Southeastern Tropical Indian Ocean Research Articles

Indo-Pacific–Induced Wave Trains during Austral Autumn and Their Effect on Australian Rainfall

Abstract During austral winter and spring, the El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD), individually or in combination, induce equivalent-barotropic Rossby wave trains, affecting midlatitude Australian rainfall. In autumn, ENSO is at its annual minimum, and the IOD has usually not developed. However, there is still a strong equivalent-barotropic Rossby wave train associated with tropical Indian Ocean sea surface temperature (SST) variability, with a pressure anomaly to the south of Australia. This wave train is similar in position, but opposite in sign, to the IOD-induced wave train in winter and spring and has little effect on Australian rainfall. This study shows that the SST in the southeastern tropical Indian Ocean (SETIO) displays a high variance during austral autumn, with a strong influence on southeast and eastern Australian rainfall. However, this influence is slightly weaker than that associated with SST to the north of Australia, which shares fluctuations with SST in the SETIO region. The SST north of Australia is coherent with a convective dipole in the tropical Pacific Ocean, which is the source of a wave train to the east of Australia influencing rainfall in eastern Australia. ENSO Modoki is a contributor to the convective dipole and as a result it exerts a weak influence on eastern Australian rainfall through the connecting north Australian SST relationship. Thus, SST to the north of Australia acts as the main agent for delivering the impact of tropical Indo-Pacific variability to eastern Australia.

An assessment of Indo-Pacific oceanic channel dynamics in the FGOALS-g2 coupled climate system model

Lag correlations of sea surface temperature anomalies (SSTAs), sea surface height anomalies (SSHAs), subsurface temperature anomalies, and surface zonal wind anomalies (SZWAs) produced by the Flexible Global Ocean-Atmosphere-Land System model: Grid-point Version 2 (FGOALS-g2) are analyzed and compared with observations. The insignificant, albeit positive, lag correlations between the SSTAs in the southeastern tropical Indian Ocean (STIO) in fall and the SSTAs in the central-eastern Pacific cold tongue in the following summer through fall are found to be not in agreement with the observational analysis. The model, however, does reproduce the significant lag correlations between the SSHAs in the STIO in fall and those in the cold tongue at the one-year time lag in the observations. These, along with the significant lag correlations between the SSTAs in the STIO in fall and the subsurface temperature anomalies in the equatorial Pacific vertical section in the following year, suggest that the Indonesian Throughflow plays an important role in propagating the Indian Ocean anomalies into the equatorial Pacific Ocean. Analyses of the interannual anomalies of the Indonesian Throughflow transport suggest that the FGOALS-g2 climate system simulates, but underestimates, the oceanic channel dynamics between the Indian and Pacific Oceans. FGOALS-g2 is shown to produce lag correlations between the SZWAs over the western equatorial Pacific in fall and the cold tongue SSTAs at the one-year time lag that are too strong to be realistic in comparison with observations. The analyses suggest that the atmospheric bridge over the Indo-Pacific Ocean is overestimated in the FGOALS-g2 coupled climate model.

Interannual Climate Variability over the Tropical Pacific Ocean Induced by the Indian Ocean Dipole through the Indonesian Throughflow

Abstract The authors’ previous dynamical study has suggested a link between the Indian and Pacific Ocean interannual climate variations through the transport variations of the Indonesian Throughflow. In this study, the consistency of this oceanic channel link with observations is investigated using correlation analyses of observed ocean temperature, sea surface height, and surface wind data. The analyses show significant lag correlations between the sea surface temperature anomalies (SSTA) in the southeastern tropical Indian Ocean in fall and those in the eastern Pacific cold tongue in the following summer through fall seasons, suggesting potential predictability of ENSO events beyond the period of 1 yr. The dynamics of this teleconnection seem not through the atmospheric bridge, because the wind anomalies in the far western equatorial Pacific in fall have insignificant correlations with the cold tongue anomalies at time lags beyond one season. Correlation analyses between the sea surface height anomalies (SSHA) in the southeastern tropical Indian Ocean and those over the Indo-Pacific basin suggest eastward propagation of the upwelling anomalies from the Indian Ocean into the equatorial Pacific Ocean through the Indonesian Seas. Correlations in the subsurface temperature in the equatorial vertical section of the Pacific Ocean confirm the propagation. In spite of the limitation of the short time series of observations available, the study seems to suggest that the ocean channel connection between the two basins is important for the evolution and predictability of ENSO.

