Temporal organization of upwelling in the southern Benguela ecosystem by resonant coastal trapped waves in the ocean and atmosphere

Abstract

The pulsing of the southern Benguela ecosystem ensures repeated replenishment of nutrients during active upwelling and phytoplankton blooming when the wind relaxes. Upwelling cycles are associated with the passage of coastal trapped waves in the shelf water. Under most circumstances, these are closely matched with an overlying coastal low. The irregular passage of coastal lows, in response to ridging high pressure cells driven by southern hemisphere mid-latitude Rossby waves, induces a system of opposing winds which propagates southwards along the west coast of southern Africa. The detail of this mechanism is examined through time-series of environmental variables over the southern Benguela shelf. The time-series displays considerable variability in the duration and the amplitude of upwelling events. An optimum pulse frequency of some 10 days is suggested. At that frequency, the free and forced modes of coastal trapped waves match and the amplitude of change between active and passive upwelling phases is greatest. Also, the 10-day pulse frequency is sufficiently short to ensure a continual resupply of nutrients and primary productivity to the fisheries.