Negative Indian Ocean Dipole Events Research Articles

HIGH-RESOLUTION SURFACE CIRCULATION OF THE BAY OF BENGAL DERIVED FROM SATELLITE OBSERVATION DATA

High-resolution surface circulation and its variability of the Bay of Bengal are derived by combining surface drifter and satellite observation data. The satellite altimetry data, satellitetracked surface drifter data and ocean surface winds from satellite scatterometers during 1993-2012 are used. The estimated velocities show good agreement with in-situ acoustic Doppler current profiler observations. The estimated velocity components are significantly correlated with monthly mean velocity components from Research Moored Array for African- Asian-Australian Monsoon Analysis and Prediction buoy data. The mean circulation exhibits the strong western boundary current, zonal currents and weak eastern boundary flow. Large spatial and temporal variations are found in the western boundary current and intense mesoscale eddy activity in the western Bay of Bengal. Significant changes in surface circulation during positive and negative Indian Ocean Dipole (IOD) events are evident. During positive IOD, the eastward equatorial jet is reversed and the western boundary current is much weakened. Meanwhile, the western boundary current is the prominent flow during negative IOD events. High eddy kinetic energy is found during strong IOD events.

Spatial and temporal variability of central Indian Ocean salinity fronts observed by SMOS

Soil Moisture and Ocean Salinity (SMOS) Sea Surface Salinity (SSS) measurements, in addition to other data, are used to study the occurrence of salinity fronts in the poorly sampled Indian Ocean, which exhibits unique seasonal wind and current reversals. The strength and extent of salinity fronts are analyzed to understand their variabilities. Salinity fronts computed from SMOS compare favorably with those computed from Argo floats data, with SMOS showing higher magnitudes and more detailed structure of the fronts than Argo. Strong salinity fronts are observed along the equatorial region, with meridional migration of the fronts following the migration of the Intertropical Convergence Zone (ITCZ) and meridional ocean currents. Linked with these fronts are less saline subsurface waters that are sustained by the ITCZ-influenced precipitation and warm near-surface layer. During the 2010 negative Indian Ocean Dipole event, the equatorial front was influenced more by the increased SSS along the equator rather than precipitation. Additional factors, such as advective processes, aid the dynamics of fronts in the tropical Indian Ocean. Zonal currents advect less saline waters from the eastern Indian Ocean into the central Indian Ocean. Additionally the Indonesian Throughflow transports less saline waters to the region south of the equator, which eventually influence SSS front dynamics.

Influences of IOD and ENSO to Indonesian Rainfall Variability: Role of Atmosphere-ocean Interaction in the Indo-pacific Sector

The relative influences of Indian Ocean dipole (IOD) and El Niño–Southern Oscillation (ENSO) on Indonesian rainfall are investigated for seasonal time scales. For the period 1960–2011, observation and reanalysis products during September to November (SON) are used to assess the impacts of ENSO and IOD in Indonesian region. Composite of SSTs and Indonesian rainfall anomalies shows detailed features in the different phases of ENSO and IOD. A distinct impact on rainfall anomalies is found during the years when an El Niño and a positive IOD event or a La Nina and a negative IOD event co-occur indicating the interplay of ENSO and IOD in generating rainfall anomalies in Indonesian region. The atmospheric circulation and sea surface temperatures associated with these responses are discussed. Using composite analysis of anomalies of rainfall, sea surface temperature (SSTs), and circulation at any atmospheric levels, it is shown that positive anomalies of rainfall over Indonesia start to be decreased when SSTs surrounding Indonesia are cool and The Walker Circulation is weakened, resulting in anomalous surface easterlies across Indonesia. The composite analysis of rainfall anomalies and the SSTs showed that rainfall variability in Indonesia is clearly influenced by IOD and ENSO phenomena. This study highlights the atmosphere–ocean interaction in Indo-Pacific sector which plays an important role on Indonesian rainfall variability.

