Indian Ocean Dipole Mode Events Research Articles

Contrasting Effects of Indian Ocean Basin and Dipole Modes on the Stratosphere

AbstractThe stratospheric impact of the sea surface temperature (SST) variability in the Indian Ocean has attracted extensive attention. Recent warming of the tropical Indian Ocean is characterized by increases in both the magnitude and zonal gradient, which is related to the variability in the positive phase of the Indian Ocean basin mode (IOBM) and Indian Ocean dipole mode (IOD). Yet, previous studies mainly focused on the stratospheric impact of the IOBM. Using the Whole Atmosphere Community Climate Model (version 4), this study explores the impact of positive IOD events on the stratosphere and compares it with the IOBM. Results show that the IOBM (IOD) serves to dry (moisten) the tropical lower stratosphere on the annual average. The moistening (drying) of the lower stratosphere in response to IOBM occurs mainly in winter and spring (autumn), while the IOD exerts a moistening effect in winter. Both modes can enhance the northern stratospheric polar vortex, and the effect of IOD is close to that of IOBM, although SST anomalies associated with the IOD are smaller. This implies that the Indian Ocean SST gradient exerts a greater influence on the Northern Hemisphere stratosphere than does the Indian Ocean SST magnitude. Moreover, both modes weaken the southern stratospheric polar vortex, but the effect of IOD is much smaller since its impact on stationary waves is weak. Contrary to the Northern Hemisphere stratosphere, the Southern Hemisphere stratosphere is more sensitive to the Indian Ocean SST magnitude than to the SST gradient.

The Effect of Local Forcing on Anomalously High Rainfall during Dry Season in Java, Indonesia

During the dry season (May to October) in Java, Indonesia, anomalously high rainfall is investigated using 37-year rainfall data from the Climate Hazards Group InfraRed Precipitation with Station data. The analysis focuses on the years having high rainfall during the dry season between 1982 and 2019. It is conducted using a combination of the presence and absence of La Niña, negative Indian Ocean Dipole Mode events, and other atmospheric/oceanic parameters, such as 2-m temperature, sea surface temperature, outgoing longwave radiation, 200 mb and 850 mb wind. The results show that the presence of both La Niña and negative Indian Ocean Dipole Mode events contributes around 39% to the high rainfall during the dry season, the presence of negative Indian Ocean Dipole Mode - 22%, the absence of both events - 22%, and the presence of La Niña - 17%. The dynamics of monsoon circulation anomaly (200 mb and 850 mb) in the southern Indian Ocean off the coast of Sumatra and Java also plays a role in the increased rainfall during the dry season in Java. This anomaly occurs due to a vortex in the southern equatorial Indian Ocean around 10⁰S, triggering the formation of double Inter-tropical Convergence Zones over the area north of the equator and the southern waters of Java. The increase in rainfall due to this local factor reaches a maximum and extends in June and October, which is associated with the strengthening of circulation anomalies in southern Java, both spatially and vertically (850 and 200 mb).

Meridional variability of atmospheric convection associated with the Indian Ocean Dipole Mode

The Indian Ocean Dipole Mode (IODM) impacts many surrounding and remote regions of the Indian Ocean, with devastating floods over East Africa but severe droughts in countries surrounding Indonesia during a positive IODM event. Understanding the dynamics is important for seasonal prediction and climate projections, but the role of meridional temperature and circulation anomalies remains unclear. Here, we show that in combination with the zonal structure of temperature and rainfall anomalies, northward contraction of the warm water pool over the eastern equatorial Indian Ocean region (EEIO) also generates an anomalous meridional cross-equatorial temperature gradient in the east. This meridional temperature gradient controls northward retreat of the atmospheric convection in association with northward cross-equatorial winds, and hence declining rainfall over the EEIO. Our results have important implications for the mean state change under greenhouse warming.

