Vertical Heat Transport by Ocean Circulation and the Role of Mechanical and Haline Forcing

Vertical transport of heat by ocean circulation is investigated using a coupled climate model and novel thermodynamic methods. Using a streamfunction in temperature–depth coordinates, cells are identified by whether they are thermally direct (flux heat upward) or indirect (flux heat downward). These cells are then projected into geographical and other thermodynamic coordinates. Three cells are identified in the model: a thermally direct cell coincident with Antarctic BottomWater, a thermally indirect deep cell coincident with the upper limb of the meridional overturning circulation, and a thermally direct shallow cell coincident with the subtropical gyres at the surface. The mechanisms maintaining the thermally indirect deep cell are investigated. Sinking water within the deep cell is more saline than that which upwells, because of the coupling between the upper limb and the subtropical gyres in a broader thermohaline circulation. Despite the higher salinity of its sinking water, the deep cell transports buoyancy downward, requiring a source of mechanical energy. Experiments run to steady state with increasing Southern Hemisphere westerlies show an increasing thermally indirect circulation. These results suggest that heat can be pumped downward by the upper limb of the meridional overturning circulation through a combination of salinity gain in the subtropics and the mechanical forcing provided by Southern Hemisphere westerly winds. 1. Vertical transport in the ocean The ocean plays a crucial role in regulating the earth’s climate, with the upper 2.5m of the ocean able to store as much heat as the entire atmosphere (Gill 1982). Over the past half century, the ocean has absorbed more than 5 times as much heat as all the other components of the earth system combined (Solomon et al. 2007). Ocean warming has caused a significant fraction of global sea level rise over the past half century (Domingues et al. 2008). Heat absorbed by the ocean today can take centuries to be rereleased into the atmosphere (Held et al. 2010) and is dependent on ocean ventilation time scales (England 1995). Regardless of future climate forcing scenarios, any warming of the ocean todaymaymaintain a significant fraction of established global temperature changes into the foreseeable future. Despite its importance in the global climate system, little attention has been directed toward understanding vertical heat transport in the ocean. In fact, the term ‘‘ocean heat transport’’ has become synonymous with meridional transport of heat by the ocean (Jayne and Marotzke 2002; Ferrari and Ferreira 2011). Classical models of ocean circulation assume that heat is fluxed downward by small-scale mixing, balancing a thermally direct overturning circulation that fluxes heat upward through upwelling of warm water and downwelling of cold water (Munk 1966; Stommel and Arons 1960; Sandstr€ om 1908). In this paradigm, heat uptake by the deep ocean would be set by the vertical temperature gradient and vertical mixing rather than circulation. Subsequent authors have queried this view, proposing that the overturning circulation of the upper 2000m is set by Southern Hemisphere winds as a result of the Drake Passage effect (Toggweiler and Samuels 1995). Corresponding author address: Jan D. Kika, University of Southampton, National Oceanography Centre, European Way, Southampton, SO14 3ZH, United Kingdom. E-mail: [email protected] OCTOBER 2013 Z IKA ET AL . 2095 DOI: 10.1175/JPO-D-12-0179.1 2013 American Meteorological Society The Drake Passage in the Southern Ocean and its consequent Antarctic Circumpolar Current permits global circulation in the upper 2000m of the ocean without vertical mixing. If circulation were truly adiabatic, cold water that downwells in the North Atlantic would upwell in the Southern Ocean at the same temperature, leading to no net vertical transport of heat. As such, adiabatic theories suggest that general circulation could play a minor role in transporting heat downward or upward in the ocean. Recent studies by Gnanadesikan et al. (2005) and Gregory (2000) have cast new light on the mechanisms of vertical heat transport operating in ocean models. Gnanadesikan et al. (2005) and Gregory (2000) find that heat is, in fact, advected downward into the deep ocean and mixed upward in the upper 2000m of coarseresolution climate models. That is, they find circulation of the upper ocean to be apparently thermally indirect. How the different components of ocean circulation such as subtropical gyres, deep overturning, and abyssal Antarctic Bottom Water (AABW) circulation flux heat vertically is an open question. Nycander et al. (2007) and Nurser and Lee (2004) present a novel formalism in this regard. They average the vertical transport in depth– density coordinates. In such coordinates, cells that flux dense water downward are thermally direct and cells that flux less-buoyant water downward are thermally indirect. Nycander et al. (2007) link thermally direct cells to buoyant processes and link thermally indirect cells to mechanical processes. Nycander et al. (2007) identify three distinct cells in an eddy permitting ocean model. They are (i) a thermally direct dense bottom cell stretching from the surface to around 5000-m depth, (ii) a thermally indirect deep cell reaching from the surface to 2000-m depth, and (iii) a thermally direct light surface cell occupying only the upper 500m of the ocean. It is as yet unclear, however, how the three thermodynamically distinct cells of Nycander et al. (2007) are connected to geographically distinct components of circulation such as the subtropical gyres and the deep overturning circulation. As previous authors have shown (Ferrari and Ferreira 2011; Nycander et al. 2007; Zika et al. 2012; D€ o€ os et al. 2012), understanding the way the ocean transports heat depends critically on the way in which ocean transport is averaged. In this study, we aim to advance the averaging methods proposed by those authors and to use such methods to diagnose the mechanisms by which ocean circulation transports heat vertically in a climate model. We will start in section 2 with a review of recent advances in our understanding and diagnosis of residual circulation. The intermediate complexity ocean model used in this study is described in section 3. In section 4, the vertical transport of the model is considered in temperature–depth coordinates and we confirm that three distinct cells exist in the model. In section 5, the three cells are projected into geographical coordinates, and in section 6 each cell is projected into salinity and density coordinates. How the different cells exchange salt within a broader thermohaline circulation is explored in section 7 by projecting the thermal overturning cells into thermohaline (temperature–salinity) coordinates. In section 8, the role of wind forcing in ultimately driving the thermally direct component of circulation is supported via wind perturbation experiments. Section 9 contains a discussion and concluding remarks aremade in section 10. 2. The residual circulation Throughout the ocean, tides, waves, and coherent recirculations move waters of different temperatures and salinities to and fro with instantaneous velocities ranging between millimeters and meters per second. Such flows can amount to very little net transport of heat or salt across latitude lines or through constant depth surfaces. Meanwhile, a background residual circulation transports properties meridionally and vertically and is characterized by velocities on the order of millimeters per second (meridionally) and micrometers per second (vertically). A common way of understanding the residual circulation is to consider a layer bounded by two surfaces of constant density from r to r 1 Dr with a thickness h. Averaging the velocity y within the isopycnal layer, one obtains the total transport of water within that layer yh (McDougall and McIntosh 1996). There may be zero mean northward velocity at constant depth (y5 0) and yet a net transport may exist within an isopycnal layer. Consider flow through a narrow channel separating two seas. Cold dense water may flow out through a channel in winter and warm light water may flow in through the channel in summer. The net transport at constant density is thus nonzero. However, if the inand outflow occur at the same depth, the transport averaged at constant depth will be zero. Alternatively, the flow may come in and out at the same density but at different depths. Thus there will be a very large mean velocity at a particular depth, but no transport at constant density. More generally then yh 61⁄4 y h. Thus, the following Reynolds decomposition is required: