North Atlantic atmospheric and ocean inter-annual variability over the past fifty years – Dominant patterns and decadal shifts

• Published in: Progress in oceanography 2015-03, Vol.132, p.197-219
• Main Authors: Hauser, Tristan, Demirov, Entcho, Zhu, Jieshun, Yashayaev, Igor
• Format: Article
• Language: English
• Subjects: Anomalies; Atmospherics; Climatology; Heat flux; Heat transfer; Marine; Oceans; Phases; Weather
• ISSN: 0079-6611; 1873-4472
• DOI: 10.1016/j.pocean.2014.10.008

•Analysis of subseasonal and interannual atmospheric variability.•Probabilistic classification of atmospheric circulation patterns.•Probabilistic description of decadal shifts in atmospheric circulation.•Analysis of ocean response to decadal shifts in atmospheric circulation.•Study of the mechanism of past multidecadal climate variability in the North Atlantic. The atmosphere and ocean of the North Atlantic have undergone significant changes in the past century. To understand these changes, their mechanisms, and their regional implications requires a quantitative understanding of processes in the coupled ocean and atmosphere system. Central to this understanding is the role played by the dominant patterns of ocean and atmospheric variability which define coherent variations in physical characteristics over large areas. Cluster analysis is used in this article to identify the patterns of the North Atlantic atmospheric variability in the subseasonal and interannual spectral intervals. Four dominant subseasonal weather regimes are defined using Bayesian Gaussian mixture models. All correlation patterns of the Sea Level Pressure (SLP) anomalies with the membership probability time series for the weather regimes show similarities with the dipole structure typical for the North Atlantic Oscillation (NAO). The SLP patterns of two of the regimes represent the opposite phases NAO+ and NAO−. The two other weather regimes, the Atlantic Ridge (AR) and Scandinavian-Greenland dipole (SG), have dipole spatial structures with the northern and southern centers of action shifted with respect to the NAO pattern. These two patterns define blocking structures over Scandinavia and near the southern tip of Greenland, respectively. The storm tracks typical for the four regimes resemble the well known paths for positive/negative phases of NAO for the NAO+/NAO− weather regimes, and paths influenced by blocking off the south Greenland tip for AR and over Scandinavia for SG. The correlation patterns of momentum and heat fluxes to the ocean for the four regimes have tripole structures with positive (warm) downward heat flux anomalies over the Subpolar North Atlantic (SPNA) for the NAO− and the AR and negative heat flux anomalies over the SPNA for the NAO+. The downward heat flux anomalies associated with the SG are negative over the Labrador Sea and positive over the eastern SPNA. The long term impact of the weather regimes on the regional climate is characterized by their distributio