Polar Pathfinder¶

Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors, Version 4 (NSIDC-0116)

Available

At a glance¶

Note on period: As of 2026-10-12, daily data cover 1978-11-01 to 2025-12-31 and weekly data 1978-11-05 to 2025-12-31 (User Guide Table 11 and the dataset page). Section 1.4.1 of the guide still gives 31 December 2024 as the end, which conflicts with the table. From 2026 there are no final data; the Arctic weekly quicklook product NSIDC-0748 continues from the week after the last final date. SMMR input before July 1987 is roughly every other day, and in the Southern Hemisphere there are gaps when SSM/I and AVHRR inputs are missing.

Overview¶

Daily Northern and Southern Hemisphere sea ice motion vectors from 1978, merged by optimal interpolation from motion derived by spatial correlation of consecutive-day imagery (AVHRR visible and infrared; SMMR, SSM/I, SSMIS, AMSR-E and AMSR2 passive microwave), IABP drifting buoys, and motion estimated from reanalysis 10 m winds. An error estimate is given for every grid cell, and the vectors are the input to NSIDC's companion Sea Ice Age product. Velocities (cm/s) are along the x and y axes of the 25 km EASE-Grid, so they must be rotated to eastward and northward components before use.

What changes with the successor¶

Polar Pathfinder v4.1 → Polar Pathfinder v4.2¶

Transition: 2026-10

With DMSP SSMIS and the NCEP/NCAR Reanalysis no longer being updated, the passive microwave input was switched to GCOM-W1 AMSR2 and the wind input to ERA5 starting with the 2025 data. The dataset version (Version 4), DOI, grid and variables are unchanged; only files from 2025 onward carry the v4.2 name. 1978–2024 (v4.1) was not reprocessed.

Sources: NSIDC-0116 V4 User Guide (updated October 2026, version history table) · NSIDC-0116 V4 dataset page

Available variables¶

Daily and weekly netCDF variables as described in the user guide. u and v are components along the EASE-Grid x and y axes, not eastward and northward. The fill value is -9999. Separate files for each input source (im_from__…) are also provided.

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

Sea ice motion (daily)¶

Sea ice motion (weekly)¶

Coordinates¶

The full variable list is in the official documentation.

How to get the data¶

• NSIDC dataset page — NSIDC-0116 Version 4 — Overview / user guide / citation
  • The user guide (updated October 2026) documents the period of each input source, the file naming convention and the error estimation method.
• NSIDC DAAC HTTPS archive — nsidc0116_icemotion_vectors_v4 — HTTPS (browser, wget or curl)
  • Requires a free NASA Earthdata Login account. For command-line downloads, store your credentials in ~/.netrc.
• NSIDC dataset page — Quicklook Arctic Weekly EASE-Grid Sea Ice Motion Vectors (NSIDC-0748) — Overview / HTTPS (Earthdata Login)
  • The Arctic weekly quicklook product covering the period after the final product (AMSR2, ERA5 and IABP buoy inputs). When the final product is extended, the quicklook coverage moves on so the two do not overlap, so switch to the final product (NSIDC-0116) for the final analysis in a paper. The dataset page (1 January 2025) and the user guide (29 April 2024) give different start dates, so check the actual file listing when downloading.

Example code¶

Assumes your Earthdata Login credentials are in ~/.netrc and that you have downloaded a Northern Hemisphere daily file from the HTTPS archive. File names follow icemotion_25km_v4.x.nc (v4.1 for 1978–2024, v4.2 from 2025); check the exact names in the directory listing. The east/north conversion follows the NSIDC help article "How to convert the horizontal and vertical components to east and north".

import glob
import numpy as np
import xarray as xr

f = sorted(glob.glob("icemotion_daily_nh_25km_*_v4.*.nc"))[-1]
ds = xr.open_dataset(f)

u = ds["u"].where(ds["u"] != -9999)          # along grid x, cm/s
v = ds["v"].where(ds["v"] != -9999)          # along grid y, cm/s
L = np.deg2rad(ds["longitude"])

