Convection and Heat Transfer in Island (Warm) Wakes

Cátia C Azevedo,Carolina M L Camargo,José Alves,Rui M A Caldeira

doi:10.1175/jpo-d-20-0103.1

Abstract

AbstractThe interaction between the incoming winds and high mountainous islands produces a wind-sheltered area on the leeward side, known as the atmospheric wake. In addition to weaker winds, the wake is also characterized by a clearing of clouds, resulting in intense solar radiation reaching the sea surface. As a consequence, a warm oceanic wake forms on the leeward side. This phenomenon, detectable from space, can extend 100 km offshore of Madeira, where the sea surface temperature can be 4°C higher than the surrounding oceanic waters. This study considers in situ, remote sensing, and ocean circulation model data to investigate the effects of the warm wake in the vertical structure of the upper ocean. To characterize the convective layer (25–70 m) developing within the oceanic wake, 200 vertical profiles of temperature, salinity, and turbulence were considered, together with the computation of the density ratio and Turner angle. In comparison with the open-ocean water column, wake waters are strongly stratified with respect to temperature, although highly unstable. The vertical profiles of salinity show distinct water parcels that sink and/or rise as a response to the intense heat fluxes. During the night, the ocean surface cools, leading to the stretching of the mixed layer, which was replicated by the ocean circulation model. In exposed, nonwake regions, however, particularly on the southeast and north coasts of the island, the stretching of the mixed layer is not detectable.

Highlights

In a strongly stratified atmosphere, a mountainous island forces the striking airflow to split, generating a wake of weak winds downstream and stronger-than-ambient winds on the flanks (i.e., Venturi effect; Xie et al 2001; Alves et al 2020)
Most observational studies are focused on enclosed seas, lakes, and lagoons (e.g., Zodiatis and Gasparini 1996; Shi and Wei 2007; Pérez-Santos et al 2014; Arnon et al 2016), and a few authors measured convection in large-scale oceanic conditions (e.g., Schmitt 1981, 2003; Holbrook et al 2003; Nagai et al 2015)
Local observations show that island wakes induce saline convection in open ocean conditions over a 24-h period

Summary

Introduction

In a strongly stratified atmosphere, a mountainous island forces the striking airflow to split, generating a wake of weak winds downstream and stronger-than-ambient winds on the flanks (i.e., Venturi effect; Xie et al 2001; Alves et al 2020). This study uses observations and numerical simulations to characterize the convective processes that develop within the warm wake of Madeira Island (Fig. 1).

Results

Conclusion