A new regional mask for Arctic sea ice trends and climatologies Walter N. Meier and J. Scott Stewart [email protected] New 2021 Arctic region mask History of old region masks Introduction Sea of Overall, Arctic sea ice is declining and Antarctic sea ice has a Bohai Japan Sea Original NASA Goddard mask near-zero trend over the past 40+ years. However, there are Pacific Ocean China • Derived in mid-1980s by 1987 distinct regional variations in these trends. Regional masks were researchers at NASA Goddard developed to investigate these trends, but the masks were created • Modified in mid-1990s in a non-rigorous fashion for defined grids at low spatial resolution; Sea of • Derived on 25 km polar and there are inconsistencies in masks, even within products at Okhotsk stereographic grid NSIDC. Here we present an improved, more accurate regional • Based on accepted general 1995 mask based on accepted standards. definitions, but boundaries are not exact Bering • No coastal sea regions within Key features of new region mask Sea Siberia Arctic Ocean • Based off of International Hydrographic Office (IHO) • Combined Kara/Barents Sea Laptev region definitions of seas – modified for use with sea ice, e.g.: East Siberian Sea Sea • Parkinson et al., JGR, 1999 • Beaufort Sea northern border at 76° N latitude, at southwest coast of Chukchi Sea St. Patrick Island Kara Alaska Sea Russia • Beaufort Sea northern border extends west at constant latitude Beaufort Central instead of diagonally across to Point Barrow as in IHO definition Sea Arctic Barents Modified NSIDC mask Canada Sea Baltic • Regions defined initially as latitude/longitude vertices and Gulf of Sea • Uses Goddard mask as its Alaska mapped into GIS shapefiles foundation Scandinavia • Subdivides some Goddard • Uses lines of constant latitude and longitude where reasonable East • Consistent, documented rules for connecting vertices and mapping Greenland regions into new regions Canadian Greenland Sea Baffin • Separate Kara and Barents shapefile onto grids Archipelago Bay • Arctic Ocean split into 2007 • Flexible to facilitate gridded to different projections, spatial resolutions Beaufort, Chukchi, East Hudson • Region shapefile polygons drawn to overlap land to allow Bay Siberia, Laptev (coastal seas) Canada & use of different land masks Europe • Northern boundary of coastal Labrador seas are arbitrary, drawn as • Consistency with previous region mask retained where it Sea straight lines in the projection makes sense Canada • Other regions and boundaries • New regions added for greater flexibility and broader Atlantic Ocean same as Goddard mask applications • Meier et al., Ann. Glaciol., 2007 USA • Gulf of Alaska, Gulf of Bothnia, Bohai Sea, Sea of Japan Gulf of St. Lawrence MASIE mask Example: Beaufort Sea Beaufort Sea September Extent, 1979-2019 • Derived to support NSIDC Multi- 2007 Mask trend: -13,600 km2/year sensor Analyzed Sea Ice Extent Boundary displaced 2007 Mask (MASIE) product east of Pt. Barrow • Used NSIDC mask as basis, but Simple straight line boundary, Beaufort regions drawn independently varying latitude 2010 Sea • Regions created as shapefiles Boundary displaced manually drawn in GIS using Ad hoc northern boundary east of Cape Bathurst straight lines on projection • Baltic Sea, Bohai Sea, and Gulf IHO-defined boundary, 2021 Mask of Alaska regions added Pt. Barrow • Baffin and Gulf of St. Lawrence Constant latitude boundary, ~76° N, Beaufort Consistent with IHO 2021 Mask trend: -12,300 km2/year merged into one region Sea • Fetterer et al., 2010; NSIDC IHO-defined boundary, Cape Bathurst Sea ice data from NOAA/NSIDC Sea Ice Concentration Climate Data Record, Version 4 IHO-defined boundary, St. Patrick Island Meier et al., in press, https://nsidc.org/data/G02202/.