Connecting synoptic atmospheric patterns to sea level oscillations at tsunami timescales

1. Connecting synoptic atmospheric patterns to sea level oscillations attsunami timescales Ivica Vilibić, Jadranka Šepić Institute of Oceanography and Fisheries, Split, Croatia • About meteotsunamis • Motivation • The Mediterranean • The World • Perspectives With a help of some French and Croatian undergraduate students.

2. About meteotsunamis • Well, I don’t know how many of you heard about meteotsunamis, destructive long ocean waves in the tsunami frequency band Ciutadella, Balearic Islands, 15 June 2006 Vela Luka, Adriatic Sea, 21 June 1978 The documented World and Med meteotsunamis in 1995 and 2015 Sea level range over frequencies at Fremantle, Australia

3. About meteotsunamis • Meteorological tsunamis are long oceanic waves that have approximately the same spatial and temporal scales as ordinary tsunamis, and can affect coastal regions in a similar destructive way, but are generated not by underwater earthquakes, volcanic explosions or landslides, but by atmospheric disturbances (hurricanes, frontal passages, squall lines, internal atmospheric waves or by jumps of atmospheric pressure). Munk [1962]: ‘…The most conspicuous thing about long waves in the open ocean is their absence’ Tsunami and meteotsunami records Meteotsunamigenic air pressure disturbances Sea level record in Split during the 1978 Vela Luka meteotsunami Tsunami and meteotsunami spectra

4. About meteotsunamis • The story of meteotsunami dynamics in a single graph Šepić et al., Sci Rep, 2015

5. Motivation • Meteotsunamis are strongly connected to specific atmospheric patterns Meteotsunamigenic synoptic pattern during the June 2014 multi-meteotsunami event. • However, there is a question about the generalisation of the meteotsunami concept to all nonseismic sea level oscillations at tsunami timescales (NSLOTT)? • What is NSLOTT contribution to overall sea level extremes? Does the synoptic conditions resemble common patterns?

6. The data • To answer on this questions, we need: • High-frequency (1 min) sea level data • Atmospheric reanalysis fields Fortunately, Intergovernmental Oceanographic Commission (IOC) established the portal a few years ago, where 1-min not checked (real-time) sea level data is available (http://www.ioc-sealevelmonitoring.org), while ECMWF has reanalysis fields (we used ERA Interim) for a long time. Sea level stations with 1-min data of a fair quality

7. The Mediterranean • First, we concentrated to the Mediterranean: • Šepić, J., Vilibić, I., Lafon, A., Macheboeuf, L., Ivanović, Z., 2015. High-frequency sea level oscillations in the Mediterranean and their connection to synoptic patterns. Progress in Oceanography, 137, 284-298. Percentage of total sea level variance explained (%) Used stations and their grouping

8. The Mediterranean • Common synoptic patterns have been found for all Mediterranean NSLOTT events, favouring the generation of strong sea level oscillations and propagating from the west to the east. • Thus, a forecast of the Mediterranean NSLOTT is allowed for. HF sea level oscillations Synoptic patterns and NSLOTT propagation Satellite imagery

9. The World • Then, we asked ourselves what is going on at the world scale: • Šepić, J., Vilibić, I., 2016. Global mapping of nonseismic sea level oscillations at tsunami timescales. Scientific Reports, under review. Pippi Långstrump painting, about NSLOTT variance and range within the total sea levels.

10. The World • There is THE correlation between zonal NSLOTT variance and range, and zonal winds at 500-hPa • That implies a predominant atmospheric mechanism responsible for the NSLOTT, already found in the meteotsunami studies and called wave-ducting! Mean wind speeds at 500-hPa level, and NSLOTT variances and maximum ranges (the size of a circle is proportional to the number of tide gauge stations in the respective zonal belt)

11. The World • There are common synoptic patterns for NSLOTT events at certain locations, but not always perfect – the picture seems far from a simple ... Mean sea level pressure (MSLP) and surface winds; temperature at 850-hPa level; winds at 500-hPa level; number of times during which minimum Richardson number between 400 and 700 hPa was lower than 0.25 at each grid point; all averaged (counted) over the 15 strongest NSLOTT events at (from left to right) Esperance (Australia), Nagasaki (Japan), Clearwater Beach (the Gulf of Mexico), and Bahia Mansa (Chile) stations.

12. Perspectives • This is a promising field in sea level research, as not much studies on nonseismic sea level oscillations at tsunami timescales have been conducted so far • Going to regional studies for different parts of the world • Going to quantifying of the source mechanism • Going towards qunatitative relationship between the source and the NSLOTT • Going towards an assessment of NSLOTT oscillation in the future climate • Going towards the NSLOTT forecast