Swadhin K. Behera

Subtropical SST dipole events in the southern Indian Ocean

Sea surface temperature (SST) anomalies in the subtropical southern Indian Ocean show interannual dipole events that are seasonally phase-locked to the austral summer. A positive phase of the event is characterized by cold SST anomalies in the eastern part i.e. off Australia and warm SST anomalies in the southwestern part, south of Madagascar. Such an event is found to produce above normal rainfall over many regions in south-central Africa. The cooling of SST in the eastern part is mainly caused by the enhanced evaporation. This is associated with stronger winds along the eastern edge of the subtropical high, which is strengthened and shifted slightly to the south during the event. On the other hand, relative decrease in the seasonal latent heat loss due to reduced evaporation dominates the warming in the southwestern part. Evolution of such subtropical dipole events shows quite a contrast to that of the tropical dipole events discovered recently in the Indian Ocean.

Interannual subsurface variability in the tropical Indian Ocean with a special emphasis on the Indian Ocean Dipole

Interannual variability in the subsurface tropical Indian Ocean (TIO) is studied using three independent data sets: satellite derived sea-level data, an ocean general circulation model simulation, and in situ upper-ocean temperature data. It is found that significant interannual variability in the TIO is confined to the north of 15°S. Unlike the Pacific Ocean, the dominant modes of interannual variability in the Indian Ocean do not show co-variability between the surface and the subsurface. In contrast to the sea-surface temperature variability dominated by the El Nino and Southern Oscillation, subsurface variability is governed by the Indian Ocean Dipole in the TIO. The dominant mode of the interannual variability in the subsurface TIO is characterized by a dipole. Its evolution is controlled by equatorial ocean dynamics forced by zonal winds in the equatorial region. The subsurface dipole provides the delayed time required to reverse the phase of the surface dipole in the following year. The second dominant mode of interannual variability shows the interesting quasi-biennial tendency. It is found that the turnabout of the phase of the subsurface dipole leads to the quasi-biennial behavior of the TIO. Analysis of in situ subsurface temperature data substantiates this finding.

Unusual ocean‐atmosphere conditions in the tropical Indian Ocean during 1994

The southeastern tropical Indian Ocean (SETIO) was characterized by unusually cold sea surface temperature (SST) and strong northwestward alongshore surface winds during 1994. Using multi-source data sets including ocean model simulation, two key processes are identified for the SETIO cooling. Entrainment cooling produced most of the negative SST anomaly near the coast whereas evaporative cooling dominated the process away from the coast. Convection was anomalously suppressed over SETIO and the divergence of moist air from the region helped the local evaporative process. This also led to anomalous moisture transports that explain the enhanced convection over the central equatorial Indian Ocean, India and East Asia. The positive feedback between the enhanced and suppressed convection regions in turn helped maintain the surface wind anomalies. These evidences clearly indicate the existence of an ocean-atmosphere coupled phenomenon in the Indian Ocean during 1994.

Coupled Ocean‐Atmosphere Variability in the Tropical Indian Ocean

The Indian Ocean Dipole (IOD) is a natural ocean-atmosphere coupled mode that plays important roles in seasonal and interannual climate variations. The coupled mode locked to boreal summer and fall is distinguished as a dipole in the SST anomalies that are coupled to zonal winds. The equatorial winds reverse their direction from westerlies to easterlies during the peak phase of the positive IOD events when SST is cool in the east and warm in the west. In response to changes in the wind, the thermocline rises in the east and subsides in the west. Subsurface equatorial long Rossby waves play a major role in strengthening SST anomalies in the central and western parts. The SINTEX-F1 coupled model results support the observational finding that these equatorial Rossby waves are coupled to the surface wind forcing associated with IOD rather than ENSO. The ENSO influence is only distinct in off-equatorial latitudes south of 10°S. Although IOD events dominate the ocean-atmosphere variability during its evolution, their less frequent occurrence compared to ENSO events leads the mode to the second seat in the interannual variability. Therefore, it is necessary to remove the most dominant uniform mode to capture the IOD statistically. The seasonally stratified correlation between the indices of IOD and ENSO peaks at 0.53 in September-November. This means that only one third of IOD events are associated with ENSO events. Since a large number of IOD events are not associated with ENSO events, the independent nature of IOD is examined using partial correlation and pure composite techniques. Through changes in atmospheric circulation and water vapor transport, a positive IOD event causes drought in Indonesia, above normal rainfall in Africa, India, Bangladesh and Vietnam, and dry as well as hot summer in Europe, Japan, Korea and East China. In the Southern Hemisphere, the positive IOD causes dry winter in Australia, and dry as well as warm conditions in Brazil. The identification of IOD events has raised a new possibility to make a real advance in the predictability of seasonal and interannual climate variations that originate in the tropics.

A CGCM Study on the Interaction between IOD and ENSO

Abstract An atmosphere–ocean coupled general circulation model known as the Scale Interaction Experiment Frontier version 1 (SINTEX-F1) model is used to understand the intrinsic variability of the Indian Ocean dipole (IOD). In addition to a globally coupled control experiment, a Pacific decoupled noENSO experiment has been conducted. In the latter, the El Nino–Southern Oscillation (ENSO) variability is suppressed by decoupling the tropical Pacific Ocean from the atmosphere. The ocean–atmosphere conditions related to the IOD are realistically simulated by both experiments including the characteristic east–west dipole in SST anomalies. This demonstrates that the dipole mode in the Indian Ocean is mainly determined by intrinsic processes within the basin. In the EOF analysis of SST anomalies from the noENSO experiment, the IOD takes the dominant seat instead of the basinwide monopole mode. Even the coupled feedback among anomalies of upper-ocean heat content, SST, wind, and Walker circulation over the Indian...

