Revisiting the Moisture Budget of the Mediterranean Region in the ERA5 Reanalysis

The climate of the Mediterranean region (MedR) is characterized by mild-wet winters and dry-hot summers. The hydrological cycle is highly sensitive to the spatio-temporal characteristics of winter storms as they move into the region, which results in a distinctive and irregular rainfall pattern (D’Agostino and Lionello 2020).

Recent studies have identified the MedR as a climate change “hotspot” (Seager et al. 2014; D’Agostino and Lionello 2020), marked by intensifying temperature and precipitation (P) reduction. This P reduction cannot be simply attributed to the thermodynamic “wet-gets-wetter” response, pointing to the importance of circulation changes. However, exact dynamical mechanisms are yet to be understood. To this end, we revisit the moisture budget in the MedR using the 5^th generation ECMWF reanalysis (ERA5) to: (1) explore how the different terms in the budget contribute to the observed climatological net P (precipitation minus evaporation (E)) patterns in the annual mean over land and sea, (2) assess the role of mean flow and transient eddies, and finally, (3) to expose any trends that might be present in the ERA5 dataset.

Moisture budget analyses reveal that most Mediterranean land areas (e.g., Europe and Turkey) have an excess of P over E. Annually averaged positive net P is particularly strong over high topographic regions (Alps and Balkans). Negative net P over the Mediterranean Sea confirms that, on average, the Mediterranean Sea is an evaporative waterbody.

Positive net P over the Mediterranean land regions is predominantly sustained by the sub-monthly transient eddies converging moisture that originate from the Mediterranean Sea. On the annual average, transient circulations (i.e., storm systems, extratropical cyclones) converge moisture over the Iberian Peninsula as well. In contrast, the annually averaged mean flow diverges moisture over most of the MedR (land and sea), except Adriatic Sea and north-western Africa (Tunisia, northern Algeria).

According to ERA5, net P in the MedR undergoes a significant decreasing trend owing to significant increase in E and a rather steady P in the 1979 – 2020 period. This is in line with previous work which show the projected drying in the MedR is already detectable (Seager et al. 2014). On a global scale, P and E within ERA5 show significant increasing trends during the analysis period. The increase in globally averaged E agrees well with the previous reanalysis product (i.e., ERA- Interim). However, the unrealistic increase in global P from ERA5 remains not well understood (Mayer et al. 2021).

 

References:

• D’Agostino, R., & Lionello, P. (2020). The atmospheric moisture budget in the Mediterranean: Mechanisms for seasonal changes in the Last Glacial Maximum and future warming scenario. Quaternary Science Reviews, 241. https://doi.org/10.1016/j.quascirev.2020.106392
• Mayer, J., Mayer, M., & Haimberger, L. (2021). Consistency and Homogeneity of Atmospheric Energy, Moisture, and Mass Budgets in ERA5. Journal of Climate, 34(10), 3955–3974. https://doi.org/10.1175/JCLI-D-20-0676.1
• Seager, R., Liu, H., Henderson, N., Simpson, I., Kelley, C., Shaw, T., Kushnir, Y., & Ting, M. (2014). Causes of increasing aridification of the mediterranean region in response to rising greenhouse gases. Journal of Climate, 27(12), 4655–4676. https://doi.org/10.1175/JCLI-D-13-00446.1