The Brazil-Malvinas Confluence : from local to global scales
The Southwest Atlantic Ocean is a key component of the global climate system. It holds the Confluence of the Brazil and Malvinas Currents (BMC), one of the highest frontal systems of the world ocean. The BMC is the encounter point of subtropical origin warm and salty waters, transported southward wi...
| Autor: | |
|---|---|
| Formato: | tesis doctoral |
| Fecha de publicación: | 2019 |
| País: | España |
| Recursos: | Universitat Politècnica de Catalunya (UPC) |
| Repositorio: | UPCommons. Portal del coneixement obert de la UPC |
| Idioma: | inglés |
| OAI Identifier: | oai:upcommons.upc.edu:2117/172603 |
| Acesso em linha: | https://hdl.handle.net/2117/172603 https://dx.doi.org/10.5821/dissertation-2117-172603 |
| Access Level: | acceso abierto |
| Palavra-chave: | Àrees temàtiques de la UPC::Enginyeria civil i ambiental |
| Resumo: | The Southwest Atlantic Ocean is a key component of the global climate system. It holds the Confluence of the Brazil and Malvinas Currents (BMC), one of the highest frontal systems of the world ocean. The BMC is the encounter point of subtropical origin warm and salty waters, transported southward within the Brazil Current, and subantarctic water flowing equatorward along the Malvinas Current. The intense mixing and cross-frontal exchanges highly transform the water masses that will finally flow southward, eastward of be subducted into the subtropical thermocline. This PhD dissertation aims at extending our knowledge of the circulation and dynamics in the BMC, placing these results in perspective from the small to the global scales. The intense property contrast found in this region resemble the intense temperature gradients between low and high latitudes. We first analyse the changes in the heat content of the atmosphere and upper ocean for the last 450 kyr and analyse the sensitivity of the system to changes in albedo, cloud cover and atmospheric and oceanic heat transports. We then describe the hydrographic conditions at the BMC during and early fall cruise. These reveal the presence of brackish river water on top the frontal system. The salinity anomaly at the surface correlates with the presence of large ageostrophic velocities along the frontal jet. In addition, the rapid evolution of these waters impinges on the thermohaline variability in the proximity of the front. The comparison of surface overview during the cruise is well represented by the reanalysis although at depth, this misses the thermohaline intrusions developed both sides the front. On the other hand, climatological data misses the numerous mesoscale features. We also examine the circulation pattern in the upper 2000 m depth. We find a relatively weak MC near 41°S, 56°W followed by its cyclonic retroflection, an intense subtropical anticyclone replacing the BC-overshoot (BCO), a subantarctic inflow near 53°W maintained both by an upstream earlier diversion of the MC and the cyclonic recirculation of the flow leaving the east along the confluence front, and on the northern extreme of the Confluence the southward flow of BC. The results in this chapter also suggest the existence of dianeutral mixing and cross-frontal exchanges. We assess the high-frequency temperature variability at this region combining sea surface temperature images with novel high-resolution SeaSoar measurements. We found spatio-temporal scales between 1.5 and 6 days and between 20 and 50 km with the shortest scales along the shelf-break BC and over the Confluence front, with the largest ones along the MC and MRC. Variability increases at the subsurface due to submesoscale thermohaline intrusions. Finally, we analyse the relative role of dianeutral mixing by small scale turbulence and isoneutal mixing induced by mesoscale eddy stirring in setting the temperature-salinity relationship in the Argentine basin using microstructure measurements along the Subtropical Front (STF). Dieneutral mixing is enhanced in the upper 500 m of the water column, especially downstream the BCO, and at depth over the shelf-break and the eastern limit of the basin at the mid-Atlantic ridge. Isoneutral diffusivity dominates below 500 m in the centre of the basin. Moreover, we determine the cross-frontal eddy-advective mass flux. These induce the subduction of about 3 Sv of mode and intermediate waters into the subtropical thermocline and the poleward transport of Upper Circumpolar Deep Water. |
|---|