Interannual to decadal sea level variability in the subpolar North Atlantic: the role of propagating signals

Abstract. The gyre-scale, dynamic sea surface height (SSH) variability
signifies the spatial redistribution of heat and freshwater in the ocean,
influencing the ocean circulation, weather, climate, sea level, and
ecosystems. It is known that the first empirical orthogonal function (EOF)
mode of the interannual SSH variability in the North Atlantic exhibits a
tripole gyre pattern, with the subtropical gyre varying out of phase with
both the subpolar gyre and the tropics, influenced by the low-frequency
North Atlantic Oscillation. Here, we show that the first EOF mode explains
the majority (60 %–90 %) of the interannual SSH variance in the Labrador and
Irminger Sea, whereas the second EOF mode is more influential in the
northeastern part of the subpolar North Atlantic (SPNA), explaining up to
60 %–80 % of the regional interannual SSH variability. We find that the two
leading modes do not represent physically independent phenomena. On the
contrary, they evolve as a quadrature pair associated with a propagation of
SSH anomalies from the eastern to the western SPNA. This is confirmed by the
complex EOF analysis, which can detect propagating (as opposed to
stationary) signals. The analysis shows that it takes about 2 years for sea
level signals to propagate from the Iceland Basin to the Labrador Sea, and
it takes 7–10 years for the entire cycle of the North Atlantic SSH tripole
to complete. The observed westward propagation of SSH anomalies is linked to
shifting wind forcing patterns and to the cyclonic pattern of the mean ocean
circulation in the SPNA. The analysis of regional surface buoyancy fluxes in
combination with the upper-ocean temperature and salinity changes suggests a
time-dependent dominance of either air–sea heat fluxes or advection in
driving the observed SSH tendencies, while the contribution of surface
freshwater fluxes (precipitation and evaporation) is negligible. We
demonstrate that the most recent cooling and freshening observed in the SPNA
since about 2010 were mostly driven by advection associated with the North
Atlantic Current. The results of this study indicate that signal propagation
is an important component of the North Atlantic SSH tripole, as it applies
to the SPNA.
