Journal Article

Effects of Southeastern Pacific Sea Surface Temperature on the Double-ITCZ Bias in NCAR CESM1

Zhang, Guang J., Song, Fengfei

2016

Overview

The double intertropical convergence zone (ITCZ) is a long-standing bias in the climatology of coupled general circulation models (CGCMs). The warm biases in southeastern Pacific (SEP) sea surface temperature (SST) are also evident in many CGCMs. In this study, the role of SEP SST in the double ITCZ is investigated by prescribing the observed SEP SST in the Community Earth System Model, version 1 (CESM1). Both the double ITCZ and dry equator problems are significantly improved with SEP SST prescribed. Both atmospheric and oceanic processes are involved in the improvements. The colder SST over the SEP decreases the precipitation, which enhances the southeasterly winds outside the prescribed SST region, cooling the ocean via increased evaporation. The enhanced descending motion over the SEP strengthens the Walker circulation. The easterly winds over the equatorial Pacific enhance upwelling and shoal the thermocline over the eastern Pacific. The changes of surface wind and wind curl lead to a weaker South Equatorial Countercurrent and stronger South Equatorial Current, preventing the warm water from expanding eastward, thereby improving both the double ITCZ and dry equator. The enhanced Walker circulation also increases the low-level wind convergence and reduces the wind speed in the tropical western Pacific, leading to warmer SST and stronger convection there. The stronger convection in turn leads to more cloud and reduces the incoming solar radiation, cooling the SST. These competing effects between radiative heat flux and latent heat flux make the atmospheric heat flux secondary to the ocean dynamics in the western Pacific warming.

• Publisher: American Meteorological Society
• Subject: Atmosphere / Atmospheric models / Bias / Climatology / Coasts / Convection / Convergence / Cooling / Easterlies / ENVIRONMENTAL SCIENCES / Equator / Equatorial countercurrents / Equatorial currents / Evaporation / Experiments / Fluctuations / General circulation models / Heat / Heat flux / Heat transfer / Intertropical convergence zone / Latent heat / Latent heat flux / Meteorology & Atmospheric Sciences / Ocean circulation / Ocean currents / Ocean dynamics / Oceans / Precipitation / Radiation / Salinity / Sea surface / Sea surface temperature / Shoals / Simulation / Solar radiation / South Equatorial Countercurrent / South Equatorial Current / Studies / Surface temperature / Surface wind / Temperature / Thermocline / Upwelling / Walker circulation / Warm water / Water temperature / Wind speed / Winds