Seasonal and inter-annual ONSET Sea Surface Temperature variability along the northern coast of the Gulf of Guinea

Abstract

The aim of this study is to characterize the coastal upwelling variability at seasonal and inter-annual time scales in the northern Gulf of Guinea (NGoG) using Sea Surface Temperature (SST) collected with autonomous "ONSET" thermometers. Results show that the ONSET SST data are suitable for numerical model evaluation, and provide relevant information in addition to satellite and reanalysis data at seasonal cycle. The minor and major coastal upwellings are present in all the products. The inter-annual SST variability is more pronounced in the western part of the region (Côte d'Ivoire and Ghana) than in the eastern part (Benin and Nigeria). The pattern differences between the west and the east of the region highlight a large spatial variability of the SST in the NGoG. Indeed, the signal of the minor upwelling season is visible only in the west of the basin, namely between Cape Palmas and Cape Three Points. We also observe a well-established thermal gradient between the western and eastern parts of the basin. This gradient is increasing from west to east during the major upwelling season, and decreasing from east to west during the rest of the year. The coastal ONSET data allow to evidence higher SST anomalies than those deduced from satellite and reanalysis products. Although the cold or warm events observed in 2008, 2010 and 2012 are well detected by all products, only the ONSET data set reveal the strong negative SST anomaly observed in 2009 along the coast of Ghana and Côte d'Ivoire.

Causes of the Northern Gulf of Guinea Cold Event in 2012

“…Although OI‐SST and ERAI are strongly correlated in the two regions (Figure 3), SST anomalies are more pronounced in OI‐SST than ERAI. It has been shown that the OI‐SST product reproduces SST variations better than ERAI in these regions (Sohou et al., 2020). The SUC and TUG regions selected are similar to those used by Roy (1995) and these two boxes are also bounded to capture the areas of strong SST cooling observed in 2012.…”

“…Previous studies have indicated that the 2012 cooling may be associated with interannual Atlantic climatic modes (Sohou et al., 2020). It is interesting to remark that the period of appearance of the strong cooling (February–June) in 2012 does not correspond with the period when the Atlantic Niño mode (or equatorial mode) normally reaches its peak amplitude (June‐July‐August).…”

“…The last product of SST used is based on three coastal stations equipped with ONSET sensors installed along the NGoG coast at Sassandra (6.08°W; 4.95°N), Takoradi (1.73°W; 4.88°N) and Cotonou (2.67°E; 6.52°N) in the framework of the Research Program in Physical Oceanography in West Africa (PROPAO). The in situ ONSET sensor data series are available over the period 2008–2012 (Sohou et al., 2020). The period 2008–2012, when all data sets are available, is used for the study period.…”

“…Located in the eastern equatorial Atlantic Ocean (15°W-15°E, 10°S-8°N), the Northern Gulf of Guinea (NGoG; Figure 1) is marked by the presence of a seasonal coastal upwelling that peak twice a year, in boreal winter (January to February) and summer (June to October) periods, from Côte d'Ivoire to Nigeria (e.g., Djakouré et al, 2014Djakouré et al, , 2017Sohou et al, 2020). The sea surface temperature (SST), widely used to characterize coastal upwelling in the NGoG, is subject to year-to-year variations that could affect the marine ecosystem (e.g., Binet & Marchal, 1993;Koné et al, 2017) and the onset of the West African monsoon (e.g., Ali et al, 2011;Lamb, 1972).…”

Coastal Upwelling Limitation by Onshore Geostrophic Flow in the Gulf of Guinea Around the Niger River Plume

“…On this period, the climatology is probably representative as the seasonal cycle is dominant in the tropical Atlantic (Xie and Carton, 2004) and the interannual climatic modes show a balanced activity (Awo et al, 2018;Lübbecke et al, 2018). Sea surface temperature (SST) measurements were collected in situ over 2008-2012 through the Research Program in Physical Oceanography in West Africa (PROPAO), which relies on a network of 4 coastal stations with ONSET sensors (Sohou et al, 2020). The monthly version of the Multi-scale Ultra-high Resolution (MUR) global satellite SST analysis is a blended product combining infrared and microwave satellite observations, with a 0.01° resolution (Chin et al, 2017).…”