CO₂‐Induced Ocean Warming of the Antarctic Continental Shelf in an Eddying Global Climate Model

Abstract

Ocean warming near the Antarctic ice shelves has critical implications for future ice sheet mass loss and global sea level rise. A global climate model with an eddying ocean is used to quantify the mechanisms contributing to ocean warming on the Antarctic continental shelf in an idealized 2xCO2 experiment. The results indicate that relatively large warm anomalies occur both in the upper 100 m and at depths above the shelf floor, which are controlled by different mechanisms. The near‐surface ocean warming is primarily a response to enhanced onshore advective heat transport across the shelf break. The deep shelf warming is initiated by onshore intrusions of relatively warm Circumpolar Deep Water (CDW), in density classes that access the shelf, as well as the reduction of the vertical mixing of heat. CO2‐induced shelf freshening influences both warming mechanisms. The shelf freshening slows vertical mixing by limiting gravitational instabilities and the upward diffusion of heat associated with CDW, resulting in the buildup of heat at depth. Meanwhile, freshening near the shelf break enhances the lateral density gradient of the Antarctic Slope Front (ASF) and disconnect isopycnals between the shelf and CDW, making cross‐ASF heat exchange more difficult. However, at several locations along the ASF, the cross‐ASF heat transport is less inhibited and heat can move onshore. Once onshore, lateral and vertical heat advection work to disperse the heat anomalies across the shelf region. Understanding the inhomogeneous Antarctic shelf warming will lead to better projections of future ice sheet mass loss.

Oceanic Heat Delivery to the Antarctic Continental Shelf: Large‐Scale, Low‐Frequency Variability

“…Recent results demonstrate the important roles of these forcings in the cross‐ACS heat transport. We also expect the choice of a fixed isobath as the boundary of the coastal control volume to introduce some bias in the heat transport calculations, because it does not always record the heat transport across the moving ASF axis (as discussed by Goddard et al, , for their simulation).…”

“…The product u ( y , z , t ) θ ( y , z , t ) is accumulated at each model time step. However, since neither the quantity θ ( y , z , t ) − θ f ( y , z , t ) nor the product u ( y , z , t ) θ f ( y , z , t ) were saved, we follow Goddard et al () and set θ f to the minimum freezing temperature found along the 1,000‐m isobath in the simulation ( θ f =−2.64 ∘ C). This means that the quantity ρ C p [ θ ( y , z ) − θ f ] must be interpreted as an upper bound (i.e., warmest value) for the melting potential of the intruding water masses.…”

The Antarctic Slope Current in a Changing Climate

“…While the surface wind stress plays a leading‐order role in determining the strength of the ASC, dense water formation on the shelf will also intensify lateral buoyancy gradients across the shelf break by increasing the density contrast with offshore water masses. This mechanism may be sensitive to changes in either the location or rates of sea ice formation, since brine rejection tends to mix dense waters throughout the water column, increasing the density at depths comparable to the offshore CDW (Goddard et al, ). In regions where the shelf fills with dense water, further production is balanced by the export of this dense water and a compensating onshore flow of lighter density classes that shoals the stratification at the shelf break.…”

“…Scientific focus on ASC eddies, and mesoscale variability in general, has intensified in recent decades largely due to their role in transporting heat across the Antarctic continental slope (e.g., Figures 3a–3c and Goddard et al, ; Stern et al, ; St‐Laurent et al, ). Initial modeling forays into this topic at relatively coarse resolution (∼4‐km horizontal grid spacing) detected CDW accessing the shelf via inertial overshoots , in which the slope current approaches a trough in the continental slope and continues onto the shelf via inertia (see Figure 3c; Dinniman et al, , ; Dinniman & Klinck, ).…”

“…CDW transport onto Antarctica's cold shelves must negotiate a sharp ASF and associated strong ASC that shield the continental shelf (see Figure 3d and section 2.1; Goddard et al, ). However, high‐resolution model simulations indicate that CDW is still able to reach the shelf in such regions via bottom‐trapped eddies that intrude beneath the ASF and onto the shelf, generated by instabilities of the ASC (see Figure 3a; Hattermann et al, ; Nøst et al, ).…”