Dynamical Aspects of the Eastern Pacific ITCZ Boundary Layer

The width and location of the intertropical convergence zone (ITCZ) has long been studied because the ITCZ greatly influences the global circulation and it is often difficult to model. The ITCZ varies considerably as a function of ocean basin and season due to complex processes involving air-sea coupling, land and terrain influences, cloud-radiative effects, and boundary layer dynamics. In this study, we focus on dynamical aspects of the boundary layer in and near the intertropical convergence zone (ITCZ) using a high-resolution, zonally symmetric, slab boundary layer numerical model on the sphere. The boundary layer zonal and meridional flow is forced by the monthly, zonally (80°W-150°W) and boundary layer (950-1000 hPa) averaged geopotential field from the Year of Tropical Convection (YOTC) reanalysis for July 2008, March 2009, and March 2010. The numerical model dynamical fields are in general agreement with the YOTC dynamical fields, producing an off-equatorial ITCZ when forced by the July 2008 geopotential, a double ITCZ when forced by the March 2009 geopotential, and a single, near-equatorial ITCZ when forced by the March 2010 geopotential. In each experiment, we compare the numerical solutions from the slab model to those produced using classical Ekman theory. In doing so, we see that horizontal advection is crucial to understanding the preferential location and shape of the ITCZ. Also, particle trajectories for each experiment highlight large boundary layer asymmetries between the southern and northern hemisphere branches of the Hadley circulation.