References

ACP

Atmospheric Chemistry and Physics

ACP

Atmos. Chem. Phys.

1680-7324

Copernicus Publications

Göttingen, Germany

10.5194/acp-13-3329-2013

A data assimilative perspective of oceanic mesoscale eddy evolution during VOCALS-REx

Subramanian

A. C.

¹ Miller

A. J.

¹ Cornuelle

B. D.

¹ Di Lorenzo

² Weller

R. A.

³ Straneo

Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA

Georgia Institute of Technology, Altlanta, Georgia, USA

Woods Hole Oceanographic Institution, MIT, 86 Water Street, MA, USA

25 03 2013

13 6 3329 3344

2013

This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/

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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 76° W, 19° S. The ROMS fits capture this eddy as an isolated rotating 3-D vortex with a strong subsurface signature in velocity, temperature and anomalously low salinity. The eddy has an average temperature anomaly of approximately −0.5 °C over a depth range from 50–600 m and features a cold anomaly of approximately −1 °C near 150 m 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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