National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2011

Ocean pre-conditioning for Cyclone Nargis in the Bay of Bengal: Interaction between Rossby waves, surface fresh waters, and sea surface temperatures

Yu, L., and M.J. McPhaden

J. Phys. Oceanogr., 41(9), 1741–1755, doi: 10.1175/2011JPO4437.1 (2011)

An in-depth data analysis was conducted to understand the occurrence of a strong sea surface temperature (SST) front in the central Bay of Bengal before the formation of Cyclone Nargis in April 2008. Nargis changed its course after encountering the front and tracked along the front until making landfall. One unique feature of this SST front was its coupling with high sea surface height anomalies (SSHA), which is unusual for a basin where SST is normally uncorrelated with SSHA. The high SSHA were associated with downwelling Rossby waves, and the interaction between downwelling and surface fresh waters was a key mechanism to account for the observed SST-SSHA coupling.

The near-surface salinity field in the Bay is characterized by strong stratification and a pronounced horizontal gradient, with low salinity in the northeast. During the passage of downwelling Rossby waves, freshening of the surface layer was observed when surface velocities were west-southward. Horizontal convergence of fresh water associated with downwelling Rossby waves increased the buoyancy of the upper layer and shoaled the mixed layer within a few meters of the surface. Surface heating trapped in the thin mixed layer caused the fresh layer to warm, while the increase in buoyancy from low-salinity waters enhanced the high SSHA associated with Rossby waves. Thus, high SST coincided with high SSHA.

The dominant role of salinity in controlling high SSHA suggests that caution should be exercised when computing hurricane heat potential in the Bay from SSHA. This situation is different from most tropical oceans where temperature has the dominant effect on SSHA.

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