How does the Mediterranean’s location dictate its dynamical response to climate change?

Climate models project the development of a future intense anomalous ridge over the Mediterranean Basin during winter that largely drives the precipitation decline for the upcoming decades. This ridge can be understood as the consequence of shifts in stationary wave structure and the atmospheric response to reduced warming of the Mediterranean Sea compared to land. However, two important dynamical aspects of the response remain unclear. First, why is the low-level response to the stationary wave changes particularly large in the Mediterranean compared to other regions? To answer this question, we use the concept of the external mode to argue that the strength of the low-level stationary wave response is inversely related to the climatological vertical wind shear. The Mediterranean during winter is characterized by relatively weak vertical shear, being situated to the east of the North Atlantic jet and to the north of the subtropical jet over Africa. Second, while the reduced warming rate of Mediterranean SSTs occurs year-round, why don’t models project anomalous high pressure in summer as well? We find that the reduced warming propagates upwards into the atmosphere to drive a circulation response in winter but not summer, and this is because of high static stability over the relatively cold water in summer and the presence of remotely forced descent from the Indian monsoon. Thus the location of the Mediterranean plays a key role for the both the seasonality and strength of the low-level circulation response.nn