Extension of summer climatic conditions into spring in the Western Mediterranean area

From a local point of view, in May/June, there is an important and positive 2m temperature trend at Palma (Mallorca), which is simultaneous and highly correlated with a strong increase in the 500hPa geopotential height. This study analyses this fact as well as the observed tendencies in a wider seasonal and geographical context. We confirm the particularly high correlation between 2m temperature and 500hPa geopotential during the warm months as opposed to the much weaker correlation in winter. This suggests that mechanisms for thermal changes act differently throughout the year in this region. Besides the direct radiative effect, warm season near-surface temperatures are linked to the presence of deep anticyclones, which effectively determine the northern edge of the Hadley cell. Accordingly, the strong warming trend in the area of Palma during the warm months of the year is purportedly related to the poleward extension of the Hadley cell. The fact that May/June shows the highest low-level temperature trend among all bi-monthly series is a common regional feature over a relatively wide area over the Western Mediterranean. Different geographical patterns emerge in other periods of the year. In July/August, the strongest low-level warming area drifts to the east-northeast, towards Ukraine and Russia. Coincidentally, the 500hPa geopotential tendencies show a coherent pattern, with an intense positive trend ridge over the Western Mediterranean area in May/June, and a displacement of this ridge to the north-east in July/August. We show the connection between 500hPa geopotential height and near-surface temperature by means of a multiple lineal regression that attributes half of the local temperature tendency in Palma to the intensification of a 500hPa ridge centred over the Western Mediterranean and surroundings.