Cool, elevated chlorophyll-a waters off northern Mozambique

Direct in-situ observations from a shallow underwater temperature recorder on the continental shelf and from a shipboard oceanographic survey, were combined with MODIS satellite data (sea surface temperature and chlorophyll-a) to assess the temporal and spatial variability of temperature and chlorophyll-a in the Mozambique Channel near the coastal town of Angoche, 16°S. Intermittent, relatively cool surface water and elevated chlorophyll-a signatures were found, indicating upwelling near Angoche over an area between 15°S and 18°S. A 5-year (2002–2007) analysis of temperature (from both in-situ and satellite) revealed two distinct periods: (1) the August–March period with highly variable intermittent “cool water” events and (2) the April–July period with little temperature variability. Generally, periods of cooling occurred at about 2 months intervals, but shorter period occurrences (8–30 days) of cool coastal events were also observed. Two possible forcing mechanisms are discussed: (1) wind derived coastal upwelling (using satellite blended sea surface wind derived from NOAA/NCDC) and (2) the effect of passing transient southward moving eddies (using sea level anomalies from AVISO altimetry). It is suggested that the cool surface, elevated chlorophyll-a waters are primed and formed by favourable wind-driven Ekman-type coastal upwelling, responding to alongshore northeasterly monsoon winds prevailing between August and March. These waters are then enhanced in chlorophyll-a and advected further offshore by anti-cyclonic/cyclonic eddy pairs interacting with the shelf.