Sea ice back-trajectory calculations

Overview

Working repository for calculation of sea ice back-trajectories using the NSIDC Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors, Version 4 (NSIDC-0116) product ("NSIDC PP").

The code in this repository was written in order to calculate back-trajectories from the northwestern Barents Sea, and the code is tailored to that application.

Scripts

Define particle tracking class.ipynb

Defines the ice_particle class used to create individual back-trajectories.

The class functions contain the code for initialization, interpolation, back-propagation, and various utility functions.

Load and prepare data.ipynb

Load sea ice drift data and land mask.

• Loads the NSIDC PP data to an xarray Dataset. Multiple files are coincatenated to a single file, and interpolation is done across gaps.
• Loads a land mask (optional) to be used to end trajectories that intersect coastlines.

Should be modified depending on system configuration/file location/desired land mask...

Data

Sea ice drift data

NSIDC PP Data were downloaded from NSIDC with Earthdata access.

Files containing daily drift vectors are on the form:

icemotion_daily_nh_25km_20120101_20121231_v4.1.nc

In the code examples, there are references to the ddir directory, which is set to my local directory containing these files. ddir should be changed to point to the location of the icemotion files.

Coastline data

The code takes a land mask as an (optional) input. The land mask, MASK, is used to terminate the trajectory calculation if the particle hits land.

In the applications here, the land mask MASK, was made from an Eastern Arctic Ocean subset of the IBCAO-v4 bathymetry, downscaled to 2 km resolution. The Svalbard archipelago was removed because we did not want to terminate trajectories intersecting land there (because we are computing backtrajectories from moorings close to land in northeastern Svalbard).

MASK can be replaced with any land mask containing x, y, is_land(x, y) and thr projection proj of x, y (see details in Load and prepare data.ipynb).

Application specifics

• Performs daily back-propagation from an initial coordinate point.
• Sea ice drift velocity at the sea ice particle coordinate is obtained by interpolating the drift components onto the coordinate

Some key dependencies

• pyproj is used for transforms between coordinate systems (e.g. WGS84 lat/lon and NSIDC PP EASE-grid x/y).
• xarray is used for handling NetCDF data.
• scipy.interpolate.griddata is used for linear interpolation between unstructured points.