Environmental Forcing of Red Tides in the Southern Benguela

Mark R Jury

doi:10.1155/2014/325321

Abstract

The Southern Benguela cape upwelling plumes have inshore wind shadows prone to red tides in late summer. Their intensity and coverage are estimated by satellite fluorescence measurements in the period 1997–2012 and qualified by in situ reports. High satellite fluorescence cases are identified at daily to seasonal time scales, and characteristics of the upper ocean and lower atmosphere are studied using third generation reanalyses. A dominant feature is easterly winds over the Cape Peninsula (34°S, 18°E) induced by a ridging anticyclone-coastal low weather pattern. Over Cape Columbine (33°S), there is a wind shadow with cyclonic wind and current shear. Composite atmospheric profiles reveal a 4°C temperature inversion near 500 m that traps a coastal wind jet >6 m/s below 200 m. The composite shelf oceanography shows a relic upwelling plume below 10 m overtopped by warmer water near the coast, providing the thermal stratification needed for biotic aggregation. Data from the IPSL5 coupled climate model over the period 1980–2080 indicates that environmental conditions favoring red tides may become more frequent.

Highlights

The Southern Benguela (31.5–34∘S, 17.5–18.5∘E) is a zone of summer upwelling and equatorward flow [1,2,3,4]
Given the peculiar sea level air pressure (SLP) pattern found in the fluorescence line height (FLH) index-to-field correlations, data from the French Institut Pierre Simon Laplace (IPSL) version 5 coupled general circulation model used for climate change projections [49] is evaluated over the period 1980–2080
The mean annual cycle is studied based on red tide index area-averaged 8-day FLH and CHL values (Figure 1(a))

Summary

Introduction

The Southern Benguela (31.5–34∘S, 17.5–18.5∘E) is a zone of summer upwelling and equatorward flow [1,2,3,4]. The upwelling is pulsed by passing weather systems and amplified by coastal low pressure cells [17,18,19] As they transit the Southern Benguela, equatorward flow becomes shallow and sheared by the topography. In addition to fluorescence proxies, satellite measured chlorophyll (CHL) indicates high biomass dinoflagellate blooms and sea surface temperature (SST) helps in establishing the background conditions that contribute to the formation of red tides during winddriven coastal upwelling relaxation. Ocean-atmosphere reanalysis products are employed here to study the environmental forcing of red tides in the Southern Benguela in February–April season in 1997–2012 This is a challenge given the coastal pulsed nature of upwelling [38,39,40,41]. Model fitting is done only to extend the MODIS record back to SeaWiFS and to establish a daily index corresponding with red tides

Data and Methods

April 2009 March 2008 March 2003

Results

Summarizing Discussion