On Coastlines and Climate: How Basin Geometry Shapes Ocean Circulation and Global Climate

Abstract

This dissertation investigates the role of ocean basin geometry in the circulation of the ocean, of the atmosphere, and in shaping the global climate system as a whole. To explore this topic, a major component of this dissertation is the development of a novel coupled climate model with configurable bathymetry, orography, and coastline geometry. Here I developed an ocean--atmosphere--sea-ice model that allows the testing of different theories for the development of major hemispheric asymmetries in the climate system. The model can be configured with a wide range of continental configurations. When configured with two ocean basins -- one that is narrow and Atlantic-like and one wide and Pacific-like -- and forced with zonally uniform atmospheric conditions in an ocean-only setup, the pole-to-pole, cross-equatorial meridional overturning circulation (MOC) and associated northern high latitude deep water formation localizes to the narrower of two ocean basins. This is similar to the ocean circulation in the real world, characterized by an Atlantic meridional overturning circulation (AMOC) and neither deep water formation nor cross equatorial overturning circulation in the Pacific. Widening the narrow basin increases the strength of the overturning circulation, in contrast to previous idealized geometry ocean-only studies. Also, the shape with which the basin widens matters for the magnitude and pattern by which the MOC strengthens. For instance, maintaining straight, meridional coastlines and widening the ocean basins results in different changes to ocean circulation than widening the ocean basin by creating slanted coastlines. I show that adding an atmosphere and a land model to form a fully-coupled model reveals vastly different results. Coupling the ocean--sea-ice model to an atmosphere--land model yields a surprising result of cross-equatorial MOC and northern deep water formation in the wide basin. MOC occurs in the wide basin, where negative surface buoyancy fluxes in the northern high latitudes allows for water transformation and deep water formation. Adding a subsurface zonal sill across the basin cuts off dense isopycnals that outcrop in the southern ocean from returning to the surface in the northern wide basin, forcing watermass transformation and deep sinking into the narrow basin. Idealized continental configurations as explored in this dissertation also allow for the study of meridional heat transport by the ocean and the atmosphere. Differences in top of atmosphere radiation between different continental geometries drive different partitioning of poleward heat transport between the ocean and the atmosphere. The total climate system heat transport also can adjust, due to changes in planetary albedo driven by cloud distribution responses to ocean circulation changes. We conclude that idealized modeling studies can illuminate key features of ocean and atmosphere circulation. These findings suggest that ocean basin geometry is of primary importance for ocean circulation and setting the mean climate state.