Mediterranean Marine heatwaves: On the comparison of the physical drivers behind the 2003 and 2015 events

Abstract

<p>Over the last decade, an intensification of extreme warm temperature events, termed as marine heatwaves (MHWs), has been reported in the Mediterranean Sea, itself a “Hot Spot” region for climate change. In the summer of 2003, a major MHW occurred in the Mediterranean with abnormal surface temperature anomalies of 2-3 Cº persisting for over a month. In 2015, an undocumented but more intense summer MHW affected almost the entire Mediterranean Sea with regional temperatures anomalies reaching 4-5 Cº. Here, we apply a MHW detection algorithm for long-lasting and large-scale summer events, on the hindcast output of a fully-coupled regional climate model (RCSM). We first examine the spatial variability and temporal evolution of both the 2003 and 2015 events. Then a basin-scale analysis of the mixed layer heat budget during each MHW is performed. The ocean and atmospheric components’ contribution is investigated separately during the onset, peak, and decay phases of both events, in order to disentangle the dominant physical processes behind each event. On the large-scale, our results indicate a key role of the wind forcing and the air-sea heat fluxes, while advection processes become more important at local scales. This study provides a comparison of the underlying mechanisms behind the two most intense MHW detected in the Mediterranean Sea during the last decade, constituting key information for the marine ecosystems of the region.</p>

Marine heatwaves in shallow coastal ecosystems are coupled with the atmosphere: Insights from half a century of daily in situ temperature records

“…Vogt et al (2022) found that the decline phase was associated with increased heat loss to the atmosphere due to increased latent heat loss and while we found a decrease in this property during the decline, there was still anomalous latent heat loss to the atmosphere. A case study of two Mediterranean MHWs also found increased latent heat loss to be associated with the decline phase, but also implicated anomalously high wind speed and sensible cooling with increased vertical diffusion (Darmaraki et al, 2020). This is in contrast to the persistent low wind speed and generally nonanomalous air-sea heat fluxes found in our results.…”

“…The anomalies observed in other variables are consistent with that expected from an atmospheric high pressure system; increased air temperature as a result of sinking air and adiabatic warming and reduced wind speeds (Ahrens, 2014) that leads to reduced latent heat flux from the ocean (Fairall et al, 2003). In the open-ocean and stratified parts of shelf-seas, reduced wind speeds associated with high pressure systems would also be anticipated to reduce vertical entrainment of cooler subsurface waters toward the surface (Simpson and Sharples, 2012), which has been identified as being an important factor in the generation and evolution of some MHWs (Fewings and Brown, 2019;Salinger et al, 2019;Darmaraki et al, 2020;Chen et al, 2021); however, the two SWMEs studied here are relatively shallow (4.5-15 m) and are likely to be vertically well-mixed (Taylor, 1981;Hunter and Tyler, 1987) such that the impact of reduced entrainment is likely to be negligible. Of these factors, the latent heat flux seems to be of most importance due to it being the most anomalous component of the air-sea heat flux budget.…”

Climatic, Decadal, and Interannual Variability in the Upper Layer of the Mediterranean Sea Using Remotely Sensed and In-Situ Data

“…In 1994, 2015, and 2018, intense summer MHWs affected almost the entire MED [81,83,84], as documented by the increasing temperature in all sub-basins (Figure 9B). The EOF2 field derived from the SST (Figure 7A) explains 13% of the total variance, showing an out-of-phase behavior between the WMED and the EMED (negative values west of 18°E and positive values on the east), a sort of dipolar configuration centered on The EOF1 of the SSS in the MED explains 28% of total variance (Table 1; Figure 4A) whereas, considering the sub-basins separately, explained variances exceed 40%, with a maximum in the EMED (66.3%).…”

Local Drivers of Extreme Upper Ocean Marine Heatwaves Assessed Using a Global Ocean Circulation Model

“…Anomalous heating due to air-sea heat flux increases stratification and is accompanied by a shoaling of the mixed layer depth, further enhancing temperature anomalies (Oliver et al, 2021). The resulting temperature increase is therefore restricted to a thinner surface layer, which explains the tendency of surface MHWs to be more responsive to changes in ASHF (Sparnocchia et al, 2006;Olita et al, 2007;Schlegel et al, 2021), increasing their frequency but decreasing their duration (Darmaraki et al, 2020). Indeed, this global study clearly shows that ASHF is the strongest driver of extreme MHW (Figure 3) in regions where the surface mixed layer is thinner (e.g., in the tropics, Figure 1).…”