Importance of Ocean Dynamics for the Skewness of the Indian Ocean Dipole Mode

AbstractInterannual anomalies of sea surface temperature (SST), wind, and cloudiness in the southeastern tropical Indian Ocean (SE-TIO) show negative skewness. In this research, asymmetry between warm and cold episodes in the SE-TIO and the importance of ocean dynamics are investigated. A coupled model simulation and observations show an asymmetric relationship between SST and the thermocline depth in the SE-TIO where SST is more sensitive to an anomalous shoaling than to deepening of the thermocline. This asymmetric thermocline feedback on SST is a result of a deep mean thermocline. Sensitivity experiments with an ocean general circulation model (OGCM) show that a negative SST skewness arises in response to sinusoidal zonal wind variations that are symmetric between the westerly and easterly phases. Heat budget analysis with an OGCM hindcast also supports the importance of ocean dynamics for SST skewness off Sumatra and Java.

Eastern Indian Ocean warming associated with the negative Indian Ocean dipole: A case study of the 2010 event

Warm sea surface temperature (SST) anomalies of more than 1 °C occurred in the southeastern tropical Indian Ocean and peaked during August to October 2010. The anomalous SST warming was associated with the negative phase of the Indian Ocean dipole (IOD) phenomenon. In this study, observational data from a moored buoy were used together with satellite and atmospheric reanalysis data sets to clarify the processes that produced the anomalously warm SST in 2010. We focused on the location 5°S, 95°E where in situ measurements of more than 10 years by a moored buoy were available. The buoy observations captured the oceanic conditions related to the anomalous warming event of 2010. Heat balance analysis demonstrated that air‐sea heat fluxes and horizontal heat advections mainly account for the mixed layer temperature variation. Reduced latent heat loss had a major role in producing the warm SST anomalies. Meridional heat advection also contributed to the warm SST anomalies where the southeastward surface current brought warmer water to the southeastern tropical Indian Ocean. The present results from the observations suggest that air‐sea heat exchanges play an active role in the SST anomalies in the southeastern tropical Indian Ocean during the negative IOD. In contrast, in the case of cold SST anomalies in the eastern Indian Ocean during the positive IOD, ocean heat advections mainly control the mixed layer temperature variation. These results suggest that the IOD includes different feedback mechanisms in its positive and negative phases.

Influences of ENSO Teleconnection on the Persistence of Sea Surface Temperature in the Tropical Indian Ocean

Abstract This study confirms a weak spring persistence barrier (SPB) of sea surface temperature anomalies (SSTAs) in the western tropical Indian Ocean (WIO), a strong fall persistence barrier (FPB) in the South China Sea (SCS), and the strongest winter persistence barrier (WPB) in the southeastern tropical Indian Ocean (SEIO). During El Niño events, a less abrupt sign reversal of SSTAs occurs in the WIO during spring, an abrupt reversal occurs in the SCS during fall, and the most abrupt reversal occurs in the SEIO during winter. The sign reversal of SSTA implies a rapid decrease in SSTA persistence, which is favorable for the occurrence of a persistence barrier. The present results indicate that a more abrupt reversal of SSTA sign generally corresponds to a more prominent persistence barrier. El Niño–induced changes in atmospheric circulation result in reduced evaporation and suppressed convection. This in turn leads to the warming over much of the TIO basin, which is an important mechanism for the abrupt switch in SSTA, from negative to positive, in the northern SCS and SEIO. The seasonal cycle of the prevailing surface winds has a strong influence on the timing of the persistence barriers in the TIO. The Indian Ocean dipole (IOD) alone can cause a weak WPB in the SEIO. El Niño events co-occurring with positive IOD further strengthen the SEIO WPB. The SEIO WPB appears to be more strongly influenced by ENSO than by the IOD. In contrast, the WIO SPB and the SCS FPB are relatively independent of the IOD.

