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

The gyre-scale, dynamic sea surface height (SSH) variabilitysignifies the spatial redistribution of heat and freshwater in the ocean,influencing the ocean circulation, weather, climate, sea level, andecosystems. It is known that the first empirical orthogonal function (EOF)mode of the interannual SSH variability in the North Atlantic exhibits atripole gyre pattern, with the subtropical gyre varying out of phase withboth the subpolar gyre and the tropics, influenced by the low-frequencyNorth Atlantic Oscillation. Here, we show that the first EOF mode explainsthe majority (60 %-90 %) of the interannual SSH variance in the Labrador andIrminger Sea, whereas the second EOF mode is more influential in thenortheastern part of the subpolar North Atlantic (SPNA), explaining up to60 %-80 % of the regional interannual SSH variability. We find that the twoleading modes do not represent physically independent phenomena. On thecontrary, they evolve as a quadrature pair associated with a propagation ofSSH anomalies from the eastern to the western SPNA. This is confirmed by thecomplex EOF analysis, which can detect propagating (as opposed tostationary) signals. The analysis shows that it takes about 2 years for sealevel signals to propagate from the Iceland Basin to the Labrador Sea, andit takes 7-10 years for the entire cycle of the North Atlantic SSH tripoleto complete. The observed westward propagation of SSH anomalies is linked toshifting wind forcing patterns and to the cyclonic pattern of the mean oceancirculation in the SPNA. The analysis of regional surface buoyancy fluxes incombination with the upper-ocean temperature and salinity changes suggests atime-dependent dominance of either air-sea heat fluxes or advection indriving the observed SSH tendencies, while the contribution of surfacefreshwater fluxes (precipitation and evaporation) is negligible. Wedemonstrate that the most recent cooling and freshening observed in the SPNAsince about 2010 were mostly driven by advection associated with the NorthAtlantic Current. The results of this study indicate that signal propagationis an important component of the North Atlantic SSH tripole, as it appliesto the SPNA.