Amundsen Sea Low (ASL) Index
The Amundsen Sea Low (ASL), also known as the Amundsen-Bellingshausen Seas Low (ABSL), is a non-stationary climatological low pressure system situtated within the Pacific Sector of the Southern Ocean. The ASL is an important feature of regional atmospheric circulation and winds and thus a key driver of the climate of West Antarctica, the Antarctica Peninsula and the Southern Ocean Pacific sector.
The data for indices which characterise the location and strength of the
ABSL ASL are made available below.
ABSL or ASL ?
Following some discussions at a dedicated ASL Workshop in LA, California (5-6 December 2013) the name Amundsen Sea Low (ASL) will be used for any future publications and presentations (rather than the Amundsen-Bellingshausen Seas Low, ABSL).
The ASL latitude and longitude is taken as the geographical location of the minimum MSLP in the ASL sector†. The study by Hosking et al. (2013) indicates that the longitudinal location of the
ABSL ASL is an important driver of climate variability over West Antarctica. The time series plot below shows how the ASL longitude has varied over the last few decades.
The ASL Relative Central Pressure Index is calculated as follows:  the representative background pressure is calculated by area-averaging the mean sea level pressure (MSLP) field over the ASL sector†;  This value is then subtracted from the actual MSLP field to give the regional pressure anomaly;  the relative central pressure is then taken as the minimum value within this regional pressure anomaly field. By calculating the depth of the ASL relative to the background pressure field we remove much of the large-scale variability which strongly modulates the ASL Actual Central Pressure Index (see below). Indicies which describe the monthly and seasonal variability of the ASL relative central pressure, and its location are constructed from ERA-Interim Reanalyses (between January 1979 — December 2012 inclusive) and can be found within the ascii files within the secton below.
The ASL Actual Central Pressure Index is taken as the minimum value within the MSLP field within the ASL sector†. This index is also provided within the ascii files below. Note that this index is strongly modulated by the Southern-Hemisphere Annual Mode across all seasons, with time series correlations significant at p < 0.01.
ABSL ASL sector is defined as 170°—290° E and 60°—75° S (indicated by the boxes in Fig. 2e-h in Hosking et al. (2013))
For more information please contact: Dr Scott Hosking
ABSL ASL indices
- The monthly ASL indices calculated from ERA-Interim Reanalysis can be found here
- The seasonal ASL indices calculated from ERA-Interim Reanalysis can be found here
- The monthly ASL indices calculated from CMIP5 coupled-climate models run as part of the Historical experiment (1979-2005) can be found here
- Python script to plot monthly ASL Longitude timeseries can be obtained from here
When using the data set in a paper, the following is the correct citation to use:
J. S. Hosking, A. Orr, G. J. Marshall, J. Turner, and T. Phillips: The influence of the Amundsen-Bellingshausen Seas Low on the climate of West Antarctica and its representation in coupled climate model simulations, Journal of Climate, 2013. [ http ]