Climatic oscillations in aerosol optical depth over tropical oceanic regions pertaining to genesis of the Indian Ocean Dipole

Data on aerosol optical depth (AOD) derived from the ocean colour sensor of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) from September 1997 to December 2010 over the western tropical Indian Ocean (WTIO) (10° S to 10° N; 50° E to 70° E) and southeastern tropical Indian Ocean (SETIO) (10° S to equator; 90° E to 110° E) were analysed with a view to understanding its response to climatic oscillations in regard to the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). This study demonstrates the existence of a bimodal distribution pattern of AOD in the atmosphere over both WTIO and SETIO, with the highest values being around 1.1 during the period of primary maximum during August over the WTIO and during October over the SETIO. A secondary maximum (∼0.9) appeared during March over both areas. In addition, the existence of a see-saw oscillation in the distribution of AOD between the atmospheric columns over the study regions was revealed, with higher values during August–December over the SETIO. AOD data over the SETIO captured very well the influence of these atmospheric modes, whereas the influence was not as significant over the WTIO. Stronger El Niño (Niño index > 0.80) events produced a significantly positive (more than +0.03) anomaly in AOD values over the SETIO during October, whereas the lone mode of IOD events and La Niña were not sufficient to induce any significant change in the aerosol distribution over the area. The mode of El Niño co-occurring with a positive IOD (PIOD) strengthens this anomalous behaviour. A significantly negative anomaly (≤0.03) in AOD was observed with concurrent La Niña (Niño index < −1.1) and negative IOD (NIOD) (dipole mode index ≤ 1.1) events. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) winds were utilized to verify these observations.

Temporal–spatial distribution of the predictability limit of monthly sea surface temperature in the global oceans

AbstractTo examine atmospheric and oceanic predictability based on nonlinear error growth dynamics, the authors introduced recently a new method using the nonlinear local Lyapunov exponent (NLLE). In this study, the NLLE method is employed to investigate the temporal–spatial distribution of the limit of sea surface temperature (SST) predictability, based on reanalysis monthly SST data. The results show that the annual mean limit of SST predictability is the greatest in the tropical central–eastern Pacific (&gt;8 months). Relatively high values were also obtained for the tropical Indian and Atlantic Oceans (5–8 months). In the northern and southern mid‐high latitude oceans, the limit of SST predictability is less than 6 months, with a minimum value of only 2–3 months. The limit of SST predictability in different oceanic regions shows significant seasonal variations, related to the persistence barriers that occur during particular seasons. In addition to the well‐known spring persistence barrier (SPB) in the tropical central–eastern Pacific, persistence barriers also occur in other ocean areas during seasons other than spring. A winter persistence barrier (WPB) exists in the southeastern tropical Indian Ocean and the northern tropical Atlantic. In the North Pacific and North Atlantic, a persistence barrier exists around July–September. These seasonal persistence barriers cause a relatively low limit of SST predictability when predictions are made across the season in which the barriers occur. In contrast, when predictions are made initiated from the season with a persistence barrier, the SST errors show rapid initial growth but slow growth in the following seasons, resulting in a relatively high limit in predictability. Analyses also indicate that the possibility of really eliminating the effects of persistence barriers on SST errors by improving ocean–atmosphere coupled general circulation models (CGCMs) or the data assimilation procedure is very low. Copyright © 2012 Royal Meteorological Society

Argo profiles variability of barrier layer in the tropical Indian Ocean and its relationship with the Indian Ocean Dipole

Interannual variability of the barrier layer (BL) in the southeastern tropical Indian Ocean (SETIO) is examined using temperature and salinity profiles derived from Argo floats since 2004. We show that a quasi‐permanent BL exists off Sumatra with a semi‐annual cycle and a maximum in November. Further, interannual variability of the BL is closely related to the Indian Ocean Dipole (IOD) with the IOD leading the BL by one month. During the 2006 positive IOD (pIOD) season, equatorial easterly‐induced upwelling Kelvin waves raise the isothermal layer (IL) off Sumatra; a salinity‐stratified mixed layer (ML) shoals due to a reduced eastward salty water transport by a weaker Wyrtki Jet, despite an offset by a reduced freshwater flux. Consequently, thinning of the BL is dominated by thinning of the IL. During the 2010 negative IOD (nIOD), similar processes operate but in an opposite direction. As thinning of the BL during a pIOD enhances the thermocline‐ML coupling, our results reveal that an IOD‐induced co‐varying BL in turn enhances the IOD positive feedbacks.

