Temperature gradients along the East African coast using 50 years of ship data
Bernardino S. Malauene1*, Patrick Vianello, Michael Roberts, Sean Fennessy, Bernadine Everett, Joseph Kamau, Damaris Mutia, Masumbuko Semba
Instituto Nacional de Investigacao Pesqueira
Av Mau Tse Tung 398 Maputo, Maputo, Mozambique Pincode: 9999 Mobile: 270646399218 E-mail: firstname.lastname@example.org
In situ observations in the Western Indian Ocean (WIO) are scarce, particularly with most coastal countries in the region not having dedicated research vessels. Over the last five decades, however, various research surveys have been undertaken by several initiatives involving local and national governments, regional and international projects or programmes, with the collected data residing in many different institutions locally and throughout the world. Despite this progress regional-scale distributional features are still poorly known. This study provides the first comprehensive assemblage of these survey data (2200 oceanographic stations) along the east African coast from Kenya (0oS) to eastern regions of South Africa (32oS). This study aimed to describe the water column in terms of temperature above benthic habitats along the East African shelf-slope (see WIOMSA WIO-Benth project). An international search for CTD and XBT cruise data between 1970 and 2020 was conducted. Data were binned in 65 latitudinal 1/2o zones along four depth strata (i.e., 0-50, 50-100, 100-200, and 200-500m). Results showed high densities of CTD and XBT data are found south of 15oS in southern Mozambique and eastern South Africa, followed by Kenya (14oS). Data are less prolific in Tanzania and northern Mozambique. This pattern was similar in all depth strata, with richer data between 100 and 500 m depths. There are higher data densities in the austral summer than winter. Data were plotted as vertical latitudinal temperature transects and horizontal maps along the East African coast for each stratum. Near-surface temperatures sampled during the satellite era were compared with MODIS-Terra SST and they manifested similar trends. Maximum surface temperatures (~30oC) were found in northern Mozambique (~10 oS) decreasing both northwards and southwards, with a minimum (15oC) in the south (>28oS) off South Africa. This indicated the strong influence in the north of the South Equatorial Current and its bifurcation. The trans-shelf thermocline shallows from north to south, particularly in the two deepest strata. Near-surface temperatures decrease by approximately 3-4oC between summer and winter while the near-seafloor temperatures show minimal variation with season, except for the near-coastal stratum (0-50 m). This graphic representation of regional-scale seawater temperatures provides a useful demonstration of condensed large-scale oceanographic patterns in the WIO.