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.
.