Sea Surface Height Variability in the Tropical Indian Ocean: Steric Contribution

Variability of Sea level and its steric contribution in the Tropical Indian Ocean (TIO) was studied based on 15 years (1993–2007) satellite altimeter observations of sea surface height (SSH) anomaly and steric height (STH) anomaly computed using temperature and salinity fields obtained from Simple Ocean Data Assimilation (SODA) product. Complex Empirical Orthogonal Function (CEOF) analysis was carried out to decompose variability of SSH and STH into various modes to examine the coherency between them. It is revealed that both the parameters exhibit variability in all the time scales. First three major modes of CEOF corresponds to 90% and 84% of the total variability of SSH and STH respectively. There exists strong coherence between the respective CEOF modes of SSH and STH. The first mode of CEOF contributes around ~50% of the total signal corresponds to the annual cycle exhibit large variability in the western Arabian Sea along the Somali and Arabia Coast, latitudinal strip between 2 and 10°N extending from Somali-coast to the west coast of India, coastal oceans around India, and the south eastern TIO. The second CEOF with 25% of total signal contains mixed signature of intra-seasonal and inter-annual periodicities. This exhibit large amplitude in the central south TIO, western and eastern parts of Equatorial Indian Ocean (EIO). Computed long term linear growth rate of sea level anomaly suggests that increase of sea level varies from small (1–3 mm yr−1) in the north TIO to large (8 mm yr−1) in the south TIO. Further analysis suggests that SSH trend in the south TIO was mostly governed by steric contribution while the variability of SSH trend in the north TIO could be explained partially by the variability in STH.

Interdecadal change in the relationship of southern China summer rainfall with tropical Indo-Pacific SST

The present study investigates the interdecadal change in the relationship between southern China (SC) summer rainfall and tropical Indo-Pacific sea surface temperature (SST). It is found that the pattern of tropical Indo-Pacific SST anomalies associated with SC summer rainfall variability tends to be opposite between the 1950–1960s and the 1980-1990s. Above-normal SC rainfall corresponds to warmer SST in the tropical southeastern Indian Ocean (SEIO) and cooler SST in the equatorial central Pacific (ECP) during the 1950–1960s but opposite SST anomalies in these regions during the 1980–1990s. A pronounced difference is also found in anomalous atmospheric circulation linking SEIO SST and SC rainfall between the two periods. In the 1950–1960s, two anomalous vertical circulations are present between ascent over SEIO and ascent over SC, with a common branch of descent over the South China Sea that is accompanied by an anomalous low-level anticyclone. In the 1980–1990s, however, a single anomalous vertical circulation directly connects ascent over SC to descent over SEIO. The change in the rainfall–SST relationship is likely related to a change in the magnitude of SEIO SST forcing and a change in the atmospheric response to the SST forcing due to different mean states. A larger SEIO SST forcing coupled with a stronger and more extensive western North Pacific subtropical high in recent decades induce circulation anomalies reaching higher latitudes, influencing SC directly. Present analysis shows that the SEIO and ECP SST anomalies can contribute to SC summer rainfall variability both independently and in concert. In comparison, there are more cases of concerted contributions due to the co-variability between the Indian and Pacific Ocean SSTs.

