Local and Remote Influences on the Heat Content of Southern Ocean Mode Water Formation Regions

Abstract

15 The Southern Ocean (SO) is a crucial region for the global ocean uptake of heat and car16 bon. There are large uncertainties in the observations of fluxes of heat and carbon be17 tween the atmosphere and the ocean mixed layer, which leads to large uncertainties in 18 the amount entering into the global overturning circulation. In order to better under19 stand where and when fluxes of heat and momentum have the largest impact on near20 surface heat content, we use an adjoint model to calculate the linear sensitivities of heat 21 content in SO mode water formation regions to surface fluxes. We find that the heat con22 tent of these regions is, in all three basins, most sensitive to recent, local heat fluxes, and 23 to non-local wind one to eight years previously. This is supported by the calculation of 24 sensitivities to potential temperature changes at constant density, which reveal the sources 25 of the mode water formation regions, and by sensitivities to potential temperature changes 26 with varying density, which reveal dynamic links with boundary current regions, the Antarc27 tic Circumpolar Current, and wave-like features. A series of forward perturbation ex28 periments in the fully non-linear model confirm that the adjoint model can accurately 29 predict linear changes in heat content of fixed volume mode water formation regions. These 30 experiments also highlight that nonlinear effects can be of importance, depending on the 31 time and region investigated, and that the contribution of volume changes to heat con32 tent changes can be as large as or larger than the contribution from temperature changes. 33 Plain Language Summary: The Southern Ocean is of crucial importance to the global 34 ocean’s uptake of carbon and heat. However, due to difficulties in making observations 35 in such a remote and hostile environment, we currently don’t know accurately how much 36 heat and carbon enters the Southern Ocean from the atmosphere. Heat from the South37 ern Ocean can get locked away for hundreds to thousands of years in the world’s deep 38 oceans, entering through a few key regions. We use a computer model to assess how the 39 heat, fresh water, and wind energy entering through the surface of the Southern Ocean 40 affects the heat of these key regions. We find that these regions are very sensitive to heat 41 coming in through the surface directly over them, and that winds across a wider area 42 of the Southern Ocean can affect the heat stored for several years. If we want to esti43 mate the heat stored in these regions more accurately, this information can be used to 44 help us decide where and when it is important to measure the winds and heat entering 45 the ocean better. 46