ESC seminar No. 15

Intermediate-depth Eddies East of Mindanao: Potential Causes

Date
May 10 (Wed.). 2006, 14:00-15:00
Place
Meeting room 1 & 2, 2F Conference Building, YES
Speaker
Pierre Dutrieux (Department of Oceanography, University of Hawaii)
Language
English

Abstract

The western equatorial Pacific has been aptly described as a “water mass crossroads” (Fine, 94), with waters of northern and southern hemisphere origin converging meridionally to feed the Indonesian Throughflow and the North Equatorial Countercurrent (NECC). The strong currents in and above the thermocline have been the subject of many observational and modelling studies (emphasizing the northern hemisphere, e.g., Qu, 98, Arruda, 03, Kashino, 99, Lukas, 91, Qiu, 96, Wijffels, 95); their characteristics are relatively well known. Below the thermocline--or below the 400-m range of most shipboard acoustic Doppler current profilers (ADCP)--much less is known about the circulation. Even the direction of mean subthermocline flow along the Mindanao coast, for example, is still open to debate. It has long been known that water properties of southern origin penetrate northward along the western boundary of the Philippine Sea, evident as a high-oxygen tongue (Reid,78); but it is not clear whether this results from a possible northward mean flow (the Mindanao Undercurrent or MUC; Hu, 91; Qu, 98) or from an eddy flux, as suggested by Wijffels, 95, or from both. The southward advection of North Pacific Intermediate Water along the Mindanao coast and into the Celebes Sea has also been observed (Bingham, 94). Recent observations have shown surprisingly strong flow beneath the thermocline in the Pacific low-latitude western boundary region east of Mindanao (Firing, 05). Maximum speeds exceed 0.5 m/s at 800 m depth, and speeds exceeding 0.2 m/s were common even below 1500 m. The spatial structure looks like that of an eddy field, not a mean flow along the coast. Whatever the mean subthermocline flow is, all evidence to date indicates that it is weak compared to the eddy variability. This conclusion is confirmed by the only moored current meter record in the region that we are aware of (Kashino, 05). Although measurements of subthermocline currents east of Mindanao are few, recent progress in numerical modelling has been dramatic, as characterized by the recent OFES runs (Masumoto et al, 04). Momentum variability analysis along with energy budgets will be used to infer some new and interesting features in this area, in an attempt to find causes of intra-seasonal time scales variability.

Contact

Hideharu Sasaki
Computational Earth Science Research Program
Earth Simulator Center
TEL: 045-778-5843
e-mail: sasaki