Thermohaline Structure of Waters in the North Atlantic in Different Phases of the Atlantic Multidecadal Oscillation

Abstract

The meridional structure of climatic trends and anomalies of potential temperature and salinity in the North Atlantic waters in different periods of the Atlantic Multidecadal Oscillation (AMO) in 1948–2017 are studied based on the EN4 and WOA2013 objective analyses data. An analysis of these different data sets allowed us to reveal almost identical patterns of variability of the thermohaline fields of the North Atlantic, which increases the reliability of the results. Long-term temperature and salinity trends calculated over the period 1948–2017 show that warming and salinization of water occur in the upper ~1 km layer of the North Atlantic. On the contrary, cooling and freshening of deep waters are observed, which is associated with the melting of the Greenland ice sheet, transport of fresher waters from the Arctic Ocean, and deepening of these cold and fresher waters into the deeper layers. Composite analysis of the zonally averaged temperature and salinity anomalies of the North Atlantic waters after removing the trends showed that in the warm AMO periods warming and salinization of waters are observed in the upper 1-km layer of the North Atlantic when compared to the cold periods based both on the EN4 and WOA2013 data. Below the 1-km layer, significant regions of cooling and freshening are observed; this distribution is more pronounced in the EN4 data. Analysis of the dynamics of zonally averaged temperature and salinity anomalies in the successive periods associated with the temporal variability of the AMO index revealed that these anomalies propagate along the zonally averaged meridional thermohaline circulation. To show this using the Institute of Numerical Mathematics Ocean Model (INMOM), the stream function of the Atlantic Meridional Overturning Circulation (AMOC) was simulated. It is shown that positive and negative anomalies of both temperature and salinity circulate along the water motion in the AMOC around its core, descending down into the deep ocean layers approximately at 60° N and ascending to the surface at 25° N, replacing each other with a period of about 60 years. It can be assumed that due to this process both the warm and cold phases of the AMO are formed.