Investigation of Wind Variability in the South Atlantic Sector of the Southern Ocean and the Influence on the Upper Ocean in a Numerical Ocean Model

Moalusi, Tumelo Comfort

Investigation of Wind Variability in the South Atlantic Sector of the Southern Ocean and the Influence on the Upper Ocean in a Numerical Ocean Model

Master Thesis

2019

Abstract

Several papers have linked climate variability in the Southern Ocean (SO) with the Southern Annular Mode (SAM), which has seen an increase in the positive phase since the mid-1960s, due to the Antarctic ozone depletion and emissions of greenhouse gases. The SAM is recognized as the main mode of atmospheric variability in the SO. The SAM index allows an understanding of the latitudinal movement (south-north) of the westerly wind belt circling Antarctica and has significant impacts on Antarctic surface temperatures, ocean circulation, and many other aspects of Southern Hemisphere climate and thus the global ocean. During negative phases of the SAM Index, westerlies intensify and move north, bringing about more (or stronger) storms, and low pressure systems over southern Australia. The changes associated with SAM forcing may have impacts on carbon uptake and storage in the SO directly through upwelling and outgassing, and indirectly, by influencing nutrient cycles and phytoplankton activity. Understanding the variability of the wind field in the SO and how it affects ocean circulation, climatic and oceanic variables is important. Thus, this thesis presents the relationship of the SAM index and the upper ocean, specifically analysing sea surface salinity (SSS), sea surface temperature (SST) and the mixed layer depth (MLD), in the Southern Atlantic sector of the SO as presented in numerical ocean models. Two resolutions of NEMO ocean model are compared: a) eddy-permitting (SATLANTIC05), b) eddy-resolving (SATLANTIC12) models, with horizontal resolutions of ½ and 1/12 °, respectively. In situ data from 2013 World Ocean Atlas is used as a benchmark for the analysis. Our model‐based analysis confirms previous studies done on the influence of the SAM on the SO, that a strong relationship exists. The SAM index is positively correlated with wind speed in the Antarctic Zone (AZ) and negatively correlated in the Subantarctic Zone (SAZ). The impacts of this is clear in the upper ocean. These correlations between SAM index and the selected variables at these selected locations confirms that the SAM index corresponds with cool surface temperatures at higher latitudes and a weak cooling at midlatitudes during positive phase, which differs regionally.

Keywords

Oceanography

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