On the major shifts in the IOD during the last century, the role of the Mascarene High displacements

ABSTRACTIn this observational research, the seasonally stratified (October to December) Indian Ocean Dipole mode (IOD) behaviour in the tropical Indian Ocean since 1870 is investigated. Three significant climate shifts manifested themselves in the Indian Ocean during the years 1918, 1961 and 1997. Each shift is preceded by a 3‐year sequence of IOD events that are unique in the entire time series. The order is such that a relatively moderate negative sea surface temperature (SST) anomaly gradient intensifies to an extreme negative IOD event which then reverses in the shift year to an extreme positive event. The last two extreme IOD events reach record‐breaking magnitudes during each shift implying intensification of the shift process with time. During the year before the shift, the Mascarene High (MH) is anomalously displaced poleward and westward, while it intensifies as it anomalously moves equatorward during the shift year. Therefore the maximum contrast of south Indian Ocean pressure pattern and hence the intensity of the MH, from one year to the other within an epoch, is achieved during the shift years. The intensity of this process has been escalating during each of the three successive shifts. Despite involving to a greater extent the IOD dynamics, these IOD shifts may primarily be an expression of the south Indian Ocean basinwide dynamics rather than a direct response to internal tropical influences.

Ocean Surface Impacts on the Seasonal-Mean Precipitation over the Tropical Indian Ocean

Abstract This study analyzes factors affecting the predictability of seasonal-mean precipitation over the tropical Indian Ocean. The analysis focuses on the contributions from the local sea surface temperature (SST) forcing in the Indian Ocean, the remote SST forcing related to ENSO in the tropical eastern Pacific, and the role of local air–sea coupling. To understand the impacts of the individual factors, the prediction skill over the tropical Indian Ocean for four model simulations, but with different treatments for the ocean, are compared. The seasonality in precipitation skill, the local precipitation–SST relationship, and prediction skill related to Indian Ocean dipole mode (IODM) are examined. It is found that the importance of the accuracy of local SST and the presence of local air–sea coupling in the Indian Ocean has a strong seasonal dependence. Accurate local SSTs are important during the boreal fall season, whereas the local air–sea coupling is important during the boreal spring. The precipitation skill over the Indian Ocean during boreal winter is primarily from ENSO. However, ENSO impacts are better realized with the inclusion of an interactive ocean. For all four seasons, the simulation without the interannual variations of local SST in the Indian Ocean shows the least precipitation skill and a much weaker seasonality. It is also found that, for the simulation where the global SSTs are relaxed to the observations and hence maintain some level of active air–sea coupling, the observed seasonal cycle of precipitation–SST relationship is reproduced reasonably well. In addition, the analysis also shows that simulations with accurate SST forcing display high precipitation skill during strong IODM events, indicating that IODM SST acts as a forcing for the atmospheric variability.

Weakening of Indian summer monsoon rainfall in warming environment

Though over a century long period (1871–2010) the Indian summer monsoon rainfall (ISMR) series is stable, it does depict the decreasing tendency during the last three decades of the 20th century. Around mid-1970s, there was a major climate shift over the globe. The average all-India surface air temperature also shows consistent rise after 1975. This unequivocal warming may have some impact on the weakening of ISMR. The reduction in seasonal rainfall is mainly contributed by the deficit rainfall over core monsoon zone which happens to be the major contributor to seasonal rainfall amount. During the period 1976–2004, the deficit (excess) monsoons have become more (less) frequent. The monsoon circulation is observed to be weakened. The mid-tropospheric gradient responsible for the maintenance of monsoon circulation has been observed to be weakened significantly as compared to 1901–1975. The warming over western equatorial Indian Ocean as well as equatorial Pacific is more pronounced after mid-70s and the co-occurrence of positive Indian Ocean Dipole Mode events and El Nino events might have reinforced the large deficit anomalies of Indian summer monsoon rainfall during 1976–2004. All these factors may contribute to the weakening of ISMR.

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.

Seasonal climate modeling over the Indian Ocean by employing a 4D‐VAR coupled data assimilation approach

We carry out the first attempt to apply an adjoint method to a coupled general circulation model (CGCM) toward enhancing a skill in seasonal climate modeling. Focusing on 10‐day mean errors of a CGCM output, we optimize the oceanic initial conditions together with the bulk adjustment factors by employing a four‐dimensional variational data assimilation approach. We perform 9‐month‐long assimilation experiments independently every 6 months between January 1990 and March 2000. When using the optimized values for the initial conditions and the adjustment factors, a set of 9‐month‐long, 10‐member ensemble simulation always displays realistic seasonal cycle and its interannual modulations over the tropical Indian Ocean (e.g., growing, mature, and decaying phases of the Indian Ocean Dipole Mode events). The optimized values of the bulk adjustment factors primarily reduce the model biases in climatological fields, while the optimization of the oceanic initial conditions largely contributes to a realistic representation of the interannual modulations of seasonal cycle. In the overlapped seasons (i.e., January–March and July–September), the ensemble mean states derived from two experiments show only slight differences in seasonal climate variations over most of the Indian Ocean. These results validate that our assimilation approach is generally effective for advancing a seasonal climate modeling and for obtaining a realistic analysis that is compatible between atmosphere and ocean.

