Indian Ocean Dipole Years Research Articles

Interannual variation of tropical cyclone energy metrics over North Indian Ocean

There is decreasing trend in the tropical cyclone (TC) number over the North Indian Ocean (NIO) in recent years, though there is increasing trend in the sea surface temperature (SST) which is one of the main environmental parameters for the development and intensification of TCs. Hence, a study has been performed to understand whether any trend exists in other TC parameters such as velocity flux (VF), accumulated cyclone energy (ACE) and power dissipation index (PDI). The interseasonal and interannual variations of VF, ACE and PDI for the NIO as a whole and Bay of Bengal (BOB) and Arabian Sea (AS) are analysed based on the data of 1990–2013 (24 years). Role of large scale features like El Nino southern oscillation (ENSO) and Indian Ocean dipole (IOD) have also been analyzed. The mean ACE per year for TCs [maximum sustained wind of 34 knots (kt) or more] over the NIO is about 13.1 × 104 kt2 including 9.5 × 104 kt2 over the BOB and 3.6 × 104 kt2 over the AS. The mean PDI per year for TCs over the NIO is about 10 × 106 kt3 including 3 × 106 kt3 over the AS and 7 × 106 kt3 over the BOB. The VF, ACE and PDI of TCs are significantly less over BOB during post-monsoon season (Oct.–Dec.) of El Nino years than in La Nina and normal years. The VF for TCs over the BOB during post-monsoon season is significantly less (higher) during positive (negative) IOD years. There is significant decreasing trend at 95 % level of confidence in ACE and PDI of TCs over AS during post-monsoon season and PDI over the BOB and NIO during pre-monsoon season mainly due to similar trend in average intensity of TCs and not due to trends in SST over Nino regions or IOD index.

Influence of the South China Sea Biweekly Sea Surface Temperature on the South China Sea Summer Monsoon Especially during the Indian Ocean Dipole

ABSTRACTThe influence of the biweekly sea surface temperature (SST) in the South China Sea (SCS) on the SCS summer monsoon, especially during the Indian Ocean Dipole (IOD) is presented using the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) SST and rainfall data for April to June from 1999 to 2013. During positive IOD (PIOD) years the biweekly SST anomalies over the SCS lead the rain anomalies by three days, with a significant correlation (r = 0.8, at the 99% confidence level), whereas during negative IOD (NIOD) years, the correlation is only 0.2. The biweekly SST is observed to influence the westward and northward propagating rainfall anomalies over the SCS and, hence, affect the SCS summer monsoon, especially during PIOD years. No such propagation was seen during NIOD years. The biweekly intraseasonal oscillation of SST in the SCS results in enhanced sea level pressure and surface shortwave radiation, especially during PIOD years. The potential findings here indicate that the biweekly SST in the SCS is strongly (weakly) influenced during PIOD (NIOD) years. Further, it is observed that SST in the SCS has a strong (weak) effect on the SCS summer monsoon by westward and northward propagation of rainfall, especially during PIOD (NIOD) years. When a PIOD or NIOD exists over the tropical Indian Ocean, the SCS SST will be strongly (r = 0.6, at the 99% confidence level) or weakly correlated with the residual index, respectively.

Ekman Pumping and Mixed Layer Depth Variability over the Indo-Pacific Oceans during the El Nino and IOD Events

The following study addresses the variability of Mixed Layer Depth (MLD) and Ekman pumping (W EK ) during the extreme El Nino and Indian Ocean Dipole (IOD) over the Indo-Pacific regions. Monthly anomalies considering the climatology of the period of 1980 through 2011 show that during the El Nino years, Ekman suction (positive Ekman pumping) is replaced with Ekman pumping (negative Ekman pumping) in the tropical eastern Pacific Ocean (88˚W-90˚W and 13˚S-15˚S) resulting in positive MLD anomalies, and the strong Ekman pumping may be the source for the deepened thermocline during El Nino. In the La Nina events shallow MLD exits in the tropical eastern Pacific Ocean, due to positive Ekman pumping. During the positive IOD (PIOD) events in the south eastern Indian Ocean (97˚E-100˚E and 2˚S-5˚S) MLD becomes shallow and positive Ekman pumping anomalies occur. During the negative IOD (NIOD) years opposite signs take place. Composite events of El Nino are compared with those of IOD, showing more Ekman pumping anomalies during IOD events as against less deviation in SST.

