Synergistic Use of Remote Sensing and Modeling to Assess an Anomalously High Chlorophyll-a Event during Summer 2015 in the South Central Red Sea

Abstract

An anomalously high chlorophyll-a (Chl-a) event (>2 mg/m 3 ) 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/m 3 ) 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.

Title: Employing Earth Observations and Artificial Intelligence to Address Key Global Environmental Challenges in Service of the SDGs

“…Notably, a lag correlation analysis revealed a statistically significant two-month positive lag correlation between dust anomalies and Chl-a anomalies using the MODIS-Aqua dataset. A similarly significant two-month negative lag correlation was identified between SST anomalies and Chl-a anomalies [199].…”

“…Using the MODIS-Aqua Chl-a product, Li et al [199] revealed a two-month lag correlation positive dust anomalies and high Chl-a anomalies, as well as a two-month lag correlation between negative SST anomalies and high Chl-a anomalies. In this work, we use the OC-CCI dataset to further test and validate the results obtained from [199].…”

“…Using the MODIS-Aqua Chl-a product, Li et al [199] revealed a two-month lag correlation positive dust anomalies and high Chl-a anomalies, as well as a two-month lag correlation between negative SST anomalies and high Chl-a anomalies. In this work, we use the OC-CCI dataset to further test and validate the results obtained from [199]. Moreover, this research explores the possible factors contributing to the influence of aeolian dust on summertime phytoplankton blooms in the Red Sea by investigating other dust-related factors, namely, wind speed, dust sources, and dust deposition type.…”

“…There has been much recent scientific interest in ocean color observations and biological productivity issues in the Red Sea region [148], [182]- [199]. The Red Sea is surrounded by deserts and other arid land and has no river inflow or stream sources.…”

“…In an attempt to better understand the relationships among oceanic circulation and atmospheric deposition on Chl-a amounts and nutrient levels in the Red Sea, a recent study examined an anomalously high Chl-a event (>2 mg/m3) that occurred in June 2015 in the South Central Red Sea (17.5° to 22°N, 37° to 42°E) [199]. The analysis suggested that a combination of factors contribute to anomalous phytoplankton events in this portion of the Red Sea basin.…”