Pacific and Indian Ocean climate signals in a tree‐ring record of Java monsoon drought

AbstractExtreme climate conditions have dramatic socio‐economic impacts on human populations across the tropics. In Indonesia, severe drought and floods have been associated with El Niño‐Southern Oscillation (ENSO) events that originate in the tropical Indo‐Pacific region. Recently, an Indian Ocean dipole mode (IOD) in sea surface temperature (SST) has been proposed as another potential cause of drought and flood extremes in western Indonesia and elsewhere around the Indian Ocean rim. The nature of such variability and its degree of independence from the ENSO system are topics of recent debate, but understanding is hampered by the scarcity of long instrumental records for the tropics. Here, we describe a tree‐ring reconstruction of the Palmer Drought Severity Index (PDSI) for Java, Indonesia, that preserves a history of ENSO and IOD‐related extremes over the past 217 years. Extreme Javan droughts correspond well to known ENSO and IOD events in recent decades, and most extreme droughts before this recent period can be explained by known ENSO episodes. Coral proxies from regions near or within the two poles of the IOD show good agreement with Javan PDSI extremes over the past ∼150 years. The El Niño of 1877, in conjunction with a positive IOD, was one of the most intense and widespread episodes of the past two centuries, based on instrumental and proxy data from across the tropical Indo‐Pacific and Asian monsoon regions. Although Java droughts typically show the expected association with El Niño‐like conditions and failed Indian monsoons, others (mainly linked to positive IOD conditions) co‐occur with a strengthened Indian monsoon, suggesting linkages between the Indian monsoon, Indonesian drought and Indian Ocean climatic variability. The close associations between the Java PDSI, ENSO and Indian Ocean climate are consistent with the hypothesis that interannual ENSO to decadal ENSO‐like modes interact to generate dipole‐like Indian Ocean variability. Copyright © 2008 Royal Meteorological Society

Global Warming and the Weakening of the Tropical Circulation

Abstract This study examines the response of the tropical atmospheric and oceanic circulation to increasing greenhouse gases using a coordinated set of twenty-first-century climate model experiments performed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The strength of the atmospheric overturning circulation decreases as the climate warms in all IPCC AR4 models, in a manner consistent with the thermodynamic scaling arguments of Held and Soden. The weakening occurs preferentially in the zonally asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component of the tropical circulation and is shown to induce substantial changes to the thermal structure and circulation of the tropical oceans. Evidence suggests that the overall circulation weakens by decreasing the frequency of strong updrafts and increasing the frequency of weak updrafts, although the robustness of this behavior across all models cannot be confirmed because of the lack of data. As the climate warms, changes in both the atmospheric and ocean circulation over the tropical Pacific Ocean resemble “El Niño–like” conditions; however, the mechanisms are shown to be distinct from those of El Niño and are reproduced in both mixed layer and full ocean dynamics coupled climate models. The character of the Indian Ocean response to global warming resembles that of Indian Ocean dipole mode events. The consensus of model results presented here is also consistent with recently detected changes in sea level pressure since the mid–nineteenth century.

Supplement to State of the Climate in 2006

Abstract The State of the Climate in 2006 report summarizes the year's weather and climate conditions, both globally and regionally. In addition, the year is placed into a long-term climatological context. Furthermore, notable events are also discussed. Overall global temperatures were fifth or sixth warmest on record, depending on the dataset, continuing an upward trend in temperatures. Many countries and regions experienced their record warmest year (or tied for warmest), including Spain, the Netherlands, the United Kingdom, and China, as well as parts of Australia and Canada. In many regions, the warmth in 2006 is statistically indistinguishable from the record warmth in 1998. However, 1998 was influenced by the unprecedented warming associated with the record 1997/98 El Niño, whereas 2006 was marked by a 2005/06 La Niña that transitioned into a weak-to-moderate 2006/07 El Niño. Consistent with the warming, sea ice extent in both polar regions reached record or near-record minima. In addition, Antarctic ozone concentrations reached an all-time minimum. Also, carbon dioxide measurements increased in the atmosphere by 2.3 parts per million (ppm) in 2006 to reach a global average of 381.1 ppm. In the global oceans, sea levels were above average for ∼80% of the ocean. The global mean sea level anomaly change of +6 mm from 2005 was the highest increase since the altimeter record began in 1993. Relative sea level change was also the highest ever recorded. Significant heat flux and current anomalies were observed in the regions of the 2006 El Niño and Indian Ocean dipole mode event. Despite the warmth around the globe, tropical cyclone counts were near average. However, Tropical Cyclone Larry made landfall in northern Australia as one of the most intense storms in decades. Following the record Atlantic hurricane season of 2005, the 2006 season was very quiet.

