Weakening of spring Wyrtki jets in the Indian Ocean during 2006–2011

Abstract

Beginning in 2006, the Indian Ocean experienced climatologically anomalous conditions due to large-scale coupled air-sea interactions that influenced the surface circulation of the equatorial Indian Ocean. Here we present evidence from observations as well as a general circulation model to demonstrate that spring Wyrtki jets (WJ) were weak during the past 6 years and were even reversed to westward flow during 2008. We note that this weakening coincided with uniformly high sea level as well as positive east to west gradient anomalies along the equatorial Indian Ocean during the month of May each year, starting in 2006. The weakened jets occur in conjunction with the latitude of zero zonal wind (LUZ) being close to the equator during these years, resulting in weaker than normal zonal winds along the equator from 2006 and onward. We find that starting in 2006, the normal tendency of westward propagation of the annual harmonic mode switches to eastward propagation, coherent with the wind forcing. In comparison to the annual harmonic component of the zonal current, the weak WJs are mainly associated with the semiannual harmonic WJs, as evident from an amplitude reduction of that mode by at least 0.3 m s⁻¹ during the post-2005 period. Our analysis demonstrates that the variance explained by the semiannual harmonic is reduced to half (30–40%) at the core of the WJ in 2006 and later years in comparison with earlier years when it was 70–80%.