Winds drive deep water formation in Gulf of Lion

By Charles Troupin, Elise Bultot and Aida Alvera Azcárate (University of Liege), Ben Loveday (Innoflair UG) and Hayley Evers-King (EUMETSAT)

The Mistral and Tramontane (or Tramuntana) are two cold, dry winds that affect the Gulf of Lion (western Mediterranean Sea, Figure 1).

The former blows from the north along the lower Rhône River valley toward the Mediterranean Sea, with a mean speed of around 50km/h and gusts above 100km/h. The latter passes between the Pyrenees and the Massif Central. Both winds can last several days, and, sometimes, more than a week.

These intense events lead to the cooling of the sea surface water, which, as it increases its density, sinks to greater depths, resulting in the formation of deep water (Western Mediterranean Deep Water, WMDW). In the Gulf of Lion, this deep water formation occurs almost each winter through dense shelf water cascading and open-sea deep convection (e.g., MEDOC Group, 1970).

Satellite data provide a synoptic view of the evolution of the surface water cooling, its intensity and spatial extent. Complemented by in situ data, we can also assess the effect of Mistral and Tramontane wind events in the water column. This case study shows several such events and focuses on the effect of the wind on the water column properties.

Effect of Mistral and Tramontane winds on Sea Surface Temperature

The domain of interest ranges from 40°N to 45°N and from 0° to 12°E, covering the Gulf of Lion. It also covers the Strait of Bonifacio, between Corsica and Sardinia, where lthe wind is forced between the mountainous island, leading to decreased sea surface temperatures (SST) to the east. While such wind events can take place at any time of the year, this case study focuses mostly on a specific event that occurred in August 2019.

When winds blow over the sea surface for a prolonged period of time, a net cooling of several degrees celsius can be observed in the SST. This is shown in Figure 2 where large cooling can be seen over the Gulf of Lion and east of the Strait of Bonifacio, is clearly evident.

The action of the winds over the sea surface also increases the roughness of the sea. This effect is visible in Sentinel-1 radar scenes, such as shown in Figure 3

If the winds are particularly strong, they can also be seen in optical imagery, as they cause waves to break over large portions of the sea. This gives rise to 'white-capping, and increases the amount of sea spray and haze; effects that can be observed as white colours in optical imagery (Figure 4).

These winds can be directly measured by the ASCAT sensors aboard Metop-A and -B (Figure 5). On 14 August (Figure 5, top panels), the area affected by the wind was clearly seen around the Gulf of Lion, with local acceleration due to Corsica and Sardinia. On 16 August (panel [c]), the situation remained similar, with the strongest wind developing southwest of the Gulf of Lion, before reversing to blow from the south (Figure 5, middle panels). On 19 August, the Mistral/Tramontane regime was again established, extending further south by 21 August (Figure 5, bottom panels).

The wind fields from satellites are provided at a spatial resolution of 12.5km, however, their main drawback is that the measurements are only available over the sea. In order to get an overview of the wind field over both land and sea, we represented the wind as obtained by the ECMWF model. The spatial resolution is 0.25° by 0.25° and there is one field every hour.

Figure 6 shows the wind field corresponding to 15 August 2019, 18:00. Not only does it show the Mistral and Tramontane wind blowing on land, then over the Gulf of Lion, but also the strong eastward wind blowing in the Strait of Bonifacio.

SST acquired by satellite is an excellent tool to study the evolution of the wind-cooling at the basin scale. By looking at the time evolution of multi-sensor Level 3 SST data we can understand the net effect a particular event has on SST. The multi sensor Level 3 SST product is used here to illustrate the utility of SST measurements.

Clouds interfere with the infrared measurements used to derive SST, so these data have gaps at level-2. The Level 3 SST data obtained from CMEMS (SST_EUR_SST_L3C_NRT_OBSERVATIONS_010_009_b) have been reconstructed using DINEOF (Data Interpolating Empirical Orthogonal Functions, Beckers and Rixen, 2003; Alvera-Azcárate et al, 2005) to provide a more complete picture of the daily SST changes due to the Mistral event.

