Sub-Antarctic Mode Waters (SAMWs) form to the north of the Antarctic Circumpolar Current (ACC) in the Indo-Pacific Ocean, whence they ventilate the ocean’s lower pycnocline and play an important role in the climate system. With a backward Lagrangian particle-tracking experiment in a data-assimilative model of the Southern Ocean (B-SOSE), we address the long-standing question of whether SAMWs originate from densification of southward-flowing subtropical waters, or lightening of northward-flowing Antarctic waters sourced by Circumpolar Deep Water (CDW) upwelling. Our analysis evidences the co-occurrence of both sources of SAMWs in all formation areas, and strong inter-basin contrasts in their relative contributions. Subtropical waters are the main precursor of Indian Ocean SAMWs (70-75% of particles) but contribute a smaller amount ($<$40%) to Pacific SAMWs, which are mainly sourced by CDW. By tracking property changes along particle trajectories, we show that SAMW formation from northern and southern sources involves contrasting drivers: subtropical source waters are cooled and densified by surface heat fluxes, and freshened by ocean mixing. Southern source waters are warmed and lightened by surface heat and freshwater fluxes, and they are made either saltier by mixing in the case of Indian SAMWs, or fresher by surface fluxes in the case of Pacific SAMWs. Our results underscore the distinct climatic impact of Indian and Pacific SAMWs, as net sources of atmospheric heat and net sinks of freshwater, respectively; a role that is conferred by the relative contributions of subtropical and Antarctic sources to their formation.

A document by Bieito Fernández Castro. Click on the document to view its contents.

Spatial variability of physical properties induced by circulation and stirring remains unaccounted for in the energy pathway of inland waters. Recent efforts in microstructure turbulence measurements have unraveled the overall energy budget in lakes. Yet, a paucity of lake-wide turbulence measurements hinders our ability to assess how representative such budgets are at the basin scale. Using an autonomous underwater glider equipped with a microstructure payload, we explored the spatial variability of turbulence in Lake Geneva. Microstructure analyses allowed turbulent dissipation rates and thermal variances estimations by fitting temperature gradient fluctuations spectra to the Batchelor spectrum. In open waters, results indicate mild turbulent dissipation rates in the surface and thermocline (~10⁻⁸ W kg⁻¹), which weaken towards the deep hypolimnion (~10⁻¹¹ – 10⁻¹⁰ W kg⁻¹). The strong thermal stratification inhibited interior mixing in the thermocline. In contrast, measurements along the coastal slope reveal a notorious enhancement of turbulent dissipation (~5×10⁻⁸ W kg⁻¹) above the sloping topography way above the known extent of the bottom boundary layer. These distinct turbulence patterns result from differing large-scale dynamics in the interior and coastal environments. Current measurements in open waters show dominant internal Poincaré waves. On the coast, three-dimensional numerical results from meteolakes.ch suggest that enhanced bottom dissipations arise from the development of centrifugal instabilities. A process driven by coastal cyclonic circulation interacting with the sloping bottom reported for the ocean but so far overlooked in large lakes. The spatially-distributed turbulence measurements we report here highlight the potential of underwater glider deployments for further lake exploration.

Bays within eastern boundary upwelling systems (EBUS) are ecological hot-spots featuring a diverse range of spatio-temporal dynamics. At the EBUSs’ poleward limit, upwelling occurs in short-lived (<1 week) pulses modulated by synoptic wind variability. The circulations in long, narrow bays can respond to these fluctuations within few hours. The short-term biological response to these pulses was investigated in two of these bays (Rias Baixas, NW-Iberia) with a two-week quasi-synoptic spatio-temporal survey in the summer 2018. A four-day-long upwelling pulse caused deep, nutrient-rich isopycnals to rise into the euphotic zone inside the bays, triggering a rapid (~1.7 days) nutrient uptake and formation of a subsurface chlorophyll maximum (~3.8 days). The phytoplankton biomass was transported rapidly toward deep, offshore waters when the winds weakened. These results suggest that high productivity in narrow bays is controlled by the transient exposure of deep, nutrient-rich waters to light during upwelling pulses.