Creates a NetCDF4 file containing daily calculations of the "Aleutian Low - Beaufort Sea Anticyclone" (Cox et al., 2019). ALBSA is a 4-pt index on the geopotential height field a 850 hPa where the points are North (“N” at 75°N/170°W), South (“S” at 50°N/170°W), East (“E” at 55°N/150°W), and West (“W” at 55°N/160°E)
ALBSA = ( E-W ) - ( N - S ) and is expressed in meters of GPH. The mean state of the wintertime (September - May) positive and negative values are as follows:
The index has been most commonly defined based on the NCEP/NCAR Reanalysis 1, but in depreciation this routine calculates ALBSA using the ECMWF ERA5 reanalysis. Testing in May 2025 shows differences between the reanalyses of mean -0.24 m, std 19.5 m, averaged for all days overlapping 1948-2025 (n = 28259). The mean absolute value of the error variance is ~0.035%. This code handles the download from ECMWF, the calculation, and the writes the new NetCDF. A histogram of the differences:
The product is generated using Copernicus Climate Change Service information (CCCS/CDS, 2023). ECMWF Reanalysis v5 (ERA5) (Hersbach, et al. 2020; Hersbach et al., 2023)
Copernicus Climate Change Service, Climate Data Store, (2023): ERA5 hourly data on pressure levels from 1940 to present. Copernicus Climate Change Service (C3S) Climate Data Store (CDS), DOI:10.243381/cds.bd0915c6
Cox, C. J., R. S. Stone, D. C. Douglas, D. M. Stanitski, and M. R. Gallagher (2019), The Aleutian Low - Beaufort Sea Anticyclone: A climate index correlated with the timing of springtime melt in the Pacific Arctic cryosphere. Geophysical Research Letters, 46(13), 7464-7473, DOI:10.1029/2019GL083306
Hersbach, H., B. Bell, P. Berrisford, G. Biavati, A. Horányi, J. Muñoz Sabater, J. Nicolas, C. Peubey, C., R. Radu, I. Rozum, D. Schepers, A. Simmons, C. Soci, D. Dee, and J.-N. Thépaut (2023): ERA5 hourly data on pressure levels from 1940 to present. Copernicus Climate Change Service (C3S) Climate Data Store (CDS), DOI:10.24381/cds.bd0915c6
Hersbach, H., B. Bell, P. Berrisford, S. Hirahara, A. Horányi, J. Muñoz-Sabater, J. Nicolas, C. Peubey, R.Radu, D. Schepers, A. Simmons, C. Soci, S. Abdalla, X. Abellan, G. Balsamo, P. Bechtold, G. Biavati, J. Bidlot, M. Bonavita, G. De Chiara, P. Dahlgren, D. Dee, M. Diamantakis, R. Dragani, J. Flemming, R. Forbes, M. Fuentes, A. Geer, L. Haimberger, S. Healy, R. J. Hogan, E. Hólm, M. Janisková, S. Keeley, P. Laloyaux, P. Lopez, C. Lupu, G. Radnoti, P. de Rosnay, I. Rozum, F. Vamborg, S. Villaume, and J.-N. Thépaut (2020) The ERA5 global reanalysis. Quarterly Journal of the Royal Meteorological Society, 146(730), 1999-2049, DOI:10.1002/qj.3803
Note on Modifications: The original API boilerplate logic (CC BY 4.0) modified to meet the needs of this project.
The code will be run annually, most likely in early June (ALBSA is most useful September - May) and the output file will replace the previous one at the Arctic Data Center. The data DOI will remain the same
- Checkout the code form this repository, including
- Read this README and the code documentation
How to run: python3 ./calc_albsa.py -p /path/to/working/directory/ -s 1940 -e 2025
Arguments: -p: path to working directory (REQUIRED) -s: starting year for the desired file contents (OPTIONAL; default to 1940) -e: ending year for the desired file contents (OPTIONAL; default to current year)
The following python packages are required, noting the tested versions
python ≥ 3.13.2
netCDF4 ≥ 1.7.2
xarray ≥ 2024.11.0
cdsapi ≥ 0.7.6
- Christopher J. Cox (NOAA/PSL) - christopher.j.cox@noaa.gov
Copyright 2025 U.S. Federal Government (in countries where recognized)
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