Can global warming affect tropical ocean heat transport?

Tropical meridional ocean heat transport is studied in six coupled ocean-atmosphere models in which atmospheric CO 2 concentration has been increased. In the Indo-Pacific, the strength of Subtropical Cells (STCs) changes in response to changes in the trade winds. However, the change is not consistent among models. In contrast, in all models the tropical Indo-Pacific heat transport remains nearly constant over time due to compensation of STC and the horizontal gyre variations. Even under strong atmospheric radiative forcing the tropical Indo-Pacific ocean heat transport trends are determined by changes in circulation, rather than changes in the stratification. Ocean heat transport in the tropical Atlantic responds to weakening of the basin-wide meridional overturning circulation (MOC). The trends in the South Atlantic STC do not affect ocean heat transport. Meridional energy transport mediates the imbalance between incoming radiation at the top of the atmosphere and outgoing long wave radiation. In the tropics, the ocean transfers most heat poleward. Elsewhere, the atmosphere dominates, although in the North Atlantic the ocean heat transport is significant as well [Trenberth and Caron, 2001]. Changes in poleward ocean heat transport (OHT) are often related to changes in the oceanic overturning cells. In particular, the basin-wide meridional overturning circulation (MOC) in the Atlantic may weaken in response to warming and freshening in the Arctic in response to rising greenhouse gas concentrations in the atmosphere [Cubasch et al. 2001]. have hardly been studied, but Merryfield and Boer [2004] report a weakening of the STCs in a coupled ocean-atmosphere model. Some conceptual models indicate that energy transport changes in the ocean and atmosphere in the tropics act in concert [Held 2001]. This is based on the tight constraint between transport by the Hadley Cells and STCs, both of which are determined by momentum transfer between ocean and atmosphere. However, Hazeleger et al. [2004] showed that the gyre heat transport can compensate substantially for changes in OHT by STCs. In contrast, radiative constraints would require that oceanic and atmospheric heat transport compensate [Bjerknes 1964]. However, trends in outgoing longwave radiation at the top of the atmosphere indicate that in a transient climate the partitioning can change [Wielicki et al. 2002]. Here we study how tropical oceanic overturning cells and meridional OHT can change when the climate changes due to rising greenhouse gasses. Previous studies focused on seasonal to decadal time scales at which the tropical dynamics is primarily wind-driven. Here we …