Abstract
An anomalously high chlorophyll-a (Chl-a) event (>2 mg/m3) during June 2015 in the South Central Red Sea (17.5° to 22°N, 37° to 42°E) was observed using Moderate Resolution Imaging Spectroradiometer (MODIS) data from the Terra and Aqua satellite platforms. This differs from the low Chl-a values (<0.5 mg/m3) usually encountered over the same region during summertime. To assess this anomaly and possible causes, we used a wide range of oceanographical and meteorological datasets, including Chl-a concentrations, sea surface temperature (SST), sea surface height (SSH), mixed layer depth (MLD), ocean current velocity and aerosol optical depth (AOD) obtained from different sensors and models. Findings confirmed this anomalous behavior in the spatial domain using Hovmöller data analysis techniques, while a time series analysis addressed monthly and daily variability. Our analysis suggests that a combination of factors controlling nutrient supply contributed to the anomalous phytoplankton growth. These factors include horizontal transfer of upwelling water through eddy circulation and possible mineral fertilization from atmospheric dust deposition. Coral reefs might have provided extra nutrient supply, yet this is out of the scope of our analysis. We thought that dust deposition from a coastal dust jet event in late June, coinciding with the phytoplankton blooms in the area under investigation, might have also contributed as shown by our AOD findings. However, a lag cross correlation showed a two- month lag between strong dust outbreak and the high Chl-a anomaly. The high Chl-a concentration at the edge of the eddy emphasizes the importance of horizontal advection in fertilizing oligotrophic (nutrient poor) Red Sea waters.
Highlights
The Red Sea is a narrow, meridionally-elongated, oceanic basin, surrounded by arid land and desert separating Northern Africa from the Arabian subcontinent [1]
Level-3 data in our analysis because using both sensors significantly improves the coverage from one sensor alone due to sun-glint avoidance and changes in cloudiness [29,30], and issues have been identified with the coverage of Moderate Resolution Imaging Spectroradiometer (MODIS)-Aqua alone, in the southern Red Sea [31]
Factors contributing to this event include upwelling processes driving the convective vertical nutrient transfer as well as eddy circulation driving the horizontal advection of nutrients
Summary
The Red Sea is a narrow, meridionally-elongated, oceanic basin, surrounded by arid land and desert separating Northern Africa from the Arabian subcontinent [1]. The Red. Sea has no river inflow or stream sources and has a high evaporation rate at more than 210 cm/yr [4]. A precipitation rate of less than 100 mm/yr [5,6], resulting in the highest salinity of any major tropical oceanic basin [4]. It is noteworthy that below the pycnocline (205–300 m), the entire Red Sea basin has water with extremely uniform temperature and salinity, with values of 21.6 and 40.6 practical salinity units (psu) ‰, respectively [8,9,10]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call