Roles of Western and Eastern Boundary Reflections in the Interannual Sea Level Variations during Negative Indian Ocean Dipole Events

AbstractThe equatorial wave dynamics of sea level variations during negative Indian Ocean dipole (nIOD) events are investigated using the LICOM ocean general circulation model forced with the European Centre for Medium-Range Weather Forecast reanalysis wind stress and heat flux from 1990 to 2001. The work is a continuation of the study by Yuan and Liu, in which the equatorial wave dynamics during positive IOD events are investigated. The model has reproduced the sea level anomalies of satellite altimeter data well. Long equatorial waves extracted from the model output suggest two kinds of negative feedback during nIOD events: the western boundary reflection and the easterly wind bursts. During the strong 1998–99 nIOD event, the downwelling anomalies in the eastern Indian Ocean are terminated by persistent and strong upwelling Kelvin waves from the western boundary, which are reflected from the wind-forced equatorial Rossby waves over the southern central Indian Ocean. During the 1996–97 nIOD, however, the reflection of upwelling anomalies at the western boundary is terminated by the arrival of downwelling equatorial Rossby waves from the eastern boundary reflection in early 1997. Therefore, the negative feedback of this nIOD event is not provided by the western boundary reflection. The downwelling anomalies in the eastern basin during the 1996–97 nIOD event are terminated by easterly wind anomalies over the equatorial Indian Ocean in early 1997. The disclosed equatorial wave dynamics are important to the simulation and prediction of IOD evolution.

Impacts of Indo-Pacific Sea Surface Temperature Anomalies on the Summer Monsoon Circulation and Heavy Precipitation over Northwest India–Pakistan Region during 2010

Abstract Quite a few studies have documented the evolution of monsoon synoptic systems and midlatitude atmospheric blocking associated with the recent heavy precipitation and floods over northwest Pakistan during 2010. This period also witnessed a very unusual Indo-Pacific sea surface temperature (SST) evolution with a strong La Niña event in the Pacific, substantial Indian Ocean warming, and a negative Indian Ocean dipole event, together with significant enhancement of precipitation over both the equatorial western Pacific Ocean and the eastern Indian Ocean. Here, the authors perform a suite of high-resolution atmospheric general circulation model experiments to investigate the influence of Indo-Pacific SST anomalies on the South Asian monsoon circulation and heavy precipitation over Pakistan and adjoining northwest India during 2010. The realistic simulation of these rainfall anomalies using observed SSTs motivated the authors to explore the specific influence of Indian Ocean and Pacific SST anomalies through additional simulation experiments. The authors find that, in addition to strengthening of the Pacific Walker circulation, the anomalous intensification of east–west circulation over the Indian Ocean in 2010 was a key element in contributing to precipitation enhancement over the northwest India–Pakistan region. It is found that the subsiding branch of the east–west circulation over the Indian Ocean induced anomalous subsidence over the western tropical Indian Ocean and played a key role in inducing northward transport of moisture and promoting generation of strong upward motion and heavy precipitation events over the northwest India–Pakistan region.

Influence of positive and negative Indian Ocean Dipoles on ENSO via the Indonesian Throughflow: Results from sensitivity experiments

The role of the Indonesian Throughflow (ITF) in the influence of the Indian Ocean Dipole (IOD) on ENSO is investigated using version 2 of the Parallel Ocean Program (POP2) ocean general circulation model. We demonstrate the results through sensitivity experiments on both positive and negative IOD events from observations and coupled general circulation model simulations. By shutting down the atmospheric bridge while maintaining the tropical oceanic channel, the IOD forcing is shown to influence the ENSO event in the following year, and the role of the ITF is emphasized. During positive IOD events, negative sea surface height anomalies (SSHAs) occur in the eastern Indian Ocean, indicating the existence of upwelling. These upwelling anomalies pass through the Indonesian seas and enter the western tropical Pacific, resulting in cold anomalies there. These cold temperature anomalies further propagate to the eastern equatorial Pacific, and ultimately induce a La Nina-like mode in the following year. In contrast, during negative IOD events, positive SSHAs are established in the eastern Indian Ocean, leading to downwelling anomalies that can also propagate into the subsurface of the western Pacific Ocean and travel further eastward. These downwelling anomalies induce negative ITF transport anomalies, and an El Nino-like mode in the tropical eastern Pacific Ocean that persists into the following year. The effects of negative and positive IOD events on ENSO via the ITF are symmetric. Finally, we also estimate the contribution of IOD forcing in explaining the Pacific variability associated with ENSO via ITF.

