing (shorter than a year) is much greater than that induced by the slowly varying wind forcing (longer than 2 year), and the center of maximum zonal current anomaly shifts from the central Pacific to the western Pacific with an increase in the timescale of wind forcing. As a result, the zonal advective feedback (the zonal advection of mean sea surface temperature by anomalous current) in a slowly varying climate system becomes weaker and barely induces a low-frequency mode such as El Niño-Southern Oscillation. On the other hand, both amplitude and zonal location of the maximum thermocline anomaly are little changed by the change in the timescale of wind forcing – confined at the strong equatorial upwelling region of the eastern Pacific. Accordingly, the thermocline feedback (the vertical advection of anomalous subsurface temperature by the mean upwelling) is more favorable to generate a low-frequency mode.The relative roles of these two feedbacks are further explored under the coupled-system context. The eigen analysis of the stripped-down version of an intermediate ocean-atmosphere coupled model shows that by altering the regime space from the weakly coupled to the strongly coupled, the dominant process that leads the leading eigen mode changes from the zonal advective feedback to the thermocline feedback, and at the same time the frequency of the leading mode also changes from the high-frequency to the low-frequency. It implies that each feedback tends to favor the different timescale coupled mode.     

Remote Efects of Tropical Cyclone Wind Forcing over the Western Pacific on the Eastern Equatorial Ocean

An ocean general circulation model(OGCM)is used to demonstrate remote efects of tropical cyclone wind(TCW)forcing in the tropical Pacific.The signature of TCW forcing is explicitly extracted using a locally weighted quadratic least-squares regression(called as LOESS)method from six-hour satellite surface wind data;the extracted TCW component can then be additionally taken into account or not in ocean modeling,allowing isolation of its efects on the ocean in a clean and clear way.In this paper,seasonally varying TCW fields in year 2008 are extracted from satellite data which are prescribed as a repeated annual cycle over the western Pacific regions of the equator(poleward of 10 N/S);two long-term OGCM experiments are performed and compared,one with the TCW forcing part included additionally and the other not.Large,persistent thermal perturbations(cooling in the mixed layer(ML)and warming in the thermocline)are induced locally in the western tropical Pacific,which are seen to spread with the mean ocean circulation pathways around the tropical basin.In particular,a remote ocean response emerges in the eastern equatorial Pacific to the prescribed of-equatorial TCW forcing,characterized by a cooling in the mixed layer and a warming in the thermocline.Heat budget analyses indicate that the vertical mixing is a dominant process responsible for the SST cooling in the eastern equatorial Pacific.Further studies are clearly needed to demonstrate the significance of these results in a coupled ocean-atmosphere modeling context.     

Remote effects of tropical cyclone wind forcing over the western Pacific on the eastern equatorial ocean

An ocean general circulation model （OGCM） is used to demonstrate remote effects of tropical cyclone wind （TCW） forcing in the tropical Pacific. The signature of TCW forcing is explicitly extracted using a locally weighted quadratic least=squares regression （called as LOESS） method from six-hour satellite surface wind data; the extracted TCW component can then be additionally taken into account or not in ocean modeling, allowing isolation of its effects on the ocean in a clean and clear way. In this paper, seasonally varying TCW fields in year 2008 are extracted from satellite data which are prescribed as a repeated annual cycle over the western Pacific regions off the equator （poleward of 10°N/S）; two long-term OGCM experiments are performed and compared, one with the TCW forcing part included additionally and the other not. Large, persistent thermal perturbations （cooling in the mixed layer （ML） and warming in the thermocline） are induced locally in the western tropical Pacific, which are seen to spread with the mean ocean circulation pathways around the tropical basin. In particular, a remote ocean response emerges in the eastern equatorial Pacific to the prescribed off-equatorial TCW forcing, characterized by a cooling in the mixed layer and a warming in the thermocline. Heat budget analyses indicate that the vertical mixing is a dominant process responsible for the SST cooling in the eastern equatorial Pacific. Further studies are clearly needed to demonstrate the significance of these results in a coupled ocean-atmosphere modeling context.     

