Observations and modeling of a California undercurrent eddy

A deep, nonlinear warm eddy advecting water that was also anomalously salti er, lower in oxygen, and higher in nutrients relative to surrounding waters was observed in moored current and temperature measurements and in hydrogr aphic data obtained at a site similar to 400 km off the coast of northern C alifornia. The eddy was reproduced using a nonlinear quasi-geostrophic mode l, initialized by an iterative procedure using time series of 2-day average d moored current measurements. The procedure demonstrates how a data assimi lative technique synthesizes and enhances the resolution of a relatively sp arse data set by incorporating time-dependence and model physics. The model forecast showed significant skill above persistence or climatology for 40 days. Our hypothesis, that the eddy was generated at the coast in winter an d subsequently moved 400 km offshore by May, is consistent with the eddy mo vement diagnosed by the model and with the observations and coastal climato logy. The model evolution significantly underpredicted the temperature anom aly in the eddy owing in part to unmodeled salinity compensation in trapped California Undercurrent water. Together, observations and model results sh ow a stable nonlinear eddy in the California Current System that transporte d water and properties southwestward through the energetic eastern boundary region. Coherent features such as this one may be a mechanism for property transfer between the eddy-rich coastal zone and the eddy desert of the eas tern North Pacific Ocean.