The Amundsen Sea Low (ASL) is a climatological low pressure center located over the extreme southern Pacific Ocean, off the coast of West Antarctica. Atmospheric variability in this region is larger than anywhere else in the Southern Hemisphere, and exhibits significant correlations with both the Southern Annular Mode (SAM) and ENSO. Given its linkages to basin-wide ENSO variability, the ASL may be thought of as a Southern Hemisphere analog to the Aleutian Low (see North Pacific Index). The ASL plays a significant role in the climate variability of West Antarctica and the adjacent oceanic environment. Due to the extreme variability in the ASL region, and the seasonal migration of the ASL's low-pressure center, defining a single index of the ASL is challenging. A consistent set of indices of the ASL, including its absolute and relative central pressure, and the latitude and longitude of these central pressures, has been derived from the ERA-Interim reanalysis. The term Amundsen-Bellingshausen Seas Low (ABSL) is another name for the ASL.
The following was contributed by Dr. Scott Hosking of the British Antarctic Survey, January, 2016:
The Amundsen Sea Low is a 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. The position and strength of the ASL are key drivers of regional change over West Antarctica (Hosking et al., 2013; Coggins and McDonald, 2015).
There has been much recent interest in the ASL because (i) rapid retreat of Pine Island and Thwaites glaciers (McMillan et al, 2015), (ii) observed rapid warming over central West Antarctica (Bromwich et al., 2012), and (iii) increase in snowfall in coastal West Antarctica (Thomas et al., 2015).
The ASL indices are derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim; Dee et al. 2011) monthly fields of sea level pressure. ERA-Interim extends back to the start of the satellite observation record, 1979 onwards.
The ASL latitude and longitude (version 2) are derived by applying a minima finding algorithm (adapted from a routine developed by SolarSoft [Freeland and Handy, 1998]) to the MSLP fields over the entire globe. Minima located outside the ASL sector region, defined as 170–298 E, 80–60 S, are discarded. Land points are also masked out. The local minimum with the largest amplitude (i.e., the deepest low) is used to define the ASL.
Version 2 differs slightly from that described in Hosking et al.  which defined the location of the ASL as the point of lowest central pressure within the ASL sector region. This updated methodology ensures that the ASL is not falsely identified in cases where there is no local minima within the defined boundaries (as is the case in some CMIP5 models). In any case, when applied to the ERA-Interim reanalysis the results are relatively insensitive to the ASL detection method employed.
The dataset website https://www.bas.ac.uk/data/absl/ has monthly, seasonal and annual data. Note that the seasonal and annual indices are computed from their respective temporally averaged two-dimensional surface pressure fields. They are not the average of the three (or twelve) points from the monthly indices. ##
ASL index Version 2 (recommended):
Hosking, J. S., A. Orr, T. J. Bracegirdle, and J. Turner (2016), Future circulation changes off West Antarctica: Sensitivity of the Amundsen Sea Low to projected anthropogenic forcing, Geophys. Res. Lett., 43, doi:10.1002/2015GL067143.
ASL index Version 1:
Hosking, J. S., A. Orr, G. J. Marshall, J. Turner, and T. Phillips (2013), The influence of the Amundsen-Bellingshausen Seas Low on the climate of West Antarctica and its representation in coupled climate model simulations, J. Clim., 26, 6633–6648, doi:10.1175/JCLI-D-12-00813.1.
Click the thumbnails to view larger sizes
|An example of seasonal mean sea level pressure around Antarctica with the location of the ASL marked with a '+' symbol. The sector used to derive the ASL indices (version 2) is enclosed by the black line box.(contributed by S. Hosking)|
|Time series of monthly mean ASL longitudinal location derived from ERA- Interim data (grey line) and the corresponding 11-month smoothed time series (black line). (contributed by S. Hosking)|
Hosking, Scott & National Center for Atmospheric Research Staff (Eds). Last modified 10 Jun 2016. "The Climate Data Guide: Amundsen Sea Low indices ." Retrieved from https://climatedataguide.ucar.edu/climate-data/amundsen-sea-low-indices.