High-frequency sea-level extremes: Global correlations to synoptic atmospheric patterns

Abstract

With the increase of available global observations at a minute resolution, emerging studies of high-frequency sea-level oscillations have already produced climatologies and explored the atmospheric processes driving the related flooding events, in a qualitative manner and/or at a very local scale. In the presented work, however, the global connections between nonseismic sea-level oscillations at tsunami timescales (NSLOTTs) and synoptic patterns are quantified for 307 tide gauge sites around the world. A global index, constructed for each site over the whole data interval as the linear combination of a prescribed set of synoptic variables extracted from the ERA5 atmospheric reanalysis, is correlated with hourly-averaged NSLOTTs. The correlation between this index and the NSLOTT ranges varies between sites but is generally higher at mid-latitudes and particularly in the Mediterranean Sea where correlations reach 0.8. Mid-troposphere wind speed is found to be the best correlated synoptic variable at roughly one third of all sites together with the mid-troposphere relative humidity. Consequently, wave-ducting is believed to be the dominant mechanism for the NSLOTT propagation and longevity. For such conditions, extreme NSLOTT events are accompanied, on average, by a very specific synoptic pattern including mid-troposphere jet streams, low-troposphere thermal fronts and poleward surface cyclones, independently of the hemisphere or the site and despite some differences in intensity. Following this study, in regions with high correlations, various applications of the index - from climate studies to operational forecasts - could lead to a broader understanding of the NSLOTT events around the world.