J. Le Sommer

Inertial Motions during the Transient Adjustment of a Density Anomaly in the Equatorial Ocean with Application to the Western Pacific Warm Pool

Abstract This paper is focused on the spontaneous transient adjustment of a buoyant lens of water with uniform density, initially at rest in the vicinity of the equator. For parameters typical of the western Pacific warm pool, the adjustment is shown to generate finite-amplitude wave motions with period ∼8 days, which are not covered by the standard theory of linear equatorial waves. This mechanism may be at the origin of inertial motions at the early stages of ENSO events in the western Pacific Ocean. The lens adjustment is studied within the 11/2-layer reduced-gravity approximation on the equatorial β plane, using the high-resolution finite-volume numerical methods that are specially designed to handle outcropping isopycnals. Under the reduced-gravity approximation, a buoyant region of light water with outcropping boundaries in the vicinity of the equator is described by two parameters: the meridional-to-zonal scale aspect ratio δ and the ratio γ of the Coriolis force to the pressure force on its meridi...

Structure and Causes of the Pulsation Mode in the Antarctic Circumpolar Current South of Australia

The subsurface variability of potential temperature and salinity south of Australia along 1308E is studied over a 25-yr period (1980‐2004). The study is done with fields provided by a global eddy-permitting model of the DRAKKARproject forcedby atmospheric reanalysis. The analysis performed by C. Sun and D. R. Watts with in situ hydrographic data is repeated. Sun andWatts haveinvestigated the EOFmodesin streamfunction space along the World Ocean Circulation Experiment (WOCE) SR3 section. In particular, they found that an EOF mode, which they called the ‘‘pulsation mode,’’ strongly dominates subsurface thermohaline variations. Here, itis found that,in themodel, an EOFmodewith spatial structuresimilar tothe Sun andWattspulsation mode dominates subsurface thermohaline variations in streamfunction space. The mode displays a maximum of variability at the Subantarctic Front (SAF) between Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The associated time series exhibits an intermittent interseasonal frequency (3‐6 months), especially during three periods (1983‐84, 1990, and 1994‐96). Some energy is also found with a 4-yr period. Further analyses reveal that the pulsation mode can also be observed in physical space. The pulsation mode is found to be related to movements of the SAF constrainedby the bathymetry of the Southeast Indian Ridge. The pulsation mode displays many similarities with cold-core eddy events rather than being related to variations of the westerly wind stress, as previously proposed. The impact of those events on SAMW properties remains unclear.