Convective modifications of water masses in the Agulhas

Abstract

The nature of convective modifications of water masses in the Agulhas Retroflection region is considered in relation to both the temperature and salinity ( TIS) properties, and in relation to the gas transfer across the air-sea interface. Data from the 1983 Agulhas Retroflection Cruise (ARC) are used along with satellite sea surface thermal data and a set of models to explore the evolution of water masses in this area. Air-sea interaction over warm pools of water isolated from the Retroflection in rings produces a cold mode water which is found extensively in the eastern South Atlantic. Quantities of a warmer South Indian Subtropical Mode Water are also formed in the Retroflection. A water mass mixing model with the inclusion of air-sea fluxes is used to quantify the possible heat, salt and gas fluxes over the Retroflection region. Heat fluxes between 125 and 610 W m −2 encompass the 225 W m −2 expected from the regional climatology. Evaporation minus precipitation results based on the most likely water masses available for modification are between 2 and 10 × 10 −5 kg s −1. This is comparable to the climatological annual values of around 6 × 10 −5 kg s −1. Consideration of oxygen and chlorofluoromethane (CFM) content of the converted layers found in the ARC data suggests that the convective changes in the surface layers are highly episodic. The CFMs and oxygen are less saturated in the converted layers than what might be expected from standard piston velocity arguments. The gas flux can be rationalized with a simple mixed-layer model, where the deep convective mixing tied to cooling events is interspaced with longer periods where the active mixing is restricted to a near surface wind mixed layer. The mixed layer comes to near equilibrium and is only periodically mixed into the deeper thermohaline converted layer. This greatly reduces the gas flux into these mode water layers, relative to what would occur in a steady convective case.