Indian Ocean warming during 1958–2004 simulated by a climate system model and its mechanism

The mechanism responsible for Indian Ocean Sea surface temperature (SST) basin-wide warming trend during 1958–2004 is studied based on both observational data analysis and numerical experiments with a climate system model FGOALS-gl. To quantitatively estimate the relative contributions of external forcing (anthropogenic and natural forcing) and internal variability, three sets of numerical experiments are conducted, viz. an all forcing run forced by both anthropogenic forcing (greenhouse gases and sulfate aerosols) and natural forcing (solar constant and volcanic aerosols), a natural forcing run driven by only natural forcing, and a pre-industrial control run. The model results are compared to the observations. The results show that the observed warming trend during 1958–2004 (0.5 K (47-year)^?1) is largely attributed to the external forcing (more than 90 % of the total trend), while the residual is attributed to the internal variability. Model results indicate that the anthropogenic forcing accounts for approximately 98.8 % contribution of the external forcing trend. Heat budget analysis shows that the surface latent heat flux due to atmosphere and surface longwave radiation, which are mainly associated with anthropogenic forcing, are in favor of the basin-wide warming trend. The basin-wide warming is not spatially uniform, but with an equatorial IOD-like pattern in climate model. The atmospheric processes, oceanic processes and climatological latent heat flux together form an equatorial IOD-like warming pattern, and the oceanic process is the most important in forming the zonal dipole pattern. Both the anthropogenic forcing and natural forcing result in easterly wind anomalies over the equator, which reduce the wind speed, thereby lead to less evaporation and warmer SST in the equatorial western basin. Based on Bjerknes feedback, the easterly wind anomalies uplift the thermocline, which is unfavorable to SST warming in the eastern basin, and contribute to SST warming via deeper thermocline in the western basin. The easterly anomalies also drive westward anomalous equatorial currents, against the eastward climatology currents, which is in favor of the SST warming in the western basin via anomalous warm advection. Therefore, both the atmospheric and oceanic processes are in favor of the IOD-like warming pattern formation over the equator.     

On the effect of ENSO precipitation anomalies in a global ocean GCM

An ocean general circulation model is used to study the influence of positive precipitation anomalies associated with El Nino and La Nina events. In this idealized model, the precipitation over the appropriate part of the equatorial Indo-Pacific region is doubled for one year. At the surface, salinity anomalies of up to –0.9 parts per thousand result from this anomalous precipitation. Perturbation surface currents ranging from 10–100% of the climatological values are induced in the tropical Indian and Pacific Oceans. A return flow is found beneath the thermocline with upwelling (downwelling) in (outside) the region of enhanced precipitation. The net effect of the precipitation anomalies is to generate a zonal overturning cell which transports fresher surface water away from the forcing region and replaces it with cooler, more saline water from below.     

Tropical–extratropical climate interaction as revealed in idealized coupled climate model experiments

Tropical–extratropical climate interactions are studied by idealized experiments with a prescribed 2°C SST anomaly at different latitude bands in a coupled climate model. Instead of focusing on intrinsic climate variability, this work investigates the mean climate adjustment to remote external forcing. The extratropical impact on tropical climate can be as strong as the tropical impact on extratropical climate, with the remote sea surface temperature (SST) response being about half the magnitude of the imposed SST change in the forcing region. The equatorward impact of extratropical climate is accomplished by both the atmospheric bridge and the oceanic tunnel. About two-thirds of the tropical SST change comes from the atmospheric bridge, while the remaining one-third comes from the oceanic tunnel. The equatorial SST increase is first driven by the reduced latent heat flux and the weakened poleward surface Ekman transport, and then enhanced by the decrease in subtropical cells’ strength and the equatorward subduction of warm anomalies. In contrast, the poleward impact of tropical climate is accomplished mainly by the atmospheric bridge, which is responsible for extratropical temperature changes in both the surface and subsurface. Sensitivity experiments also show the dominant role of the Southern Hemisphere oceans in the tropical climate change. CCR contribution number 829; DAS-PKU contribution number 002.     