Impact of southeast Indian Ocean sea surface temperature anomalies on monsoon-ENSO-dipole variability in a coupled ocean–atmosphere model

Recent studies show that SouthEast Indian Ocean (SEIO) SSTs are a highly significant precursor of transitions of the whole monsoon-El Nino-Southern Oscillation (ENSO) system during recent decades. However, the reasons for this specific interannual variability have not yet been identified unequivocally from the observations. Among these, the possibility of SEIO SST-driven variability in the monsoon-ENSO system is investigated here by inserting positive/negative SEIO temperature anomalies in the February’s restart files of a state-of-the-art coupled General Circulation Model (GCM) for 49 years of a control simulation. For each year of the control simulation, the model was then integrated for a 1-year period in fully coupled mode. These experiments show that Indian Summer Monsoon (ISM) and tropical Indian Ocean Dipole Mode (IODM) events are significantly influenced by the SEIO temperature perturbations inserted in the mixed layer of the coupled GCM several months before. A warm SEIO perturbation, inserted in late boreal winter, slowly propagates northward during the following seasons, implies enhanced ISM rainfall and finally triggers a negative IODM pattern during boreal fall in agreement with observations. A reversed evolution is simulated for a cold SEIO perturbation. It is shown that the life cycle of the simulated SEIO signal is driven by the positive wind-evaporation-SST, coastal upwelling and wind-thermocline-SST feedbacks. Further diagnosis of the sensitivity experiments suggests that stronger ISM and IODM variabilities are generated by excluding the El Nino years of the control simulation or when the initial background state in the SEIO is warmer. This finding confirms that IODM events may be triggered by multiple factors, other than ENSO, including subtropical SEIO SST anomalies. However, the ENSO mode does not react significantly to the SEIO temperature perturbation in the perturbed runs even though the simulated Pacific pattern agrees with the observations during boreal fall. These discrepancies with the observations may be linked to model biases in the Pacific and to the too strong ENSO simulated by this coupled GCM. These modeling evidences confirm that subtropical Indian Ocean SST anomalies generated by Mascarene high pulses during austral summer are a significant precursor of both ISM and IODM events occuring several months later.

Southern meridional atmospheric circulation associated with IOD

Using the monthly wind and sea surface temperature (SST) data, southern meridional atmospheric circulation cells associated with the Indian Ocean Dipole Mode (IOD) events in the Indian Ocean are for the first time described and examined. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. During the four different phases of the positive IOD events, the anomalous meridional Hadley circulation over the western Indian Ocean shows that the air rises in the tropics, flows poleward in the upper troposphere, sinks in the subtropics, and returns back to the tropics in the lower troposphere. The anomalous Hadley circulation over the eastern Indian Ocean is opposite to that over the western Indian Ocean. During positive IOD events, the meridional Hadley circulation over the eastern Indian Ocean is weakened while it is strengthened over the western Indian Ocean. Correlation analysis between the IOD index and the indices of the Hadley cells also proves that, the atmospheric circulation patterns are evident in every IOD event over the period of record.

Remote origins of interannual variability in the Indonesian Throughflow region from data and a global Parallel Ocean Program simulation

The mean and interannual variability of the thermal structure of the World Ocean Circulation Experiment (WOCE) repeat IX1‐expendable bathythermograph (XBT) transect between Java and Western Australia were compared statistically for the years 1987–1997 with concurrent, co‐located output from a global eddy‐permitting configuration of the Parallel Ocean Program (POP) model forced with realistic surface fluxes. Dominant variability at long timescales for both model and data in the southern IX1 region was associated with Pacific El Niño–Southern Oscillation (ENSO) events; at the northern end it was due to remote equatorial Indian Ocean forcing and Indian Ocean Dipole Mode events. In the Indo‐Pacific domain the model reproduced the structure and magnitude of observed low‐frequency variability. Event analyses following the warm ENSO phase showed low‐frequency off‐equatorial Rossby waves interacting with the North Pacific western maritime boundary to reflect onto the equator and excite a coastally trapped response that propagated through the Indonesian seas and along the northwest coast of Australia. In turn, the signal progressively propagated away from this coast as free baroclinic Rossby waves to 90°E. Cross‐spectral analyses confirmed that on interannual timescales, both off‐equatorial and equatorial signals remotely forced in the Pacific were largely responsible for the strong observed and modeled variability at the southern end of IX1.

