Elements of the dynamical response to climate change over the Mediterranean

Future climate simulations indicate that the Mediterranean Basin will experience large low-level circulation changes during winter, characterized by a strong anomalous ridge that drives a regional precipitation decline. Previous research highlighted how shifts in stationary wave structure and the atmospheric response to reduced warming of the Mediterranean Sea compared to land could explain the development of this anomalous pressure high. Here, we expand on these results and provide new arguments for why and how the Mediterranean is projected to experience large circulation changes during winter. First, we find that zonal asymmetries in the vertical structure of stationary waves are important to explain the enhanced circulation response in the region, and that these asymmetries are related through the external mode to the vertical structure of the mean zonal wind. Second, in winter, the Mediterranean is located just to the north of the Hadley cell edge and consequently relatively free of large-scale descent; together with low near-surface static stability above the sea, this allows the weaker warming trend above the sea to propagate to the low troposphere and trigger a major circulation response. During summer, however, remotely-forced descent and strong static stability prevent the cooling anomaly from expanding upwards. Most of the inter-model scatter in the projected low-level circulation response is related to the spread in upper-tropospheric dynamical trends. Importantly, because climate models exhibit too much vertical coherence over the Mediterranean, our results suggest they overestimate the sensitivity of Mediterranean near-surface circulation to large-scale dynamical changes.