The global ocean circulation on a retrograde rotating earth

Abstract

To understand the three-dimensional ocean circulation patterns that have occurred in past continental geometries, it is crucial to study the role of the present-day continental geometry and surface (wind stress and buoyancy) forcing on the present-day global ocean circulation. This circulation, often referred to as the Conveyor state, is characterized by an Atlantic Meridional Overturning Circulation (MOC) with deep water formation at northern latitudes and the absence of such deep water formation in the North Pacific. This MOC asymmetry is often attributed to the difference in surface freshwater flux: the North Atlantic is a basin with net evaporation, while the North Pacific receives net precipitation. This issue is revisited in this paper by considering the global ocean circulation on a retrograde rotating earth, computing an equilibrium state of the coupled atmosphere-ocean-land surface-sea ice model CCSM3. The Atlantic-Pacific asymmetry in surface freshwater flux is indeed reversed but the ocean circulation pattern is not an Inverse Conveyor state (with deep water formation in the North Pacific) as there is strong and highly variable deep water formation in the North Atlantic. Using a fully-implicit, global ocean-only model also the stability properties of the Atlantic MOC on a retrograde rotating earth are investigated, showing a similar regime of multiple equilibria as in the present-day case. These results demonstrate that the present-day asymmetry in surface freshwater flux is not a crucial factor for the Atlantic-Pacific asymmetry in the global MOC

The climate of a retrograde rotating earth

“…The direction of the simulated changes is similar to the outcome of experiments by Smith et al (2008) and Kamphuis et al (2011), but in general the circulation changes found in our model are more similar to the results of Smith et al (2008) than to the results of Kamphuis et al (2011). Both studies showed a weakening of the Atlantic MOC together with a surface freshening in the North Atlantic and saltier surface conditions in the North Pacific.…”

“…An important and outstanding question of physical oceanography is why the North Atlantic is so disproportionate in its production of deep water, with very little or no deep water formed in the North Pacific. Kamphuis et al (2011) suggested that the more southward continental boundary of the Pacific limits deep water formation as compared to the Atlantic, which extends well into the Arctic. Other studies (e.g., Warren, 1983) have suggested that the freshening of the Pacific relative to the Atlantic stabilizes the overturning pattern.…”

“…Both studies showed a weakening of the Atlantic MOC together with a surface freshening in the North Atlantic and saltier surface conditions in the North Pacific. Whereas Smith et al (2008) also showed a strong MOC in the Pacific, Kamphuis et al (2011) obtain a state with a relative weak MOC both in the Atlantic and Pacific.…”

“…Smith et al (2008) found an inverse conveyor belt circulation with strong deep water formation in the North Pacific in a simulation with the climate model FAMOUS (Smith, 2012). Using another model, Kamphuis et al (2011), meanwhile, found that deep water formation in the North Atlantic weakened in their simulations, and intensified intermediate water formation became evident in the North Pacific. This study did not, however, show evidence of a complete reversal in the role of the subpolar North Atlantic and Pacific for deep water formation, leading to the conclusion that an altered net freshwater flux is not sufficient to obtain the reversal, but that the continental geometry is crucial for the pattern of the overturning circulation.…”

“…In the new climate, changes in wind direction lead to changes in ocean circulation, precipitation patterns, and storm track locations in ways that inform our understanding of hemispheric and longitudinal asymmetries of the present climate system, such as the position of the tropical rain bands, or structure of the storm tracks. Two studies (Smith et al, 2008;Kamphuis et al, 2011) have explored the consequences of a retrograde rotation of the Earth with ocean circulation questions in mind. But even for these more-limited-in-scope studies, the authors come to quite different findings.…”