On topography-related dynamics in the Argentine Basin

The Argentine Basin (>5000 m) is home to rather unique oceanic structures of prime importance to the global circulation. In this thesis, we focus on elements of circulation (ocean waves and currents) constrained by topography. To the west, the Patagonian continental shelf of variable slope and direction hosts a variety of topographic waves. By combining outputs of a high-resolution reanalysis (GLORYS12), in-situ and satellite data, two types of sub-inertial waves were documented, fast and slow waves. Fast waves propagating along the Patagonian shelf break, with phase velocities between 1 m/s and 7 m/s, periods ranging from 5 to 130 days and wavelengths from 1,200 to 12,000 km. Their characteristics depend upon stratification, slope, shelf width, latitude and mean flow. High-frequency waves with periods between 5 and 40 days are often locally forced by strong variations of the zonal wind south of 47°S(Poli et al.,2020), whereas lower-frequency waves with periods between 40 and 130 days propagate from the equatorial Pacific to the tropical Atlantic (22°S, Poli et al.,2022). The Madden-Julian Oscillation (MJO) plays a key role in forcing these low-frequency waves in two ways. (a) By an oceanic teleconnection involving equatorial Kelvin waves reaching the American west coast, and (b) by an atmospheric teleconnection reinforcing southerly winds in the southeast Pacific. In addition, local winds, which are not necessarily linked to the MJO, modulate and trigger low-frequency waves (40-130 days period) in specific locations, such as the Brazil-Malvinas confluence and the Drake Passage. All these waves have an impact on along-shore currents : during the positive phase, the near-surface current is increased by around 0.1 m/s. In addition, these waves contribute to the supply of nutrients sustaining the high primary production on the Patagonian shelf. Slow waves propagate in the core of the Malvinas Current with phase velocities between 0.10 and 0.30 m/s, periods around 20, 60 and 100 days, and wavelengths ranging between 450 and 1200 km. These waves were tracked back to Drake Passage and the Malvinas Escarpment. In the center of the basin, the Zapiola anticyclone is located above a sedimentary deposit and displays an intensity equivalent to that of the strongest ocean currents (>100 Sv) with enhanced bottom velocities reaching 0.1 m/s. Using GLORYS12 reanalysis, we constructed a time series of Zapiola transport. Transport reaches extreme values ranging from -18 Sv to 250 Sv. The extrema show a seasonal distribution, with a majority of weak events occuring in austral winter. When the Zapiola anticyclone collapses, it becomes more permeable to the influences of surrounding waters, and the water characteristics in the center of the Zapiola are modified. In particular, cyclonic eddies originating from the polar front penetrate up to the center. During strong events, wind stress curl is reinforced and turbulent kinetic energy is enhanced around the anticyclone. The region around the Zapiola anticyclone exhibits a multi-year modulation, with periods of 4-5 years (1993-1997, 1998-2003 and 2004-2009) of low (high) salinity corresponding to low (high) transport. Over the last 27 years, waters of the Argentine basin became warmer and saltier in the first 2000 m of the water column. These trends are concomitant with an increase of turbulent kinetic energy in the south of the basin which is probably associated with the southward migration of the subtropical front (Poli et al., 2023).