In the Gulf of Eilat, unique hydrographic and oceanographic processes occur, significantly affecting the functioning of the Gulf's distinctive ecological system and the biogeochemical cycles at varying temporal and spatial scales. The number of observations and sampling across the Gulf is limited, and therefore, much of the understanding of the flow regime is based on theoretical and hydrodynamic models of the Gulf.
In general, the water column in the Gulf of Eilat is warm throughout the year, with seasonal variations in water temperature (20 – 29°C), evaporation rates (2 m/year), salinity (40.3 – 41.6 ‰), and total alkalinity (~2500 µmol/kg) that are relatively high. Chlorophyll a concentrations range from 0.03 to 1.95 µg/L, and primary production ranges from 0.1 to 1.9 mg C m-2 d-1, with the maximum chlorophyll depth (deep chlorophyll max) reaching depths of 80-90 meters. The entire water column is saturated with oxygen (~100%). The photic zone extends from 80 to 115 meters. The residence time of water is estimated to be about two years. In terms of heat flux, during the winter months, the Gulf's water loses heat to the atmosphere (Kimor & Golandsky, 1977; Labiosa et al., 2003; Stambler, 2006; Lazar et al., 2008; THETIS, 2011; Gildor, 2014).
Throughout most of the year, water exchange occurs between the Gulf of Eilat and the Red Sea, with the flow in the Straits of Tiran (252 m) being bidirectional: relatively fresh and warm water flows from the northern Red Sea into the Gulf of Eilat from the surface (in the upper layer), while cold and saline water flows from the Gulf of Eilat into the Red Sea from below (at depth). The annual average flow is less than 0.02 million m³ per second. Flow in the Gulf is driven by several factors: (a) heat fluxes and evaporation from the Gulf surface; (b) hydrographic conditions in the northern Red Sea; (c) tides; (d) wind (surface flow). The first two factors are responsible for the flow regime throughout the Gulf. Wind directly influences only the upper layer of water (Gildor, 2014).
During the spring and summer months, a significant amount of heat enters the Gulf through advection, originating from the advection of warm water entering from the northern Red Sea, moving northward until it reaches the northern Gulf of Eilat (contrary to the northern winds), thus contributing to the warming of the surface layer. In contrast, during the fall and winter months, the Gulf's water loses heat to the atmosphere and cools. As a result of this cooling, the water becomes denser, and the stratification formed during the spring and summer months is gradually destroyed, with the thermocline deepening and creating deep vertical mixing in the water column, ranging between 250 and 850 meters (Genin et al., 1995). Even at the end of winter, there is an influx of surface waters from the northern Red Sea into the Gulf of Eilat, but their temperature is relatively low and close to the Gulf's water temperatures. Consequently, the heat advection through the current exchange is relatively low, and the incoming water mixes (vertically) with the Gulf's water through heat loss (Biton & Gildor, 2011; THETIS, 2011, Gildor, 2014).
Seasonal changes in hydrography directly affect biological and chemical processes in the Gulf. During the months of May–September, when the water column is stratified, the photic zone is nutrient-poor. In late winter and autumn, vertical mixing brings nutrient-rich deep waters into the photic zone. This process leads to a "spring bloom," characterized by an increase in primary production and phytoplankton, followed by an increase in zooplankton biomass. During this period, peaks are also observed in the general particulate flux and organic and carbon particulate flux in the water column. This dynamic will be of higher intensity as the mixing duration increases and the mixing depth becomes deeper (Lindell & Post, 1995; Labiosa et al., 2003; Lazar et al., 2008; Chernihovsky et al., 2020; Torfstein et al., 2020).
Surface flow driven by northern winds in the Gulf of Eilat creates eddies along the main axis of the Gulf at different times (Biton & Gildor, 2011; Afargan & Gildor, 2015). Additionally, at the northeastern end of the Gulf, northern winds cause upwelling of nutrient-rich deep waters, leading to an increase in chlorophyll concentrations along the coast (Labiosa et al., 2003). Tidal currents in the Gulf of Eilat are directly influenced by tidal conditions in the Red Sea (Monismith & Genin, 2004; Gildor, 2014).
References:
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