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Amundsen Sea Low (ASL) index


Over the last 50 years West Antarctica has experienced one of the largest increases in regional temperature on Earth. Observed spatial trend patterns in surface temperature and sea ice extent indicate that these are likely linked to changes in the Amundsen Seas Low (ASL): a highly dynamic and mobile climatological low pressure system located in the Pacific sector of the Southern Ocean. In this sector, variability in sea-level pressure is greater than anywhere in the Southern Hemisphere making it challenging to isolate local fluctuations in the ASL from larger-scale shifts in atmospheric pressure. BAS scientists have demonstrated (Hosking et al., 2013) that the position and strength of the ASL are crucial for understanding regional change. Furthermore, the state-of-the-art climate models that best simulate the ASL provide a distinctly more realistic surface climate and sea ice extent in the region. As a major driver of West Antarctic climate variability, identifying the spread of ASL realisations across all the climate models is fundamental for reducing uncertainties in West Antarctica's past and improving predictions of future change.

Indices which characterise the location and strength of the ASL are made available below.

ABSL or ASL? In the paper referenced below, we used the term Amundsen-Bellingshausen Seas Low (ABSL). However, following a dedicated ASL workshop, the term Amundsen Sea Low (ASL) will be used for our future publications and presentations.

ASL Indices

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 ASL is an important driver of climate variability over West Antarctica.

The ASL Relative Central Pressure Index is calculated as follows: [1] the representative background pressure is calculated by area-averaging the mean sea level pressure (MSLP) field over the ASL sector; [2] This value is then subtracted from the actual MSLP field to give the regional pressure anomaly; [3] 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. Indicies which describe the monthly and seasonal variability of the ASL relative central pressure, and its location are constructed from ERA-Interim Reanalyses 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.

  † The ASL sector is defined as 170°—290° E and 75°—60° S (indicated by the boxes in Fig. 2e-h in Hosking et al. (2013))

For more information please contact: Dr Scott Hosking

Download 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


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 ]