Spatial and seasonal responses of precipitation in the Ganges and Brahmaputra river basins to ENSO and Indian Ocean dipole modes: implications for flooding and drought

Abstract. We evaluated the spatial and seasonal responses of precipitation in the Ganges and Brahmaputra basins as modulated by the El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) modes using Global Precipitation Climatology Centre (GPCC) full data reanalysis of monthly global land-surface precipitation data from 1901 to 2010 with a spatial resolution of 0.5° × 0.5°. The GPCC monthly total precipitation climatology targeting the period 1951–2000 was used to compute gridded monthly anomalies for the entire time period. The gridded monthly anomalies were averaged for the years influenced by combinations of climate modes. Occurrences of El Niño alone significantly reduce (88% of the long-term average (LTA)) precipitation during the monsoon months in the western and southeastern Ganges Basin. In contrast, occurrences of La Niña and co-occurrences of La Niña and negative IOD events significantly enhance (110 and 109% of LTA in the Ganges and Brahmaputra Basin, respectively) precipitation across both basins. When El Niño co-occurs with positive IOD events, the impacts of El Niño on the basins' precipitation diminishes. When there is no active ENSO or IOD events (occurring in 41 out of 110 years), precipitation remains below average (95% of LTA) in the agriculturally intensive areas of Haryana, Uttar Pradesh, Rajasthan, Madhya Pradesh, and Western Nepal in the Ganges Basin, whereas precipitation remains average to above average (104% of LTA) across the Brahmaputra Basin. This pattern implies that a regular water deficit is likely, especially in the Ganges Basin, with implications for the agriculture sector due to its reliance on consistent rainfall for successful production. Historically, major droughts occurred during El Niño and co-occurrences of El Niño and positive IOD events, while major flooding occurred during La Niña and co-occurrences of La Niña and negative IOD events in the basins. This observational analysis will facilitate well-informed decision making in minimizing natural hazard risks and climate impacts on agriculture, and supports development of strategies ensuring optimized use of water resources in best management practice under a changing climate.

Satellite and Argo Observed Surface Salinity Variations in the Tropical Indian Ocean and Their Association with the Indian Ocean Dipole Mode

Abstract This study investigates sea surface salinity (SSS) variations in the tropical Indian Ocean (IO) using the Aquarius/Satelite de Aplicaciones Cientificas-D (SAC-D) and the Soil Moisture and Ocean Salinity (SMOS) satellite data and the Argo observations during July 2010–July 2014. Compared to the Argo observations, the satellite datasets generally provide SSS maps with higher space–time resolution, particularly in the regions where Argo floats are sparse. Both Aquarius and SMOS well captured the SSS variations associated with the Indian Ocean dipole (IOD) mode. Significant SSS changes occurred in the central equatorial IO, along the Java–Sumatra coast, and south of the equatorial IO, due to ocean circulation variations. During the negative IOD events in 2010, 2013, and 2014, westerly wind anomalies strengthened along the equator, weakening coastal upwelling off Java and Sumatra and decreasing SSS. South of the equatorial IO, an anomalous cyclonic gyre changed the tropical circulation, which favored the eastward high-salinity tongue along the equator and the westward low-saline tongue in the south. An upwelling Rossby wave favored the increase of SSS farther to the south. During the positive IOD events in 2011 and 2012, the above-mentioned processes reversed, although the decrease of SSS was weaker in magnitude.

