The Impacts of SST-Nudging on Performance of Community Earth System Model (CESM) in Representing the Euro-Mediterranean Climate

The Mediterranean Basin, including the Mediterranean Sea and the surrounding countries, is referred to as a hotspot in terms of climate change, primarily because of a basin-wide drying trend projected for its future. The Mediterranean Sea plays an important role in the climate of the basin through air-sea interactions, and it is, therefore, important to understand how it is coupled with global as well as regional atmosphere. Coarse resolution fully coupled Earth System Models (ESM) show inaccurate results in terms of sea surface temperature (SST) and precipitation over the Mediterranean Sea and Europe. Better representation of the Mediterranean Sea SST (MedSST) by ESMs is a critical issue for the Euro-Mediterranean climate.

In this study, we conduct three simulations using the fully-coupled Community Earth System Model (CESM): i) a historical control simulation integrated for the 1850-2014 period subject to anthropogenic forcings; ii) a Mediterranean Pacemaker-I (MedP-I) experiment where MedSST is nudged to the monthly Extended Reconstructed SST (ERSST) starting from 1880; and iii) a Mediterranean Pacemaker-II (MedP-II) experiment where the MedSST is nudged to the Optimum Interpolation SST (OISST)  starting from 1980. In both pacemaker experiments, in comparison with the control simulation, nudging of the MedSST affects the poleward energy flux transported by the atmospheric latent and dry heat, and changes the total meridional energy flux by more than ±0.1 PW over lower latitudes. Similarly, net radiation flux at the surface is changed by about ±2 W/m2 over the Mediterranean Basin. The fidelity of the nudging method was investigated by comparing solutions from MedP-I and MedP-II with respective fields from the control simulation and those from observations, i.e., World Ocean Atlas, Hadley Centre Sea Ice and SST, Climate Prediction Center, and European Observations for the 1981 - 2010 period. The control simulation shows higher surface temperatures than observations and overestimates the total precipitation over Euro-Mediterranean and Turkey. In contrast, both MedP-I and MedP-II show improvements in reproducing total precipitation over the Euro-Mediterranean region, Turkey, and at the entrance of the Gibraltar Strait. While MedP-I has improvements over the northeast Europe and the southern Mediterranean Basin regarding the surface temperatures, MedP-II has some improvements over Turkey and at the coastal areas of the Mediterranean Sea. MedP-II has more improvements for the SST and sea surface salinity (SSS) values over the Mediterranean Sea and the Black Sea compared to MedP-I. Additionally, MedP-II has a better representation of the North Atlantic SSS bias compared to the control simulation, while both MedP-I and MedP-II have some SST improvements for different areas over the North Atlantic. Core climate indices defined by the European Climate Assessment and Dataset project are calculated using simulated daily parameters and results are compared with the Global Land Data Assimilation System dataset. Accordingly, MedP-II is found to have improvements over more areas, especially for the indices calculated by using daily precipitation. Overall, we conclude that Mediterranean Sea Pacemaker simulations improve our understanding of how the Mediterranean Sea impacts the surface temperature and precipitation over the Euro-Mediterranean.