2.1.1 Currents
To characterize ocean currents, we used the horizontal surface velocity components from HYCOM (CDAM, 2015). HYCOM is a primitive equation ocean general circulation model that remains isopycnic in the open stratified ocean, smoothly reverting to a terrain-following coordinate near the bottom and z-level coordinates in the mixed layer and/or unstratified seas (Bleck, 2002). HYCOM provides different vertical mixing schemes for the surface mixed layer and the interior diapycnal mixing, parameterizing the contribution of background internal wave breaking, shear instability mixing, and double diffusion. Processes are parameterized in the surface boundary layer, including wind-driven mixing, surface buoyancy fluxes, and convective instability (Chassignet et al., 2003; Halliwell, 2004; Large et al., 1997). HYCOM simulations include the Navy Coupled Ocean Data Assimilation data (NCODA) (Cummings, 2006; Cummings & Smedstad, 2013). We used the eastward (u -component) and northward (v -component) surface current velocity from the HYCOM + NCODA GOFS 3.1 reanalysis from 1994 to 2015, and analysis GLBv0.08 experiment from 2016 to 2018, which has a temporal resolution of 3 hours and a spatial resolution of 1/12o. In the rest of the article, we refer to this velocity time series as HYCOM. HYCOM is forced by the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), described in the next section. We calculated daily means for the current data to create the 25-year climatology. A critically important aspect to this study which is based on climatological currents: Duran et al. (2018) showed that climatological ocean currents preserve the ensemble-mean Lagrangian transport patterns of the instantaneous velocity, thus using a climatological velocity to study instantaneous Lagrangian patterns produces meaningful results; more details are presented in section 2.2.1.
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