On the Second-Year Warming in Late 2019 over the Tropical Pacific and Its Attribution to an Indian Ocean Dipole Event※

After its maturity, El Ni?o usually decays rapidly in the following summer and evolves into a La Ni?a pattern. However, this was not the case for the 2018/19 El Ni?o event. Based on multiple reanalysis data sets, the space-time evolution and triggering mechanism for the unusual second-year warming in late 2019, after the 2018/19 El Ni?o event, are investigated in the tropical Pacific. After a short decaying period associated with the 2018/19 El Ni?o condition, positive sea surface temperature anomalies (SSTAs) re-intensified in the eastern equatorial Pacific in late 2019. Compared with the composite pattern of El Ni?o in the following year, two key differences are evident in the evolution of SSTAs in 2019. First, is the persistence of the surface warming over the central equatorial Pacific in May, and second, is the re-intensification of the positive SSTAs over the eastern equatorial Pacific in September. Observational results suggest that the re-intensification of anomalous westerly winds over the western and central Pacific, induced remotely by an extreme Indian Ocean Dipole (IOD) event, acted as a triggering mechanism for the second-year warming in late 2019. That is, the IOD-related cold SSTAs in the eastern Indian Ocean established and sustained anomalous surface westerly winds over the western equatorial Pacific, which induced downwelling Kelvin waves propagating eastward along the equator. At the same time, the subsurface ocean provided plenty of warm water in the western and central equatorial Pacific. Mixed-layer heat budget analyses further confirm that positive zonal advection, induced by the anomalous westerly winds, and thermocline feedback played important roles in leading to the second-year warming in late 2019. This study provides new insights into the processes responsible for the diversity of El Ni?o evolution, which is important for improving the physical understanding and seasonal prediction of El Ni?o events.     

On the mechanisms in a tropical ocean–global atmosphere coupled general circulation model. Part I: mean state and the seasonal cycle

 The mechanisms responsible for the mean state and the seasonal and interannual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global atmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest. Departures from the mean state are characterized by oscillations with dominant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evolved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Convergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the latitude of organized convention. In turn, the stronger oceanic subsidence reinforces equatorward convergence of water masses at the thermocline depth which, being not balanced by equatorial upwelling, deepens the equatorial thermocline. An equivalent view is that the deepening of the thermocline proceeds from the weakening of the meridional draining of near-surface equatorial waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appear at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacific eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. The whole oscillation is synchronized by the apparent meridional motion of the sun, through the seasonal oscillation of the ITCZ. This possible mechanism is partly supported by the observed seasonal reversal of vorticity between the equator and the ITCZ, and by observational evidence of eastward propagating subsurface temperature anomalies at the thermocline level. Received: 7 April 1997 / Accepted: 15 July 1998     

Response of the Tropical Indian Ocean to Greenhouse Gases and Aerosol Forcing in the GFDL CM3 Coupled Climate Model

The response of the tropical Indian Ocean (TIO) to greenhouse gases (GHGs) and aerosols are investigated based on historical single-forcing and all-forcing simulations using the Geophysical Fluid Dynamics Laboratory Climate Model, version 3 (GFDL CM3). Results reveal a positive Indian Ocean Dipole (pIOD)-like pattern in GHG forcing but a negative Indian Ocean Dipole (nIOD)-like pattern in aerosol forcing. The GHG-induced pIOD-like pattern features less (more) sea surface temperature (SST) warming over the southeastern (western) TIO, accompanied by equatorial easterly anomalies, as well as a shallower thermocline off Sumatra. The aerosol-induced nIOD-like pattern displays the reverse features, characterized by less (more) SST cooling over the southeastern (western) TIO, anomalous equatorial westerlies, and a deeper thermocline off Sumatra. Although the aerosol-induced pattern appears to resemble a reversal of the GHG-induced pattern, there is a strong asymmetry in the SST changes over the southeastern TIO, where the cooling responding to aerosol forcing exceeds the warming in response to GHG forcing, and a negative SST residual is thus produced. A mixed-layer heat budget analysis suggests that the negative SST residual results mainly from asymmetric responses of shortwave radiation, zonal advection, and diffusion to GHGs and aerosols. For comparison, the formation processes for the negative SST skewness over the southeastern TIO between the internal pIOD and nIOD are also discussed.     

