Mesoscale structures viewed by SAR and AVHRR near the Canary islands

E D Barton,E G Mitchelson-Jacob,H Dodds,P Flament

doi:10.3989/scimar.2001.65s1167

Abstract

Comparison of Synthetic Aperture Radar and Advanced Very High Resolution Radiometry images of sea surface backscatter and temperature during the peak of the summer Trade winds reveals many aspects of the regional oceanography of the Canary Islands. A strong correspondence occurs between the SAR and AVHRR signals. The generally uniform wind field is perturbed as it flows past the islands producing regions of calm immediately downstream. These are bounded by lines of strong horizontal wind shear that coincide with temperature fronts between warmer lee waters and the cooler surrounding ocean. Weaker winds prevail up to 50 km downstream while enhanced wind speed on the boundaries of the lee can extend more than 150 km. Lee waves are excited in the atmospheric inversion layer where the wind passes over abrupt island topography. Quantitative estimates of wind speed made with the CMOD4 algorithm are unreliable in the downstream region because wind direction is variable and unknown and because spatial gradients in air-sea temperature difference affect atmospheric boundary layer stability. A large anticyclone south of Tenerife strongly influenced the estimated wind speed probably because higher ocean temperatures in its centre caused atmospheric instability and increased radar backscatter. The temperature fronts marking boundaries of the upwelling filament and strong eddies observed in AVHRR appear as lines of current shear in the SAR images.

Highlights

The equatorward flows of the trade winds and of the Canary current are interrupted by the abrupt topography of the Canaries archipelago
Intensification of wind speed appears south of Gran Canaria, where the winds were in excess of 15 m s-1
The results clearly show that synthetic aperture radar (SAR) imagery is capable of detecting both wind and near surface current features in an energetic area like the Canary islands region

Summary

Introduction

The equatorward flows of the trade winds and of the Canary current are interrupted by the abrupt topography of the Canaries archipelago. The islands rise from the deep seabed at depths in excess of 3000 m to similar heights above sea level (Fig. 1a). Downstream of the islands both atmospheric and oceanic disturbances are produced. Oceanic eddies spun off from the islands have been reported by Arístegui et al (1994, 1997) and Barton et al. (1998), while Chopra (1973) reported an atmospheric von Karman vortex street in cloud formations downwind of the islands. March to September (Fig. 1b) are capped by an atmospheric temperature inversion between 400 and 1000 m (Naya, 1984). The wind ascending the northern slopes of the islands often forms a layer of stratocumulus at the base of the inversion. The air is prevented from rising further by the stable inversion layer and so is diverted around the island flanks. Extensive cloud rarely forms, because of subsidence down the lee slopes

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Discussion

Conclusion