Altimetry of the Arctic Ocean and Subpolar Seas: Ocean Surface Topography Science Team Report: 2016-2020

Sinead Farrell (University of Maryland, United States)

CoAuthors

Kyle Duncan (University of Maryland, USA); John Kuhn (NOAA/NESDIS/STAR Laboratory for Satellite Altimetry, USA)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Salient results from the 2017-2020 OSTST PIs

Presentation type: Type Forum

Our knowledge of sea surface topography is limited in the Arctic Ocean and subpolar seas, due to the constant presence of sea ice. As a result of the permanent ice cover, knowledge of the mean sea surface and bathymetric features of the Arctic Ocean is limited, as is detailed knowledge of geostrophic circulation, and its variability. Knowledge of sea surface height (SSH) is critical for deriving sea ice freeboard, and hence ice thickness from satellite altimeter data, where freeboard is defined as the difference between sea ice elevation and the local SSH. Beyond this, measuring SSH in the polar oceans provides a means of conducting a number of geodetic and oceanographic investigations such as monitoring variability in dynamic ocean topography, changes in significant wave height, and the impacts of increasing storms on the marginal ice zone. In support of the NOAA/NASA Ocean Surface Topography Science Team (OSTST), we completed a number of critical studies of the Arctic Ocean and subpolar seas during the project period 2016-2020. Under this project we analyzed Envisat, ICESat, CryoSat-2, SARAL/AltiKa and Sentinel-3A/B altimeter data in the Arctic Ocean, as well as Jason-1,-2, and -3 measurements in the ice-free subpolar seas south of the marginal ice zone.
First, we conducted an analysis of the existing, state-of-the-art mean sea surface (MSS) models for the Arctic Ocean to understand the impact of remaining errors and omissions on the derivation of cryospheric and oceanic parameters, such as freeboard and mean dynamic topography. We found that a major advancement in mapping the Arctic Ocean MSS was a result of including novel CryoSat-2 radar altimeter measurements of sea surface height (Skourup et al., 2017). The latest MSS models, that incorporate CryoSat-2 sea surface height measurements, show improved definition of features of the marine gravity field. Furthermore, the inclusion of CryoSat-2 data beyond the previously available ICESat and Envisat data, is important since it extends the coverage of the MSS field north from 86oN to 88oN. We found that sea ice freeboard retrievals, derived from both airborne/satellite altimeters, are impacted by the choice of MSS model used in the retrieval algorithm (Skourup et al., 2017). Although widely used in previous studies, the EGM2008 geoid is not recommended for use in the freeboard retrieval algorithm, since it results in large freeboard errors across the central Arctic Ocean.
Second, as sea ice extent in the Arctic has diminished over the past several decades, we investigated if wave heights increased and if the subpolar regions have become stormier. Our study assessed significant wave height (SWH) just south of the ice edge in the Bering Sea. We collated all available daily radar altimeter measurements over a 21-year period spanning 2000-2020 to characterize the seasonal cycle, decadal trends, and inter-annual variability in SWH. SWH in the Bering Sea is dominated by interannual variability, where mean (modal) SWH is 3.08 m (2.25 m) in winter. Over the study period 1% of observations showed SWH > 7.5 m. Phenomenal sea states (SWH > 14 m) in the Bering Sea in winter are common. There were ten phenomenal sea state events during the study period, and the largest SWH recorded in the Bering Sea, measuring 15.2 m, occurred on December 14, 2015. The majority (90%) of these phenomenal sea state events have occurred in the last decade. Extremely stormy winter seas were defined as those with SWH > 9 m. Winter storms increased in the Bering Sea during the 20-year study period.
Third, we provided critical mission support and scientific guidance to two key polar altimeter missions: The NASA ICESat-2 Mission, and the ESA/EU Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) Mission. Long term, uninterrupted observations of the polar oceans, including sea ice and ice-shelf elevation, are of utmost importance for climate and sea-level related scientific studies, and provide critical support to operational ice services and decision-makers (Kern et al., 2020). Continuation of this vital time series is dependent of the successful, and timely, launch of the ESA/EU Copernicus CRISTAL mission, since there is no other proposed, planned or approved altimetric mission to observed beyond 81.5° N/S.
Fourth, to enable data discovery, easy access, and broader usage of high-latitude satellite data products, we partnered with the NOAA Southwest Fisheries Science Center to develop NOAA PolarWatch. PolarWatch (polarwatch.noaa.gov) is a new node of the NOAA CoastWatch/OceanWatch program. It delivers multi-sensor physical and biological ocean remote sensing data in support of broad applications in the Arctic and Southern Oceans. It includes an ability to search and filter satellite datasets, providing data previews in polar-stereographic projection.
 
Sinead Farrell
University of Maryland
United States
sinead.farrell@noaa.gov