Tropical cyclones (TCs) have been known for long to result in a strong cooling of the oceanic upper layer. In situ measurements from ship then using buoy arrays and more recently satellite observations e.g. showed that they generate a cold wake with maximum amplitude in their right-rear quadrant (in the Northern Hemisphere) which can lower the sea surface temperature (SST) by 5 to 6 ∘C. This surface cooling can have a strong feedback effect on the cyclone development and intensity as SST controls for a large part the surface enthalpy flux e.g.. This typical ocean response (and feedbacks) has motivated many studies documenting the processes at play and the factors controlling its amplitude.

The oceanic processes at the origin of the cooling have been investigated in case studies involving simulations or based on observations . Turbulence injected into the oceanic mixed layer (OML) by the surface wind stress results in a very strong vertical shear at the base of the OML with respect to the underlying thermocline. For cyclones with a translation speed above ∼1–2 m s-1, near-inertial currents act to reinforce this shear and establish a vertical oscillation with a period slightly below the inertial period and amplitude of 20 to 40 m . The first upwelling of this series of alternate vertical motions contributes to cool the upper layer by bringing colder water from below the thermocline into the OML. The resulting change in the mixed-layer depth (MLD) can further alter the OML budget and its response to the atmospheric forcing. This turbulent mixing usually accounts for most of the cooling effect in strong cyclones (Category 2 or above in the Saffir–Simpson scale). A global study based on the simulation of more than 3000 TCs evaluated its contribution to 56 % of the SST cooling on average within 200 km of the TC track. This contribution varies depending on the TC intensity from ∼30 % of the total cooling for the weakest cyclones to more than 90 % for the most intense ones . Surface heat fluxes also act to cool the upper-ocean layer mainly through the latent heat flux. They account for the remaining part of the surface cooling, which is up to 70 % for the weakest TCs but less then 10 % for the TCs of intensity 4 or above. Lateral advection also contributes as a secondary process, accounting for nearly 10 % of the total cooling for the strongest TCs and a larger proportion for weaker ones. It may especially reinforce the asymmetry of the cold wake originating from the shear-driven turbulence at the base of the OML e.g..

Cyclone intensity and translation speed are key criteria controlling the cooling magnitude and its spatial extent. More intense cyclones generate stronger cooling, and slow-moving cyclones produce stronger cooling for a given intensity . Slow-moving cyclones also produce cold wakes of a larger extent and more symmetrical because the upwelling has more time to settle .

The amplitude and spatial extent of the cold wake also depend on the ocean preconditioning with a strong influence of the pre-storm SST, upper-layer stratification, and MLD. The amplitude of the cooling has been shown to vary by 1 order of magnitude depending on the ocean state prior to the cyclone passage . In strong TCs, a shallower MLD or stronger stratification favour more intense cooling as the turbulent mixing at the base of the OML more efficiently brings colder water upwards . Colder SSTs or lower ocean heat content (generally defined as the temperature integral from the surface down to the 26 ∘C isotherm depth; ) result in weaker TCs and associated weaker cooling.

Upper-layer oceanic features like fronts or eddies also have the potential to modulate the upper-ocean response to a TC. Statistical studies based on 30 to 3000 cases showed that both anticyclonic and cyclonic eddies can influence the TC development through the upper-ocean feedback . Anticyclonic eddies deepen the OML and limit the SST cooling by insulating the warm surface layer against storm-induced upwelling and mixing. Conversely, cyclonic eddies are associated with shallower OML and lead to stronger surface cooling.

The surface cooling can also be altered by salinity stratification. Salinity-induced barrier layers (BLs; ) form in the presence of less saline water at the surface due to rain freshening, run-off, or horizontal advection. They uncouple the OML, driven by salinity, from the isothermal surface layer and form an intermediate barrier layer (at least 5 m deep) that is more saline than the OML and warmer than the water below the thermocline. Two recent studies based on climatological datasets and idealized coupled simulations gave consistent results and showed that the effect of BLs strongly depend on the intensity (or stage) of the TC. For a TC of weak intensity, where most of the cooling arises from the surface heat fluxes, the BL shoals the OML and isolates it from the colder water below, thus making the surface heat extraction more efficient. For a TC of stronger intensity, where most of the cooling is due to shear instability and mixing at the base of the OML, the presence of the BL mitigates the efficiency of the mixing.

