Controls on the ocean response to idealized Antarctic meltwater input

Abstract

Antarctic meltwater is expected to increase throughout the coming centuries and impact sea level, ocean circulation, and the coupled climate evolution. This motivates interest in understanding the ocean response to Antarctic freshwater injection, including potential sources of uncertainty. In this study, we use idealized single-basin ocean simulations with meltwater input to examine the dependence of ocean transport and the timescales of the adjustment of regional sea level patterns on: (a) the model resolution and parameter values such as the mesoscale eddy Gent-McWilliams parameterization and vertical diffusivity, thereby partially addressing structural and parametric uncertainty; and (b) the depth of meltwater forcing, which must be prescribed both in our experiments and in most comprehensive climate model simulations, due to a lack of dynamic coupling with an ice sheet model. We find distinct sea level adjustment timescales and changes in the upper and abyssal cells depending on the depth of input, including a near total shutdown of the abyssal cell which only occurs with meltwater injection at the surface. We additionally find correlations between the ocean response to meltwater and the background stratification in each control simulation, which depends on the model resolution and parameter values. These results indicate that, in addition to uncertainty in how ocean models interact with fluxes from ice sheets, the ocean physics and simulated preindustrial state substantially influence the dynamic ocean response to projected ice shelf meltwater fluxes.

Figures (14)

• Figure 1: Numerical model set-up. (a): The single basin bathymetry, identical to Eisenman2024TheFluxes, with linearly sloping continental shelves and a re-entrant channel with a ridge. (b): The values of the $\kappa_{GM/Redi}$ parameter in the simulations. (c) Schematic of experiments as described in text. Note that partway through the spin-up runs (shown in blue and orange), we switched from a linear free surface to a non-linear free surface, because the latter, which is less commonly used, perfectly conserves tracers. For the $\frac{1}{4}^{\circ}$ run, this switch was done at year 3414 (of the total additional 5726 year spin-up), while for the $1^{\circ}$ runs, this was performed at the time of branching. (d) The different vertical diffusivity profiles considered. For both the GM parameter and the vertical diffusivity, unless the parameter is specified in text to be changed, the default values highlighted in orange are used in the coarse simulation. (e): The zonal wind stress profile, primarily applied in the Southern Ocean region. (f)/(g): The sea surface temperature (SST) and sea surface salinity (SSS) relaxation profiles utilized in the spin-up runs.
• Figure 2: The circulation strength, both the upper cell and the abyssal cell, in the control simulations for the perturbed parameter ensemble. (a) for different $\Delta \kappa_v$ values; (b) for different $\kappa_{GM/Redi}$ values; (c) for the $\frac{1}{4}^{\circ}$ resolution.
• Figure 3: Meridional overturning streamfunction in the control runs: (a)-(h) with varying GM/Redi parameter ($\kappa_{\textrm{GM}}, \kappa_{\textrm{Redi}}$) as indicated in Figure \ref{['fig:model_setup']}b, with the parameter value indicated in each subplot title; (i)-(p) changing the vertical diffusivity as indicated in Figure \ref{['fig:model_setup']}d, with the value in the subfigure title indicating the shift from the default profile (uniformly over the whole depth); (q): the $\frac{1}{4}^{\circ}$ run; and (r): the 1-degree run with default parameter values (see Figure \ref{['fig:model_setup']}).
• Figure 4: Meridional overturning streamfunction in the surface perturbation runs: (a)-(h) with varying GM/Redi parameter ($\kappa_{\textrm{GM}}, \kappa_{\textrm{Redi}}$) as indicated in Figure \ref{['fig:model_setup']}b, with the parameter value indicated in each subplot title; (i)-(p) changing the vertical diffusivity as indicated in Figure \ref{['fig:model_setup']}d, with the value in the subfigure title indicating the shift from the default profile (uniformly over the whole depth); (q): the $\frac{1}{4}^{\circ}$ run; and (r): the 1-degree run with default parameter values (see Figure \ref{['fig:model_setup']}).
• Figure 5: Meridional overturning streamfunction in the deep perturbation runs: (a)-(h) with varying GM/Redi parameter ($\kappa_{\textrm{GM}}, \kappa_{\textrm{Redi}}$) as indicated in Figure \ref{['fig:model_setup']}b, with the parameter value used indicated in each subplot title; (i)-(p) changing the vertical diffusivity as indicated in Figure \ref{['fig:model_setup']}d, with the value in the subfigure title indicating the shift from the default profile (uniformly over the whole depth); (q): the $\frac{1}{4}^{\circ}$ run; and (r): the 1-degree run with default parameter values (see Figure \ref{['fig:model_setup']}).