Impacts of IOD, ENSO and ENSO Modoki on the Australian Winter Wheat Yields in Recent Decades.

Impacts of the Indian Ocean Dipole (IOD), two different types of El Niño/Southern Oscillation (ENSO): canonical ENSO and ENSO Modoki, on the year-to-year winter wheat yield variations in Australia have been investigated. It is found that IOD plays a dominant role in the recent three decades; the wheat yield is reduced (increased) by −28.4% (12.8%) in the positive (negative) IOD years. Although the canonical ENSO appears to be responsible for the wheat yield variations, its influences are largely counted by IOD owing to their frequent co-occurrence. In contrast, the ENSO Modoki may have its distinct impacts on the wheat yield variations, but they are much smaller compared to those of IOD. Both the observed April-May and the predicted September-November IOD indices by the SINTEX-F ocean-atmosphere coupled model initialized on April 1st just before the sowing season explain ~15% of the observed year-to-year wheat yield variances. The present study may lead to a possible scheme for predicting wheat yield variations in Australia in advance by use of simple climate mode indices.

Seasonal-to-Interannual Time-Scale Dynamics of the Equatorial Undercurrent in the Indian Ocean

AbstractThis paper investigates the structure and dynamics of the Equatorial Undercurrent (EUC) of the Indian Ocean by analyzing in situ observations and reanalysis data and performing ocean model experiments using an ocean general circulation model and a linear continuously stratified ocean model. The results show that the EUC regularly occurs in each boreal winter and spring, particularly during February and April, consistent with existing studies. The EUC generally has a core depth near the 20°C isotherm and can be present across the equatorial basin. The EUC reappears during summer–fall of most years, with core depth located at different longitudes and depths. In the western basin, the EUC results primarily from equatorial Kelvin and Rossby waves directly forced by equatorial easterly winds. In the central and eastern basin, however, reflected Rossby waves from the eastern boundary play a crucial role. While the first two baroclinic modes make the largest contribution, intermediate modes 3–8 are also important. The summer–fall EUC tends to occur in the western basin but exhibits obvious interannual variability in the eastern basin. During positive Indian Ocean dipole (IOD) years, the eastern basin EUC results largely from Rossby waves reflected from the eastern boundary, with directly forced Kelvin and Rossby waves also having significant contributions. However, the eastern basin EUC disappears during negative IOD and normal years because westerly wind anomalies force a westward pressure gradient force and thus westward subsurface current, which cancels the eastward subsurface flow induced by eastern boundary–reflected Rossby waves. Interannual variability of zonal equatorial wind that drives the EUC variability is dominated by the zonal sea surface temperature (SST) gradients associated with IOD and is much less influenced by equatorial wind associated with Indian monsoon rainfall strength.

Tropical Indian Ocean subsurface temperature variability and the forcing mechanisms

The first two leading modes of interannual variability of sea surface temperature in the Tropical Indian Ocean (TIO) are governed by El Nino Southern Oscillation and Indian Ocean Dipole (IOD) respectively. TIO subsurface however does not co-vary with the surface. The patterns of the first mode of TIO subsurface temperature variability and their vertical structure are found to closely resemble the patterns of IOD and El Nino co-occurrence years. These co-occurrence years are characterized by a north–south subsurface dipole rather than a conventional IOD forced east–west dipole. This subsurface dipole is forced by wind stress curl anomalies, driven mainly by meridional shear in the zonal wind anomalies. A new subsurface dipole index (SDI) has been defined in this study to quantify the intensity of the north–south dipole mode. The SDI peaks during December to February (DJF), a season after the dipole mode index peaks. It is found that this subsurface north–south dipole is a manifestation of the internal mode of variability of the Indian Ocean forced by IOD but modulated by Pacific forcing. The seasonal evolution of thermocline, subsurface temperature and the corresponding leading modes of variability further support this hypothesis. Positive wind stress curl anomalies in the south and negative wind stress curl anomalies in the north of 5°S force (or intensify) downwelling and upwelling waves respectively during DJF. These waves induce strong subsurface warming in the south and cooling in the north (especially during DJF) and assist the formation and/or maintenance of the north–south subsurface dipole. A thick barrier layer forms in the southern TIO, supporting the long persistence of anomalous subsurface warming. To the best of our knowledge the existence of such north–south subsurface dipole in TIO is being reported for the first time.

