Dynamical Importance of the Trade Wind Inversion in Suppressing the Southeast Pacific ITCZ

Sea surface temperature (SST) gradients are a primary driver of low-level wind convergence in the Inter-Tropical Convergence Zone (ITCZ) through their hydrostatic relationship to the surface pressure force (PGF). To what extent boundary layer virtual temperature gradients have an effect on ITCZ convergence through their modulation of the surface PGF is not well understood. In this study, we investigate the reasons for the observed northern hemisphere position of the east Pacific ITCZ using a slab boundary layer model (SBLM) driven by different approximations of the surface and boundary layer virtual temperature field within the surface PGF forcing. SBLM simulations using the entire boundary layer virtual temperature profile produce a realistic northern hemisphere ITCZ. However, SST-only simulations produce excessive equatorial divergence and southern hemisphere convergence resulting in a double ITCZ-like structure. Subsequent investigations of virtual temperature gradients highlight the importance of temperature gradients weakening with height more strongly from the equator to 15 degrees south just below the trade wind inversion (TWI). Our interpretation is that the equatorial cold tongue induces relatively high surface pressure and a double ITCZ-like convergence. At the same time, relatively high surface pressure spreads out in the southern hemisphere due to longwave cooling at the top of stratocumulus clouds just below the TWI. Together, the equatorial cold tongue and the TWI/stratocumulus clouds enable a more northern hemisphere dominant ITCZ. Thus, we provide evidence of a dynamical link between double ITCZs and low clouds, which both continue to be problematic in Earth system models.