Coupling ocean–atmosphere intensity determines ocean chlorophyll-induced SST change in the tropical Pacific

Phytoplankton pigments (e.g., chlorophyll-a) absorb solar radiation in the upper ocean and induce a pronounced radiant heating effect (chlorophyll effect) on the climate. However, the ocean chlorophyll-induced heating effect on the mean climate state in the tropical Pacific has not been understood well. Here, a hybrid coupled model (HCM) of the atmosphere, ocean physics and biogeochemistry is used to investigate the chlorophyll effect on sea surface temperature (SST) in the eastern equatorial Pacific; a tunable coefficient, α, is introduced to represent the coupling intensity between the atmosphere and ocean in the HCM. The modeling results show that the chlorophyll effect on the mean-state SST is sensitively dependent on α (the coupling intensity). At weakly represented coupling intensity (0 ≤ α < 1.01), the chlorophyll effect tends to induce an SST cooling in the eastern equatorial Pacific, whereas an SST warming emerges at the strongly represented coupling intensity (α ≥ 1.01). Thus, a threshold exists for the coupling intensity (about α = 1.01) at which the sign of SST responses can change. Mechanisms and processes are illustrated to understand the different SST responses. In the weak coupling cases, indirect dynamical cooling processes (the adjustment of ocean circulation, enhanced vertical mixing, and upwelling) tend to dominate the SST cooling. In the strong coupling cases, the persistent warming induced by chlorophyll in the southern subtropical Pacific tends to induce cross-equatorial northerly winds, which shifts to anomalous westerly winds in the eastern equatorial Pacific, consequently reducing the evaporative cooling and weakening indirect dynamical cooling; eventually, SST warming maintains in the eastern equatorial Pacific. These results provide new insights into the biogeochemical feedback on the climate and bio-physical interactions in the tropical Pacific.

     

CMIP5模式对拉尼娜生命史模拟能力的评估

利用19个CMIP5模式输出资料,评估模式对于拉尼娜事件特殊生命史发展过程的模拟能力。评估结果显示,仅有少数模式可以很好地再现拉尼娜事件缓慢衰减并再次增强的生命史发展过程,而多数模式中拉尼娜事件持续衰减直至消亡。观测分析结果表明,一个可能导致拉尼娜再次增强的原因是风场强迫作用下的海洋赤道波动过程。模拟能力较好的模式可以建立起"SST—对流—风场"正反馈过程,使得拉尼娜事件再次发展。而模拟能力较弱的模式中正反馈过程无法建立,因此拉尼娜事件最终消亡。另一个可能导致拉尼娜事件再次增强的原因是海洋平均经圈环流的作用。模拟能力较好的模式可以很好地模拟出气候态海洋经圈环流强度,因此海洋平均经向冷平流会帮助赤道地区负海温距平再次增强。而模拟能力较弱的模式中海洋经圈环流强度较弱,因此赤道地区负海温距平持续衰减,最终回归到气候态。     

