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

Abstract

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.

Observed mechanisms activating the recent subpolar North Atlantic Warming since 2016

“…The sea-surface height anomaly map highlighting the 2017–2021 period shows, however, an indication that the low sea-surface height anomaly that dominated the eastern SPNA during the 2012–2016 period has propagated westwards, figure 3 b . This westward propagation of sea-surface height anomalies is thought to be linked to shifting wind patterns and to the cyclonic time-mean ocean circulation in the SPNA, as shown recently by Volkov et al [ 43 ]. The eastern and western boundary are now experiencing anomalous sea-surface height increase and decrease, respectively, that is particularly enhanced at the intergyre boundary, as visible from the difference between the two periods ( figure 3 c ).…”

Spatial and temporal variations in sea surface pCO2 and air-sea flux of CO2 in the Bering Sea revealed by satellite-based data during 2003–2019

“…Due to the large uncertainty of various salinity datasets in the Bering Sea, the input data produced low-quality results, so we removed the salinity parameter. The SSH usually contains contributions from high-and low-frequency changes in steric (temperature and salinity) signals and ocean circulation (Meyssignac and Cazenave, 2012;Zhou et al, 2012;Volkov et al, 2022). Here, we used SSH to relatively optimize the performance of our algorithm in the lack of high-quality salinity data.…”

What Forcing Mechanisms Affect the Interannual Sea Level Co‐Variability Between the Northeast and Southeast Coasts of the United States?

“…As the Florida Current becomes the Gulf Stream and flows offshore south of Cape Hatteras, it has less impact on sea-level variations along the U.S. Northeast Coast north of Cape Hatteras than the Southeast Coast. In the North Atlantic, there is an out-of-phase variability between the subtropical and subpolar gyres, which is part of the so called North Atlantic sea surface height (SSH)/sea surface temperature (SST) tripole (e.g., Volkov et al, 2019Volkov et al, , 2022Volkov et al, , 2023. Similar to Frederikse et al (2017) for the U.S. Northeast Coast, Volkov et al (2019) showed that sea level along the U.S. Southeast and Gulf Coasts is correlated with steric sea level in the North Atlantic subtropical gyre.…”