Publication Date

2017-05-12

Availability

Open access

Embargo Period

2017-05-12

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2017-04-06

First Committee Member

William E. Johns

Second Committee Member

Igor Kamenkovich

Third Committee Member

Benjamin Kirtman

Fourth Committee Member

Rick Lumpkin

Abstract

An eastward subsurface current, the Equatorial Undercurrent (EUC), is part of the equatorial oceanic circulation in all oceans: permanent in the Atlantic and Pacific and seasonally present in the Indian due to the monsoonal wind circulation. This work focuses on the seasonal variability of the Atlantic EUC that supplies the equatorial upwelling with colder, salty and nutrient-rich waters, being therefore of significant importance for the surface heat budget and primary productivity in the eastern equatorial Atlantic and the Gulf of Guinea. The present study aims to answer two major questions: (i) what is the seasonal upwelling transport in the Eastern Tropical Atlantic (ETA) that is related to the EUC transport and (ii) what is the forcing that controls the seasonality of the EUC. Methodologically, the first question is addressed in view of the seasonal mass balance while the second question is addressed by the seasonal momentum balance, both estimated with a unique data set of observations collected over the last 15 years, especially during the Tropical Atlantic Climate Experiment (TACE). Time series from the Prediction and Research Mooring Array in the Tropical Atlantic (PIRATA) and moorings that were deployed during TACE, observations from research cruises, Argo profiling floats measurements, drifter and altimetry data provide the core of the observational data used in this study. The 5-year daily output of a regional high resolution (1/4ºx1/4º) model simulation of the Tropical Atlantic is also used as a test-bed for various methodological sensitivity tests and as a comprehensive tool, when observations are not sufficient to resolve certain aspects of the balances. There are significant differences between the western and the eastern parts of the ETA basin, in terms of the roles of zonal (mainly related to the EUC downstream mass losses) and meridional convergence in supplying the observed seasonal upwelling. In the western ETA, the upwelling shows a semiannual character with maxima occurring in July and December, while in the eastern region the upwelling cycle is mainly annual with a maximum in August. In both regions, the 50-300m EUC supplies significant part of the upwelling: 60% in the western box and 90% in the eastern box of the upwelling transport are supplied by zonal convergence, approximately ¾ of which is attributed to the EUC in both boxes. However, the near surface meridional mass divergence (0-30m) drives the near surface upwelling in both western and eastern boxes, according to the traditional view of the equatorial upwelling, confirmed by observations (drifters) and the model. From a momentum balance perspective, mid-basin in the ETA at 10ºW, non-linear advection terms are very important and seasonal changes in EUC strength can only be accurately reproduced at 10ºW when the non-linear advection terms are considered, in contrast with the linear dynamics that have been used to explain the EUC seasonality in numerous studies. On the other hand, below the EUC core, the seasonal zonal momentum balance is mainly linear, with the zonal pressure gradient being primarily responsible for accelerating and decelerating the lower EUC. Although the observational estimates of the mass and momentum balances involve methodological challenges, the model's ability to reproduce accurately the seasonal cycle of the EUC helps significantly in understanding the main processes controlling the seasonal cycles of upwelling and the EUC in the ETA. This work is one of the few observational studies that quantifies the seasonal cycle of upwelling in the ETA, one of the few attempts to study the zonal momentum balance on a seasonal time scale, and the first observationally-based zonal momentum balance study in the equatorial Atlantic.

Keywords

equatorial Atlantic; EUC; upwelling; seasonal mass balance; seasonal momentum balance

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