On the annual cycle over the atlantic sector: the relative role of land and ocean
This study investigates how local and remote forcings shape the annual cycle of surface temperature and precipitation in the tropical Atlantic ocean, Africa, and South America.We use an atmospheric general circulation model with prescribed SST or coupled to a slab ocean model, and prescribe insolation over land and SST---or, in the coupled case, insolation and ocean heat transport convergence (OHTC)---independently of each other. The annual cycle in a simulation in which only one forcing varies annually indicates the importance of that particular forcing for the observed annual cycle. Additional experiments in which we specify the elevated condensational heating in selected regions are used to distinguish between circulations driven by elevated heating and circulations driven by boundary layer processes.SST determines the location of the maritime ITCZ and influences precipitation in equatorial coastal regions and the Sahel. The circulation generated by the ITCZ elevated heating generates the low-level convergence and precipitation response in the coastal regions. Changes in subtropical SST are advected inland and force changes in sea level pressure, low-level convergence, and precipitation in the Sahel.The Atlantic ITCZ intensity is modulated by the precipitation intensity in Africa and South America. Reduced land precipitation cools the upper troposphere and enhances the convective available potential energy in the ITCZ region, thereby making precipitation more intense.In coupled experiments, annual variations in the continental climate impact the position of the Atlantic ITCZ. The annual changes in Saharan surface temperature affect the strength of the northern subtropical Trades, while changes in continental precipitation affect the surface wind in proximity of the ITCZ. Both processes trigger a coupled response in the equatorial Atlantic surface wind, the ITCZ, and the cross-equatorial SST gradient, whose net effect is the meridional displacement of the ITCZ.It is shown that the annual cycles of precipitation and temperature over the continents are as important as the annual cycle of insolation over the ocean and of OHTC in forcing the annual march of the Atlantic ITCZ seen in the control simulation.
- Atmospheric sciences