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 east Pacific Inter-Tropical Convergence Zone (ITCZ) through their hydrostatic relationship to the surface pressure gradient force (PGF). However, the surface PGF may not always align with SST gradients due to variations in boundary layer temperature gradients with height, that is, the boundary layer contribution to the surface PGF. In this study, we investigate the observed northern hemisphere position of the east Pacific ITCZ using a slab boundary layer model (SBLM) driven by different approximations of the boundary layer virtual temperature field. 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 latitudinally confined double ITCZ-like structure. Observed virtual temperature gradients highlight the importance of northward temperature gradients strengthening with height from the equator to 15 degrees S below the trade wind inversion (TWI). Our interpretation is that the equatorial cold tongue induces relatively weak high surface pressure and double ITCZ-like convergence because the resulting layer of cold air is shallow. Concurrently, relatively strong high surface pressure spreads out in the southern hemisphere due to interactions between stratocumulus clouds and the ocean surface. 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 the equatorial cold tongue, low clouds, and double ITCZs, which continue to be problematic in Earth system models. State-of-the-art climate models have been plagued by biases in the Inter-Tropical Convergence Zone (ITCZ), where the trade winds converge and the world's most intense rainfall occurs. Climate models often produce one ITCZ in each hemisphere, a double ITCZ, when there is nearly always one ITCZ observed in the northern hemisphere. In this study, we investigate why the northern hemisphere ITCZ dominates over the east Pacific Ocean using an idealized model driven by observed southern and northern hemisphere contrasts in: (a) sea surface temperature (SST) only and (b) both SST and atmospheric temperature. Experiments driven by only SST contrasts produce a double ITCZ-like structure that is reminiscent of climate model double ITCZ biases. In observations, a cold tongue of ocean water on the equator induces relatively weak high surface pressure and a double ITCZ-like wind convergence. At the same time, relatively strong high surface pressure spreads out in the southern hemisphere due to stratocumulus clouds and the ocean surface. Together, the equatorial cold tongue and stratocumulus clouds enable a more northern hemisphere dominant ITCZ. This study provides a dynamical link between the equatorial cold tongue, low clouds, and double ITCZs, which continue to be problematic in models. East Pacific ITCZ surface wind convergence is strongly controlled by SST and boundary layer (BL) horizontal temperature gradients SST gradients overemphasize the equatorial cold tongue leading to excessive equatorial divergence and latitudinally confined double ITCZs BL temperature gradients show a shallow cold tongue and deep cold air below the trade wind inversion are key to maintaining a northern ITCZ