Subramanian, A. C., A. J. Miller, B. D. Cornuelle, E. Di Lorenzo, R. A. Weller and F. Straneo, 2013:
A data assimilative perspective of oceanic mesoscale eddy
evolution during VOCALS-REx
Atmospheric Chemistry and Physics, 13, 3329-3344.
Abstract.
Oceanic observations collected during the VOCALS-REx cruise time period, 1-30 November
2008, are assimilated into a regional ocean model (ROMS) using 4DVAR and then analyzed for
their dynamics. Nonlinearities in the system prevent a complete 30-day fit, so two 15-day fits for
1-15 November and 16-30 November are executed using the available observations of hydrographic
temperature and salinity, along with satellite fields of SST and sea-level height anomaly. The fits
converge and reduce the cost function significantly, and the results indicated that ROMS is able to
successfully reproduce both large-scale and smaller-scale features of the flows observed during the
VOCALS-REx cruise. Particular attention is focused on an intensively studied eddy at 76W, 19S.
The ROMS fits capture this eddy as an isolated rotating 3-D vortex with a strong subsurface signa
ture in velocity, temperature and anomalously low salinity. The eddy has an average temperature
anomaly of approximately -0.5C over a depth range from 50-600m and features a cold anomaly
of approximately -1C near 150m depth. The eddy moves northwestward and elongates during
the second 15-day fit. It exhibits a strong signature in the Okubo-Weiss parameter, which indicates
significant nonlinearity in its evolution. The heat balance for the period of the cruise from the ocean
state estimate reveals that the horizontal advection and the vertical mixing processes are the dominant
terms that balance the temperature tendency of the upper layer of the ocean locally in time and
space. Areal averages around the eddies, for a 15-day period during the cruise, suggest that vertical
mixing processes generally balance the surface heating. Although, this indicates only a small role
for lateral advective processes in this region during this period, this quasi-instantaneous heat budget
analysis cannot be extended to interpret the seasonal or long-term upper ocean heat budget in this
region.
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