Local and remote influences on the heat content of Southern Ocean mode water formation regions.

Abstract

The Southern Ocean (SO) is a crucial region for the global ocean uptake of heat and carbon. There are large uncertainties in the observations of fluxes of heat and carbon between the atmosphere and the ocean mixed layer, which leads to large uncertainties in the amount entering into the global overturning circulation. In order to better understand where and when fluxes of heat and momentum have the largest impact on near-surface heat content, we use an adjoint model to calculate the linear sensitivities of heat content in SO mode water formation regions to surface fluxes. We find that the heat content of these regions is most sensitive to recent, local heat fluxes, and to non-local wind stress fluxes on the order of one to eight years previously. This is supported by the calculation of sensitivities to kinematic potential temperature changes, which reveal the sources of the mode water formation regions, and by sensitivities to dynamic potential temperature changes, which reveal dynamic links with boundary currents, the ACC, Kelvin and Rossby waves. A series of forward perturbation experiments in the fully non-linear model confirm that the adjoint model can accurately predict linear changes in heat content of fixed volume mode water formation regions. These experiments also highlight that nonlinear effects can also be of importance, depending on the time and region investigated, and that the contribution of volume changes to heat content changes can be as large or larger than temperature changes.

Subtropical Contribution to Sub‐Antarctic Mode Waters

“…The along‐ACC advection of warm, salty and nutrient poor waters of subtropical origin described here provides a dynamic link between the western boundary currents and the SAMW formation regions, consistent with the delayed connections (with a lag of a few years) between them identified with an adjoint ocean model (Boland et al., 2021). Our findings suggest that this long‐range connection is of primary relevance for SAMW dynamics and their physical and biogeochemical characteristics, potentially influencing the drawdown of heat, carbon and nutrients into the global pycnocline.…”

The observed spatiotemporal variability of Antarctic Winter Water

“…WW profiles are largely constrained to the extent of the PF, as per the WW definition (Section 2.2; Table 1), but there are regions with WW detected northward of the PF. In the Pacific sector of the SO, WW is consistently detected north of the PF throughout the seasonal cycle (Figures 3a-3d); SAMW, similar to WW, forms via buoyancy loss and drives ML deepening, which takes place in the vicinity of the ACC and northwards (Boland et al, 2021;Cerovečki et al, 2013;Meijers et al, 2019;Wang et al, 2022). Hence, the detection of WW north of the PF in the Pacific sector where Shallow Salinity Minimum Water (Karstensen, 2004) is observed in a region transitioning from a beta density regime to an alpha density regime (Roquet et al, 2022) (see their Figure 2b).…”

Untangling local and remote influences in two major petrel habitats in the oligotrophic Southern Ocean

“…An adjoint model then calculates the sensitivity of that integral quantity to input variables such as surface wind stress, net heat flux, and tracer concentration at each model grid cell and timestep. As such, adjoint models allow users to calculate how the quantity of interest depends on both nearby and remote grid cells across a wide range of lead times (e.g., Boland et al, 2021; Jones et al, 2018). We define our quantity of interest as the October‐to‐February integrated passive tracer concentration:where the integrand is the passive tracer concentration, which depends on both position r and time t , and the volume integral over either the SIO or the CPO.…”