A scale-dependent analysis of the barotropic vorticity budget in a
global ocean simulation
Abstract
The climatological mean barotropic vorticity budget is analyzed to
investigate the relative importance of surface wind stress, topography
and nonlinear advection in dynamical balances in a global ocean
simulation. In addition to a pronounced regional variability in
vorticity balances, the relative magnitudes of vorticity budget terms
strongly depend on the length-scale of interest. To carry out a
length-scale dependent vorticity analysis in different ocean basins,
vorticity budget terms are spatially filtered by employing the
coarse-graining technique. At length-scales greater than 10o (or roughly
1000 km), the dynamics closely follow the Topographic-Sverdrup balance
in which bottom pressure torque, surface wind stress curl and planetary
vorticity advection terms are in balance. In contrast, when including
all length-scales resolved by the model, bottom pressure torque and
nonlinear advection terms dominate the vorticity budget
(Topographic-Nonlinear balance), which suggests a prominent role of
oceanic eddies, which are of Ο(10-100) km in size, and the associated
bottom pressure anomalies in local vorticity balances at length-scales
smaller than 1000 km. Overall, there is a transition from the
Topographic-Nonlinear regime at scales smaller than 10o to the
Topographic-Sverdrup regime at length-scales greater than 10o. These
dynamical balances hold across all ocean basins; however,
interpretations of the dominant vorticity balances depend on the level
of spatial filtering or the effective model resolution. On the other
hand, the contribution of bottom and lateral friction terms in the
barotropic vorticity budget remains small and is significant only near
sea-land boundaries, where bottom stress and horizontal friction
generally peak.