Interannual modulation and its dynamics of the mesoscale eddy variability in the southeastern tropical Indian Ocean

[1] Interannual modulation of the mesoscale eddy activity at the intraseasonal time scale in the southeastern tropical Indian Ocean (SETIO) and a possible mechanism responsible for the modulation are investigated using the results from a high-resolution oceanic general circulation model (OGCM). The model reproduces reasonably well the observed intraseasonal variability in the SETIO and its interannual modulation. It is shown that the simulated intraseasonal eddies are generated by baroclinic instability. The magnitude of the eddy activity in the SETIO changes year by year. From a composite analysis classified into cases in which significant eddy activity cooccurred with/without Indian Ocean Dipole (SETIO cooling) events, the meridional gradient of the heat content anomaly in the region south of Java Island is enhanced in both cases. This meridional gradient is generated by the negative heat content anomaly off Sumatra and Java in the SETIO cooling case and the positive heat content anomaly along 14°S, as well as the weak negative anomaly in the coastal region, in the no SETIO cooling case. The anomalous positive heat content along 14°S in the no SETIO cooling case is originated from the western tropical Pacific and it takes about half a year to reach the SETIO. These interannual variations in the upper ocean heat content anomaly modulate the mesoscale eddy activity in the SETIO through enhancement of the baroclinic energy conversion, with the lag of 3 months. The energy budget analysis and the simple stability analysis also confirm this result.

The critical role of the boreal summer mean state in the development of the IOD

[1] Boreal summer is a critical season for the rapid development of the Indian Ocean Dipole (IOD). In this study, three factors related to the boreal summer mean state are proposed to be important for the rapid development of the IOD, by strengthening the equatorial zonal wind anomaly and thus the dynamic Bjerknes feedback. Firstly, as part of the Indo‐Pacific warm pool, the high mean SST in the southeasterntropicalIndianOcean(SEIO)actsasanessential prerequisite for the development of anomalous convection. Secondly, the maximum of the suppressed precipitation in response to a cold SST anomaly (SSTA) in the SEIO, shifts northward towards the equator because the mean precipitation is equatorially trapped in boreal summer. Thirdly, the monsoonal easterly shear in boreal summer promotes an enhanced, more equatorially symmetric low‐level Rossby wave response to a prescribed equatorially asymmetric heating over the SEIO. The above three processes promote a greater equatorial zonal wind response and thus a greater Bjerknes feedback, as well as a greater IOD development during boreal summer. Citation: Xiang, B., W. Yu, T. Li, and B. Wang (2011), The critical role of the boreal summer mean state in the development of the IOD, Geophys. Res. Lett., 38, L02710, doi:10.1029/2010GL045851.

Possible interannual to interdecadal variabilities of the Indonesian throughflow water pathways in the Indian Ocean

A few possible interannual to interdecadal pathways of the Indonesian throughflow (ITF) water in the Indian Ocean are found from ocean reanalysis data provided by the European Centre for Medium‐Range Weather Forecasts under the name Ocean Re‐Analysis version S3 (ORA‐S3). The data was used in an off‐line mode to evolve a prognostic passive tracer. The tracer sources were prescribed at the ITF entrance channels and the spreading of the ITF in the Indian Ocean were tracked using interannually varying circulations for two separate episodes of 21 years spanning from 1960 to 1980 and from 1980 to 2000. A climatological run of 21 years was carried out for each time period and the differences in tracer spreading between the control and the climatological runs were computed. The empirical orthogonal function (EOF) analysis shows that the dominant mode of the interannual ITF pathway variability in the Indian Ocean is focused on the ITF entrance region into the southeastern tropical Indian Ocean. The first three EOFs collectively explain a total of 50% of the interannual pathway variability out of which EOF‐1 and 2 are significantly correlated with the Indian Ocean Dipole/Zonal Mode (IODZM). The IODZM‐related meridional circulation anomalies cause the ITF axis from the entrance region to be relatively closer to 10°S than the climatological axis which is shifted southward to 15°S∼20°S. The ITF pathways in the Indian Ocean show decadal shifts in the axis with a more direct departure from the entrance region to the western Indian Ocean prior to 1980, whereas a southwestward shift, subduction, and northwestward thermocline trajectories from the entrance region is seen after 1980. The dramatic shift in the surface meridional velocities in the ITF entrance region revealed from the ORA‐S3 data is a potential cause for the interdecadal changes in ITF pathways. The interannual pathways retrieved in this study thus complement the mean pathways of the ITF that were reported in previous studies. The variability in the pathways we found has implications for the crucial region of the southwestern thermocline ridge and the associated ocean‐atmosphere response.