Indian Ocean Dipole mode events and austral surface air temperature anomalies

The impact of Indian Ocean Dipole (IOD) mode events on austral surface air temperature (SAT) variability was studied both by statistical analysis of observed/assimilated data and experiments with a mechanistic baroclinic atmospheric model. During the period of analysis (January 1958–December 1999), IOD events had the strongest impact on SAT anomalies during austral spring and hence, the analysis was focussed on this season. IOD events induced large scale, intercontinental correlations of SAT anomalies amongst Australia, Africa and South America. Surface temperature consistently rose (fell) abnormally and coherently in the subtropical regions of these continents during positive (negative) IOD events. Variability during non-IOD years was considerably weaker than during IOD years over these regions. Analysis of stream function anomalies at the 200 hPa level (source: NCEP/NCAR reanalysis) revealed a Rossby-wave train extending from the eastern Indian Ocean into the subtropical regions of the Pacific and Atlantic oceans. Further, the diagnosed Rossby-wave activity flux emanated from the eastern Indian Ocean and propagated along the subtropical and subpolar jet streams qualitatively in agreement with linear wave dynamics. Experiments with idealized forcing in a primitive equation mechanistic atmospheric model suggested that tropical convective anomalies in the Indian Ocean during IOD events likely affects the austral subtropics through stationary Rossby-wave propagation.

Mechanisms for anomalous warming in the western Indian Ocean during dipole mode events

During a typical Indian Ocean Dipole mode (IOD) event the weakening and reversal of winds in the central equatorial Indian Ocean lead to the development of unusually warm sea surface temperatures (SSTs) in the western Indian Ocean. Here we report an analysis of model results and observations for the 1990–1999 period, showing the anomalous warming that occurred during 1997–1998 and 1994–1995 IOD events in the western Indian Ocean. It was primarily the result of the internal oceanic processes within the Arabian Sea rather than a direct response to external forcing from the eastern Indian Ocean as previously suggested. We compare the evolution of subsurface temperatures in the Arabian Sea just prior to (1996–1997: Representative of a normal year) and during the IOD event (1997–1998) to infer the physical processes responsible for the anomalous warming during the IOD events. In a normal year such as 1996 the existence of a unique open ocean upwelling feature, the Arabian Sea Dome (ASD), raises the thermocline in the southern Arabian Sea, a result of the positive wind stress curl of the winter monsoon. During an IOD year (1997), with the collapse of the monsoon wind system in the Arabian Sea the cold tongue, the strip of cool water indicative of ASD that normally occupies the southern Arabian Sea, fails to develop. As a result, the thermocline remains deep, which in turn enhances the heat content of the upper ocean, leading to warm SSTs. Volume‐integrated heat budget terms in the ASD region illustrate the role of horizontal and vertical advection in the upper ocean warming. An implication is that SSTs remain higher than 28°C in the southern Arabian Sea, thereby supporting large‐scale deep convection in the atmosphere during the winter monsoon. A band of deep convective cloud forms directly over the warm ASD region during the IOD events, which bring greater than average rainfall over the southern Arabian Sea and east Africa, whereas convection is suppressed over the cold ASD region during the normal years resulting in less than average rainfall.

Impact of salinity on the 1997 Indian Ocean dipole event in a numerical experiment