Variability of zonal currents in the eastern equatorial Indian Ocean on seasonal to interannual time scales

This study examines equatorial zonal current variations in the upper layers of eastern Indian Ocean in relation to variations in the Indian Ocean Dipole (IOD). The analysis utilizes data from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) and the European Centre for Medium-Range Weather Forecasts-Ocean Reanalysis System 4 (ECMWF-ORAS4). Surface currents are characterized by semiannual eastward flowing Wyrtki jets along the equator in boreal spring and fall, forced by westerly monsoon transition winds. The fall jet intensifies during negative IOD (NIOD) events when westerlies are anomalously strong but significantly weakens during positive IOD (PIOD) events when westerlies are anomalously weak. As zonal wind stress weakens during PIOD events, sea surface height becomes unusually low in the eastern basin and high in the west, setting up an anomalous pressure force that drives increased eastward transport in the thermocline. Opposite tendencies are evident during NIOD events in response to intensified equatorial westerlies. Current transport adjustments to anomalous zonal wind forcing during IOD events extend into the following year, consistent with the cycling of equatorial wave energy around the basin. A surface layer mass budget calculation for the eastern sea surface temperature (SST) pole of the IOD indicates upwelling of ∼2.9±0.7 Sv during normal periods, increasing by 40–50% during PIOD events and reducing effectively to zero during NIOD events. IOD-related variations in Wyrtki jet and thermocline transports are major influences on these upwelling rates and associated water mass transformations, which vary consistently with SST changes.

SMOS Mission Reveals the Salinity Structure of the Indian Ocean Dipole

This letter reports the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) observations of the sea surface salinity (SSS) structure during an Indian Ocean Dipole (IOD) event. Comparisons with Argo data show that the SMOS satellite is able to resolve the observed SSS pattern in the Indian Ocean despite some challenges in the northern Indian Ocean. Results of box averages for the Java Sumatra Coast (JSC) and South Central Indian Ocean (SCIO) regions show low SSS anomalies in the former and high SSS anomalies in the latter during the 2010 negative IOD event. Analyses of salt flux and salt budget terms suggest that, in the JSC region, salt tendency is an interplay between freshwater forcing and horizontal advection terms, with increased precipitation having a higher impact in driving SSS anomalies than advection. In the SCIO region, advection seems to be more important than the freshwater forcing term.

The asymmetric influence of the positive and negative IOD events on China's rainfall.

Rainfall anomalies over southern China are found to be asymmetricly influenced by the Indian Ocean Dipole (IOD), with a far stronger influence from positive IOD (pIOD) events. A greater convection anomaly and an equivalent-barotropic Rossby wave train response occurs during pIOD events than during negative IOD (nIOD) events. Over the Bay of Bengal (BOB) and South China Sea (SCS), an associated low-level anomalous anticyclone strengthens the southwesterlies during boreal fall (September, October and November, SON), when a pIOD matures. The increased moisture flux gives rise to the anomalously high rainfall over southern China. During its developing phase (boreal summer, June, July, and August, JJA), the influence of a pIOD event on the contemporaneous rainfall over southern China is weak, but a JJA pIOD index is highly correlated with fall rainfall. Therefore, this index can serve as a potential predictor for variations of boreal fall rainfall over southern China.

Effect of nonlinear advection on the Indian Ocean diploe asymmetry

The asymmetry of sea surface temperature anomaly (SSTA) amplitudes between the positive and negative phases of the Indian Ocean dipole (IOD) are studied. The dynamic effects on it are analyzed using a hybrid coordinate ocean model (HYCOM). It suggests that the IOD is still asymmetric even when forced by a symmetric wind stress, and the asymmetry of the SSTA in the eastern pole is strong while that in the western pole is almost insignificant during the mature phase (September-November (SON)). Thus, the IOD asymmetry is primarily caused by the asymmetry in the IODE. A heat budget analysis is also conducted for the mixed-layer temperature in the eastern Indian Ocean (IODE), which indicates that a nonlinear ocean advection cools both the positive and negative IOD events. Therefore, the nonlinear ocean advection is responsible for the asymmetry of the IOD.

