Slower long-term coastal warming drives dampened trends in coastal
marine heatwave exposure
Abstract
Long-term temperature changes drive coastal Marine Heat Waves (MHW)
trends globally. Here, we provide a more comprehensive global analysis
of cross-shore gradients of MHW and SST changes using an ensemble of
three satellite SST products during recent decades. Our analysis reveals
depressed onshore SST trends in more than 2/3 of coastal pixels,
including both eastern and western boundary current systems. These were
well correlated with depressed trends of MHW exposure and severity,
ranging from a -2 to -10 decrease in MHW days per decade and a –2.5 to
–15°C.days per decade decrease in cumulative intensity. Results were
consistent across all satellite products, indicating that these
cross-shore gradients are a robust feature of observations. ERA
reanalysis data shows that neither air-sea heat fluxes nor wind driven
upwelling were found to be consistent drivers. Global ocean circulation
models (OFAM3 and ACCESS-OM2) have limited ability to simulate the
depressed onshore trends. A heat budget analysis performed in the
Chilean coast region, where models agree with observations, showed that
the gradient of temperature change was controlled by an onshore increase
of longwave radiative cooling, despite an increase in upwelling. This
highlights the complexity of small-scale coastal ocean-atmosphere
feedbacks, which coarser resolution climate models do not resolve. Here,
we show that global coastal regions may act as thermal refugia for
marine ecosystems from aspects of climate change and pulsative (MHW)
changes. Contrary to the literature, our results suggest that driving
mechanisms are region dependant, stressing the necessity to improve
climate models resolution.