

        {\section*{Definitions}}
        \emph{MLD}: mixed layer depth defined with density threshold\\
        \emph{MLD series}: MLD average series in the subpolar gyre box\\ (see Figure 1)\\
        \emph{sigma2}: Potential density with reference pressure of 2000 dbar\\
        \emph{tos}: Ocean surface temperature\\
        \emph{sos}: Ocean surface salinity