Impacts of ENSO and IOD on tropical cyclone activity in the Bay of Bengal

The impacts of El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on tropical cyclone (TC) activity (intensity, frequency, genesis location, track and average lifetime) in the Bay of Bengal (BoB) are studied for the period 1891–2007 using cyclone e-Atlas of India Meteorological Department, Nino3.4 Index, Oceanic Nino Index and Dipole Mode Index (DMI). TCs in the present study include cyclones with maximum sustainable wind (MSW) ≥34 knots (referred as cyclonic storms) and severe cyclonic storms with MSW ≥48 knots. The study shows a total of 502 TCs over BoB during the 117-year study period at the rate of 4.29 TC per year. Seven-year running mean of TCs for the period 1891–2007 shows a decreasing trend. Correlation between Nino3.4 Index and DMI for the 117-year period is significant and positive and the significance level is higher (lower) for the period with higher (lower) TC frequencies. One-third monthly interval analysis for the 117-year period indicates first third (1–10) of November as the most favoured period of TC formation over BoB. 117-year study period is divided into years of ENSO (El Nino, La Nina and neutral ENSO) and IOD (+ve IOD, −ve IOD and no IOD) categories. Maximum frequency of TC is observed during La Nina years, −ve IOD years and also when La Nina co-occurred with −ve IOD. More severe cyclones are formed during La Nina and +ve IOD years. Genesis location of TCs indicates that during La Nina (El Nino) years, the TCs are oriented in the south-east–north-west (south-west–north-east) direction. TCs in no IOD and −ve (+ve) IOD years are more (less) in northern BoB (north of 15° N), while in southern BoB (south of 15° N), TCs are more (less) during no IOD and +ve (−ve) IOD condition. BoB is divided into four quadrants, and number of TCs in each quadrant is computed under different ENSO–IOD events. Peak direction of track movement is observed as north-east followed by north–north-west which is corroborated from the dissipation of TCs in the specific quadrant. Total TC tracks in the peak direction of track movement are maximum during El Nino and no IOD years. The study reveals that TCs with shorter lifetime are observed during El Nino and −ve IOD years, while TCs with longer lifetime are observed during La Nina, neutral ENSO and +ve IOD years. The decade with maximum TC formation is observed as 1921–1930, and the impacts of ENSO and IOD on decadal variability are distinctly observed.

Relative role of El Niño and IOD forcing on the southern tropical Indian Ocean Rossby waves

AbstractThe role of local air‐sea interactions over the tropical Indian Ocean (TIO) and remote forcing from the tropical Pacific Ocean in the formation and maintenance of southern TIO Rossby waves during El Niño and positive Indian Ocean Dipole (IOD) years is investigated. These Rossby waves are significantly intensified during the El Niño and IOD cooccurrence years, as compared to those during pure El Niño or IOD years. Coupled ocean‐atmosphere model sensitivity experiments reveal that air‐sea coupled processes in the TIO are responsible for the Rossby wave formation and its maintenance from boreal summer to fall, while remote forcing from the Pacific intensifies and maintains these waves up to the following spring. During the cooccurrence years, the Rossby waves are generated by both the persistent equatorial easterlies and off‐equatorial wind stress curl. During pure El Niño years, however, only off‐equatorial wind stress curl exists to drive weak Rossby wave. Asymmetric heating associated with IOD and the mean background easterly vertical wind shear (in the northern hemisphere) during summer and fall excite two symmetric anticyclones in both sides of the equator as atmospheric Rossby wave response, which are responsible for the anomalous equatorial surface easterlies. In contrast, symmetric heat sink over the Maritime Continent in winter associated with El Niño‐induced subsidence and mean easterly vertical shear (in southern hemisphere) are responsible for strong anticyclone in the southern TIO, which supports off‐equatorial wind stress curl.

