Mechanisms of winter precipitation variability in the European-Mediterranean region associated with the North Atlantic Oscillation

• Main Authors: Seager, Richard, Liu, Haibo, Kushnir, Yochanan, Osborn, Timothy J., Simpson, Isla R., Kelley, Colin R., Nakamura, Jennifer A.
• Format: Text
• Language: unknown
• Published: Columbia University 2020
• Subjects: Rain and rainfall; Hydrometeorology; North Atlantic oscillation; Climatic changes; North Atlantic
• Online Access: https://dx.doi.org/10.7916/d8-0w54-qq43; https://academiccommons.columbia.edu/doi/10.7916/d8-0w54-qq43

The physical mechanisms whereby the mean and transient circulation anomalies associated with the North Atlantic Oscillation (NAO) drive winter mean precipitation anomalies across the North Atlantic, Europe and the Mediterranean are investigated using the European Centre for Medium Range Weather Forecasts Interim Reanalysis. A moisture budget decomposition is used to identify the contribution of the anomalies in evaporation, the mean flow, storm tracks and the role of moisture convergence and advection. Over the eastern North Atlantic, Europe and the Mediterranean, precipitation anomalies are primarily driven by the mean flow anomalies with, for a positive NAO, anomalous moist advection causing enhanced precipitation in the northern British Isles and Scandinavia and anomalous mean flow moisture divergence causing drying over continental Europe and the Mediterranean region. Transient eddy moisture fluxes work primarily to oppose the anomalies in precipitation minus evaporation generated by the mean flow but shifts in storm track location and intensity help explain regional details of the precipitation anomaly pattern. The extreme seasonal precipitation anomalies that occurred during the two winters with the most positive (1988/89) and negative (2009/10) NAO indices are also explained by NAO-associated mean flow moisture convergence anomalies.