Interactions between mesoscale eddy variability and Indian Ocean dipole events in the Southeastern tropical Indian Ocean—case studies for 1994 and 1997/1998

Interannual modulation of mesoscale eddy activity at the intraseasonal timescale in the southeastern tropical Indian Ocean and its relation to the Indian Ocean dipole mode (IOD) events are investigated using results from a high-resolution ocean general circulation model. The model reproduces observed characteristics of the intraseasonal variability and its interannual modulation fairly well, with large variances of the intraseasonal variability during the 1994 and 1997/1998 IOD events. Large negative temperature anomaly off the coasts of Java and the Lesser Sunda Islands in boreal summer, due to seasonal variation and interannual anomaly, extended further to the east in 1994, and the associated strong Indonesian throughflow enhanced the baroclinic instability in the upper layer, generating anomalously large mesoscale eddy activity. The eddy heat transport, in turn, significantly affected decaying phase of the 1994 IOD event. On the other hand, the development of the cold region off the Java Island associated with the 1997/1998 IOD event occurred in boreal winter, causing weaker baroclinic instability and hence weaker eddy activity off Java. This led to little influence on the heat budget in the southeastern tropical Indian Ocean for the 1997/1998 IOD event.

The crucial role of ocean–atmosphere coupling on the Indian monsoon anomalous response during dipole events

This paper examines an issue concerning the simulation of anomalously wet Indian summer monsoons like 1994 which co-occurred with strong positive Indian Ocean Dipole (IOD) conditions in the tropical Indian Ocean. Contrary to observations it has been noticed that standalone atmospheric general circulation models (AGCM) forced with observed SST boundary condition, consistently depicted a decrease of the summer monsoon rainfall during 1994 over the Indian region. Given the ocean–atmosphere coupling during IOD events, we have examined whether the failure of standalone AGCM simulations in capturing wet Indian monsoons like 1994 can be remedied by including a simple form of coupling that allows the monsoon circulation to dynamically interact with the IOD anomalies. With this view, we have performed a suite of simulations by coupling an AGCM to a slab-ocean model with spatially varying mixed-layer-depth (MLD) specified from observations for the 1994 IOD; as well as four other cases (1983, 1997, 2006, 2007). The specification of spatially varying MLD from observations allows us to constrain the model to observed IOD conditions. It is seen that the inclusion of coupling significantly improves the large-scale circulation response by strengthening the monsoon cross-equatorial flow; leading to precipitation enhancement over the subcontinent and rainfall decrease over south-eastern tropical Indian Ocean—in a manner broadly consistent with observations. A plausible physical mechanism is suggested to explain the monsoonal response in the coupled frame-work. These results warrant the need for improved monsoon simulations with fully coupled models to be able to better capture the observed monsoon interannual variability.

A numerical investigation of eddy-induced chlorophyll bloom in the southeastern tropical Indian Ocean during Indian Ocean Dipole—2006

An eddy-resolving coupled physical–biological model is used to study the effect of cyclonic eddy in enhancing offshore chlorophyll-a (Chl-a) bloom in the southeastern tropical Indian Ocean during boreal summer–fall 2006. The results demonstrate that the offshore Chl-a blooms are markedly coincident with the high eddy kinetic energy. Moreover, the vertical variations in Chl-a, nitrate, temperature, and mixed-layer depth (MLD) strongly imply that the cyclonic eddies induce surface Chl-a bloom through the injection of nutrient-rich water into the upper layer. Interestingly, we found that the surface bloom only occurs when the deep Chl-a maximum is located within the MLD. On the other hand, the response of subsurface Chl-a to the eddy pumping is remarkable, although it is hardly observable at the surface.

