Estimating Relative and Absolute Sea Level Rise at

Estimating Relative and Absolute Sea Level Rise at Ny-Ålesund with satellite altimetry and in-situ observations Stefano Vignudelli 1 and Francesco De Biasio 2 1 National Research Council of Italy - Institute of Biophysics, Pisa, Italy 2 National Research Council of Italy - Institute of Polar Sciences, Venice, Italy We compare sea level measurements from the Ny-Ålesund tide gauge with the climate -oriented, altimeter-derived sea level, Copernicus Climate Change Service (C 3 S) gridded product and with the along track data from the only Cryo. Sat-2 mission 30/04/2021 Objectives: • to verify if the differences between satellite radar altimetry observations and tide gauge measurements can be used as a proxy of vertical ground movement (as in Vignudelli et al. [2018] and De Biasio et al. [2020]) • validation of the results with ground vertical displacements estimated using Global Positioning System (GPS) data from the stations close to NyÅlesund • to explore how the synergy with along track high resolution Cryo. Sat-2 data might help to detail the sea ice impact on the observations of the relative and absolute sea level rise around Svalbard • to assess the performances of the existing gridded data moving from offshore to near coasts EGU 2021 1

Ny-Ålesund (78° 55’ N, 11° 56’ E), Spitzbergen Island (Svalbard), is a hot spot for the Arctic Amplification: the Arctic is warming twice as fast as the global average. Dedicated observations from polar satellite altimetry (Cryo. Sat-2) Long time series of sea level (SL) from tide gauge (TG) Long time series of vertical land motion from GPS and DORIS stations collocated with the TG 30/04/2021 EGU 2021 2

Ny-Ålesund • Blue box: observations of along-track CS -2 are here • Orange circles: grid point of C 3 S SLA dataset for comparison • Red triangle: AVISO DAC has been picked here to correct tide gauge SL for comparison with altimetry SLA • Yellow circle: Ny-Ålesund tide gauge and GPS/DORIS location 30/04/2021 EGU 2021 3

Relative Sea Level Rise (RSLR) at Ny-Ålesund • Tide gauge operational since 1976 • Available observations sampled every 10‘ (Kartverket, Norway) 1992 -ongoing • monthly means from PSMSL 1 1976 -ongoing • Monthly means derived from 10‘ data 1992 ongoing 1. PSMSL: Permanent Service for Mean Sea Level 30/04/2021 EGU 2021 4

Global Positioning System at NyÅlesund • Gps NYA 1 • Gps NYAL • Doris • the terrain is moving up at around 9 mm/y, much more than the estimated GMSL rise (3. 4 mm/y) Is the some acceleration in the last years ? 30/04/2021 EGU 2021 5

Cryo. Sat-2 at Ny-Ålesund • High revisiting rates from satellite altimetry missions • Along–track data from Cryo. Sat-2 SAR altimetry mission (2010 -ongoing) processed by ESA@ • The temporal sampling is irregular in the Svalbard coastal zone, as there is switching from SAR to SARIN mode 30/04/2021 EGU 2021 6

Copernicus C 3 S gridded SLA daily mean Copernicus C 3 S gridded dataset of (daily) SLA: seasonal SLA field 1993 -2020. • Global coverage • Resolution ¼ of a degree • Dual mission • 1993 -ongoing • Climate study - oriented dataset 30/04/2021 C 3 S SLA Climatological fields February-March EGU 2021 August. September 7

To derive VLM from Altimetry SLA and tide gauge RSLR The VLM can be calculated as: VLMsea level = ASLR-RSLR from satellite altimetry and tide gauge observations, as well as with direct measurements of GPS (VLMgps) • The VLM calculated from the difference of the time series of altimetry absolute sea level and tide gauge relative sea level is in agreement with that observed by GPS • We found: VLMsea level – VLMgps = 1. 6 mm/y • C 3 S and Cryo. Sat-2 VLMsea level in good agreement (using only collocated observations for all datasets!!) 30/04/2021 Source Altimetry ASLR Trends (mm/y) C 3 S gridded 5. 61 ± 3. 22 CS 2 along track 5. 21 ± 5. 09 Tide gauge RSLR Monthly means from -5. 67 ± 3. 40 10’ observations (NY 2) VLMsea level ALT-TG (ASLRRSLR) ALT C 3 S-TG 11. 28 ± 2. 32 ALT CS 2 -TG 10. 89 ± 4. 53 GPS NYA 1 9. 28 ± 0. 28 VLMgps Δ = 10. 89 - 9. 28 ~ 1. 61 mm/y EGU 2021 8

Cryo. Sat-2 along track 20 Hz @gpod (ESA) • To explore how the synergy with along track high resolution Cryo. Sat-2… • SLA in the coastal zone potentially degraded as land enters radar footprint • Tracks are not orthogonal to the land. Reduced improvement due to SAR mode 30/04/2021 EGU 2021 9

Cryo. Sat-2 along track 20 Hz @gpod (ESA) To explore how the synergy with along track high resolution Cryo. Sat-2 data might help to detail the sea ice impact on the observation of relative and absolute sea level rise around Svalbard • Peakiness is quite low in this area all year round: sea ice barely present 30/04/2021 EGU 2021 10

Cryo. Sat-2 along track 20 Hz @gpod (ESA) • To explore how the synergy with along track high resolution Cryo. Sat-2… • Importance of data editing 30/04/2021 EGU 2021 11

Cryo. Sat-2 along track 20 Hz @gpod (ESA) • To explore how the synergy with along track high resolution Cryo. Sat-2… • SLA vs dist 2 coast: higher variability in the last 5 km, with exceedingly high values, and high negative variability from 6 km and up • SLA vs Pulse_Peakiness: high variability below 0. 1 peak. 30/04/2021 EGU 2021 12

Cryo. Sat-2 along track 20 Hz @gpod (ESA) • Cryo. Sat-2 has better coverage with respect to traditional altimeters, as its orbit drifts with a period of 369 days, thus tracks are spread on wider areas • 20 Hz high frequency data permit the sampling of narrow straits otherwise invisible to 1 Hz traditional altimetry, going closer to coast with few data lost 30/04/2021 EGU 2021 13

To assess the performances of the gridded data moving from offshore to near coasts • to assess the performances of the gridded data moving from offshore to near coasts, we have calculated the ASL trend in six equal latlon boxes at increasing distance from the coast (25123 km). Trends sligthly increase with increasing distance 30/04/2021 EGU 2021 Distance to coast of C 3 S area (km) 1993 -2020 trend (mm/y) 123. 2 2. 21 ± 0. 82 102. 5 2. 06 ± 0. 75 82. 1 1. 95 ± 0. 73 62. 0 1. 79 ± 0. 72 42. 9 1. 62 ± 0. 79 25. 5 1. 60 ± 0. 92 14