Seasonal eddy kinetic energy modulations along the North Equatorial Countercurrent in the western Pacific

Abstract

Seasonal eddy kinetic energy (EKE) variability and its associated eddy energy conversion processes in the western tropical Pacific are investigated using satellite altimeter observations and a global, eddy‐resolving, ocean general circulation model (OGCM). Both the altimeter‐observed sea surface height anomalies and the OGCM simulation show an area with enhanced EKE east of the Mindanao Island centered around 133°E and 5°N. This enhanced EKE area corresponds to the location of the quasi‐stationary meander of the North Equatorial Countercurrent (NECC) and is bordered to the south by the Halmahera Eddy. The mesoscale EKE in this area exhibits a clear seasonality, strong in summer (July–August) and weak in winter (November–January), and much of this seasonality is confined to the upper 200 m layer. An investigation into the upper ocean eddy energetics based on the OGCM simulation reveals that the areal barotropic eddy energy conversion rate has an annual cycle similar to the EKE variations, while the areal baroclinic eddy energy conversion is found to be much smaller that the barotropic conversion rate and exhibits no clear seasonal changes. This indicates that the EKE variations are largely controlled by barotropic conversion of the seasonally varying regional circulation. By examining the seasonal background circulation changes, we find that the amplification of the barotropic eddy energy conversion rate in July–August is related to the seasonal evolution of the Mindanao Current and the New Guinea Coastal Current that amplifies the curvature and amplitude of the quasi‐stationary meander of the NECC and results in an elevated EKE level through increased regional barotropic conversion.

Hemispheric Asymmetry of the Pacific Shallow Meridional Overturning Circulation

“…Because of the interference between the surface and subsurface branches, the linear trends of the two branches are partially cancelling. This explains the strongly reduced trends that we find for the modeled STC branches at 5° N. The origin of this vertical interference is thought to be the high eddy activity across 5° N (Chen et al, ). At 5° S, on the other hand, the two branches can be clearly separated by means of the decadal trends.…”

“…During the recent (multi)decadal period, the SH STC appears to have played the most prominent role in the observed change of the tropical Pacific background state as the STC at 5° S increases at double the rate compared to 5° N. The reduced variability at 5° N relative to 5° S is not found to be due to reduced forcing but is related to the enhanced vertical interference which appears to be induced by the high eddy activity that is present across the basin at this latitude (Chen et al, ; Holmes et al, ). It is interesting to note that a similar hemispheric difference is found when the STCs are defined at 9° latitude, although the underlying physics are thought to be quite distinct from those occurring at 5° latitude, as this indicates the asymmetry is not sensitive to the latitude of the STC definition.…”

Mooring observations of equatorial currents in the upper 1000 m of the western Pacific Ocean during 2014

“…F. W. Wang et al (Intraseasonal Variability of the Surface Zonal Currents in the Western Tropical Pacific Ocean: Characteristics and Mechanisms, manuscript submitted to Journal of Physical Oceanography , 2016) found that the primary mechanism for ISV is the influence of internal variability related to mesoscale eddies at local scale (<200 km), and the influence of intraseasonal wind forcing at large scale (>500 km). In the NECC and SEC regions, barotropic instability induced by horizontal shears of flows is the major cause for the mesoscale eddy variability [ Zhao et al ., ; Chen et al ., ]. For intermediate currents, they are suggested to arise from the rectification of deep equatorial intraseasonal variability [ Hua et al ., ; Ascani et al ., ].…”

Interannual and interdecadal variability of the North Equatorial Countercurrent in the Western Pacific

“…[] for comprehensive reviews on the NEC). The NECC, on the other hand, serves as a confluent “return flow” for the low‐latitude western boundary currents: the Mindanao Current (MC) off the Philippine coast and the New Guinea Coastal Current (NGCC) off the New Guinea coast [ Lukas et al ., ; Gordon and Fine , ; Kashino et al ., ; Tozuka et al ., ; Hsin and Qiu , ; Chen et al ., ]. Located across the latitudinal center of the western Pacific warm pool, the NECC has been observed and modeled to play important roles in the maintenance and heat budget variability of the warm pool [ Meyers and Donguy , ; Picaut and Delcroix , ; Clement et al ., ].…”