A Workshop on the Amundsen Sea Low (ASL), was held at University of California, Los Angeles (UCLA) on December 5-6th, 2013. The ASL Workshop was part of a response to the World Climate Research Programme WCRP’s recently established Polar Climate Predictability Initiative (PCPI). The PCPI is a sub-initiative of the “Cryosphere in a Changing Climate” Grand Challenge led by CliC (Climate and Cryosphere). Its main goal is to advance our understanding of the sources of polar climate predictability on a range of timescales ranging from seasonal to multi-decadal. This page provides information for meeting participants.
Improving polar climate predictability in the Southern Hemisphere requires a better understanding of the connections between the jets and the non-zonal circulation couple to the surface climate system. The Amundsen Sea Low represents a very important component of the non-zonal circulation with significant influence in the Pacific sector of the high Southern Latitudes and links to the Tropical Pacific. There is then a need to evaluate our understanding of this system, to identify where gaps in our knowledge exist and to suggest how these gaps might be filled. To address these needs, the Workshop is designed around a group of five questions that have to be answered with the aim of increasing understanding of the ASL. The questions are
1. What is the variability of the Amundsen Sea Low in observations? Dr. John Turner
2. What is the impact of the ASL on the climate in Antarctica? – Dr. Ryan Fogt
3. What are the links between the ASL and the extra-polar regions, in particular the Tropics? - Dr. David Schneider
4. How can we expect the ASL to change – in CMIP5 model projections? Scott Hosking
5. What do we know about ASL variability over the past millennium? Dan Dixon
For each question a keynote speaker has been invited to give a review talk. Each presentation is subsequently the subject of extensive discussion by all workshop participants.
Potential outcomes of the Workshop include an assessment of the state of the knowledge about the ASL as well as future directions for research.
UCLA Department of Geography,
1255 Bunche Hall – RM 1221D
Los Angeles, CA 90095
|# Early Career PhD.|
Thursday 5th December, 2013
|9:00 – 10:30 a.m.||
Session 1 – The variability of the Amundsen Sea Low in observations
|10:30 – 11:00 a.m.||Coffee Break|
|11:00 – 12:30 p.m.||Session 2 – The impact of the Amundsen Sea Low on the climate of Antarctica (Ryan Fogt)|
|12:30 – 2:00 p.m.||Lunch|
|2:00 – 3:30 p.m.||Session 3 – Links between the Amundsen Sea Low and extra-polar regions (David Schneider)|
|3:30 – 4:00 p.m.||Coffee Break|
|4:00 – 5:00 p.m.||Discussion/Summary/etc. (Everyone)|
Friday 6th December, 2013
|9:00 – 10:30 a.m.||Variability in the Amundsen Sea Low over the past millennium (Dan Dixon)|
|10:30 – 11:00 a.m||Coffee Break|
|11:00 a.m. -12:30 p.m||Expected (Projected?) changes in the Amundsen Sea Low to – in CMIP5 projections (Scott Hosking)|
|12:30 – 2:00 p.m.||Lunch|
|2:00 – 3:30 p.m.||Discussion/Wrap up/etc. (Everyone)|
The variability of the Amundsen Sea Low in observations (John Turner)
The atmospheric reanalysis data sets are the primary tool that we have to investigate variability and change in the Amundsen Sea Low (ASL) over recent decades. These show that within the circumpolar trough (60 – 70° S) there are three climatological areas of low pressure, with the centre off West Antarctica constituting the ASL. The ASL is clearly present because of the large number of synoptic and mesoscale low pressure systems in the area that have either moved into the region from the north or developed in the highly baroclinic zone of the circumpolar trough. However, the exact relationship between the climatological ASL and the location and depth of the individual lows is still not fully understood.
The ASL is a highly variable feature in the monthly mean MSLP pressure fields. Sometimes it has a very clear centre, while on other occasions there is a very weak pressure field from the Antarctic Peninsula to the Ross Sea. This is consistent with the ASL being coincident with the region of the ‘pole of variability’, where the inter-annual variability of the MSLP is greater than any other location in the Southern Hemisphere.
The location of the ASL can fairly easily be identified as the lowest MSLP in the West Antarctic sector. However, there is more of a debate over what constitutes the depth of the low. The MSLP at the centre of the ASL can be taken, but this is strongly influenced by the Southern Annular Mode and the Semi-Annual Oscillation. An alternative is to subtract the regional mean MSLP.
The centre of the low is more easterly (westerly) in summer (winter) and more northerly (southerly) in summer (winter). There is close agreement between the surface location of the ASL and the 500 hPa geopotential height trough, and both move westwards between summer and winter.
The absolute depth of the ASL has a semi-annual form, with the lowest pressure in October. If the regional mean pressure is removed from the absolute pressure then the ASL relative depth has an annual cycle with the lowest values in winter. There has been no significant change in the depth of the ASL for the year as a whole. However, it has deepened significantly in September, increasing the southerly flow over the Ross Sea and increasing the sea ice extent in this region at the time of maximum ice extent.
The ASL is believed to be present because of the form of the orography of the Antarctic continent and it has been suggested that it can be regarded as a separation vortex off Victoria Land. The zonal location of the ASL is certainly significantly correlated with the 500 hPa zonal wind.