SMOS reveals the signature of Indian Ocean Dipole events

The tropical Indian Ocean experiences an interannual mode of climatic variability, known as the Indian Ocean Dipole (IOD). The signature of this variability in ocean salinity is hypothesized based on modeling and assimilation studies, on account of scanty observations. Soil Moisture and Ocean Salinity (SMOS) satellite has been designed to take up the challenge of sea surface salinity remote sensing. We show that SMOS data can be used to infer the pattern of salinity variability linked with the IOD events. The core of maximum variability is located in the central tropical basin, south of the equator. This region is anomalously salty during the 2010 negative IOD event, and anomalously fresh during the 2011 positive IOD event. The peak-to-peak anomaly exceeds one salinity unit, between late 2010 and late 2011. In conjunction with other observational datasets, SMOS data allow us to draw the salt budget of the area. It turns out that the horizontal advection is the main driver of salinity anomalies. This finding is confirmed by the analysis of the outputs of a numerical model. This study shows that the advent of SMOS makes it feasible the quantitative assessment of the mechanisms of ocean surface salinity variability in the tropical basins, at interannual timescales.

The Association of Tropical and Extratropical Climate Modes to Atmospheric Blocking across Southeastern Australia

Abstract Relationships of the Indian Ocean dipole (IOD), El Niño–Southern Oscillation (ENSO), and the southern annular mode (SAM) with atmospheric blocking are investigated using a linear framework over the austral autumn–spring (cool) seasons for southeast Australia (SEA). Positive blocking events occurring at 130°–140°E increase the likelihood of cutoff low pressure systems developing that generate significant rainfall totals across SEA. In mid to late austral autumn (April–May), blocking is coherent with negative IOD events. During this season, a negative IOD event and blocking are associated with warm sea surface temperature anomalies in the eastern tropical Indian Ocean and a blocking high pressure cell south of Australia. An anomalous cyclonic pressure center over southern Australia directs tropical moisture flux anomalies to the region. Despite this, only a small portion of a negative IOD's impact on SEA rainfall comes through blocking events. During austral winter, ENSO is coherent with blocking; anomalous tropical moisture fluxes from the western Pacific during a La Niña merge with anomalous cyclonic flows centered over SEA, delivering enhanced rainfall via cutoff lows. The low pressure cell constitutes a center of the Southern Oscillation associated with ENSO. This ENSO-blocking coherence is considerably weaker in austral spring, whereby circulation anomalies associated with blocking resemble a SAM-like pattern. As such, a large portion of the SAM's impact on SEA spring rainfall occurs in conjunction with blocking events. The relative importance of associations between the dominant climate modes and blocking in generating the drought-breaking cool season precipitation in 2010 across SEA is discussed.

Asymmetry in the IOD and ENSO Teleconnection in a CMIP5 Model Ensemble and Its Relevance to Regional Rainfall

Abstract Recent studies have shown that the impact of the Indian Ocean dipole (IOD) on southern Australia occurs via equivalent barotropic Rossby wave trains triggered by convective heating in the tropical Indian Ocean. Furthermore, the El Niño–Southern Oscillation (ENSO) influence on southern Australian climate is exerted through the same pathway during austral spring. It is also noted that positive phase [positive IOD (pIOD) and El Niño] events have a much larger impact associated with their respective skewness. These phenomena play a significant role in the region's rainfall reduction in recent decades, and it is essential that climate models used for future projections simulate these features. Here, the authors demonstrate that climate models do indeed simulate a greater climatic impact on Australia for pIOD events than for negative IOD (nIOD) events, but this asymmetric impact is distorted by an exaggerated influence of La Niña emanating from the Pacific. The distortion results from biases in the Pacific in two respects. First, the tropical and extratropical response to La Niña is situated unrealistically too far westward and hence too close to Australia, leading to an overly strong impact on southeast Australia that shows up through the nIOD–La Niña coherence. Second, the majority of models simulate a positive sea surface temperature skewness in the eastern Pacific that is too weak, overestimating the impact of La Niña relative to that of El Niño. As such, the impact of the positive asymmetry in the IOD only becomes apparent when the impact of ENSO is removed. This model bias needs to be taken into account when analyzing projections of regional Australian climate change.

