Decadal variability of the Southern Hemisphere subtropical gyres

Abstract

Between 1971 and 2010, studies showed that 93% of the energy imbalance was absorbed by the oceans, and more specifically 30-40% of the excess is absorbed in the Southern Hemisphere Oceans between 30°S and 40°S. Therefore, in this warming climate, the Southern Ocean should be a primary focus of studies. The three Southern Hemisphere subtropical gyres in the Pacific, Indian and Atlantic oceans dominate much of the climate locally and globally. With increasing westerlies and positive phases of the Southern Annular Mode (SAM), it is likely that the Southern Hemisphere gyres are changing. This thesis investigates the temporal and spatial variability of the three gyres and evaluates the response to changes in wind stress, dynamic height, sea surface height and the climate indices. All data is examined at three different latitudinal bands (20°S, 30°S and 40°S) within the gyre in each basin. Variability of sea surface height anomalies attained from satellite altimetry is observed between 1993-2015, along with dynamic height anomaly, calculated using Argos temperature and salinity profiles collected between 2004-2015. Wind stress is averaged globally at the latitude bands using Japanese 55-year Reanalysis (JRA-55), which is also used to calculate the volume of Ekman transport towards the interiors of the gyres. Absolute geostrophic volume transport is calculated meridionally from dynamic height with ‘Level of No Motion’ assumed at 2000 meters depth, then dynamic height is depth integrated, and averaged over longitude to examine basin totals temporally. Meridional surface velocity is computed by interpolating sea surface height with Mean Dynamic Tomography. The wind stress, volume transport and surface velocity time-series are the primary focus of this study. Basin-scale transports showed greatly varying results, with occasions of correspondence, interspersed by phases of inconsistency. Wind stress patterns showed mid-latitude winds at 30°S became fully westward for the study period, and the westerly winds at 40°S showed a slight decreasing trend in strength, whereas westerly winds at 55°S begin increasing towards the end of the study period. The main findings of this study are that the gyres do not show a consistent signal between the basins. Relationship between zonal winds, subsurface ocean volume transport and surface ocean velocity is predominantly strong, however, in the case of strong anomalies in sea surface height and/or dynamic height anomaly, these seem to dominate the signals. Southern gyre boundaries at 40°S show great variability, from northward transport, to decreasing southward transport, to increasing southward transport. The results showed frequent fluctuations, and inconsistent patterns for much of the study period, which made analysis of causes difficult. Consequently it is evident that further research/statistical breakdown must conducted to attain a clearer image of the fluctuations and their frequencies within the three gyres. Nonetheless, the results of this study show large-scale temporal and spatial changes in the Southern Hemisphere subtropical gyres.