Onset and Decline Rates of Marine Heatwaves: Global Trends, Seasonal Forecasts and Marine Management

Claire M Spillman,Grant A Smith,Alistair J Hobday,Jason R Hartog

doi:10.3389/fclim.2021.801217

Claire M Spillman, Grant A Smith + Show 2 more

Open Access

https://doi.org/10.3389/fclim.2021.801217

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Abstract

Changing ocean conditions due to anthropogenic climate change, particularly the increasing severity and frequency of extreme events, are a growing concern for a range of marine sectors. Here we explore the global trends in marine heatwaves (MHWs), specifically onset and decline rates, two metrics which describe how quickly a MHW will emerge or disappear from a location. These rates determine the reaction window—the start of a MHW event to peak MHW temperatures—and the coping window—time from peak temperatures to the end of an event—two important time periods relevant to a marine decision-maker. We show that MHW onset and decline rates are fastest in dynamic ocean regions and that overall, the global trend in onset rate is greater than the global trend in decline rate. We map ocean regions where these rates are changing together with forecast skill from a seasonal dynamical model (ACCESS-S). This analysis highlights areas where the length of the preparation window for impending MHWs is increased by using forecasts, and areas where marine decision-makers should be prepared for rapid responses based on realtime observations as MHWs evolve. In regions such as south Africa and Kerguelen, northwest Atlantic, northwest Pacific, southwest South Atlantic and off Australian east coast where rapid median onset and decline rates are observed, there is also a positive trend in onset and decline rates i.e., MHWs are developing and declining more rapidly. This will be a concern for many decision-makers operating in these regions.

Highlights

The world ocean is warming rapidly, resulting in rapid environmental change (World Meteorological Organization, 2021)
Areas where 90 or more marine heatwaves (MHWs) events have occurred during this period include the northwest Atlantic, southwest Atlantic, eastern and south Africa, Bay of Bengal and the tropical West Pacific
Median reaction and coping windows have similar geographic patterns, coping windows are generally longer, with ∼88% of ocean grid cells having a longer coping window than reaction window. This asymmetry reveals that the decay of MHW events takes more time than the initial accumulation of heat to peak MHW intensity

Summary

Introduction

The world ocean is warming rapidly, resulting in rapid environmental change (World Meteorological Organization, 2021). Climate change projections (IPCC, 2019) point to an increasing frequency and intensity of many types of extreme events this century, driven by a warming ocean and atmosphere. Onset and Decline of Marine Heatwaves have dramatic environmental effects (e.g., Smale et al, 2019), which complicates environmental management and decisionmaking (e.g., Caputi et al, 2019) and disrupts economic systems and societies (e.g., Dunstan et al, 2018; Bellquist et al, 2021) Extreme events such as marine heatwaves (MHWs) represent a stress test for environmental management (Barbeaux et al, 2020; Holbrook et al, 2020), where MHWs are currently defined as five or more consecutive days of sea surface temperatures above the 90th percentile (Hobday et al, 2016a). The pace of change means unprecedented events, outside historical baselines or experience with increased severity and frequency and reduced recovery time between events, means the ocean cannot be used and managed as in the past (Hobday et al, 2018b)

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