# Northern EASE-Grid → eastward (E) and northward (N) components (Southern: E = u cosL - v sinL, N = u sinL + v cosL)
east = u * np.cos(L) + v * np.sin(L)
north = -u * np.sin(L) + v * np.cos(L)

err = ds["icemotion_error_estimate"]
good = (err >= 0) & (err < 1000)             # drop coastal-suspect (negative) and distant-input (+1000) cells
speed = np.hypot(east, north).where(good)
print(float(speed.isel(time=0).mean()), "cm/s")

Practical tips¶

• u and v are components along the EASE-Grid x and y axes, not eastward and northward. Rotate them to east/north using longitude before comparing with wind or current data or plotting arrows on a map. The signs in the conversion differ between hemispheres (see the NSIDC help article).
• icemotion_error_estimate mixes flags with the values themselves: 1000 is added when the closest input vector is more than 1250 km away, and suspect vectors within about 25 km of the coast are made negative. Filter these cells out before computing means or trends.
• The passive microwave correlation method needs open expanses of ice, so there are few motion estimates where land and sea are interleaved, such as the Canadian Archipelago. Missing values do not mean there is no ice, and estimates are also poor in highly deforming ice and near the ice edge.
• The mix of inputs changes over time (AVHRR 1981–2000, SSM/I 1987–2006, SSMIS 2007–2024, AMSR-E 2002–2011, AMSR2 2025 onward, NCEP/NCAR winds to 2024, ERA5 winds from 2025). In particular, the satellite and wind inputs both change on 2025-01-01, so check whether any shift across that date in a long-term trend is real.
• IABP buoy input is sparse in the eastern Arctic and is not used in the Southern Hemisphere. Southern Hemisphere motion relies on satellite and wind input only, and has gaps when SSM/I and AVHRR are missing.
• EASE-Grid 1.0 (EPSG:3408/3409) is defined on a sphere, so overlaying it directly on EASE-Grid 2.0 data (EPSG:6931/6932, WGS 84) misplaces features. Regrid when combining with other sea ice datasets.

Notices and change history¶

References for your paper¶

Dataset citation¶

NSIDC-0116 Version 4 — DOI 10.5067/INAWUWO7QH7B

@misc{nsidc0116_v4,
  author    = {Tschudi, M. and Meier, W. N. and Stewart, J. S.},
  title     = {Polar Pathfinder Daily 25 km {EASE-Grid} Sea Ice Motion Vectors ({NSIDC-0116}, Version 4)},
  year      = {2019},
  publisher = {NASA National Snow and Ice Data Center Distributed Active Archive Center},
  address   = {Boulder, Colorado USA},
  doi       = {10.5067/INAWUWO7QH7B}
}

Quicklook extension — NSIDC-0748 Version 1 (if you used the period after the final product) — DOI 10.5067/O0XI8PPYEZJ6

Reference papers¶

Version 4 methods and enhancements (recommended in the user guide) — DOI 10.5194/tc-14-1519-2020

@article{tschudi2020motion,
  author  = {Tschudi, Mark A. and Meier, Walter N. and Stewart, J. Scott},
  title   = {An enhancement to sea ice motion and age products at the {National Snow and Ice Data Center} ({NSIDC})},
  journal = {The Cryosphere},
  volume  = {14},
  number  = {5},
  pages   = {1519--1536},
  year    = {2020},
  doi     = {10.5194/tc-14-1519-2020}
}

ERA5 (wind input from 2025, cited in the user guide) — DOI 10.1002/qj.3803

Acknowledgements¶

NSIDC requires citation of the dataset as a condition of use. Give the period, hemisphere and access date you used, and if you filled Arctic data from 2026 onward with the quicklook product (NSIDC-0748), cite its DOI separately.

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• NSIDC-0116 V4 User Guide
• NSIDC-0748 quicklook User Guide
• How to convert the horizontal and vertical components to east and north (NSIDC Help Center)
• NSIDC DAAC programmatic data access guide (Earthdata Login)
• EASE-Grid conversion tools (easegrids-easeconv)