Influence of a southern shift of the ITCZ from Quick Scatterometer data on the Pacific North Equatorial Countercurrent

Abstract

By analyzing the climatologically averaged wind stress during 2000–2007, it is found that the easterly wind stress in the northern tropical Pacific Ocean from Quick Scatterometer (QSCAT) data was stronger than those from Tropical Atmosphere Ocean (TAO) data and from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis I. As a result, the Intertropical Convergence Zone (ITCZ) in the Pacific Ocean is more southward in the QSCAT data than in the NCEP/NCAR data. Relative to the NCEP wind, the southern shift of the ITCZ in the QSCAT data led to negative anomaly of wind stress curl north of a latitude of 6°N. The negative anomaly results in downward Ekman pumping in the central Pacific. The excessive local strong easterly wind also contributes to the downward Ekman pumping. This downward Ekman pumping suppresses the thermocline ridge, reduces the meridional thermocline slope and weakens the North Equatorial Countercurrent (NECC). These effects were confirmed by numerical experiments using two independent ocean general circulation models (OGCMs). Furthermore, the excessive equatorial easterly wind stress was also found to contribute to the weaker NECC in the OGCMs. A comparison between the simulations and observation data indicates that the stronger zonal wind stress and its southern shift of QSCAT data in the ITCZ region yield the maximum strength of the simulated NECC only 33% of the magnitude derived from observation data and even led to a “missing” NECC in the western Pacific.