Geostrophy assessment and momentum balance of the global oceans in a
tide- and eddy-resolving model
Abstract
Current satellite altimeters map sea surface height (SSH) with an
effective spatial scale of O(100 km) and, as a result, surface ocean
velocity can be appropriately estimated from merged SSH fields by
assuming geostrophic equilibrium. The validity of the geostrophic
assumption down to the spatial scale of O(10 km) that will be newly
resolved by the next generation of satellite altimeters, such as the
Surface Water Ocean Topography (SWOT) mission, remains unknown. In this
study, the accuracy of geostrophy for the estimation of surface currents
from a knowledge of instantaneous sea level is quantified using the
hourly fields from a tide- and eddy-resolving global numerical
simulation. Geostrophic balance is found to be the leading-order balance
in frontal regions characterized by large kinetic energy, such as the
western boundary currents and the Antarctic Circumpolar Current.
Everywhere else, the ageostrophic flow is of comparable or larger
amplitude than the total flow. As expected, the validity of geostrophy
is shown to improve at low frequencies (typically <0.5 cpd).
Global estimates of the horizontal momentum budget reveal that the
tropical and mid-latitude regions where geostrophic balance fails are
dominated by fast (e.g., semidiurnal and supertidal) unbalanced motions
and turbulent stress divergence terms rather than higher-order
geostrophic terms. These findings indicate that the estimation of
velocity from geostrophy applied on SWOT raw sea level maps may be
challenging away from energetic areas.