Spatio-temporal variability of eddies in the vicinity of the Gulf Stream

Abstract

The atmospheric mass distribution over the North Atlantic generates a latitudinal pressure gradient over the North Atlantic. This pressure gradient fluctuates around a reference value. This phenomenon is called the North Atlantic Oscillation (NAO) and is a cause of disturbance of the Gulf Stream (GS). Several studies demonstrated that there is a correlation between the strength of the NAO and the position and transport of the GS.

In this paper, we studied the impact of mesoscale eddies on the NAO-GS correlation. To do so, we took back the output of sea surface height (SSH) from the DRAKKAR project. This output consists of the 5-days average SSH from the year 1970 to 2015. Thanks to the SSH, we detected and tracked all the eddies in the domain [70 W, 30 W] X [30N, 50N] and got their spatio-temporal variations. The tracking procedure was computed using an existing software called "py-eddy-tracker".

The characteristics of the eddies have been analyzed and compared to those from a previous study called "Global observations of nonlinear mesoscale eddies" by Chelton et al. . By analyzing characteristics such as the distances covered by the eddies, we had a first hint that the eddies do not carry the energy from the NAO signal to the GS. Then we selected the eddies that could potentially carry the signal from the NAO to the GS and compared their characteristics to a NAO index that represents the strength of the pressure gradient. To do so, we compared the eddies to the East of the domain to the NAO signal and found the correlation coefficients between the NAO index and the different characteristics of the eddies. It appeared from this comparison that the NAO signal does not affect these eddies.\\ Then we compared the NAO signal to the eddies that are to the West of the domain, near the GS. The comparison was made by considering a time lag of 0, 1 and 2 years between the NAO signal and the characteristics of the eddies. Here again, the correlation coefficients were insignificant. Finally we compared the eddies to the GS position, intensity and transport using 3 indices that were computed in "Reconstruction of the Gulf Stream from 1940 to the Present and Correlation with the North Atlantic Oscillation" from Watelet et al. . The results showed that the eddies are likely to be correlated to the GS. This meant that they are generated by the GS instead of being linked to the NAO. This study enhanced the assumption that the energy is transferred from the NAO to the GS through higher scales phenomena such as Rossby waves.