Intraseasonal variability of Indian Ocean sea surface temperature during boreal winter: Madden‐Julian Oscillation versus submonthly forcing and processes

Abstract

Intraseasonal variability of Indian Ocean sea surface temperature (SST) during boreal winter is investigated by analyzing available data and a suite of solutions to an ocean general circulation model for 1998–2004. This period covers the QuikSCAT and Tropical Rainfall Measuring Mission (TRMM) observations. Impacts of the 30–90 day and 10–30 day atmospheric intraseasonal oscillations (ISOs) are examined separately, with the former dominated by the Madden‐Julian Oscillation (MJO) and the latter dominated by convectively coupled Rossby and Kelvin waves. The maximum variation of intraseasonal SST occurs at 10°S–2°S in the wintertime Intertropical Convergence Zone (ITCZ), where the mixed layer is thin and intraseasonal wind speed reaches its maximum. The observed maximum warming (cooling) averaged over (60°E–85°E, 10°S–3°S) is 1.13°C (−0.97°C) for the period of interest, with a standard deviation of 0.39°C in winter. This SST change is forced predominantly by the MJO. While the MJO causes a basin‐wide cooling (warming) in the ITCZ region, submonthly ISOs cause a more complex SST structure that propagates southwestward in the western‐central basin and southeastward in the eastern ocean. On both the MJO and submonthly timescales, winds are the deterministic factor for the SST variability. Short‐wave radiation generally plays a secondary role, and effects of precipitation are negligible. The dominant role of winds results roughly equally from wind speed and stress forcing. Wind speed affects SST by altering turbulent heat fluxes and entrainment cooling. Wind stress affects SST via several local and remote oceanic processes.