Guinea Gulf SST anomalies impact on Mediterranean 2003 summer: a numerical sensitivity analysis

Several anomalous warm summers occurred in the Mediterranean and southern Europe in the last 30 years, with heat-wave events of different intensity and length. However, the heat-wave occurred in 2003 (the most extreme in 500 years) was far the longest and warmest occurred, with more than 30,000 fatalities in Western Europe, and it has been located on top of all natural disasters of the year. In this respect, heat wave events represent one of the worst weather-related catastrophe, therefore an extensive effort has been provided in order to understand direct and/or indirect physical mechanisms responsible for them. The West African Monsoon (WAM) is a well known phenomenon playing an important role in determining summer conditions in the Mediterranean basin. Its fluctuations are known to produce significant changes not only at regional scale, but also on the large scale circulation. WAM variability is partly connected to the anomalous warming of the sea surface in the Guinea Gulf near the equator, while a link between the Guinean coastal region and the Sahel region exists due to both thermodynamic and dynamical processes. As part of the air in the rising branch of the Hadley circulation, which is located over the Sahel in late July and August, subsides further north over the Mediterranean influencing the temperature and precipitation, and giving rise to anomalous summertime climate. A southward shift of the monsoonal activity is related to cooler and wetter conditions over the central and western Mediterranean sea in mid-late summer. While the summer is hot and dry over the Mediterranean sea when WAM activity is anomaly shifted to the north. Using SST anomalies patterns in the Guinea Gulf we force the RAMS (Regional Atmospheric Modelling System) for a sensitivity study of their impact on the dynamical patterns during summer 2003. We perform several seasonal (April – September) simulations using perturbed (positive and negative) SST in the Guinea Gulf, and compare results with results from a control run. Local precipitation and circulation patterns over the Guinea Gulf and Sahel regions, as well as Jet Streak position and strength over Mediterranean sea – Central Europe have been analyzed, showing a clear link between their variability and the perturbed SST field. The study reveals a long distance linkage, a sort of “weak ties”, which connects the monsoon local dynamics to mid – latitude large scale circulation through the Jet Stream position acting on a wide range of time scales.