A comparison of surface eddy kinetic energy and Reynolds stresses in the Gulf Stream and the Kuroshio Current systems from merged TOPEX/Poseidon and ERS‐1/2 altimetric data

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Abstract
Analyses of surface eddy kinetic energy (EKE) inferred from 5 years of merged TOPEX/Poseidon and ERS‐1/2 altimetric data are presented in the Gulf Stream (GS) and the Kuroshio (KS) Current systems, focusing on anisotropy and EKE seasonal, interannual, and long‐term variations, as well as Reynolds stresses. For both GS and KS a descriptive analysis of the EKE field shows that it is anisotropic along the path of the currents and assesses the close relationship between the spatial distribution of EKE high levels and bottom topography. A striking ocean feature is found in the GS, shaped as a zonal “double‐blade” structure in the EKE field, centered at 37°N, 71.5°W, that likely corresponds to a regular and permanent lateral displacement of the mean path of the current at this geographical location. Maps of EKE seasonal means for years 1993, 1996, and 1997 provide a detailed description of the surface variability (meanders, pinched‐off eddies, etc.) and show strong interannual variations in the KS. EKE monthly mean changes in the KS reveal a well‐marked seasonal cycle during the first 3 years (1993–1995), with EKE peaking in July/August. This is followed by a constant increase of its level, from the beginning of year 1996 till the beginning of 1997. During this increase the zonal velocity variance 〈u′2〉 is up to 30% higher than its meridional counterpart, suggesting a zonal acceleration of the eastward flow. EKE seasonal changes are also found in the KS Current (30°–35°N, 130°–140°E) and also show a regular annual cycle during the first 3 years, which precedes by 2 months the EKE seasonal cycle in the KS Extension (maximum in May/June). The maximum in EKE in both regions precedes by 2 months the maximum surface transport found by Qiu et al. [1991] and Zlotnicki [1991], occurring in July and late September, respectively. No such annual cycle is found in the GS region during the first 3 years. Year 1996 is anomalous in both current systems, with mean EKE level that increases by more than 20% compared to the other years. Also, EKE monthly mean changes in the KS reveal a well‐marked positive tendency in the eastern part of the current of more than 300 cm2 s−2 yr−1 and a negative tendency both in its eastern part and in the KS Current. This feature is absent from the GS. At last, eddy/mean flow interaction is presented. Both the spatial averaging of turbulent quantities and the relative contribution of the different terms of the horizontal momentum fluxes are discussed. A tentative explanation is given for the anomalously large 〈u′2〉 at the end of 1996 in the KS Extension.