A Short Surface Pathway of the Subsurface Indonesian Throughflow Water from the Java Coast Associated with Upwelling, Ekman Transport, and Subduction

A surface pathway of the subsurface Indonesian Throughflow (ITF) in the southeastern Indian Ocean is proposed using a combined analysis of Lagrangian particles and passive tracers derived from two independent tools: an Ocean General Circulation Model (OGCM) and Simple Ocean Data Assimilation (SODA.2.0.2) reanalysis data. This newly suggested pathway follows the processes in succession as upwelling in the south Java coast, offshore Ekman drift and subduction into the thermocline centered on 20∘S. The upwelling of subsurface ITF along the south Java coast is found to occur from August to October. Upon surfacing, the ITF advects southwestward being trapped in the surface Ekman layer for an approximate period of 260 days and reaches the southeastern tropical Indian Ocean subduction zone centered on 20∘S which is demarcated by the Zero Wind Stress Curl (ZWSC) and subducts there. The particle trajectory revealed that during the subduction within the ZWSC region, the surface eastward flow above 120 m depth carries the particle about 10∘ to the east and westward flow below this depth carries the particle to the western Indian Ocean along the thermocline. These pathways are confirmed by a series of tracer experiments using SODA reanalysis data. The effects of vertical mixing and entrainment on the surfacing of the ITF at south Java coast were identified.

Significant Influence of the Boreal Summer Monsoon Flow on the Indian Ocean Response during Dipole Events

Abstract A majority of positive Indian Ocean dipole (IOD) events in the last 50 years were accompanied by enhanced summer monsoon circulation and above-normal precipitation over central-north India. Given that IODs peak during boreal autumn following the summer monsoon season, this study examines the role of the summer monsoon flow on the Indian Ocean (IO) response using a suite of ocean model experiments and supplementary data diagnostics. The present results indicate that, if the summer monsoon Hadley-type circulation strengthens during positive IOD events, then the strong off-equatorial southeasterly winds over the northern flanks of the intensified Australian high can effectively promote upwelling in the southeastern tropical Indian Ocean and amplify the zonal gradient of the IO heat content response. While it is noted that a strong monsoon cross-equatorial flow by itself may not generate a dipolelike response, a strengthening (weakening) of monsoon easterlies to the south of the equator during positive IOD events tends to reinforce (hinder) the zonal gradient of the upper-ocean heat content response. The findings show that an intensification of monsoonal winds during positive IOD periods produces nonlinear amplification of easterly wind stress anomalies to the south of the equator because of the nonlinear dependence of wind stress on wind speed. It is noted that such an off-equatorial intensification of easterlies over the SH enhances upwelling in the eastern IO off Sumatra–Java, and the thermocline shoaling provides a zonal pressure gradient, which drives anomalous eastward equatorial undercurrents (EUC) in the subsurface. Furthermore, the combination of positive IOD and stronger-than-normal monsoonal flow favors intensification of shallow transient meridional overturning circulation in the eastern IO and enhances the feed of cold subsurface off-equatorial waters to the EUC.

A model study on oceanic processes during the Indian Ocean Dipole termination

In this study, the processes affecting the temperature variability over the Southeastern Tropical Indian Ocean (STIO) during 1958-2000, accomplishing the positive and negative Indian Ocean Dipole (IOD) events are analyzed. The upper ocean heat budget analysis of the STIO has been carried out to understand the oceanic process during the termination of the recent strongest IOD events. The three recent strongest positive IOD events (1961, 1994 and 1997) and a strong negative IOD event (1996) are considered for detailed analysis. The heat budget analysis revealed that the positive net-surface heat flux and vertical advection played dominant roles in the termination of 1997 IOD event, whereas horizontal and vertical advections are responsible for the termination of IOD events during 1961 and 1994. The anomalous negative surface heat flux and horizontal advection caused the dipole termination during the negative dipole year 1996. The findings are well supported by the analysis of anomaly correlation between model upper ocean heat content tendency and heat budget components. Significant intra-seasonal oscillations (ISOs) in sea surface temperature (SST) anomaly are seen during the initial phase of termination in the eastern equatorial Indian Ocean during 1961 and 1994 IOD events. The influence of ISOs in SST is not so evident during the IOD termination in 1997. It is found that the termination processes have started more than a month prior to the actual IOD termination.