North Indian Ocean tropical cyclone activities influenced by the Indian Ocean Dipole mode

Using Joint Typhoon Warning Center tropical cyclone (TC) track data over the North Indian Ocean (NIO), National Centers for Environmental Prediction monthly reanalysis wind and outgoing long-wave radiation data, and National Oceanic and Atmospheric Administration sea surface temperature data from 1981 to 2010, spatiotemporal distributions of NIO TC activity and relationships with local sea surface temperature (SST) were studied with statistical diagnosis methods. Results of empirical orthogonal function (EOF) analysis of NIO TC occurrence frequency show that the EOF1 mode, which accounts for 16% of total variance, consistently represents variations of TC occurrence frequency over the whole NIO basin. However, spatial distributions of EOF1 mode are not uniform, mainly indicating variations of westward-moving TCs in the Bay of Bengal. The prevailing TC activity variation mode oscillates significantly on a quasi-5 year interannual time scale. NIO TC activity is notably influenced by the Indian Ocean dipole (IOD) mode. When the Indian Ocean is in a positive (negative) phase of the IOD, NIO SST anomalies are warm in the west (east) and cold in the east (west), which can weaken (strengthen) convection over the Bay of Bengal and eastern Arabian Sea, and cause anticyclonic (cyclonic) atmospheric circulation anomalies at low levels. This results in less (more) TC genesis and reduced (increased) opportunities for TC occurrence in the NIO. In addition, positive (negative) IOD events may strengthen (weaken) westerly steering flow over the Bay of Bengal, which further leads to fewer (more) westward-moving TCs which appear in regions west of 90°E in that bay.

Effects of climate variability on the distribution and fishing conditions of yellowfin tuna (Thunnus albacares) in the western Indian Ocean

Variations in the abundance and distribution of pelagic tuna populations have been associated with large-scale climate indices such as the Southern Oscillation Index in the Pacific Ocean and the North Atlantic Oscillation in the Atlantic Ocean. Similarly to the Pacific and Atlantic, variability in the distribution and catch rates of tuna species have also been observed in association with the Indian Ocean Dipole (IOD), a basin-scale pattern of sea surface and subsurface temperatures that affect climate in the Indian Ocean. The environmental processes associated with the IOD that drive variability in tuna populations, however, are largely unexplored. To better understand these processes, we investigated longline catch rates of yellowfin tuna and their distributions in the western Indian Ocean in relation to IOD events, sea surface water temperatures (SST) and estimates of net primary productivity (NPP). Catch per unit effort (CPUE) was observed to be negatively correlated to the IOD with a periodicity centred around 4 years. During positive IOD events, SSTs were relatively higher, NPP was lower, CPUE decreased and catch distributions were restricted to the northern and western margins of the western Indian Ocean. During negative IOD events, lower SSTs and higher NPP were associated with increasing CPUE, particularly in the Arabian Sea and seas surrounding Madagascar, and catches expanded into central regions of the western Indian Ocean. These findings provide preliminary insights into some of the key environmental features driving the distribution of yellowfin tuna in the western Indian Ocean and associated variability in fisheries catches.

