The Gulf of Aden Intermediate Water Intrusion Regulates the Southern Red Sea Summer Phytoplankton Blooms

Abstract

Knowledge on large-scale biological processes in the southern Red Sea is relatively limited, primarily due to the scarce in situ, and satellite-derived chlorophyll-a (Chl-a) datasets. During summer, adverse atmospheric conditions in the southern Red Sea (haze and clouds) have long severely limited the retrieval of satellite ocean colour observations. Recently, a new merged ocean colour product developed by the European Space Agency (ESA)—the Ocean Color Climate Change Initiative (OC-CCI)—has substantially improved the southern Red Sea coverage of Chl-a, allowing the discovery of unexpected intense summer blooms. Here we provide the first detailed description of their spatiotemporal distribution and report the mechanisms regulating them. During summer, the monsoon-driven wind reversal modifies the circulation dynamics at the Bab-el-Mandeb strait, leading to a subsurface influx of colder, fresher, nutrient-rich water from the Indian Ocean. Using satellite observations, model simulation outputs, and in situ datasets, we track the pathway of this intrusion into the extensive shallow areas and coral reef complexes along the basin’s shores. We also provide statistical evidence that the subsurface intrusion plays a key role in the development of the southern Red Sea phytoplankton blooms.

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

“…Figure 2 also demonstrates the coverage differences between the blended OC-CCI dataset and the MODIS-Aqua dataset alone. White regions with missing data are much more prevalent in MODIS-Aqua plots for the SRS and SCRS and have been discussed by previous authors [26,30,31]. It is noteworthy that an anomalous Chl-a outbreak was quite evident from both datasets in region 3 (SCRS) near 19.5 • N to 17.5 • N during the summer of 2015.…”

“…The anomalous feature of interest appears in the SCRS in the summer of 2015 as a band of enhanced Chl-a values (warm colors) that is easier to identify in the MODIS-Aqua data, but is also apparent in the OC-CCI dataset. While other researchers have identified peaks in Chl-a concentrations in May or June [17,30,31], especially in the southern Red Sea, it is both the timing and magnitude of this anomaly that attracted our interest.…”

“…However, it is important to recognize that the mission objective for SeaWiFS was an error less than 35%, so the MODIS-Terra Chl-a results are acceptable [28]. We also include both MODIS-Terra and MODIS-Aqua 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 MODIS-Aqua alone, particularly in the southern Red Sea [31]. In this work MODIS-Terra and Aqua Level-3 Chl-a data version 2014 at 4-km resolution are used [32,33] ( Table 2).…”

“…Critical steps for achieving data consistency include band-shifting and empirical bias correction [34][35][36]. The OC-CCI dataset has been used by other researchers in the Red Sea and shows good consistency with in situ observations along with substantially improved coverage, particularly for the southern region [26,30,31]. In this work, we used daily and five-day composites of Level-3 OC-CCI Chl-a data at 4 km resolution from 25 to 29 June 2015 [37] (Table 2).…”

“…Both sets of panels in Figure 2 show the strong seasonal pattern in Chl-a in the Red Sea with increased concentrations in boreal winter (December-January) and decreased concentrations in boreal summer (June-August) [17,22,30,31]. The anomalous feature of interest appears in the SCRS in the summer of 2015 as a band of enhanced Chl-a values (warm colors) that is easier to identify in the MODIS-Aqua data, but is also apparent in the OC-CCI dataset.…”