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Amundsen Sea Low (ASL) index

Background

Amundsen Sea Low (ASL)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 that the position and strength of the ASL are crucial for understanding regional change (Hosking et al., 2013). Furthermore, the state-of-the-art climate models which best simulate the ASL produce a better representation of the regional surface climate. 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 (Hosking et al., 2016). Additional information on these indices can be found on the Climate Data Guide.

Two sets of indices which characterise the location and strength of the ASL are made available below.

ABSL or ASL? In Hosking et al. (2013) we used the term Amundsen-Bellingshausen Seas Low (ABSL). However, following a dedicated ASL workshop, the term Amundsen Sea Low (ASL) is used for more recent publications.

ASL Indices

Amundsen Sea Low (ASL) longitude monthly time-series Hosking et al. (2013) found that the location of the ASL is an important driver of climate variability over West Antarctica. Following this study, an updated set of indices were developed which includes a more robust way to identify the ASL latitude and longitude (see Version 2 below for more details).

The ASL Actual Central Pressure Index is simply defined as the pressure at the ASL location. Note that the ASL actual central pressure is strongly modulated by the Southern-Hemisphere Annual Mode (SAM) across all seasons, with time series correlations significant at p<0.01. For this reason we derive an alternative measure called the "Relative Central Pressure".

The ASL Relative Central Pressure is essentially a regional pressure anomaly. It is calculated by subtracting the ASL actual central pressure from the area-averaged pressure over the ASL domain (domains are specified below).

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.

For more information, or to request that these indices are updated to include more recent data, please contact: Dr Scott Hosking

➤ Version 2 (Recommended)

In version 2 the ASL latitude and longitude are identified using a minima finding algorithm within the ASL sector (here defined as 170°—298° E, 80°—60° S). This methodology is more robust at identifying the ASL compared to version 1 (see Hosking et al., 2016).

When using the version 2 dataset in a paper, the following is the correct citation to use:

Hosking, J. S., Orr, A., Bracegirdle, T. J., Turner, J. (2016). Future circulation changes off West Antarctica: Sensitivity of the Amundsen Sea Low to projected anthropogenic forcing.. Geophysical Research Letters, 43, doi:10.1002/2015GL067143

➤ Version 1

In the original ASL indices (now known as version 1) the ASL latitude and longitude are identified at the geographical location of minimum pressure in the ASL sector (here defined as 170°—290° E, 75°—60° S; note that this is different to the region used in version 2).

When using the version 1 dataset in a paper, the following is the correct citation to use:

Hosking, J. S., Orr, A., Marshall, G. J., Turner, J., and Phillips, T. (2013). 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, 26(17):6633-6648. doi:10.1175/JCLI-D-12-00813.1