The animation (Figure 7) shows the gradual cooling in the Gulf of Lion and also the effect of the channelled winds through the Strait of Bonifacio.

The difference in SST between 17 August and 10 August 2019 (Figure 8), indicates a generalised cooling over the whole northwestern Mediterranean Sea, with a more marked amplitude (up to 4.5°C) in the Gulf of Lion and east of the strait.

Mistral winds and the formation of deep water masses

Persistent winds do not only affect the temperature of the sea surface. In the the Gulf of Lion, wind-induced surface cooling also leads to the formation of deep waters. During strong Mistral/Tramontane events it is possible to use in situ data to observe how the temperature changes at depth.

Between 13 to 23 August 2019 two such wind events took place see Figure 5). The effects of these events were captured by several moorings in the region (Figure 9), in particular the platforms located at 42.067°N, 4.671°E (WMO code: 6100002) and at 41.893°N, 3.628°E (WMO code 6100196)

Among many variables acquired by these moorings, wind (intensity and direction) and sea water temperature are the most relevant in this case. The wind time series for the two stations during the month of August 2019 (Figure 10) highlights two successive periods of Mistral/Tramontane winds, with a maximal wind speed larger close to 15m/s.

The first event took place from 13 to 17 August , the second from 19 to 23 August. At station 6100196, located nearer to the coast, the wind is more dominantly from the north, while at station no. 6100002, it blows more from a northwest direction. For both stations, the wind events are consistent in terms of duration and amplitude.

The temperature evolution (Figure 11) can be split into five phases:

1. A general warming until August, during which the maximal temperature (25.8°C) was reached.
2. A first cooling period, related to the first wind event. The lowest temperature (21.6°C) was reached at the end of the period.
3. An intermediate period of warming, corresponding to wind blowing from the south.
4. A second, weaker, cooling period, related to the second, shorter, wind event .
5. A warming period corresponding to weaker winds in the region of study.

In order to better visualise the relationship between the temperature and the wind (direction and intensity), the feather plot (Figure 10) is re-drawn using temperature set the colour of the bars. The progressive cooling in water temperature when the persistent north west winds occur is clearly shown.

The in situ observations of temperature can be compared to the original and reconstructed satellite sea surface temperature at the same location (Figure 13). The overall evolution is similar in each case, with the in situ observations generally warmer than the satellite counterparts, and showing the intra-diurnal variability.

The PROVOR CTS3-DO Profiling Float (WMO code 6902937, see Figure 9) acquired several temperature profiles in the region and period of interest (Figure 14).

On 10 August, the temperature in the surface layer exceeded 25°C and two thermoclines can be identified: the first at a depth of 5m, the other at about 25m. On 15 August, the profile has a different shape: the surface layer has eroded, the mixed layer extends up to 35m deep, while the surface temperature decreased by 2.6°C, with respect to the previous profile (five days earlier).

Deep water circulation is an important process determining physical oceanography in the Mediterranean Sea, and, thus, represents a key component of the knowledge necessary for modelling oceanographic processing in this region, and managing their resultant impacts on human activities. The Mediterranean Sea also offers an excellent and accessible region in which to study the process of deep water formation. This process also occurs in the North Atlantic and Southern Oceans, where it plays a key role in global climate as part of the oceans thermohaline circulation.

References

Alvera-Azcárate, A., Barth, A., Rixen, M. and Beckers, J.M. (2005). Reconstruction of incomplete oceanographic data sets using Empirical Orthogonal Functions. Application to the Adriatic Sea surface temperature, Ocean Modelling, 9:325–346. DOI: 10.1016/j.ocemod.2004.08.001

Beckers, J.M. and Rixen, M. (2003). EOF Calculations and Data Filling from Incomplete Oceanographic Datasets, Journal of Atmospheric and Ocean Technology, 20(12): 1839–1856.

DOI: 10.1175/1520-0426(2003)020<1839:ECADFF>2.0.CO;2

MEDOC Group (1970). Observation of formation of Deep Water in the Mediterranean Sea, Nature, 227: 1037–1040. DOI: 10.1038/2271037a0