Modeling winter rainfall in Northwest India using a hidden Markov model: understanding occurrence of different states and their dynamical connections

A multiscale-modeling framework for daily rainfall is considered and diagnostic results are presented for an application to the winter season in Northwest India. The daily rainfall process is considered to follow a hidden Markov model (HMM), with the hidden states assumed to be an unknown random function of slowly varying climatic modulation of the winter jet stream and moisture transport dynamics. The data used are from 14 stations over Satluj River basin in winter (December–January–February–March). The period considered is 1977/78–2005/06. The HMM identifies four discrete weather states, which are used to describe daily rainfall variability over study region. Each state was found to be associated with a distinct atmospheric circulation pattern, with the driest and drier states, State 1 and 2 respectively, characterized by a lack of synoptic wave activity. In contrast, the wetter and wettest states, States 3 and 4 respectively, are characterized by a zonally oriented wave train extending across Eurasia between 20N and 40N, identified with ‘western disturbances’ (WD). The occurrence of State 4 is strongly conditioned by the El Nino and Indian Ocean Dipole (IOD) phenomena in winter, which is demonstrated using large-scale correlation maps based on mean sea level pressure and sea surface temperature. This suggests that there is a tendency of higher frequency of the wet days and intense WD activities in winter during El Nino and positive IOD years. These findings, derived from daily rainfall station records, help clarify the sequence of Northern Hemisphere mid-latitude storms bringing winter rainfall over Northwest India, and their association with potentially predictable low frequency modes on seasonal time scales and longer.

Indian Ocean Dipole and southern high latitude precipitation: possible links

Southern high latitude precipitation during austral spring in relation to the Indian Ocean Dipole (IOD) and ENSO is investigated in the present study. Both the IOD and ENSO generate Rossby waves trains that create positive and negative pressure anomalies. These anomalous pressure centres generate meridional moisture fluxes that impact the precipitation. Influence of the IOD is detected mainly in the Ross sea region, where the southward moisture transport induced by the low pressure cell enhances precipitation. During strong IOD years, east Antarctica near 100°E, is also characterised by enhanced precipitation induced by the southward moisture transport by a high pressure cell located south of Australia. In the Dronning Maud Land, precipitation is linked to the moisture advection through the Atlantic during ENSO years and not during the IOD years.

The role of Arabian Sea in the evolution of Indian Ocean Dipole

AbstractAn Indian Ocean Dipole (IOD) event often evolves without any forcing from the Pacific. This study demonstrates that Arabian Sea spring warming and associated summer barrier layer (BL) play an important role in the evolution of such IODs. We propose an alternate mechanism for IOD initiation. Anomalous Walker circulation over the Indian Ocean during IOD years forced by Arabian Sea warming in spring and summer plays a crucial role in initiating the easterly wind anomalies in the eastern equatorial Indian Ocean. The western warming and the associated convection induce easterly wind anomalies much before the significant eastern cooling during IOD years, which strongly supports the role of western warming in the IOD evolution. IOD years are characterized by anomalous thickening of summer BL over the Arabian Sea. The anomalous BL formation during IOD years is attributed to the anomalous negative Ekman pumping velocity in the Arabian Sea. This negative Ekman pumping velocity forced downwelling deepens isothermal layer depth and thickens the BL. In addition to this, strong precipitation during IOD years strengthened the stratification and supported the BL formation. These favourable conditions for BL formation are absent during El Niño years. Anomalous BL is mainly responsible for the deepening of thermocline and subsurface warming in the Arabian Sea during IOD years.

