CSIR NET Dec 2015 Q137: Physical Oceanography

The Wyrtki jet is a strong, eastward-flowing surface current along the equator in the Indian Ocean that develops twice a year, during the spring (April-May) and fall (October-November) inter-monsoon transition periods. During these transition seasons, the Indian Ocean experiences a brief but well-organized period of westerly (eastward-blowing) winds along the equator, distinct from the southwesterly monsoon winds of summer and the northeasterly winds of winter, which are more meridionally oriented and less conducive to driving a strong zonal equatorial current. These organized equatorial westerlies during spring and fall directly drive the fast, narrow, eastward-flowing Wyrtki jet via wind stress forcing along the equatorial waveguide, matching option 2.

Option 1 (spring and fall due to organized easterly winds) is incorrect regarding wind direction; it is specifically westerly (not easterly) winds during the transition seasons that drive the eastward Wyrtki jet. Option 2 is correct: the Wyrtki jet forms during the spring and fall inter-monsoon periods, driven by organized equatorial westerly winds unique to those transition seasons. Option 3 (summer due to southwesterly winds) is incorrect because the summer monsoon's southwesterly winds have a strong meridional (cross-equatorial) component and do not produce the same organized zonal equatorial wind forcing needed for the Wyrtki jet, which is instead suppressed during the summer monsoon. Option 4 (winter due to northeasterly winds) is similarly incorrect, as the winter monsoon's northeasterly winds are also not the organized equatorial westerlies responsible for driving the Wyrtki jet.