The impact of the Amundsen Sea Low on the climate of Antarctica (Ryan Fogt)
Due to the lack of surface-based reliable in situ observations in the Amundsen, Bellingshausen, and Ross Seas, the impact of the Amundsen Sea Low (ASL; also called the Amundsen-Bellingshausen Seas Low) on the regional climate is just beginning to be better understood. The majority of recent work has examined the impact on temperature and precipitation across West Antarctica and the Antarctic Peninsula, and sea ice concentration in the Ross, Amundsen, and Bellingshausen Seas. Notably, the main impacts on these variables are directly tied to variations in the location and intensity of the ASL. In particular, the latitudinal location of the ASL has the most consistent and persistent influence, often leading to differences in temperature, sea ice, and precipitation from near and along the Antarctic Peninsula to portions of the Siple Coast of West Antarctica, bordering the Ross Ice Shelf and Ross Sea. The impacts are less sensitive to the depth and latitudinal position. Sea ice changes are largely consistent with atmospheric wind changes, and these partially explain the growth of sea ice in the Ross Sea and decreases in the Amundsen and Bellingshausen Seas. It considering these impacts, it is important to remove the influence of the Southern Annular Mode, which has a strong connection to especially the depth of the ASL on interannual timescales.
In addition to the location and intensity, it is also important to consider the breadth—the overall spatial extent of the ASL. During times when La Niña events occur with a positive phase of the Southern Annular Mode (or El Niño events with a negative phase of the Southern Annular Mode), recent work focusing on austral spring has demonstrated that the spatial extent of the ASL is greater than when these climate modes influence the ABSL in isolation. The greater breadth of the ASL during these conditions in austral spring leads to uniform impacts on temperature, wind, and pressure across the Antarctic Peninsula. Otherwise, SAM events tend to influence the northeastern Peninsula, while ENSO events typically have a pronounced influence across the western Peninsula.
This presentation will highlight the current impacts of the ASL as detailed above, and pose several questions for future research, especially considering non-linear impacts and feedbacks that may exist between the atmosphere-ice-ocean which can influence the ASL and its affect on the regional climate.
What are the impacts of tropical variability and trends on the Amundsen Sea Low? (David Schneider)
The Amundsen-Bellingshausen Sea (ABS) region exhibits some of the largest interannual atmospheric circulation variability in the Southern Hemisphere (SH), due in part to orographic forcing and in part to its location in the South Pacific, where atmospheric waves associated with ENSO variability have a year-round influence. Two atmospheric circulation indices, the SAM index and the Southern Oscillation Index (SOI), have significant correlations with the ASL. In September-October-November (SON), the SAM and SOI are independent, that is, they are uncorrelated. In SON (and other non-summer seasons, not shown), the SAM is largely an atmospheric mode, exhibiting little correlation with SSTs in the tropics or even the mid-latitudes. The most energetic Rossby waves associated with ENSO variability in the SH occur in SON, and hence the strongest correlations between ENSO variability and the ASL generally occur in SON. In its positive phase (by convention, associated with a La Niña event), in SON the SOI is associated with a deeper ASL and with warm air advection towards the Antarctic Peninsula and West Antarctica. However, from SON to December-January-February (DJF) the sign of the SOI correlation with respect to air temperature anomalies over Antarctica reverses in many locations. Given the persistent interannual ENSO signals in the ABS, it is natural to ask how much influence the tropics have had on atmospheric circulation trends and related climate changes in this region. A number of recent studies have suggested that tropical teleconnections have contributed to atmospheric warming West Antarctica and the Peninsula, and to sea ice loss in the ABS. However, a different line of research suggests that polar stratospheric depletion has been the major driver of atmospheric circulation trends in the SH over the past few decades. Results from a of series experiments with the Community Atmosphere Model, version 4 (CAM4) suggest that it is likely that the observed circulation trends in the ABS have been driven by a combination of factors, including tropical SSTs and stratospheric ozone depletion.
Variability in the Amundsen Sea Low over the past millennium (Dan Dixon)
West Antarctic ice core sea-salt concentrations have proven to be the best proxy thus far for sea level pressure (SLP) in the Amundsen Sea region. Wind strength in the Amundsen Sea region is directly correlated to sea level pressure (SLP). The climatological low pressure system in this area is commonly referred to as the Amundsen Sea low (ASL) or Amundsen-Bellingshausen Seas low (ABSL). Correlations between West Antarctic ice core sea-salt and seasonal SLP from atmospheric reanalyses have revealed strongest associations in the area of the ASL in spring (SON). Previous research by Kreutz et al. (2000) focused on developing an ASL proxy from the Siple Dome ice core sea-salt record. Their investigation found a negative association between Siple Dome sea-salt concentrations and SLP in the Amundsen Sea region but did not find a significant relationship with sea ice extent. Subsequent researchers, using a suite of West Antarctic ice cores, have revealed a significant (>95%) association between sea-salt concentrations and sea ice extent at several sites (Dixon et al., 2005; Kaspari et al., 2005). This presentation will focus on the methods and challenges of developing an ASL proxy using a suite of 12 extremely well-dated high-resolution West Antarctic ice cores.
Projections of the Amundsen Sea Low up to 2100 (Scott Hosking, Andrew Orr, Tom Bracegirdle, Gareth Marshall, John Turner)
The location and strength of the Amundsen Sea Low (ASL) are highly variable year-to-year. Previous work has shown (by firstly removing the underlying changes from modes of large-scale variability) that both these quantities play important roles in controlling West Antarctic surface climate variability. To assess how the location and strength of the ASL will change between now and the end of the century in response to increasing greenhouse gases (GHGs) and stratospheric ozone recovery, we use the ~50 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Firstly, to assess the impact of increasing GHGs alone, we compare 50-year PDFs for the period 2051-2100 between Representative Concentration Pathways (RCP) 2.6 and 8.5 (here the ozone forcing within models is largely the same). Secondly, to assess the impact of ozone recovery alone, we use the Historical and RCP 2.6 experiments to compare between the periods 1981-2030 (ozone hole) and 2051-2100 (ozone recovered). In this latter comparison we assume that the climate response due to ozone recovery is considerably greater relative to that of increasing GHGs.