Role of thermocline–SST coupling in the evolution of IOD events and their regional impacts

The evolution of sea surface temperature (SST) and thermocline (represented by 20 °C isotherm depth, D20) in the east equatorial Indian Ocean (EIO) associated with the Indian Ocean Dipole (IOD) years is studied for the period of 50 years from 1958 to 2007. A new IOD index based on combined anomalies of surface winds, D20 and SST over the equatorial Indian Ocean is defined to identify strong and weak IOD events. It is found that the evolution of strong IOD events is driven by ocean dynamics in the form of thermocline–SST coupling and is strongly interactive with the atmosphere, whereas the weak IOD events are mere response to surface winds without such dynamical coupling. The easterly wind anomalies extend up to the western equatorial Indian Ocean (WIO) during strong IOD years and support enhanced EIO air–sea interactions. On the other hand, the evolution of zonal wind anomalies is weak during the weak IOD years. Thermocline–SST coupling is robust in both EIO and WIO during strong IOD years, which is primarily responsible for the enhanced SST gradient, strong enough to establish anomalous Walker circulation within the Indian Ocean. The strong convection over the WIO associated with the Indian Ocean Walker cell triggers a secondary cell with subsidence over the African landmass. This double cell structure over the equatorial Indian Ocean is not reported before. Such double cell structure is not evident in weak IOD years and instead the convection over WIO extends up to African landmass. These are well supported by the spatial pattern of anomalous precipitable water during strong and weak IOD years. Strengthening of monsoon flow and local Hadley cell associated with strong IOD events enhances precipitation over the Indian subcontinent, whereas weak IOD years have less impact on the Indian summer monsoon circulation and rainfall. Analysis reveals that the EIO thermocline index and combined index could be potential predictors for the central Indian rainfall during summer.

Episodic phytoplankton bloom events in the Bay of Bengal triggered by multiple forcings

Two episodic phytoplankton bloom events were identified in the southeastern and southwestern Bay of Bengal in January 1998 and December 2005, respectively, from SeaWiFS-derived chlorophyll-a concentration data collected between September 1997 and December 2010. Possible causes were examined using time series datasets of satellite-derived sea surface height anomalies, sea surface temperature, wind stress and Ekman pumping velocity data. It was found that strong cyclonic eddies with long residence time transported cold, nutrient-rich water from the subsurface layer to the euphotic zone, and were responsible for sustaining the continuous growth of phytoplankton that led to the two aforementioned phytoplankton blooms. Moreover, regarding the event in December 2005, three tropical cyclones passed through the region of the phytoplankton bloom and could have been additional factors that enhanced the bloom. Regarding the event in January 1998, transportation from the adjacent Andaman Sea by the geostrophic current and an anomalous zonal easterly wind contributed to the phytoplankton bloom in the southeastern Bay of Bengal. In addition, both blooms occurred during Indian Ocean Dipole (IOD) years. The absence of the second downwelling Kelvin waves in positive IOD year of 1997–1998 favouring the long residence of the strong cyclonic eddy in the eastern bay, and the strong wind-induced mixing in negative IOD year of 2005 might contribute to enhancing productivity.

Influence of ENSO and of the Indian Ocean Dipole on the Indian summer monsoon variability

Indian summer monsoon (ISM) variability is forced from external factors (like the El Nino Southern Oscillation, ENSO) but it contains also an internal component that tends to reduce its potential for predictability. Large-scale and local monsoon indices based on precipitation and atmospheric circulation parameters are used as a measure of ISM variability. In a 9-members ensemble of AMIP-type experiments (with same boundary SST forcing and different initial conditions) their potential predictability is comparable using both local and large-scale monsoon indices. In the sample analyzed, about half of more predictable monsoon years coincide with El Nino and/or positive Indian Ocean Dipole (IOD) events. Summer monsoon characteristics during ENSO and IOD years are analyzed through composites computed over a three years period (i.e. one year before and one year after the event peak) to investigate the mutual relationship between the events lagged in time. The connection between ISM and IOD is mostly confined in the summer and autumn, while that with ENSO is stronger and extends more in time. In the coupled model results the IOD influence on the monsoon is large, even because in the model IOD events are intense and easily reproduced due to a strong air-sea feedback in the eastern side of the basin. Monsoon seasons preceding or following an El Nino or a La Nina event are not exactly symmetric, even in terms of their biennial character. In most of the cases, both in reanalysis and model, El Nino and positive IOD events tend to co-occur with larger anomalies either in the Indo-Pacific ocean sector or over India, while La Nina and negative IOD do not. From the observed record, the ENSO-IOD correlation is positive strong and significant since mid-60s and it may correspond with either strong or weak ENSO-monsoon relationship and with strong or weak IOD-monsoon relationship. A main difference between those periods is the relationship between Indian monsoon rainfall and SST in other ocean basins rather than the Indo-Pacific sector alone.

