Role of synoptic activity on projected changes in upwelling-favourable winds at the ocean\u2019s eastern boundaries

Catalina Aguirre,René D. Garreaud,Maisa Rojas,David A. Rahn

doi:10.1038/s41612-019-0101-9

Abstract

The climate of the ocean’s eastern boundaries is strongly influenced by subtropical anticyclones, which drive a surface wind stress that promotes coastal upwelling of nutrient-rich subsurface water that supports high primary productivity and an abundance of food resources. Understanding the projected response of upwelling-favourable winds to climate change has broad implications for coastal biogeochemistry, ecology, and fisheries. Here we use a reanalysis, an ensemble of global climate simulations, and an objective algorithm to track anticyclones to investigate the projected changes in upwelling-favourable wind events at the California, Canary, Humboldt, and Benguela coastal upwelling systems. Except for the north Pacific, we find consistent poleward shifts of mean and upper percentile daily winds over the ocean basins. We propose that extratropical, synoptic-scale migratory anticyclones that force intense coastal upwelling events—which become more frequent at higher latitudes and less frequent at lower latitudes in the future—play an important role in the projected changes in upwelling-favourable wind events in these coastal upwelling systems. These changes complement large-scale processes such as the poleward shift of the subtropical ridge (STR) and stationary subtropical highs. Hence, both extratropical and tropical processes need to be considered to fully explain projected changes at the coastal upwelling systems under anthropogenic climate change.

Highlights

Subtropical anticyclones are important features that influence the climate of the ocean eastern boundaries
They show that changes of surface winds in a warmer climate will result in an increase of the summer mean upwelling-favourable winds at the poleward portions of the present-day EBUS6,7
Climate change projections indicate a poleward shift of stationary anticyclones at subtropical latitudes.[7]

Summary

Introduction

Subtropical anticyclones are important features that influence the climate of the ocean eastern boundaries These high-pressure systems drive predominately alongshore equatorward winds[1] (Fig. 1). Previous studies have investigated projected changes in EBUS under warming climate scenarios using an ensemble of ocean–atmosphere coupled models. They show that changes of surface winds in a warmer climate will result in an increase of the summer mean upwelling-favourable winds at the poleward portions of the present-day EBUS6,7 (except in the California system). The latitudinal pattern of changes in upwelling-favourable winds along the EBUS is more consistent with a poleward displacement of the subtropical anticyclones and not Bakun’s hypothesis.[7] These previous studies have focused on changes of the summer mean (or median) upwelling, but not on the change in the full distribution of daily wind. The results indicated that there were fewer weak events and more moderate to intense events without a significant change in the extremes.[8]

Methods

Results

Conclusion