What sets the heat content of Southern Ocean mode

Abstract

14 The Southern Ocean (SO) is a crucial region for the global ocean uptake of heat and car15 bon. There are large uncertainties in the observations of fluxes of heat and carbon be16 tween the atmosphere and the ocean mixed layer, which leads to large uncertainties in 17 the amount entering into the global overturning circulation. In order to better under18 stand where and when fluxes of heat and momentum have the largest impact on near19 surface heat content, we use an adjoint model to calculate the linear sensitivities of heat 20 content in SO mode water formation regions to surface fluxes. We find that the heat con21 tent of these regions is most sensitive to recent, local heat fluxes, and to non-local wind 22 stress fluxes on the order of one to eight years previously. This is supported by the cal23 culation of sensitivities to kinematic potential temperature changes, which reveal the sources 24 of the mode water formation regions, and by sensitivities to dynamic potential temper25 ature changes, which reveal dynamic links with boundary currents, the ACC, Kelvin and 26 Rossby waves. A series of forward perturbation experiments in the fully non-linear model 27 confirm that the adjoint model can accurately predict linear changes in heat content of 28 fixed volume mode water formation regions. These experiments also highlight that non29 linear effects can also be of importance, depending on the time and region investigated, 30 and that the contribution of volume changes to heat content changes can be as large or 31 larger than temperature changes. 32 The Southern Ocean is of crucial importance to the global ocean’s uptake of car33 bon and heat. However, due to difficulties in making observations in such a remote and 34 hostile environment, we currently don’t know accurately how much heat and carbon en35 ters the Southern Ocean from the atmosphere. Heat from the Southern Ocean gets locked 36 away for hundreds to thousands of years in the globe’s deep oceans, entering through a 37 few key regions. We use a computer model to assess how the heat, fresh water, and wind 38 energy entering through the surface of the Southern Ocean affects the heat of these key 39 regions. We find that these regions are very sensitive to heat coming in through the sur40 face directly over them, and that winds across a wider area of the Southern Ocean can 41 affect the heat in them for several years. If we want to estimate the heat in these regions 42 accurately, this information can be used to help us decide where and when it is impor43 tant to measure heat and winds better. 44