The influence of the Deep Western Boundary Current on Meridional Overturning Circulation on South Atlantic

Author(s)

Giovana Jeremias Andrade

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto Oceanográfico (IO/DIDC)
• Defense date: 2019-06-07

Examining board members

Olga Tiemi Sato; Ricardo de Camargo; Wandrey de Bortoli Watanabe

• Advisor: Olga Tiemi Sato

Abstract

The Deep Western Boundary Current (DWBC) is the oceanic current responsible for the transport of a significant portion of the deep volume of the Atlantic as part of the Meridional Overturning Circulation (MOC). The main objective of this work is to investigate what are the dominant scales of DWBC variability, how its intensity varies along the western boundary of the South Atlantic and what is its effective contribution is to the MOC. Meridional velocity data obtained between 2012-2014 by CPIES current meters were used. In addition, the outputs of the model HYbrid Coordinate Ocean Models (HYCOM) and the Estimating the Circulation & Climate of the Ocean (ECCO) have been used. The analyzes of both models have been made in the period between 1992-2015, at latitudes 10°S, 14°S, 18°S, 22°S, 26°S, 30°S e 34.5°S and between the longitudes of 55°W e 32°W. The results obtained by the statistic model SkillScore were from 0,90 and 0,81 (HYCOM) and 0,93 and 0,92 (ECCO) for temperature and salinity, respectively. The mean velocity obtained from CPIES-AA was 2,83±2,9 cm.s-1 and 1,48±1,8 cm.s-1 by CPIES-BB. The mean velocity obtained by HYCOM at the same points were -2,48±3,5 cm.s-1 and -1,43±2,7 cm.s-1 and by ECCO were - 1,78±1,5 cm.s-1 and -0,85±0,4 cm.s-1, respectively. The DWBC volume transport integrated average decreases along the course of the South Atlantic with the DWBC centered between 2000 and 2500 m. Over the 22 years of temporal series data, HYCOM showed that only in the latitudes of 10°S and 18°S there is a tendency of volume transport increase of -0,054 Sv/year and -0,083 Sv/year, respectively, and the other latitudes indicate a tendency of transport reduction. We have seen that as the DWBC moves along the South Atlantic, it loses intensity, variability, as well as its transportation volume. When analyzing the ECCO, we have observed at all latitudes a tendency to transport increase, except at 30°S, where there is a tendency to volume transport reduction -0,014 Sv/year. After analysis, it was identified a annual cycle of DWBC. The annual signal was removed, resulting in transport anomaly. To eliminate the high frequency signals, a 25 month Hamming filter was applied to the data. The results of HYCOM at the latitudes 10°S and 22°S showed a pattern of increased transport between 1995-1998, 2004-2007 and 2009-2012. At 34.5°S there was an increase in transport between 1995-1997, 1999-2001, 2003-2004, 2007-2009 and 2011-2014. From the ECCO we have seen that at the latitude of 10°S there has been an increase in transport after the year 2003. At 22°S and 34.5°S there was an increase between 1995-1999 and after 2005. The variation percentage explained by the annual or semi-annual period is less than 19% (HYCOM) and 19% (ECCO) each. The period of the highest energy concentration was the same for the 3 latitudes, that is, between 50-160 days for both models. The spectral analysis of the variance from the integrated transport volume of DWBC indicates a peak energy centered around 120-160 days (HYCOM) and 130-160 days (ECCO) at 3 latitudes. HYCOM analysis showed that at latitudes 10°S, 22°S and 34.5°S the correlation between CCOP and AMOC was r=0,7, r=0,5 and r=0,4, correspondingly. ECCO presented similar results to HYCOM, in which the correlation at 10°S was r=-0,7, 22°S and 34.5°S was r=-0,4, respectively. We can assume that at 10°S, when a transport increase of CCOP occurs, a transport increase of the AMOC occurs at the same time. (AU)

• FAPESP's process: 17/09859-5 - The influence of the Deep Western Boundary Current in the Meridional Overturning Circulation in the South Atlantic
• Grantee: Giovana Jeremias Andrade
• Support Opportunities: Scholarships in Brazil - Master