The ocean's role in the tropical-extratropical interaction in an idealized AOGCM

A number of studies show that extratropical forcing (e.g., change in AMOC, ice cover, or aerosol loading) can significantly modify the tropical circulation and precipitation by changing, from the global perspective, the interhemispheric heat content asymmetry (e.g., ITCZ shifts toward the warmer hemisphere). The ocean heat content is the dominant element of the Earth's heat budget, hence this study explores how changes in ocean circulation influence the tropical-extratropical interaction and the hemispheric asymmetry of the climate.

We use an intermediate complexity AOGCM derived from NOAA/GFDL CM2.0 to examine the influence of slanted tropical ocean coastlines and the opening of a circumpolar channel on the tropical climate, and its exchange with extratropics and the interhemispheric processes. Our simplified numerical setup has longitudinal sector geometry with a flat-bottom basin and no mountains. We primarily focus on the dynamical aspects thus the computational burden is further reduced by using gray radiative transfer in the atmosphere (e.g., there is no water vapor feedback), and clouds are removed.

Westward slanted tropical coastlines induce substantial hemispheric asymmetry in an experiment with closed basin of uniform zonal width. Ocean upwelling enables the wind-evaporation-SST feedback that tries to anchor ITCZ north of the equator. However, the western boundary configuration causes preferential outflow of the surface tropical waters into the Northern Hemisphere. This gradually induces the strongest deep-water formation in the Southern Hemisphere, southward (northward) ocean (atmosphere) heat transport across the equator, and compensating modification of Hadley circulation that positions ITCZ in the Southern Hemisphere with the dominant ocean heat content.

The opening of an idealized Drake Passage enables the formation of a strong circumpolar current and shifts the deep-water production in the Northern Hemisphere. This pushes the climate into a state with northward (southward) ocean (atmosphere) heat transport across the equator and ITCZ shifts back in the Northern Hemisphere that now has a bigger ocean heat content. A set of experiments with various sill depths examines details of such influence of the extratropical ocean circulation on tropical climate. This is contrasted with the experiments with a closed basin that show that a modest hemispheric asymmetry in the tropics results in a substantial hemispheric asymmetry in the extratropics and reverse interhemispheric pathways.