Drifter‐derived maps of lateral diffusivity in the Pacific and Atlantic Oceans in relation to surface circulation patterns

Abstract

[1] Maps of apparent lateral diffusivity K derived from surface drifters in the Pacific and Atlantic Oceans are analyzed together with mean surface circulation patterns. A remarkable feature of the diffusivity map in the Atlantic was proven to be trans-ocean belts of enhanced K in the thirties latitudes. It was shown that the North Atlantic belt is caused by the meandering and eddy-producing Azores Current, while its counterpart in the South Atlantic results from northwestward and westward drift of warm eddies produced by the Agulhas Retroflection. A puzzling feature of the diffusivity map in the Pacific was a zonally elongated tongue of high K at the center of a subtropical gyre in the Southern Hemisphere beginning from the western boundary. A similar tongue observed at the center of a subtropical gyre of the North Pacific is known to result from baroclinic instability in the seasonal North Subtropical Countercurrent (NSTCC). Both tongues were characterized by a strong and identical seasonal cycle of the eddy kinetic energy (EKE), which persuaded us to suggest an identity for the formation mechanisms that would imply the existence of southern counterpart of NSTCC: the would-be South Subtropical Countercurrent (SSTCC). A seasonal-latitudinal dependence of the mean zonal current and EKE as well as maps of the mean zonal velocity were derived from drifters that provided some evidence in favor of the existence of this SSTCC.

Measuring surface currents with Surface Velocity Program drifters: the instrument, its data, and some recent results

“…These observations would be invaluable for measuring mixed layer heat content variability, which can be poorly correlated with SST changes (Kelly, 2004) Atlantic (Largier and Boyd, 2001), tropical Pacific (Grodsky and Carton, 2001b;Johnson, 2001;Yaremchuk and Qu, 2004), central subtropical Pacific Flament et al, 2001), western Pacific , North Pacific (Rabinovich and Thomson, 2001), tropical Indian (Grodsky and Carton, 2001a) and Southern (Zhou et al, 2002) oceans. Several recent studies have also focused on marginal seas including the Japan/East (Lee et al, 2000), South China (Centurioni et al, 2004) and Black Zhurbas et al, 2004) seas.…”

“…More recently, Zhurbas and Oh (2004) mapped the global surface diffusivity using a methodology designed to avoid contaminat- underestimates EKE in the Gulf Stream extension and ocean interior, perhaps due to the climatological forcing employed or the lack of vertical shear in the bulk mixed layer (Garraffo et al, 2001). Similarly, the 1/10 • -resolution Parallel Ocean Program (POP) model does not reproduce the observed Lagrangian scales, due to the model being too hydrodynamically stable in the surface layer (McClean et al, 2002).…”

Investigating the Eddy Diffusivity Concept in the Coastal Ocean

“…Alternatively, eddy diffusivities can be obtained by computing the correlation between the velocity and displacement for an ensemble of Lagrangian particles (Davis 1991;Zhurbas and Oh 2004) or by integrating the velocity autocorrelation function (Davis 1991;Sallee et al 2008;Griesel et al 2010). Diffusivity can also be estimated based on two-particle (or relative) statistics (LaCasce and Ohlmann 2003;Poje et al 2014) as well as inferred from the observed or numerically simulated tracer distributions (Sundermeyer and Ledwell 2001;Nakamura 1996Nakamura , 2001Abernathey and Marshall 2013).…”

Connection between encounter volume and diffusivity in geophysical flows

“…A number of diffusivity estimates other than Okubo's have been made for the Gulf Stream extension region (e.g., Zhurbas and Oh, 2004;LaCasce, 2008;Rypina et al, 2012;Abernathey and Marshall, 2013;Klocker and Abernathey, 2014;or Cole et al, 2015). These estimates are based on surface drifters (Zhurbas and Oh, 2004;LaCasce, 2008;Rypina et al, 2012), satellite-observed velocity fields (Abernathey and Marshall, 2013;Klocker and Abernathey, 2014;Rypina et al, 2012), and Argo float observations (Cole et al, 2015), and they use either the spread of drifters or the evolution of simulated or observed tracer fields to deduce diffusivity.…”

“…These estimates are based on surface drifters (Zhurbas and Oh, 2004;LaCasce, 2008;Rypina et al, 2012), satellite-observed velocity fields (Abernathey and Marshall, 2013;Klocker and Abernathey, 2014;Rypina et al, 2012), and Argo float observations (Cole et al, 2015), and they use either the spread of drifters or the evolution of simulated or observed tracer fields to deduce diffusivity. The resulting diffusivities are spatially varying and span 2 orders of magnitude, from 2 × 10 4 m 2 s −1 in the most energetic regions in the immediate vicinity of the Gulf Stream and its extension, to 10 3 m 2 s −1 in less energetic areas, to 200 m 2 s −1 in the coastal areas of the Slope Sea.…”