The annual cycle of surface eddy kinetic energy and its influence on eddy momentum fluxes as inferred from altimeter data

Abstract

Abstract:The annual cycle of surface eddy kinetic energy (EKE) and its influence on eddy momentum fluxes are investigated using an updated record of satellite altimeter data. It is found that there is a phase difference between the annual cycles of EKE in the western boundary current regions and EKE in the interior of the subtropical gyres, suggesting that different mechanisms may be at work in different parts of the subtropical gyres. The annual cycles of EKE averaged in the two hemispheres are found to be of similar magnitude but in opposite phase. As a result, the globally-averaged EKE shows little seasonal variability. The longer record of altimeter data used in this study has brought out a clearer and simpler picture of eddy momentum fluxes in the Gulf Stream and Kuroshio Extensions: eddies in both regions systematically flux westerly momentum into the western boundary current jets. Considerable seasonal variations in eddy momentum fluxes are found in the western boundary current regions, which potentially play an important role in modulating the strength of the western boundary currents and their associated recirculation gyres on the seasonal time scale.

On the Seasonal Eddy Variability in the Kuroshio Extension

“…The EWW exhibits overwhelming negative values, although mingled with positive patches over the region. This indicates that wind forcing plays an important role to spin down the geostrophic eddies, consistent with previous studies (Hughes and Wilson 2008;Scott and Xu 2009;Zhai et al 2012;Xu et al 2016). The negative EWW also presents a well-defined seasonal cycle whose amplitude tends to be larger during the winter season than summer, indicating that more EKE is damped during winter.…”

“…As we reviewed in the introduction, several previous studies have found positive correlations of wind forcing and EKE in low-eddy-energy ocean sectors (Frankignoul and Müller 1979;White and Heywood 1995;Pujol and Larnicol 2005;Yang et al 2013). However, the scenario of direct eddy generation by local wind forcing is questioned in recent studies of wind work estimates based on finescale scatterometer surface stress observations as well as state-of-the-art coupled oceanatmosphere models (Duhaut and Straub 2006;Hughes and Wilson 2008;Scott and Xu 2009;Zhai et al 2012;Xu et al 2016;Renault et al 2016). In these studies, the authors found that atmospheric wind acts as an ''eddy FIG.…”

“…Dynamically, the seasonal modulation of the flow pattern and thermal structure could lead to large changes in the stability properties of the ocean, leaving imprints on the seasonal variation of the regional mesoscale eddy field. Early studies based on short-record altimetry data have reported that the eddy kinetic energy (EKE) level in the Kuroshio Extension peaks in summer and reaches its minimum in winter (Tai and White 1990;Stammer and Wunsch 1999;Ducet and Le Traon 2001;Scharffenberg and Stammer 2010); this has been recently reconfirmed by Zhai (2017) using updated records from 1993 to 2016. So far, the mechanism underlying the seasonal eddy variabilities in the Kuroshio Extension region is still unclear.…”

Spatiotemporal Variability of the Global Ocean Internal Processes Inferred from Satellite Observations

“…Thanks to the accumulating altimeter-derived geostrophic current data, the seasonal variability of the ocean KE has been identified and examined in many regions in the ocean (e.g., Eden and Böning 2002;Jouanno et al 2012;Rieck et al 2015;von Appen et al 2016;Zhai 2017;Kang and Curchitser 2017;Yang and Liang 2018). Figure 11 shows the horizontal distributions of EKE anomalies in the four seasons averaged over the 1994-2015 period.…”

On the Role of the Kuroshio Extension Bimodality in Modulating the Surface Eddy Kinetic Energy Seasonal Variability

“…Besides, the surface EKE in the KE region yields a remarkable seasonal variability: the EKE reaches the maximum in summer and the minimum in winter (Ducet & Le Traon, 2001;Ji et al, 2018;Scharffenberg & Stammer, 2010;Yang & Liang, 2018;Zhai, 2017). Yang and Liang (2018) suggested that, in contrast to the KE EKE decadal variability, the baroclinic instability is responsible for its seasonal variability.…”