Connections between the Long-Period Variability Modes of Both Temperature and Depth of the Upper Mixed Layer of the North Atlantic and the Climate Variability Indices

Abstract

The interannual–multidecadal variability of the temperature and depth of the upper mixed layer (UML) in the North Atlantic (NA) is analyzed on the basis of the ORA-S3 ocean reanalysis data for 1959–2011. A large part of the UML in the NA is characterized by warming and thinning in all seasons in the period under study. After removing a linear trend, the UML temperature and depth anomalies in individual seasons are decomposed into empirical orthogonal functions (EOF). It is found that the three leading EOFs describe more than 50% of the total variability of the UML temperature and depth. The structure of the first EOF is horseshoe-shaped; this EOF represents coherent changes in the UML temperature and depth throughout the NA, which manifest themselves the year round. This mode corresponds to the Atlantic multidecadal oscillation. The spatial structure of the second EOF in the winter–spring period is a tripole and is caused by the North Atlantic Oscillation (NAO). The time coefficient of the second EOF of the UML temperature in the NA and the NAO index strongly correlate both synchronously and when the NAO index is 11 years ahead. The second EOF of the UML temperature in the summer–autumn period is associated with the Atlantic meridional mode. The third EOF is typical for the UML temperature fluctuations in January and corresponds to the East Atlantic Pattern.