Thermally Driven Exchanges between a Coral Reef and the Adjoining Ocean

TLDR

Exchange flows across coral reefs are driven by onshore–offshore temperature gradients, with vertical shear scaling with convective velocity and bottom slope, and the resulting thermal siphon may be a common feature of nearshore oceanography. The study presents hydrographic observations over a fringing coral reef in the Gulf of Aqaba to analyze thermally driven exchange flows. Observations show that heating produces offshore surface and onshore deep exchange, cooling reverses this pattern, summer conditions yield a diurnally reversing flow, winter shows only cooling, and when scaled by ΔV the cross‑shore profiles collapse onto a single curve, with turbulence weakening the shear yet overall transport agreeing with previous studies.

Abstract

Abstract In this paper hydrographic observations made over a fringing coral reef at the northern end of the Gulf of Aqaba near Eilat, Israel, are discussed. These data show exchange flows driven by the onshore–offshore temperature gradients that develop because shallow regions near shore experience larger temperature changes than do deeper regions offshore when subjected to the same rate of heating or cooling. Under heating conditions, the resulting vertically sheared exchange flow is offshore at the surface and onshore at depth, whereas when cooling dominates, the pattern is reversed. For summer conditions, heating and cooling are both important and a diurnally reversing exchange flow is observed. During winter conditions, heating occupies a relatively small fraction of the day, and only the cooling flow is observed. When scaled by ΔV, the observed profiles of the cross-shore during cooling velocity collapse onto a single curve. The value of ΔV depends on the convective velocity scale uf and the bottom slope β through the inertial scaling, ΔV ∼ β−1/3uf first proposed by Phillips in the 1960s as a model of buoyancy-driven flow in the Red Sea. However, it is found that turbulent stresses associated with the longshore tidal flows and unsteadiness due to the periodic nature of the buoyancy forcing can act to weaken the sheared exchange flow. Nonetheless, the measured exchange flow transport agrees well with previous field and laboratory work. The paper is concluded by noting that the “thermal siphon” observed on the Eilat reef may be a relatively generic feature of the nearshore physical oceanography of reefs and coastal oceans in general.