Asymmetry in the tropical Indian Ocean subsurface temperature variability

The empirical orthogonal function (EOF) analysis of subsurface temperature shows a dominant north-south mode of interannual variability in the Tropical Indian Ocean (TIO) at around 100 m depth (thermocline). This subsurface mode (SSM) of variability evolves in September-November (SON) as a response to Indian Ocean Dipole and intensifies during December-February (DJF) reinforced by El Niño and Southern Oscillation (ENSO) forcing. The asymmetry in the evolution of positive and negative phases of SSM and its impacts on the modulation of surface features are studied. The asymmetry in the representation of anomalous surface winds along the equator and off-equatorial wind stress curl anomalies are primarily responsible for maintaining the asymmetry in the subsurface temperature through positive and negative phases of the SSM. During the positive phase of SSM, downwelling Rossby waves generated by anticyclonic wind stress curl propagate towards the southwestern TIO (SWTIO), the thermocline ridge region of mean upwelling. The warmer subsurface water associated with the downwelling Rossby waves upwells in the region of mean upwelling and warms the surface resulting in strong subsurface-surface coupling. Such interaction processes are however weak during the negative phase of SSM. The asymmetry in the subsurface-surface interaction during the two phases of SSM and its impact on the modulation of surface features of TIO are also reported. In addition to the ENSO forcing, self-maintenance of SSM during DJF season is evident in the positive SSM (PSSM) years through modulation of subsurface surface coupling and air-sea coupling. This positive feedback during PSSM years is maintained by the deepening thermocline, warm SSTs and convection. The asymmetry in the thermocline evolution is more evident in the SWTIO and southern TIO.     

The Kelvin Wave Processes in the Equatorial Indian Ocean during the 2006–2008 IOD Events

The present study investigates the role of Kelvin wave propagations along the equatorial Indian Ocean during the 2006–2008 Indian Ocean Dipole (IOD). The 2006 IOD lasted for seven months, developing in May and reaching its peak in December, while the 2007 and 2008 IODs were short-lived events, beginning in early May and ending abruptly in September, with much weaker amplitudes. Associated with the above IODs, the impulses of the sea surface height (SSH) anomalies reflect the forcing from an intraseasonal time scale, which was important to the evolution of IODs in 2007 and 2008. At the thermocline depth, dominated by the propagation of Kelvin waves, the warming/cooling temperature signals could reach the surface at a particular time. When the force is strong and the local thermocline condition is favorable, the incoming Kelvin waves dramatically impact the sea surface temperature (SST) in the eastern equatorial Indian Ocean. In July 2007 and late July 2008, the downwelling Kelvin waves, triggered by the Madden-Julian Oscillation (MJO) in the eastern and central equatorial Indian Ocean, suppressed the thermocline in the Sumatra and the Java coast and terminated the IOD, which made those events short-lived and no longer persist into the boreal fall season as the canonical IOD does.     

Why is Indian Ocean warming consistently?

Observations have shown that the Indian Ocean is consistently warming and its warm pool is expanding, particularly in the recent decades. This paper attempts to investigate the reason behind these observations. Under global warming scenario, it is expected that the greenhouse gas induced changes in air–sea fluxes will enhance the warming. Surprisingly, it is found that the net surface heat fluxes over Indian Ocean warm pool (IOWP) region alone cannot explain the consistent warming. The warm pool area anomaly of IOWP is strongly correlated with the sea surface height anomaly, suggesting an important role played by the ocean advection processes in warming and expansion of IOWP. The structure of lead/lag correlations further suggests that Oceanic Rossby waves might be involved in the warming. Using heat budget analysis of several Ocean data assimilation products, it is shown that the net surface heat flux (advection) alone tends to cool (warm) the Ocean. Based on above observations, we propose an ocean-atmosphere coupled positive feedback mechanism for explaining the consistent warming and expansion of IOWP. Warming over IOWP induces an enhancement of convection in central equatorial Indian ocean, which causes anomalous easterlies along the equator. Anomalous easterlies in turn excite frequent Indian ocean Dipole events and cause anti-cyclonic wind stress curl in south-east and north-east equatorial Indian ocean. The anomalous wind stress curl triggers anomalous downwelling oceanic Rossby waves, thereby deepening the thermocline and resulting in advection of warm waters towards western Indian ocean. This acts as a positive feedback and results in more warming and westward expansion of IOWP.