The upper-ocean salinity changes in the wake of a TC have motivated fewer studies than the surface cooling because of their indirect effect on the cyclone intensity. However, they can modulate the subsurface stratification and enhance or weaken the mixing or heat extraction. Sea surface salinity (SSS) changes result from the competing effects of precipitation and vertical shear and mixing near the base of the OML bringing saltier water up to the surface. Rain rates in TCs are a maximum of 35 to 50 km away from the storm centre, and their magnitudes range from 3 to 12 mm h-1 on average depending on the cyclone intensity . In a study of more than 800 TCs based on satellite observations and reanalysis of the upper-ocean characteristics, showed that, despite strong precipitation and neglecting the effect of evaporation, the wake of TCs is marked by salinization. Strong mixing brings saltier water from below the OML, overcompensating the freshening effect of rain. Note that, in this study, the SST cooling is attributed entirely to the turbulent mixing. A recent and more realistic study using in situ observations brought contrast to that. It showed that the freshening effect of strong precipitation dominates the salinizing due to mixing and evaporation at least in a 150 km radius around the cyclone centre .

The cooling of the oceanic upper layer by extratropical storms or hybrid cyclones like medicanes has motivated fewer studies than for TCs probably because their magnitude is much smaller and there is supposedly no or very little feedback on the atmosphere. Indeed, coupled simulations of mid-latitude storms in the North Atlantic showed that, in contrast with TCs where the cooling can reach several degrees, the mean effect is below 1 ∘C . A recent statistical study on North Pacific storms based on satellite observations and reanalysis over 20 years also gave a mean SST cooling of 1 order of magnitude smaller than what is obtained for TCs . Surface heat fluxes and turbulent mixing contributed equally to the cooling. Finally, a study of a strong storm in the Gulf of Lion in May 2005 using a coupled ocean–atmosphere–wave simulation obtained a surface cooling of 2 ∘C over a large area . During this storm, the major contributor to the OML cooling was the vertical mixing enhanced by the strong surface stress (68 %) with secondary contributions from the latent and sensible heat fluxes (15 % and 7.5 % of the cooling). These contributions are similar to those obtained in TCs of Category 2 or above.

In the present study, we use a coupled ocean–atmosphere kilometre-scale simulation to investigate the impact of the short but intense medicane Qendresa (7 November 2014) on the OML. Comparing the atmospheric processes of medicanes with those of TCs is a tempting but challenging undertaking. Indeed, no systematic statistical study provides, to the best of our knowledge, a comprehensive assessment of the processes at play according to the cyclone intensity, especially for categories 1 and 2, that could be compared with medicanes. Conversely, as detailed here above, the impact of TCs on the upper ocean and the corresponding mechanisms were the subject of systematic studies scanning the different TC categories e.g.. We thus take advantage of this knowledge to contrast the coupling mechanisms and oceanic upper-layer processes in a medicane with those of TCs.

A first study of the life cycle and atmospheric processes of this medicane, including the assessment of the impact of the SST cooling on the atmosphere, showed that the surface cooling is at least 1 order of magnitude lower than in typical TCs . The present study aims to investigate (i) how the surface cooling and/or salinity changes obtained in this case compare with changes observed in TCs or mid-latitude storms, (ii) whether the OML processes are similar to those of TCs, and (iii) whether the characteristics of the changes obtained here are modulated by the atmospheric forcing or by the central Mediterranean oceanic conditions prior to the medicane. Section 2 summarizes the case study and presents the oceanic conditions before the event and the numerical tools used in this work. The results, with the evolution of the ocean and an analysis of the role of the atmospheric forcing and of the mechanisms at play, are given in Sect. 3. Section 4 presents the impact of the oceanic conditions on the cooling and salinity changes in different areas. These results are discussed and conclusions are given in Sect. 5.

