The Roles of Continental Geometry and Ocean Heat Transport on the Annual and Seasonal Mean ITCZ

The intertropical convergence zone (ITCZ) is a key climatic feature that is integrally connected to the energy and water cycles of the Earth system. The energetic framework has been used to explain shifts in the ITCZ position in response to internal variability and external forcings by relating the net energy input (NEI) into the atmospheric column to the divergence of the vertically integrated energy flux by the Hadley circulation. In the energetic framework, continental geometry may affect the ITCZ position through its influence on the NEI into the atmosphere, given that surface fluxes over land are constrained to zero on time scales longer than a few days. To understand how different land configurations affect the annual and seasonal mean ITCZ position, we perform simulations using an idealized aquaplanet model with a slab ocean, where land and ocean differ only by the mixed-layer depth of the slab ocean. The model is run with different zonally symmetric configurations of Northern Hemispheric (NH) land that extends poleward from southern boundaries at various latitudes. The annual mean ITCZ is calculated using several predictors to relate the ITCZ to the dynamics and energetics. The predictors agree well in how the ITCZ position varies with the latitudinal extent of the continent. In all the simulations, a Southern Hemispheric annual mean ITCZ is observed, which decreases in poleward extent as the southern boundary of the land is moved further poleward. Interestingly, the global mean surface temperature increases as the land extent is reduced perhaps due to a lack of other radiative feedbacks in the model setup. During NH summer, simulations with land in the tropics exhibit a double ITCZ structure. In contrast, in NH winter, simulations with more poleward southern boundaries of land produce double ITCZs. To understand the seasonal structure of the ITCZ, we relate differences in the surface energy budget with changes in the vertical and horizontal circulation using diagnostics of energy and dynamics. As ocean heat transport (OHT) also influences the spatial distribution of energy and the NEI into the atmospheric column, simulations with varying magnitude and direction of OHT are also performed for each continental geometry to explore the influence of the ocean energy flux in combination with a continent on the ITCZ structure.