Impact of Indian Ocean Dipole and El Niño/Southern Oscillation wind‐forcing on the Wyrtki jets

Interannual variability of the Wyrtki jets is studied in the context of Indian Ocean Dipole (IOD) and El Niño and Southern Oscillation (ENSO) wind‐forcing using a three dimensional numerical ocean model and observations. The boreal fall (October–November) Wyrtki jet is more significantly affected than the boreal spring (April–May) Wyrtki jet since both the IOD and ENSO tend to peak toward the end of the calendar year. Various statistical methods are used in an attempt to separate the impacts of the IOD and ENSO on these jets, with emphasis on the fall jet. The first two modes of an Empirical Orthogonal Function (EOF) decomposition account for about 90% and 85% of variability in zonal currents and wind stress respectively along the equator in the Indian Ocean, but EOF analysis does not cleanly separate out IOD and ENSO forcing and response. Partial correlation analysis reveals that IOD wind‐forcing and zonal equatorial current response are stronger on average than for ENSO and extend further west across the basin. Composite analysis of IOD only, ENSO only, and combined IOD and ENSO years provides a complementary definition of the relative contributions of these two phenomena on Wyrtki jet variability and in general is consistent with the results of the partial correlation analysis.

ENSO, IOD and Indian Summer Monsoon in NCEP climate forecast system

El Nino-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Indian Summer Monsoon rainfall features are explored statistically and dynamically using National Centers for Environment Prediction (NCEP) Climate Forecast System (CFSv1) freerun in relation to observations. The 100 years of freerun provides a sufficiently long homogeneous data set to find out the mean state, periodicity, coherence among these climatic events and also the influence of ENSO and IOD on the Indian monsoon. Differences in the occurrence of seasonal precipitation between the observations and CFS freerun are examined as a coupled ocean–atmosphere system. CFS simulated ENSO and IOD patterns and their associated tropical Walker and regional Hadley circulation in pure ENSO (PEN), pure IOD (PIO) and coexisting ENSO-IOD (PEI) events have some similarity to the observations. PEN composites are much closer to the observation as compared to PIO and PEI composites, which suggest a better ENSO prediction and its associated teleconnections as compared to IOD and combined phenomenon. Similar to the observation, the model simulation also show that the decrease in the Indian summer monsoon rainfall during ENSO phases is associated with a descending motion of anomalous Walker circulation and the increase in the Indian summer monsoon rainfall during IOD phase is associated with the ascending branch of anomalous regional Hadley circulation. During co-existing ENSO and IOD years, however, the fate of Indian summer monsoon is dictated by the combined influence of both of them. The shift in the anomalous descending and ascending branches of the Walker and Hadley circulation may be somewhat attributed to the cold (warm) bias over eastern (western) equatorial Indian Ocean basin, respectively in the model. This study will be useful for identifying some of the limitations of the CFS model and consequently it will be helpful in improving the model to unravel the realistic coupled ocean–atmosphere interactions for the better prediction of Indian Summer Monsoon.

The influences of ENSO on tropical cyclone activity in the Bay of Bengal during October–December

The El Niño–Southern Oscillation (ENSO) influence on tropical cyclone (TC) activity (frequency, genesis location, and intensity) in the Bay of Bengal (BoB) during the primary TC peak season (October–December) are studied for the period of 1993–2010. The study shows that during primary TC peak season, accumulated cyclone energy in the BoB is negatively correlated with Niño3.4 sea surface temperature anomaly. Under La Niña regime number of extreme TC cases (wind speed >64 kt) increases significantly in the BoB during the primary TC peak season. The analysis further shows that negative Indian Ocean dipole year is also favorable for extreme TC activity in the BoB during the primary TC peak season. The existence of low‐level cyclonic (anticyclonic) vorticity, enhanced (suppressed) convection, and high (low) tropical cyclonic heat potential (TCHP) in the BoB provides favorable (unfavorable) conditions for the TC activity under La Niña (El Niño) regimes together with weak vertical wind shear and high sea surface temperature (SST). The genesis location of TC shifts to the east (west) of 87°E in the BoB during La Niña (El Niño) regime due to the variability in convective activity. The probable reason for the intense TC during a La Niña regime is likely explained in terms of longer track for TCs over warm SST and high TCHP due to eastward shifting of genesis location together with other favorable conditions. The variability of Madden‐Julian Oscillation and its influence on TC activity in the BoB during La Niña and El Niño regime are also examined.

