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Title: Significant influence of the boreal summer monsoon flow on the Indian Ocean response during dipole events
Authors: Krishnan R
Swapna P 
Keywords: Cross-equatorial flow;Equatorial undercurrents;Equatorial water;Heat contents;Indian ocean;Indian ocean dipoles;Meridional overturning circulations;Nonlinear amplification;Nonlinear dependence;North India;Ocean heat content;Ocean model;Sumatra;Summer monsoon;Supplementary data;Thermocline shoaling;Tropical Indian ocean;Wind speed, Amplification;Carbon fiber reinforced plastics;Enthalpy;Wind stress, Oceanography, air-sea interaction;atmosphere-ocean coupling;atmosphere-ocean system;Hadley cell;meridional circulation;monsoon;nonlinearity;precipitation (climatology);sea surface temperature;upwelling;wind stress;wind velocity;zonal flow, Indian Ocean;Indian Ocean (Tropical)
Issue Date: 2009
Publisher: Journal of Climate
American Meteorological Society
Source: Volume no: 22
Issue no: 21
Abstract: A majority of positive Indian Ocean dipole (IOD) events in the last 50 years were accompanied by enhanced summer monsoon circulation and above-normal precipitation over central-north India. Given that IODs peak during boreal autumn following the summer monsoon season, this study examines the role of the summer monsoon flow on the Indian Ocean (IO) response using a suite of ocean model experiments and supplementary data diagnostics. The present results indicate that, if the summer monsoon Hadley-type circulation strengthens during positive IOD events, then the strong off-equatorial southeasterly winds over the northern flanks of the intensified Australian high can effectively promote upwelling in the southeastern tropical Indian Ocean and amplify the zonal gradient of the IO heat content response. While it is noted that a strong monsoon cross-equatorial flow by itself may not generate a dipolelike response, a strengthening (weakening) of monsoon easterlies to the south of the equator during positive IOD events tends to reinforce (hinder) the zonal gradient of the upper-ocean heat content response. The findings show that an intensification of monsoonal winds during positive IOD periods produces nonlinear amplification of easterly wind stress anomalies to the south of the equator because of the nonlinear dependence of wind stress on wind speed. It is noted that such an off-equatorial intensification of easterlies over the SH enhances upwelling in the eastern IO off Sumatra-Java, and the thermocline shoaling provides a zonal pressure gradient, which drives anomalous eastward equatorial undercurrents (EUC) in the subsurface. Furthermore, the combination of positive IOD and stronger-than-normal monsoonal flow favors intensification of shallow transient meridional overturning circulation in the eastern IO and enhances the feed of cold subsurface off-equatorial waters to the EUC.
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