The role of salinity during the onset and growth of the 1997 Indian Ocean Dipole Mode event is explored with an ocean general circulation model thoroughly validated to observations. In fall 1997, anomalous easterlies drive an Indian Ocean equatorial circulation similar to that regularly observed in the Pacific Ocean: a South Equatorial Current (SEC) near the equator, an Equatorial Under Current (EUC) subsurface, and intense upwelling in the eastern part of the basin. The SEC transports westward the eastern Indian Ocean fresh pool creating, along the equator, a shallow salinity stratification favoring the creation of a barrier layer. In our experiment, the shallow top of the thermocline limits the barrier layer thickness to about 10 m; nevertheless, salinity has a significant impact on sea surface temperature (SST). Indeed, the shallow salinity stratification along the equator traps the wind forcing in a thin surface mixed‐layer. The reduction of wind momentum penetration decreases the deceleration of the EUC, and the greater amplitude of wind momentum input in surface layers strengthens the SEC. This intensification of the equatorial zonal circulation increases the Sumatra upwelling and its associated meridional circulation. This strengthening of the whole equatorial circulation shifts upward the thermohaline structure and reinforces the cold SST anomaly off Sumatra by about 20%. Overall, the effect of salinity on the 1997 Indian Ocean Dipole is to reinforce the oceanic anomalies favoring a strengthening of the air‐sea interactions.

Structure of SST and Surface Wind Variability during Indian Ocean Dipole Mode Events: COADS Observations*

A study of the detailed spatiotemporal characteristics of the Indian Ocean dipole (IOD) mode in SST and surface winds using available observations from 1958 till 1997 is reported. The analysis is used to address several of the controversial issues regarding the IOD. One key finding of this study is that interdecadal fluctuations contribute strongly to tropical Indian Ocean (TIO) SST variability; in SST anomalies (SSTA) interdecadal variance is as strong as interannual variance. Over both the western and eastern TIO, an accelerated warming of SST after the mid-1970s is apparent. The lack of anticorrelation between western and eastern TIO SSTA occurs only in this latter half of the analysis period. In order to examine the hypothesis that the IOD is a part of ENSO evolution in the TIO, the temporal

The interannual variability in the tropical Indian Ocean as simulated by a CGCM

The interannual variability in the tropical Indian Ocean, and in particular the Indian Ocean dipole mode (IODM), is investigated using both observations and a multi-decadal simulations performed by the coupled atmosphere–ocean general circulation model SINTEX. Overall, the characteristics of the simulated IODM are close to the features of the observed mode. Evidence of significant correlations between sea level pressure anomalies in the southeastern Indian Ocean and sea surface temperature anomalies in the tropical Indian and Pacific Oceans have been found both in observations and a multi-decadal simulation. In particular, a positive SLP anomaly in the southeastern part of the basin seems to produce favorable conditions for the development of an IODM event. The role played by the ocean dynamics both in the developing and closing phases of the IODM events is also investigated. Our results suggest that, during the developing phase, the heat content and SST variability associated with the IODM are influenced by a local response of the ocean to the winds, and a remote response with the excitation of Kelvin and Rossby waves. Ocean wave dynamics appear to be important also during the dying phase of the IODM, when equatorial downwelling Kelvin waves transport positive heat content anomalies from the western to the eastern part of the basin, suppressing the zonal heat content anomaly gradient. The results obtained from the model suggest a mechanism for the IODM. This mechanism is generally consistent with the characteristics of the observed IODM. Furthermore, it might give some clue in understanding the correlation between IODM and ENSO activity found both in the model and in the observations.

Possible impacts of Indian Ocean Dipole mode events on global climate

Impacts of Indian Ocean Dipole mode (IOD) events on global climate are estimated by correlation/regression analysis. The analysis examined land rain and temperature and 3-dimensional atmospheric variables for a 42 yr period from January 1958 to December 1999. The correlation between IOD and the El Nino Southern Oscillation (ENSO) is accounted for using the multiple regression technique. We used partial correlation coefficients to describe the unique contribution of IOD to climate variability, independent of ENSO. In the Indian Ocean rim countries, IOD is associated with significant temperature and rain variability manifesting 2 large-scale patterns. In one, land tem- perature and rain are anomalously high over countries west of the Indian Ocean and anomalously low to its east. In the second pattern, enhanced rainfall is found over the Asian monsoon trough, extending from Pakistan up to southern China. Also noted are IOD impacts on several regions remote from the Indian Ocean. Strong correlation is found over Europe, northeast Asia, North and South America and South Africa concurrent with IOD events. Over these regions, positive IOD events are associated with warm land surface anomalies and reduced rainfall. The troposphere above the Indian Ocean exhibits strong variability during IOD events characterized by the following structures: (1) a Walker cell anomaly over the equator; (2) a deep modulation of monsoon westerlies; and (3) a Hadley cell anomaly over the Bay of Bengal. In the extratropics, IOD is associated with equivalent barotropic geopotential anomalies. These assume annular structure in the northern hemisphere, but Rossby wave train structure in the southern hemisphere.