Global Trends in Marine Heatwaves and Cold Spells: The Impacts of Fixed Versus Changing Baselines
Stephen M. Chiswell
Abstract
Abstract Marine heatwaves (MHW) and cold spells (MCS) are considered to be anomalously warm or cool events in sea surface temperature that are commonly defined relative to a long‐term fixed baseline. As a result, because of climate warming, it is often concluded that MHW will become stronger, longer, and more frequent in the future, whereas MCS will become weaker, shorter, and less frequent. Here, it is argued that in order to understand the physics of MHW/MCS, it is important to remove the climate signal when detecting events. The 1982–2021 National Oceanic and Atmospheric Administration optimum interpolation sea surface temperature data set (NOAA OISST V2) is used to show that using a fixed baseline leads on average to overestimating MHW and underestimating MCS strengths between 1982 and 2021 compared to using a linear trend to remove the climate signal. If a linear trend is used to remove the climate signal, global trends over the 40‐year record indicate a shift to weaker MHW and MCS in the tropics, especially in the eastern Pacific Ocean, and a shift to stronger MHW and MCS in the northern Pacific and Atlantic Oceans. The trends in the tropics are likely related to El Niño conditions predominating during the first half of the record and La Niña conditions predominating during the second half of the record. The trends at higher latitudes are consistent with changes in the North Atlantic and Atlantic Multidecadal Oscillations.