Seasonal Climate Predictability in a Coupled OAGCM Using a Different Approach for Ensemble Forecasts

Abstract Predictabilities of tropical climate signals are investigated using a relatively high resolution Scale Interaction Experiment–Frontier Research Center for Global Change (FRCGC) coupled GCM (SINTEX-F). Five ensemble forecast members are generated by perturbing the model’s coupling physics, which accounts for the uncertainties of both initial conditions and model physics. Because of the model’s good performance in simulating the climatology and ENSO in the tropical Pacific, a simple coupled SST-nudging scheme generates realistic thermocline and surface wind variations in the equatorial Pacific. Several westerly and easterly wind bursts in the western Pacific are also captured. Hindcast results for the period 1982–2001 show a high predictability of ENSO. All past El Nino and La Nina events, including the strongest 1997/98 warm episode, are successfully predicted with the anomaly correlation coefficient (ACC) skill scores above 0.7 at the 12-month lead time. The predicted signals of some particular e...

Influence of the state of the Indian Ocean Dipole on the following year’s El Niño

El Nino-Southern Oscillation (ENSO) consists of irregular episodes of warm El Nino and cold La Nina conditions in the tropical Pacific Ocean(1), with significant global socio-economic and environmental impacts(1). Nevertheless, forecasting ENSO at lead times longer than a few months remains a challenge(2,3). Like the Pacific Ocean, the Indian Ocean also shows interannual climate fluctuations, which are known as the Indian Ocean Dipole(4,5). Positive phases of the Indian Ocean Dipole tend to co-occur with El Nino, and negative phases with La Nina(6-9). Here we show using a simple forecast model that in addition to this link, a negative phase of the Indian Ocean Dipole anomaly is an efficient predictor of El Nino 14 months before its peak, and similarly, a positive phase in the Indian Ocean Dipole often precedes La Nina. Observations and model analyses suggest that the Indian Ocean Dipole modulates the strength of the Walker circulation in autumn. The quick demise of the Indian Ocean Dipole anomaly in November-December then induces a sudden collapse of anomalous zonal winds over the Pacific Ocean, which leads to the development of El Nino/La Nina. Our study suggests that improvements in the observing system in the Indian Ocean region and better simulations of its interannual climate variability will benefit ENSO forecasts.

Extended ENSO Predictions Using a Fully Coupled Ocean–Atmosphere Model

Abstract Using a fully coupled global ocean–atmosphere general circulation model assimilating only sea surface temperature, the authors found for the first time that several El Nino–Southern Oscillation (ENSO) events over the past two decades can be predicted at lead times of up to 2 yr. The El Nino condition in the 1997/98 winter can be predicted to some extent up to about a 1½-yr lead but with a weak intensity and large phase delay in the prediction of the onset of this exceptionally strong event. This is attributed to the influence of active and intensive stochastic westerly wind bursts during late 1996 to mid-1997, which are generally unpredictable at seasonal time scales. The cold signals in the 1984/85 and 1999/2000 winters during the peak phases of the past two long-lasting La Nina events are predicted well up to a 2-yr lead. Amazingly, the mild El Nino–like event of 2002/03 is also predicted well up to a 2-yr lead, suggesting a link between the prolonged El Nino and the tropical Pacific decadal va...

Interaction between El Niño and Extreme Indian Ocean Dipole

Abstract Climate variability in the tropical Indo-Pacific sector has undergone dramatic changes under global ocean warming. Extreme Indian Ocean dipole (IOD) events occurred repeatedly in recent decades with an unprecedented series of three consecutive episodes during 2006–08, causing vast climate and socioeconomic effects worldwide and weakening the historic El Nino–Indian monsoon relationship. Major attention has been paid to the El Nino influence on the Indian Ocean, but how the IOD influences El Nino and its predictability remained an important issue to be understood. On the basis of various forecast experiments activating and suppressing air–sea coupling in the individual tropical ocean basins using a state-of-the-art coupled ocean–atmosphere model with demonstrated predictive capability, the present study shows that the extreme IOD plays a key role in driving the 1994 pseudo–El Nino, in contrast with traditional El Nino theory. The pseudo–El Nino is more frequently observed in recent decades, coinci...

The Role of the Western Arabian Sea Upwelling in Indian Monsoon Rainfall Variability

Abstract The Indian summer monsoon rainfall has complex, regionally heterogeneous, interannual variations with huge socioeconomic impacts, but the underlying mechanisms remain uncertain. The upwelling along the Somalia and Oman coasts starts in late spring, peaks during the summer monsoon, and strongly cools the sea surface temperature (SST) in the western Arabian Sea. They restrict the westward extent of the Indian Ocean warm pool, which is the main moisture source for the monsoon rainfall. Thus, variations of the Somalia–Oman upwelling can have significant impacts on the moisture transport toward India. Here the authors use both observations and an advanced coupled atmosphere–ocean general circulation model to show that a decrease in upwelling strengthens monsoon rainfall along the west coast of India by increasing the SST along the Somalia–Oman coasts, and thus local evaporation and water vapor transport toward the Indian Western Ghats (mountains). Further observational analysis reveals that such decre...