A modified method to estimate eddy diffusivity in the North Pacific using altimeter eddy statistics

Abstract

The method proposed by Stammer (1998) is modifi ed using eddy statistics from altimeter observation to obtain more realistic eddy diffusivity ( K ) for the North Pacifi c. Compared with original estimates, the modifi ed K has remarkably reduced values in the Kuroshio Extension (KE) and North Equatorial Counter Current (NECC) regions, but slightly enhanced values in the Subtropical Counter Current (STCC) region. In strong eastward fl ow areas like the KE and NECC, owing to a large difference between mean fl ow velocity and propagation velocity of mesoscale eddies, tracers inside the mesoscale eddies are transported outside rapidly by advection, and mixing length L is hence strongly suppressed. The low eddy probability ( P ) is also responsible for the reduced K in the NECC area. In the STCC region, however, L is mildly suppressed and P is very high, so K there is enhanced. The zonally-averaged K has two peaks with comparable magnitudes, in the latitude bands of the STCC and KE. In the core of KE, because of the reduced values of P and L , the zonally-averaged K is a minimum. Zonally-integrated eddy heat transport in the KE band, calculated based on the modifi ed K , is much closer to the results of previous independent research, indicating the robustness of our modifi ed K . The map of modifi ed K provides useful information for modeling studies in the North Pacifi c.

Spatial structure and temporal variability of the zonal flow in the Luzon Strait

“…This westward propagation was more obvious in the distance‐time plot of regressed SLA along the 20.5°N latitude (Figure a). The westward propagation speed was estimated to be 9.2 cm/s, close to the earlier estimates of eddy propagation speeds in this region [ Yang et al ., ; Zhang et al ., ]. It was also consistent with the phase speed of the first‐mode baroclinic Rossby waves at the same latitudes [ Chelton et al ., ].…”