We present here a summary of the November 2014 medicane, the oceanic conditions that pre-exist in the central Mediterranean, and the ocean–atmosphere simulating configuration used in this study.

To quantify the role of oceanic preconditioning and precipitation in the oceanic response to the medicane, we define different zones within the SFA. The EDDY zone is defined as the area of the ABV cyclonic eddy using an SST of 20.5 ∘C as a threshold (1005 grid points). We also define a heavy rain zone as the area, in the Strait of Sicily, where integrated water flux at 00:00 UTC on 8 November reaches 90 mm of water. This heavy rain (HR hereafter) zone includes 2383 grid points, and its mean integrated water flux at 12:00 UTC on 8 November is 136 mm. Note that it is not entirely within the SFA (Fig. ). Finally, a reference zone includes the grid points of the SFA that are not part of either the EDDY or the HR zone (REF hereafter, 4343 grid points). The objective here is to evaluate whether the characteristics of the central Mediterranean (bathymetry, dynamics) or heavy rain control the oceanic response to the event.

Comparing a medicane with low-intensity TCs from the viewpoint of the upper-layer oceanic processes proves to be relevant and reveals many similarities and a few differences. According to its maximum sustained wind, the medicane studied here is comparable to TCs of Category 1 on the Saffir–Simpson scale. Its duration is nevertheless much shorter than a typical TC with wind energy flux above 0.2 kg s-3 for a few hours only between 07:00 and 13:00 UTC on 7 November and upward heat fluxes above 400 W m-2 for 30 h between the beginning of the event and 07:00 UTC on 8 November. The mean surface cooling on the SFA is therefore less than -0.6 ∘C, which is significantly lower than the typical cooling of weak TCs. The dominant cooling mechanism is the surface forcing (at the origin of a -0.45 ∘C cooling), while the turbulent mixing accounts for less than -0.1 ∘C. These two mechanisms do not act simultaneously: the surface forcing starts earlier, peaks at 09:00 UTC on 7 November, and stays above -400 W m-2 for more than 24 h in total; the turbulent mixing increases until 12:00 UTC on 7 November and drops rapidly in the afternoon of 7 November. This may show that surface forcing has a much larger effect than turbulent mixing on the OML temperature. Yet the cooling effect of the surface fluxes is at the upper threshold of the typical range given by , while the cooling effect of the turbulent mixing is always weak even at the peak intensity of the event. This discrepancy can be explained by the effect of heavy rainfall in the first hours of the event, which create or deepen an existing BL. Indeed, comparing the cooling of two areas forced by similar wind stress and heat fluxes but very different water fluxes show that the presence of the BL results in a 37 % increase in the effect of the surface forcing and 64 % decrease in the effect of the turbulent mixing.

A diagram based on the simulated mean profiles and MLD values summarizes the upper-layer evolution throughout the event and the impact of heavy rain (Fig. ). Before the event, both REF and HR zones have similar profiles in temperature and salinity (dotted black lines, Fig. a and b), with an MLD of 37 m controlled by temperature. During the first part of the event, the temperature evolution is the same in REF and HR and is mainly due to the surface heat extraction (Fig. a, dashed line). The salinity decrease in the OML is mainly due to the rain–evaporation balance at the surface and is larger in HR close to the surface (Fig. b, blue versus black dashed lines). As a result, the MLD is governed by temperature in REF and increases to 42 m (dashed black line, Fig. c), while it is controlled by salinity in HR and slightly decreases to 36 m (dashed blue line, Fig. c). During the second part of the event, the temperature and salinity evolutions in REF are very similar to those of the first part except that mixing close to the OML bottom and horizontal advection result in a stronger cooling between 25 and 50 m (solid black lines, Fig. a). In HR conversely, the slightly thinner OML due to the surface freshening which occurred during the first part of the event makes the OML more sensitive to surface forcing. The effect of precipitation on salinity is amplified with a strong decrease at the surface (solid blue line, Fig. b), and this salinity change controls the change in density with a corresponding decrease at the surface and large shoaling of the MLD to 22 m (solid blue line, Fig. c). Heavy precipitation also acts on the surface temperature during the second half of the event (Fig. a; see also Fig. ) even though it had been occurring since the beginning of the event. This delayed effect is due to the positive feedback of a salinity-induced thinning of the OML, which settles after a few hours only. Heavy convective precipitation is usual during the development phase of medicanes before the wind reaches its maximum speed, and the preconditioning effect of the oceanic upper layer we obtain here is probably rather typical. Mediterranean cyclones in general come with heavy rain, which are prone to alter the response of the oceanic upper layer as observed here. On the OML salinity, the effects of both the surface forcing and the turbulent mixing are enhanced by the BL.

