The University of Southampton
University of Southampton Institutional Repository

The effect of large-scale interannual variations in the Gulf of Guinea

The effect of large-scale interannual variations in the Gulf of Guinea
The effect of large-scale interannual variations in the Gulf of Guinea
The Gulf of Guinea is an important region of tropical Atlantic variability, and embedded within it are the Atlantic Ni~no and the seasonal tropical instability waves. Although, the dynamics of this region are accepted to modulate the monsoon system by the scientific community, it is still debated what controls temperatures and the exact coupling mechanism with the atmosphere. Here, I present the results of temperature advection, variability of the Equatorial Undercurrent (EUC) in relation to atmospheric forcing, influence of Gulf of Guinea sea surface temperature (SST) to rainfall variability over West Africa and the Sahel, spatio-temporal variability of nonseasonal ocean-atmosphere processes and the coupling of winds and SST. The central aim is to understand how anomalous currents advect temperatures, and how anomalous oceanic temperatures respond to the atmosphere to cause coupled variability.

In order to understand the enigmatic influence of temperature advection by tropical instability waves (TIW), I examined the influence of anomalous currents and temperatures associated with the waves. Results show that TIWs advect temperatures zonally and warm the Gulf of Guinea during the boreal summer, and at the central equatorial Atlantic, they advect temperatures to the north. The EUC, represented by salinity maximum is investigated from four regions to cover the northern, central and southern branches of the current. West of the Gulf of Guinea, anomalous northward currents associated with TIWs destabilise the vertical positioning of the EUC core during the boreal summer. In the Gulf of Guinea, where bathymetry, river discharge and winds vary seasonally, vertical fluctuations are apparent every three months and the shallowing of the EUC affects the upper layer temperatures.

The Gulf of Guinea SST correlates well with variability of rainfall over West Africa and the Sahel, where cold (warm) temperatures in 2005 (2007) explained the causes of drought (flooding) that occurred in those years respectively. Strongest intraseasonal SST variability is observed at the Guinea Dome and the Gulf of Guinea region and both occur due to seasonally varying northeasterlies and southeasterlies. However, north of 18N, zonal tropospheric winds associated with Madden-Julian Oscillation appear to alter the changes on surface winds. This connection between surface and atmospheric winds explains the inability of the winds to covary interannually with SST at the TIWs frontal zones. Investigations presented in this thesis increase our understanding of tropical Atlantic variability and results therein are useful as background conditions for coupled ocean-atmosphere models.
Muhammed, Ibrahim
eb878479-a67b-4cae-bc0a-192872564d87
Muhammed, Ibrahim
eb878479-a67b-4cae-bc0a-192872564d87
Quartly, Graham
fdd4fb80-e9e4-4e54-aa67-7c382343961a
Challenor, Peter
a7e71e56-8391-442c-b140-6e4b90c33547

Muhammed, Ibrahim (2011) The effect of large-scale interannual variations in the Gulf of Guinea University of Southampton, School of Ocean and Earth Science, Doctoral Thesis , 289pp.

Record type: Thesis (Doctoral)

Abstract

The Gulf of Guinea is an important region of tropical Atlantic variability, and embedded within it are the Atlantic Ni~no and the seasonal tropical instability waves. Although, the dynamics of this region are accepted to modulate the monsoon system by the scientific community, it is still debated what controls temperatures and the exact coupling mechanism with the atmosphere. Here, I present the results of temperature advection, variability of the Equatorial Undercurrent (EUC) in relation to atmospheric forcing, influence of Gulf of Guinea sea surface temperature (SST) to rainfall variability over West Africa and the Sahel, spatio-temporal variability of nonseasonal ocean-atmosphere processes and the coupling of winds and SST. The central aim is to understand how anomalous currents advect temperatures, and how anomalous oceanic temperatures respond to the atmosphere to cause coupled variability.

In order to understand the enigmatic influence of temperature advection by tropical instability waves (TIW), I examined the influence of anomalous currents and temperatures associated with the waves. Results show that TIWs advect temperatures zonally and warm the Gulf of Guinea during the boreal summer, and at the central equatorial Atlantic, they advect temperatures to the north. The EUC, represented by salinity maximum is investigated from four regions to cover the northern, central and southern branches of the current. West of the Gulf of Guinea, anomalous northward currents associated with TIWs destabilise the vertical positioning of the EUC core during the boreal summer. In the Gulf of Guinea, where bathymetry, river discharge and winds vary seasonally, vertical fluctuations are apparent every three months and the shallowing of the EUC affects the upper layer temperatures.

The Gulf of Guinea SST correlates well with variability of rainfall over West Africa and the Sahel, where cold (warm) temperatures in 2005 (2007) explained the causes of drought (flooding) that occurred in those years respectively. Strongest intraseasonal SST variability is observed at the Guinea Dome and the Gulf of Guinea region and both occur due to seasonally varying northeasterlies and southeasterlies. However, north of 18N, zonal tropospheric winds associated with Madden-Julian Oscillation appear to alter the changes on surface winds. This connection between surface and atmospheric winds explains the inability of the winds to covary interannually with SST at the TIWs frontal zones. Investigations presented in this thesis increase our understanding of tropical Atlantic variability and results therein are useful as background conditions for coupled ocean-atmosphere models.

PDF Muhammed_PhD_2011.pdf - Other
Download (20MB)

More information

Published date: October 2011
Organisations: University of Southampton, Physical Oceanography

Identifiers

Local EPrints ID: 209795
URI: http://eprints.soton.ac.uk/id/eprint/209795
PURE UUID: c79eb0d4-8ccc-4008-ac57-913d158cd264

Catalogue record

Date deposited: 01 Feb 2012 13:23
Last modified: 18 Jul 2017 10:46

Export record

Contributors

Author: Ibrahim Muhammed
Thesis advisor: Graham Quartly
Thesis advisor: Peter Challenor

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×