Troposphere–Planetary Boundary Layer Interactions and the Evolution of Ocean Surface Density: Lessons from Red Sea Corals

A time series of oxygen isotope (d18O) measurements of a coral from the northern Red Sea (RS) is presented and used as a direct proxy for water surface density rs. With a relatively constant subsurface density, the generated surface density time series closely tracks water mass formation variability. Two positive and two negative high-amplitude rs anomalies are studied, and associated atmospheric and oceanic data are analyzed to understand large-scale ocean–atmosphere processes. The dominant process is lower-tropospheric subsidence. It dries the boundary layer (BL), increases surface evaporation and rs, and ultimately drives water mass formation. The main cause of the subsidence is a temperature increase along the principal axis of the RS from the eastern Mediterranean to the convergence zone (CZ, 188–228N). The increase entails isentropes sloping down to the southeast, crossing pressure surfaces due to their tilt. With nearly adiabatic flow along the principal axis, air parcels move roughly along the sloped isentropes toward ever increasing pressure; that is, they subside. The subsidence supplies the northern RS BL with upstream air from higher altitudes (where humidity is low), thus drying the BL. The resultant high evaporation north of ;258N (with low evaporation minus precipitation south of ;228N) depresses sea levels in the northern RS, and thus drives the RS thermohaline circulation and controls water mass formation. Excluded from the presented picture, but probably very important, is mixing with the hot, dry air of the surrounding deserts.