The Thermal Structure of the Upper Ocean

Abstract

The salient feature of the oceanic thermal structure is a remarkably shallow thermocline, especially in the Tropics and subtropics. What factors determine its depth? Theories for the deep thermohaline circulation provide an answer that depends on oceanic diffusivity, but they deny the surface winds an explicit role. Theories for the shallow ventilated thermocline take into account the influence of the wind explicitly, but only if the thermal structure in the absence of any winds, the thermal structure along the eastern boundary, is given. To complete and marry the existing theories for the oceanic thermal structure, this paper invokes the constraint of a balanced heat budget for the ocean. The oceanic heat gain occurs primarily in the upwelling zones of the Tropics and subtropics and depends strongly on oceanic conditions, specifically the depth of the thermocline. The heat gain is large when the thermocline is shallow but is small when the thermocline is deep. The constraint of a balanced heat budget therefore implies that an increase in heat loss in high latitudes can result in a shoaling of the tropical thermocline; a decrease in heat loss can cause a deepening of the thermocline. Calculations with an idealized general circulation model of the ocean confirm these inferences. Arguments based on a balanced heat budget yield an expression for the depth of the thermocline in terms of parameters such as the imposed surface winds, the surface temperature gradient, and the oceanic diffusivity. These arguments in effect bridge the theories for the ventilated thermocline and the thermohaline circulation so that previous scaling arguments are recovered as special cases of a general result.

ENSO Suppression due to Weakening of the North Atlantic Thermohaline Circulation*

“…Further studies will disentangle the atmospheric and oceanic teleconnections in more detail. We have discussed only one controlling factor of Pacific thermocline changes (the remote effect due to global baroclinic ocean adjustment), while neglectingfor reasons of simplicity-the importance of wind changes in spinning up the ocean thermocline structure (Boccaletti et al 2004). Further studies will elucidate the importance of wind changes on the thermocline structure in the Pacific and hence on ENSO variability on multidecadal to millennial time scales.…”

Climate Fluctuations of Tropical Coupled Systems—The Role of Ocean Dynamics

“…Closer examination of model results indicates that substantial diapycnal transformation is occurring along the flow (Rodgers et al 2003). The model studies of Boccaletti et al (2004) demonstrate that increasing the background thermal diffusivity within the ocean has a strong impact on the heat transport as well as the overall structure of the cold tongue. The McCreary and Lu (1994) view of the STC envisions water beneath the thermocline that does not surface in the Tropics.…”

“…Boccaletti et al (2004) frame the problem in a slightly different fashion by asking the question of how the heat transport by STCs can work as a constraint for the equatorial thermocline depth, but also find that the connection between the heat gain in the ocean in the equatorial cold tongue and its return to the atmosphere in the subtropics is strong. This implies that a large component of the net heat transport by the ocean is accomplished by the STCs.…”

The dynamics of waves interacting with the Equatorial Undercurrent

“…the discussion in Cronin and McPhaden (1997)), we restrict our attention to the eastward heat advection. Note that the rate of heat energy transport through the surface responds mainly to changes in the sea-surface temperature (see Haney 1971, Boccaletti et al 2004, Boccaletti 2005, Wallcraft et al 2008, with the dimensional ocean-atmosphere heat flux Q well-approximated as…”