Proxy-based assessment of Mediterranean meteotsunamis in present and future climates

1. Proxy-based assessment of strength and frequency of the Mediterranean meteotsunamis in present and future climates Ivica Vilibić1, Jadranka Šepić1 , Natalija Dunić1, Florence Sevault2, Sebastian Monserrat3, Gabriel Jorda3, Paola Vulinović4 1 Institute of Oceanography and Fisheries, Split, Croatia 2 CNRM-UMR3589, CNRS/Météo-France, Toulouse, France 4 University of the Balearic Islands, Palma de Mallorca, Spain 7 Student at Faculty of Science, University of Split, Split, Croatia • About meteotsunamis • Connection to synoptics • Meteotsunami index • Meteotsunamis in future climate • Concluding remarks

2. 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). Vela Luka, Adriatic Sea, 21 June 1978 Ciutadella, Balearic Islands, 15 June 2006 Sea level record in Split during the 1978 Vela Luka meteotsunami Sea level range over frequencies at Fremantle, Australia (courtesy of C. Pattiaratchi) Munk [1962]: ‘…The most conspicuous thing about long waves in the open ocean is their absence’

3. About meteotsunamis • The story of meteotsunami dynamics in a single graph. Šepić et al., Sci Rep, 2015 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

4. About meteotsunamis • Meteotsunami („šćiga”) in Vela Luka, Croatia, on 21 June 1978 Wave heights were up to 6 m, typical period was about 20 min! 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

5. Connection to synoptics • Average of 15 strongest events per station Šepić et al., PO, 2015 • It has been found that the Mediterranean meteotsunamis are occurring during specific synoptic conditions, where wave ducting is maintained by (i) a stable lower troposphere with a dry and warm air moving from subtropical regional towards the poles, overtopped by (ii) instable strong mid-troposphere jet. 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

6. Connection to synoptics • The Mediterranean event of 23-27 June 2014 was connected to a propagating upper troposphere atmospheric storm, which were favourable for generation of ducted intense air pressure oscillations. Šepić et al., Sci. Rep., 2015 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

7. Meteotsunami index • That allows for quantification of the connection and construction of the meteotsunami index. • Done for the Balearic Islands (Šepić et al., GRL, 2016). meteotsunami index  regression coefficients are obtained through least-squares applied on the high-frequency sea level wave height data • The 95 percentile in the index can be used as a threshold for tsunamigenic synoptic conditions, while only 20% of the conditions (20 days) are collocated with moderate and strong meteotsunamis (4 events > 100 cm). 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

8. Meteotsunami index • That partially saves the day, as mesoscale meteorological models are still far from reliable forecasting of meteotsunamigenic air pressure disturbances • 18:06 B C Skaramock, 2004 Boothbay Harbor (ME,USA) meteotsunami of 28 Oct 2008 X Horvath and Vilibić (NH, 2014): ”The success of the WRF modelappears limited to very short-range forecast, most advancedparameterizations, and veryhigh-resolution grid spacing.” 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

9. Meteotsunamis in future climate Construction of the synoptic meteotsunami index for future climate: • Verification of present climate simulation (HIST, MED-11_CNRM-CM5 historical run) on evaluation simulation (EVAL, ERAINT_evaluation run) and ERA Interim reanalysis (ERA). • Computation of the index for RegCM future scenarios (MED-11_CNRM-CM5 RCP2.6, MED-11_CNRM-CM5 RCP4.5 and MED-11_CNRM-CM5 RCP8.5 scenario runs). 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

10. Meteotsunamis in future climate Verification of the index: • Synoptic patterns (winds at 500 mb, inflow of warm air at 850 hPa) are preserved • Seasonal cycle is preserved • Known meteotsunami events in Ciutadella (the Balearic Islands) are associated with very high synoptic meteotsunamic index (mostly above 95 percentile) 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

11. Meteotsunamis in future climate Trends of the index: • Not significant for RCP2.6 and RCP4.5 scenarios. • Significant increase in RCP8.5 scenario: The increase in number of yearly days with tsunamigenic synoptic conditions from 20 in 2000s to 25 in 2090s. • The increase is largest during the summer (May-August), when destructive meteotsunamis are occurring the most. 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

12. Meteotsunamis in future climate Properties of future meteotsunamis: • A small change in seasonal cycle: RCP4.5 peak moved from July to August, an increase in spring peak (RCP8.5), ... • The least change in number of days with meteotsunamis may be found in the wintertime (September-March). • The 2071-2100 vs. HIST increase in the total number of days with meteotsunamis in the RCP2.6 and RCP4.5 scenarios is not larger than 12%. • The total number of days with meteotsunamis in the RCP8.5 scenario is larger for 13%, 17% and 32% than in the HIST run in 2011-2040, 2041-2070 and 2017-2100, respectively. 11th HyMeX Workshop Lecce, 29 May – 2 June 2018

13. Concluding remarks • Meteotsunamis are highly mesoscale phenomenon, still far from proper reproduction of their atmospheric properties on the climate (multi-decadal) timescale. • They are highly correlated with specific synoptic conditions (at least on the Balearic Islands), allowing for construction of a proxy: synoptic meteotsunami index. • The index has been estimated for future climate scenarios. • No significant changes have been found for RCP2.6 and RCP4.5 scenarios, but 30% increase of meteotsunamigenic synoptic conditions is foreseen in the RCP8.5 scenario, concentrated in summer months when meteotsunamis are known to occur. Future steps: • Extending analyses for other Mediterranean and World Ocean areas. • Using an ensemble of climate models, to allow for uncertainty estimations. • Using the methodology for closing sea level budget, which do not consider high-frequency sea level oscillations. 11th HyMeX Workshop Lecce, 29 May – 2 June 2018