Effects of Mesoscale Eddies on Subduction and Distribution of Subtropical Mode Water in an Eddy-Resolving OGCM of the Western North Pacific

Abstract

The effects of mesoscale eddies on the subduction and distribution of the North Pacific Subtropical Mode Water (STMW) are investigated using an eddy-resolving ocean general circulation model (OGCM). First, the subduction rate is calculated and the contribution of eddies to the subduction of STMW is estimated. It is found that eddy subduction significantly contributes to the total subduction of STMW. Second, eddy thickness transport and diapycnal flux are directly diagnosed to investigate the large-scale eddy-induced transport process of STMW. The large southward eddy thickness transport in the STMW core density is consistent with eddy subduction. The eddy transport on the isopycnal surface of STMW is directed down the thickness gradient and traverses the mean flow. The meridional eddy transport streamfunction indicates two eddy circulation cells south of 30°N, associated with the circulation of STMW. These cells flatten density surfaces, similar to the effect of the Gent and McWilliams (GM) scheme. The subducted STMW is gradually dissipated to lower or higher densities in the main thermocline, basically by vertical diffusion. Finally, local processes of eddy subduction and transport of STMW are explored using an anticyclonic eddy. Results imply two possible local processes of the eddy subduction of STMW. One is the destruction of a potential vorticity (PV) gradient by eddy mixing, where the PV gradient is due to winter deep mixed layer formation. The other is the southward translation of anticyclonic eddies that accompany low PV.

Energy pathways and structures of oceanic eddies from the ECCO2 state estimate and simplified models

“…For example, the eddy field can influence water mass transformation by contributing to the total subduction and ventilation rates (e.g. Gebbie, 2007;Nishikawa et al, 2010;Lachkar et al, 2009). Also, eddies are key in balancing the heat budget in the overflow regions and the deep convection sites (e.g.…”

Mesoscale eddy effects on the subduction of North Pacific mode waters

“…The effects of mesoscale eddies on mode water formation have received increasing attention over the past decade due to the concurrent increase of remotely sensed and in situ observations [Uehara et al, 2003;Qiu and Chen, 2006;Qiu et al, 2007;Oka et al, 2009], and finer resolution OGCMs which resolve these processes [Qu et al, 2002;Nishikawa et al, 2010;Oka and Qiu, 2012]. Marshall [1997] suggested that in strong frontal regions where intense baroclinic instability occurs (e.g., in the Kuroshio Extension region), an ensemble of eddies provides advection via the ''bolus velocity.''…”

“…Using the output of a global high-resolution OGCM, Qu et al [2002] pointed out that mesoscale eddies enhance the annual subduction rate of North Pacific subtropical mode water (STMW) and central mode water (CMW) by up to 100 m/yr, or 34% of the total subduction. Recently, Nishikawa et al [2010] estimated that eddies contribute some 50% of the total mode water subduction rate based on an eddy-resolving OGCM (1/12 3 1/18 ) simulation of the western North Pacific. From a climatological viewpoint, subduction is limited to the intersections of the winter mixed layer depth (MLD) front and outcrop lines [Xie et al, 2000], but eddies may broaden the horizontal extent of the subduction sites.…”

Four-dimensional variational ocean reanalysis: a 30-year high-resolution dataset in the western North Pacific (FORA-WNP30)

“…Therefore, spatial and temporal variability of the surface mixed layer depth (MLD) and water masses is one of the important metrics for assessing the reproduction of the surface layer variability (e.g., Toyoda et al 2015a). In addition, it has been indicated that smaller scale (eddy) processes are essential for the reproduction of MLD and surface-intermediate water masses (e.g., Ishizaki and Ishikawa 2004;Tsujino and Yasuda 2004;Nishikawa et al 2010). Thus, the present product (FORA-WNP30) is expected to effectively reproduce the MLD-water mass variability by adopting a high-resolution OGCM.…”