Publications

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2013
Subramanian, AC, Miller AJ, Cornuelle BD, Di Lorenzo E, Weller RA, Straneo F.  2013.  A data assimilative perspective of oceanic mesoscale eddy evolution during VOCALS-REx. Atmospheric Chemistry and Physics. 13:3329-3344.   10.5194/acp-13-3329-2013   AbstractWebsite

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 degrees W, 19 degrees 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 degrees C over a depth range from 50-600 m and features a cold anomaly of approximately -1 degrees 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 non-linearity 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.

2010
Kim, SY, Cornuelle BD, Terrill EJ.  2010.  Decomposing observations of high-frequency radar-derived surface currents by their forcing mechanisms: Locally wind-driven surface currents. Journal of Geophysical Research-Oceans. 115   10.1029/2010jc006223   AbstractWebsite

The wind impulse response function and transfer function for high-frequency radar-derived surface currents off southern San Diego are calculated using several local wind observations. The spatial map of the transfer function reflects the influence of the coast on wind-current dynamics. Near the coast (within 20 km from the shoreline), the amplitudes of the transfer function at inertial and diurnal frequencies are reduced due to effects of coastline and bottom bathymetry. Meanwhile, the amplitude of low-frequency currents increases near the coast, which is attributed to the local geostrophic balance between cross-shore pressure gradients against the coast and currents. Locally wind-driven surface currents are estimated from the data-derived response function, and their power spectrum shows a strong diurnal peak superposed on a red spectrum, similar to the spectra of observed winds. Current magnitudes and veering angles to a quasi-steady wind are typically 2-5% of the wind speed and vary 50 degrees-90 degrees to the right of the wind, respectively. A wind skill map is introduced to present the fractional variance of surface currents explained by local winds as a verification tool for wind data quality and relevance. Moreover, the transfer functions in summer and winter are presented to examine the seasonal variation in ocean surface current response to the wind associated with stratification change.

2009
Kim, SY, Cornuelle BD, Terrill EJ.  2009.  Anisotropic Response of Surface Currents to the Wind in a Coastal Region. Journal of Physical Oceanography. 39:1512-1533.   10.1175/2009JPO4013.1   Abstract

Analysis of coastal surface currents measured off the coast of San Diego for two years suggests an anisotropic and asymmetric response to the wind, probably as a result of bottom/coastline boundary effects, including pressure gradients. In a linear regression, the statistically estimated anisotropic response explains approximately 20% more surface current variance than an isotropic wind-ocean response model. After steady wind forcing for three days, the isotropic surface current response veers 42 degrees +/- 2 degrees to the right of the wind regardless of wind direction, whereas the anisotropic analysis suggests that the upcoast (onshore) wind stress generates surface currents with 10 degrees +/- 4 degrees (71 degrees +/- 3 degrees) to the right of the wind direction. The anisotropic response thus reflects the dominance of alongshore currents in this coastal region. Both analyses yield wind-driven currents with 3%-5% of the wind speed, as expected. In addition, nonlinear isotropic and anisotropic response functions are considered, and the asymmetric current responses to the wind are examined. These results provide a comprehensive statistical model of the wind-driven currents in the coastal region, which has not been well identified in previous field studies, but is qualitatively consistent with descriptions of the current response in coastal ocean models.