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Temperatures in the Mediterranean: after the heat wave of 2003 a return to normality in 2004
By Isabelle Charon, Dominique Obaton, Jérôme Sureau from Mercator Océan
Last year … the heat wave in Europe
In this bulletin:
Last summer, exceptionally high temperatures were recorded in France throughout the summer. This anomaly in air temperatures also had an impact on the ocean. In the last bulletin, we have seen that surface waters were clearly warmer than normal in July 2003, while July 2004 was more normal.
During August, these results are confirmed: 2004 meteorological situations are normal for August, while in 2003, the Azores anticyclone had maintained a heat wave over all this period time.
We can have an idea of the mean meteorological situation by plotting month-averaged wind stress: wind stress computed from the surface wind derived from the atmospheric model is used directly to force our oceanic model at the surface:
Wind stress : August 2003 average
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Wind stress : August 2004 average
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Notable differences can be observed from one year to another:
- In the Gulf of Lion, stress generated by the Mistral and the Tramontane are very weak in 2003
- In the Tyrrhenian Sea, traditional strong westerly wind between Corsican, Sardinia and the italian coasts are very weaker in 2003.
- Less occurence of the North-West wind from Sicily Channel to Afro-Sicilian basin.
- Notable weaker wind in the Ionian Sea.
The Meltem in the Aegean Sea is on the other hand very present during the two years, with a stronger North component in 2003. The situation in the East-Mediterranean is quite similar from one year to another.
These anomalies have direct consequences on the high-resolution Mediterranean prototype (PSY2) behaviour. Below are presented the August-averaged sea surface temperatures maps both in 2003 and 2004. Current vectors, also month-averaged, are superimposed on the SST.
Surface current and temperature : August 2003 average
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Surface current and temperature : August 2004 average
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What is striking are the very higher temperatures in 2003 than in 2004 over a large part of the basin:
- West of the basin, water circulation is different: in the Gulf of Lion, the mixing and upwellings generally created by the North winds are weaked in 2003. The Ligurian-Provencal current is reinforced along the Gulf. On another hand a water blocking phenomena is taking place in the South of Balearic Islands (the Algerian current keeps close to the coast). Consequently, a large anticyclonic eddy is observed in the Balearic Basin where surface waters are trapped and then warmed up. Yet, the North Balearic front does not extend beyond 5E along 41N with a weak temperature gradient. The temperature gradient between the Alboran Sea and the Algerian Basin reaches 8° instead of 5° in 2004.
- In the Tyrrhenian Sea, if the structures are rather similar between the two years (cyclonic circulation along the coast and warm waters trapping in the South Italy), the temperatures are warmer from 2 to 3°C in 2003. This warming can be partially explained by the lack of westerly wind between Corsican and Sardinia. In 2004, these winds generate a large upwelling downstream the islands, with temperatures weaker from 3 to 5°C than over the rest of the Tyrrhenian Sea. Furthermore, the upwelling south of Sardinia is not present in 2003 and the mixing with Algerian current waters which normally allows the cooling from the South Tyrrhenian does not happen.
- In the African-Sicilian Basin, circulation along Sicily is reversed -presence of a large cyclonic eddy, preventing cool water diffusion towards South. Temperature discrepancies reach there 1 to 2°C.
- The situation in the East of the Basin is on another hand quite similar for the two years: the Aegean Sea is the only place in Mediterranean where we can observe temperatures colder in 2003 than in 2004. This is due to the anticyclonic meteorological situation favouring North winds (Meltem).
We present the 30/08/2004 Mercator forecast surface temperature and the high-resolution Safo (satellites and in situ data combination) temperature at the same day.
Mercator surface temperature : 30 August 2004
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Safo surface temperature : 30 August 2004
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Structures are quite similar, even if a few details can be noticed:
- Well marked upwelling in the Gulf of Lion: Mercator accentuates excessively the Rhone plume. Upwellings along the West Corsican coasts and Sardinia are also overestimated.
- The Balearic front is well positionned and the Algerian current circumvolutions are quite wel reproduced, even if the temperatures along the Algerian coasts are slightly overestimated by Mercator.
- The two edies in the Alboran Sea are well visible.
- The Tyrrhenian Sea eddies and the cool waters outflow in the Sicily channel are well described.
- The Ligurian-Provencal current is well positionned and its width is correct.
- In the Adriatic Sea, temperatures are slightly underestimated but the front between Italy and Lybia, which separates Tunisian warm waters from cool Ionian Sea waters is strikingly well positionned.
- In the East of the Basin, temperatures are also quite similar to the observed temperatures: warm on the Middle East. Upwellings along the Turkey coast are slightly overestimated by Mercator.
By comparing with Levitus climatology, it appears that surface temperature (and at deeper levels) is still warm this year. However it is difficult to conclude to a warming because Levitus climatology is based upon seasonal averages in locations where observations are rare.
To sum up, summer 2004 is quite conform to the normal. It's too early to evaluate the consequences of the last summer heat wave on the Mediterranean. Anyway, our PSY2 prototype had a quite good behaviour, reproducing correctly the currents and surface water transports all over the basin.
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