Why is Indian Ocean warming consistently?

Abstract

Observations have shown that the Indian Ocean is consistently warming and its warm pool is expanding, particularly in the recent decades. This paper attempts to investigate the reason behind these observations. Under global warming scenario, it is expected that the greenhouse gas induced changes in air-sea fluxes will enhance the warming. Surprisingly, it is found that the net surface heat fluxes over Indian Ocean warm pool (IOWP) region alone cannot explain the consistent warming. The warm pool area anomaly of IOWP is strongly correlated with the sea surface height anomaly, suggesting an important role played by the ocean advection processes in warming and expansion of IOWP. The structure of lead/lag correlations further suggests that Oceanic Rossby waves might be involved in the warming. Using heat budget analysis of several Ocean data assimilation products, it is shown that the net surface heat flux (advection) alone tends to cool (warm) the Ocean. Based on above observations, we propose an ocean-atmosphere coupled positive feedback 710 Climatic Change (2012) 110:709-719 mechanism for explaining the consistent warming and expansion of IOWP. Warming over IOWP induces an enhancement of convection in central equatorial Indian ocean, which causes anomalous easterlies along the equator. Anomalous easterlies in turn excite frequent Indian ocean Dipole events and cause anti-cyclonic wind stress curl in south-east and north-east equatorial Indian ocean. The anomalous wind stress curl triggers anomalous downwelling oceanic Rossby waves, thereby deepening the thermocline and resulting in advection of warm waters towards western Indian ocean. This acts as a positive feedback and results in more warming and westward expansion of IOWP.

Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

“…Meanwhile, the central-east Indian Ocean is characterized by a warm pool with SSTs greater than 28.0°C. Though previous studies demonstrate a basin-wide warming in the Indian Ocean during the past half century 21,22 , our analysis using long-term SST datasets show that over an extended period of 112 years, the western Indian Ocean (WIO, 50-65°E, 5°S-10°N) has experienced anomalous warming of 1.2°C (Fig. 2), which is 0.5°C greater than the warming over the warm pool region 28 .…”

“…However, the zonal direction of the wind anomalies near the equator appears different in the observations and the model. This is not an unexpected result as the periods under consideration and the model simulations are different, since the observed winds are available only for 1948-2012, a period during which the Indian Ocean experienced a basin-wide warming 21,22 .…”

“…Even though a warming trend over the Indian Ocean may result in an increased supply of moisture locally over the ocean, it is not necessary that the surplus moisture is carried on to the peninsula 29 . The sustained Indian Ocean warming in the west has led to a greater westward extension of this Indian Ocean warm pool in recent decades 21,28 . The expansion of the Indian Ocean warm pool can in turn modify the land-sea thermal gradient, which could modulate the strength and flow of the monsoon circulation and the moisture laden winds towards the South Asian subcontinent.…”

“…This implies that the relationship between a warm ocean and a weak monsoon is not linear. Since the ocean warming is a slow process, the heat has to build-up over several decades to make a substantial difference and reach the warm pool SST values (28.0°C) 21,28 . The sustained western Indian Ocean warming has°C Changing land-sea thermal contrast.…”

“…An increase in the land-sea thermal contrast should ideally increase the strength of the monsoon [17][18][19] . Also, the sea surface temperatures (SST) over the Indian Ocean have shown a warming trend over the last several decades [20][21][22] , potentially enhancing the supply of moisture to the monsoon regions and increasing the rainfall amounts 23 . In a recent review of climate change and the South Asian monsoon, Turner and Annamalai 13 point out that though the land-sea contrast and the increased moisture availability should favour an increased monsoon rainfall, the observations do not provide any convincing evidence for such a positive trend.…”