The nonlinear impact of surface forcing changes on bottom water
formation and overturning in the Southern Ocean
Abstract
Two coupled climate models, differing primarily in horizontal resolution
and treatment of mesoscale eddies, were used to assess the impact of
perturbations in wind stress and Antarctic ice sheet (AIS) melting on
the Southern Ocean meridional overturning circulation (SO MOC), which
plays an important role in global climate regulation. The largest impact
is found in the SO MOC lower limb, associated with the formation of
Antarctic Bottom Water (AABW), which in both models is enhanced by wind
and weakened by AIS meltwater perturbations. Even though both models
under the AIS melting perturbation show similar AABW transport
reductions of 4-5 Sv (50-60%), the volume deflation of AABW south of
30˚S is four times greater in the higher resolution simulation (-20 vs
-5 Sv). Water mass transformation (WMT) analysis reveals that
surface-forced dense water formation on the Antarctic shelf is absent in
the higher resolution and reduced by half in the lower resolution model
in response to the increased AIS melting. However, the decline of the
AABW volume (and its inter-model difference) far exceeds the
surface-forced WMT changes alone, which indicates that the divergent
model responses arise from interactions between changes in surface
forcing and interior mixing processes. This model divergence
demonstrates an important source of uncertainty in climate modeling, and
indicates that accurate shelf processes together with scenarios
accounting for AIS melting are necessary for robust projections of the
deep ocean’s response to anthropogenic forcing and role as the largest
sink in Earth’s energy budget.