Footprints of IOD and ENSO in the Kenyan coral record

[1] Low-frequency variations in the seasonally phase-locked signal of the Indian Ocean Dipole (IOD) and the Pacific El Nino/Southern Oscillation (ENSO) recorded in 115-year coralδ18O variability from Kenyan coast, the tropical western Indian Ocean, were investigated. A comparison of the monthly coral δ18O corresponding to IOD and ENSO years shows that Kenyan coral distinctly records the East African Short Rain anomaly related to the IOD variability in January, a few months after the Short Rain peak due to oceanographic condition. On the other hand, the ENSO-induced signals do not appear clearly as the positive sea surface temperature (SST) and rainfall anomalies in the monthly coral record. Moreover, annual mean coralδ18O and ENSO show only a weak coherence at the periodicity of 4 to 5 years. These results support the suggestion that the IOD is the dominant climate mode rather than ENSO in the Kenyan coast. The coral records indicate that the negative IOD- like anomalously cold SST condition in the western Indian Ocean precedes the evolution of the Pacific El Nino by one year. The anomalously cold SST condition was prominent in the late 19th century, but weakened in the 20th century. This retreat of the cold SST condition due to warming of the western tropical Indian Ocean may influence the nature of the Pacific ENSO.

Impact on the chlorophyll concentration in the Bay of Bengal and Arabian Sea during Indian Ocean dipole mode

The chlorophyll (Chl) concentration in the seawater of the Bay of Bengal (BoB) and the Arabian Sea (AS) changes most during the Indian Ocean dipole (IOD) mode event. The empirical orthogonal function (EOF) of several oceanographic and atmospheric data was studied, using 9 years (1998–2006) of reanalysed satellite data. The variation of Chl during the period of September to November (SON) over the 9 years has been studied in this article. It has been found that significant enhancement of Chl in the BoB takes place during IOD years owing to the surface wind action, that is, wind-stress curl (WC), which favours the upwelling process, whereas the AS shows a decline in concentration of Chl owing to a reduced open ocean upwelling process.

IOD influence on the early winter tibetan plateau snow cover: diagnostic analyses and an AGCM simulation

Using diagnostic analyses and an AGCM simulation, the detailed mechanism of Indian Ocean Dipole (IOD) influence on the early winter Tibetan Plateau snow cover (EWTPSC) is clarified. In early winter of pure positive IOD years with no co-occurrence of El Nino, the anomalous dipole diabatic heating over the tropical Indian Ocean excites the baroclinic response in the tropics. Since both baroclinic and barotropic components of the basic zonal wind over the Arabian Peninsula increase dramatically in early winter due to the equatorward retreat of the westerly jet, the baroclinic mode excites the barotropic Rossby wave that propagates northeastward and induces a barotropic cyclonic anomaly north of India. This enables the moisture transport cyclonically from the northern Indian Ocean toward the Tibetan Plateau. The convergence of moisture over the plateau explains the positive influence of IOD on the EWTPSC. In contrast, the basic zonal wind over the Arabian Peninsula is weak in autumn. This is not favorable for excitation of the barotropic Rossby wave and teleconnection, even though the IOD-related diabatic heating anomaly in autumn similar to that in early winter exists. This result explains the insignificant (significant positive) partial correlation between IOD and the autumn (early winter) Tibetan Plateau snow cover after excluding the influence of ENSO. The sensitivity experiment forced by the IOD-related SST anomaly within the tropical Indian Ocean well reproduces the baroclinic response in the tropics, the teleconnection from the Arabian Peninsula, and the increased moisture supply to the Tibetan Plateau. Also, the seasonality of the atmospheric response to the IOD is simulated.