An Asymmetry in the IOD and ENSO Teleconnection Pathway and Its Impact on Australian Climate

Abstract Recent research has shown that the climatic impact from El Niño–Southern Oscillation (ENSO) on middle latitudes west of the western Pacific (e.g., southeast Australia) during austral spring (September–November) is conducted via the tropical Indian Ocean (TIO). However, it is not clear whether this impact pathway is symmetric about the positive and negative phases of ENSO and the Indian Ocean dipole (IOD). It is shown that a strong asymmetry does exist. For ENSO, only the impact from El Niño is conducted through the TIO pathway; the impact from La Niña is delivered through the Pacific–South America pattern. For the IOD, a greater convection anomaly and wave train response occurs during positive IOD (pIOD) events than during negative IOD (nIOD) events. This “impact asymmetry” is consistent with the positive skewness of the IOD, principally due to a negative skewness of sea surface temperature (SST) anomalies in the east IOD (IODE) pole. In the IODE region, convection anomalies are more sensitive to a per unit change of cold SST anomalies than to the same unit change of warm SST anomalies. This study shows that the IOD skewness occurs despite the greater damping, rather than due to a breakdown of this damping as suggested by previous studies. This IOD impact asymmetry provides an explanation for much of the reduction in spring rainfall over southeast Australia during the 2000s. Key to this rainfall reduction is the increased occurrences of pIOD events, more so than the lack of nIOD events.

Sea surface salinity variability during the Indian Ocean Dipole and ENSO events in the tropical Indian Ocean

An ocean reanalysis that covers the period from 1871 to 2008 is used to analyze the interannual variability of sea surface salinity (SSS) in the tropical Indian Ocean. The reanalysis SSS and the SSS anomaly patterns during Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO) events are compared with patterns from Argo SSS data. The mean seasonal SSS variation is large in the northern Bay of Bengal compared with variations in the Arabian Sea and equatorial Indian Ocean. During a positive IOD event, positive SSS anomalies are found along the Sumatra coast that are due to the combination of wind‐driven upwelling of subsurface high‐salinity waters, enhanced evaporation, and anomalous surface circulation. The opposite is true, to a lesser extent, during negative IOD events. A dipole mode index for salinity (DMIS) based on SSS data and a new index based on the average of salinity in a region off the coast of Sumatra are introduced to monitor SSS variability during IOD and ENSO events. The impact of concomitant El Niño events on a positive IOD event is large with freshening (a negative SSS anomaly) in the equatorial Indian Ocean and salting (positive SSS anomaly) off the southern Sumatra coast. The (impact of) intense freshening reaches into the southwestern tropical Indian Ocean. The impact of concomitant La Niña with negative IOD is also large with an intense freshening in the southeastern Arabian Sea and salting off the northern Sumatra coast.

Interactions of ENSO, the IOD, and the SAM in CMIP3 Models

Abstract Simulations of individual global climate drivers using models from the Coupled Model Intercomparison Project phase 3(CMIP3) have been examined; however, the relationship among them has not been assessed. This is carried out to address several important issues, including the likelihood of the southern annular mode (SAM) forcing Indian Ocean dipole (IOD) events and the possible impact of the IOD on El Niño–Southern Oscillation (ENSO) events. Several conclusions emerge from statistics based on multimodel outputs. First, ENSO signals project strongly onto the SAM, although ENSO-forced signals tend to peak before ENSO. This feature is similar to the situation associated with the IOD. The IOD-induced signal over southern Australia, through stationary equivalent Rossby barotropic wave trains, peak before the IOD itself. Second, there is no control by the SAM on the IOD, in contrast to what has been suggested previously. Indeed, no model produces a SAM–IOD relationship that supports a positive (negative) SAM driving a positive (negative) IOD event. This is the case even in models that do not simulate a statistically significant relationship between ENSO and the IOD. Third, the IOD does have an impact on ENSO. The relationship between ENSO and the IOD in the majority of models is far weaker than the observed. However, the ENSO’s influence on the IOD is boosted by a spurious oceanic teleconnection, whereby ENSO discharge–recharge signals transmit to the Sumatra–Java coast, generating thermocline anomalies and changing IOD properties. Without the spurious oceanic teleconnection, the influence of the IOD on ENSO is comparable to the impact of ENSO on the IOD. Other model deficiencies are discussed.