Joint Seminar: Why the simulated Atlantic ITCZ is often south of the equator and how it matters

Even though the observed Atlantic ITCZ is rarely found south of the equator, the simulated ITCZ in GCMs favors this location in boreal spring (MAM). This occurs even in AGCMs forced with observed SST, indicative of an atmospheric origin. Analysis of CMIP5 experiments and observations suggests that several factors contribute to this pervasive model shortcoming. One is the underrepresentation of eastward moving convective systems over equatorial South America. In observations, these disturbances typically originate over the western Amazon region and travel all the way to the central equatorial Atlantic. This contributes to centering the ITCZ over the equator despite the maximum SST being located at 5S during MAM. Another factor, in coupled GCMs, is the warm SST bias off southwestern Africa. GCM experiments suggest that SST biases in the region have a considerable influence on the position of the Atlantic ITCZ.

 

Errors in the representation of the Atlantic ITCZ also have wider implications. Analysis of monthly mean fields indicates that in both observations and GCMs the latitude of the Atlantic ITCZ is closely linked to the strength of the equatorial zonal surface winds such that the winds are weakest when the ITCZ is furthest south. Thus the erroneous southward shift of the ITCZ is consistent with the ubiquitous westerly wind bias in GCMs. In addition to its consequences for GCM biases, the link between ITCZ and equatorial surface winds also has implications for interannual variability in the region. Since the latitude of the Atlantic ITCZ is sensitive to the interhemispheric SST gradient, subtropical and extratropical influences may cause a weakening or strengthening of equatorial winds and thus excite the zonal model of Atlantic variability.

Datum

08.07.2015

Uhrzeit

13:30 Uhr

Ort

Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg

ReferentIn

Ingo Richter, Japan-Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan

Chair

Bjorn Stevens

Zur Übersicht