Intensification of marine heatwaves when compounding with atmospheric heat waves

In the last two decades, the Mediterranean Sea has experienced an accelerated warming, widely associated by the scientific community with anthropogenic climate change. A manifestation of that warming has been the extremization of marine heatwaves, both in frequency and intensity. At the same time, the intensification of atmospheric heat waves has also been observed, being more intense and extensive in the Mediterranean basin. This increased frequency of both phenomena has most likely increased their interaction or, at least, their spatio-temporal coincidence.
Although extreme atmospheric climatic events compounding has increasingly received more attention from the scientific community in the recent years, less interest has been paid to the compounding or concurrence of marine and atmospheric extreme events. With this gap in mind, we wanted to take a further step in our research line on climate change in the Mediterranean, especially in terms of sea temperature warming and extreme events, and investigate what the impact of the concurrence of both marine and atmospheric temperature extremes could be.
Hence, we developed a new methodology for detecting heat waves that would consider the spatio-temporal evolution of such events, see table 1. In a first step all grid points temperature values were checked looking for heatwave conditions. Then, daily spatial continuity was assessed by checking if HW candidate grid points belonged to a cluster of contiguous points, with a minimum extension of 5% of the Mediterranean basin being this threshold based on previous studies of the authors.
AHW: In each gridpoint: daily max T2m > P95 for three or more consecutive days (P95 corresponds to the period May-September 1961-1990) MHW: In each gridpoint, daily mean SST > P90 for at least five consecutive days (P90 corresponds to the period January-December 1961-1990) |
AHWD and MHWD candidate grid points belong to a cluster of contiguous points occupying at least 5% of the Mediterranean, filtering out smaller spatial extension groups |
Find start/end date and characterise main metrics for the spatiotemporal continuous AHWDs/MHWDs constituting an AHW/MHW event |
Table 1. AHW and MHW detection algorithm process
HWs concurrence is defined as the coincidence of AHW and MHW conditions in the same grid point at least for three days. Both types of events can last for a longer period or have a greater spatial extent than the concurrence itself, so single events can go through concurrent and non-concurrent phases during their life cycles and spatial extent or could be concurrent more than one HW event. Then, different concurrent types can be observed a) fully non-concurrent events, b) partially non-concurrent ones, namely the non-concurrent times and areas of concurrent events and c) concurrent events, defined as the phase in space of the event where both phenomena coexist at the same time. Once HWs concurrent events were found, their main characteristics were computed for the three different phases.
From the analysis of our results, it follows that concurrence in the Mediterranean Sea shows a clear increasing trend in the last decades while stand-alone events have become progressively less frequent. The HW concurrency frequency trend has increased since the last decade of the 20th century, with approximately 77% of the days of the May-September period experiencing AHW being concurrent and 30% of days of concurrent MHWs.
The HWs concurrence has an impact on their characteristics, especially for MHWs mainly due to changes in air-sea heat fluxes. While AHWs do not show relevant changes in intensity, the opposite happens for MHWs. Mean and maximum intensity of the MHW concurrent phase show higher values, with mean value 0.7–0.8 °C higher than those of fully non-concurrent events. Partially non-concurrent points intensity is located at an intermediate range between the other two categories with higher values significantly higher than the ones for fully non-concurrent events. From these results, the greatest impact of the concurrence is the intensification of MHWs, both in their mean and maximum intensity values and in the speed of their intensification.
This MHW intensification, on top of the observed climate change-induced one, appears to be a transient local phenomenon in the concurrency area. It has also been observed that MHWs intensity decreases progressively with distance to the closer concurrent area, reinforcing the local character of the MHW intensification associated with the concurrence. The influence of this local intensification on the already observed long-term intensification of MHWs needs to be assessed to better account for the climate change impacts analysis, as the concurrence will probably increase in the near future and thus exacerbate the impacts on mediterranean climate and biodiversity.
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