Indian Ocean Heating – More Evidence Of The Dominate Role Of Regional Circulation Patterns

Suryachandra A. Rao, Ashish R. Dhakate, Subodh K. Saha, Somnath Mahapatra, Hemantkumar S. Chaudhari, Samir Pokhrel and Sobhan K. Sahu, 2012, Why is Indian Ocean warming consistently?
Climatic Change Volume 110, Numbers 3-4, 709-719, DOI: 10.1007/s10584-011-0121-x

The abstract reads [highlight added]

“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 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.”

As the authors succinctly conclude in the final section of their paper

This study explains the consistent warming in last two decades.”

My only issue with their study that the Indian Ocean has not been consistently warming. At the top of this post is the latest surface temperature anomaly analysis for this region. While parts of the southern part of the ocean are warmer than average, the northern part is not. In the Rao et al 2012 paper shows no warming since the late 1990s – and perhaps even earlier in the 1990s (see their Figure 1).

In any case, warming that has occurred in this region can be explained by regional circulation changes, and while human climate forcings certainly could be playing an important role (e.g. the heterogeneous heating from black carbon – see Figure 1 bottom in Matsui and Pielke, 2006), a global average warming is not the most important effect (if it has any appreciable effect at all). Moreover, the multi-decadal global model predictions, to my knowledge,  have failed to skillfully predict the changes observed in the Indian Ocean.

 source of image

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