The 2011 marine heat wave in Cockburn Sound, southwest Australia

T H Rose,G Botting,D A Smale

doi:10.5194/os-8-545-2012

Abstract

Abstract. Over 2000 km of Western Australian coastline experienced a significant marine heat wave in February and March 2011. Seawater temperature anomalies of +2–4 °C were recorded at a number of locations, and satellite-derived SSTs (sea surface temperatures) were the highest on record. Here, we present seawater temperatures from southwestern Australia and describe, in detail, the marine climatology of Cockburn Sound, a large, multiple-use coastal embayment. We compared temperature and dissolved oxygen levels in 2011 with data from routine monitoring conducted from 2002–2010. A significant warming event, 2–4 °C in magnitude, persisted for > 8 weeks, and seawater temperatures at 10 to 20 m depth were significantly higher than those recorded in the previous 9 yr. Dissolved oxygen levels were depressed at most monitoring sites, being ~ 2 mg l−1 lower than usual in early March 2011. Ecological responses to short-term extreme events are poorly understood, but evidence from elsewhere along the Western Australian coastline suggests that the heat wave was associated with high rates of coral bleaching; fish, invertebrate and macroalgae mortalities; and algal blooms. However, there is a paucity of historical information on ecologically-sensitive habitats and taxa in Cockburn Sound, so that formal examinations of biological responses to the heat wave were not possible. The 2011 heat wave provided insights into conditions that may become more prevalent in Cockburn Sound, and elsewhere, if the intensity and frequency of short-term extreme events increases as predicted.

Highlights

The frequency and intensity of short-term thermal events (i.e. “heat waves”) is very likely to increase as a consequence of anthropogenic climate change (Solomon et al, 2007)
This, in turn, fed warm tropical waters into the poleward flowing Leeuwin Current, which strongly influences the oceanography and biology of southwestern Australia as it flows along its coastline (Gaughan and Fletcher, 1997; Koslow et al, 2008)
Increased flow of the Leeuwin Current was evidenced by anomalously high sea level readings in temperate regions (e.g. Fremantle Port, see Gaughan and Fletcher, 1997 for further details) and resulted in increased transport of warm water along the coastline of Western Australia (Fig. 1)

Summary

Introduction

The frequency and intensity of short-term thermal events (i.e. “heat waves”) is very likely to increase as a consequence of anthropogenic climate change (Solomon et al, 2007). Increased flow of the Leeuwin Current was evidenced by anomalously high sea level readings in temperate regions (e.g. Fremantle Port, see Gaughan and Fletcher, 1997 for further details) and resulted in increased transport of warm water along the coastline of Western Australia (Fig. 1). We present detailed in situ environmental data collected from the semi-enclosed embayment of Cockburn Sound (32.1706◦ S, 115.7239◦ E) to characterise the heat wave event in an important, multipleuse coastal system

Study area

Monitoring data

Results and discussion