Lateral advection plays a secondary but significant role in cooling or warming the OML. In particular, strong upper-layer horizontal currents develop close to the cyclone centre in the ABV eddy. Colder water is shifted southwards, reinforcing the cooling, and warmer water is brought towards the eddy centre by converging motions from the surrounding area with deeper OML. In addition, due to shallower MLD and weaker 100 m stratification in the eddy, the effect of the surface heat fluxes is lower, while the effect of the turbulent mixing is strengthened. As a result, the cooling in the ABV eddy is dampened with respect to the surroundings. The peculiar dynamics of the Strait of Sicily, with its strong currents and the regular presence of eddies, is able to modulate the impact of cyclones on the ocean.

The present study is in strong contrast with the case study of a strong storm with comparable sustained wind speed in the north-western Mediterranean . The cooling obtained by these authors was between -1.5 and -2 ∘C over the large area of the Gulf of Lion. A precise comparison of the two events and of their impact on the oceanic upper layer is beyond the scope of the present study, but several factors can partly explain the stronger cooling they obtained. First, the duration of the storm was longer than Qendresa with strong wind stress and heat fluxes from 4 until 6 May. The track of the storm was also favourable to a stronger impact on the ocean since it looped for 3 d over the Gulf of Lion. Second, the MLD in May in the Gulf of Lion is usually shallower than in November in the Strait of Sicily with values in the range of 10–30 m . More precisely, observations of the Lion buoy show that prior to the storm, the temperature at 10 m was -0.3 ∘C below the SST taken at 2 m; see. No measurements were available at that time at depths between 10 and 200 m, but observations on the same date in 2012 show a -0.6 ∘C difference between the temperature at 15 m and the SST and a -1 ∘C difference at 35 m. The upper layer was thus likely strongly stratified before the storm, and this favours the cooling by turbulent mixing. As far as we can judge, atmospheric and oceanic conditions acted together to enhance the impact of the turbulent mixing, and this resulted in a stronger cooling.

This study is based on a single case, and our results should be confirmed by simulating more events. A broader study involving a statistically significant number of Mediterranean cyclones is nevertheless still challenging. Intense cyclones like the present case occur one to two times per year e.g., and our results show that high-resolution oceanic models are necessary to represent the fine-scale features modulating the oceanic response. The atmospheric forcing should also be well resolved to realistically reproduce the intensity and variations of the surface input. The simple comparison with the oceanic impacts obtained by shows that Mediterranean storms are diverse and, depending on the area and season concerned, can lead to different oceanic responses due to the different processes. Previous studies at the climatological scale show that the Strait of Sicily is among the preferred areas for medicane formation . The processes exposed here, especially the time lag between intense rainfall occurring at the beginning of the event and maximum intensity of heat and wind stress, are characteristic of medicanes . We are thus convinced that the enhanced effect of the surface forcing and the reduced effect of the turbulent mixing due to the rain-induced barrier layer are rather widespread under medicanes. The ABV eddy, which significantly reduces the cooling due to colder SSTs and shallower MLD, is not always present. Yet both cyclonic and anticyclonic structures are common in the central Mediterranean and able to modulate, as was shown here, the oceanic response to medicanes.