# Presentation: 077597

## Climatic Impacts on West Antarctica

**David H. Bromwich and Ryan L. Fogt**
- Polar Meteorology Group, Byrd Polar Research Center
- The Ohio State University, Columbus, Ohio
*and*
- Atmospheric Sciences Program, Department of Geography
- The Ohio State University, Columbus, Ohio

## Impacts on the WAIS Climate

- Current research suggests that two main processes affect the WAIS climate
- The El-Niño Southern Oscillation (ENSO)
- The Antarctic Oscillation (AA0), a.k.a. the Southern Annular Mode (SAM)
- Perhaps the largest global climate change signals over the last 50 yr are found in the Antarctic Peninsula, a region affected by both ENSO and the AAO.

## ENSO

- The El-Niño Southern Oscillation (ENSO) strongly influences climate conditions in West Antarctica [*Smith and Stammerjohn *1996; *Turner * 2004; *Cullather et al. *1996]
- The tropical ENSO signal is propagated to the South Pacific via a wave-train of alternating positive-negative geopotential height anomalies, termed the Pacific South American pattern (PSA).
- Enhanced blocking occurs in the South Pacific during an El-Niño event, changing the circulation of the region [*Renwick *1998]
- The ENSO teleconnection to the South Pacific has shown considerable decadal variability, with the strongest relationship found in the 1990s [e.g., *Fogt and Bromwich* 2005]
- Observations also depict the decadal variability
- ENSO is typically monitored by the Southern Oscillation Index (SOI)

## Previous Research on ENSO Decadal Variability in West Antarctica (75-90oS, 120-180oW)

- Modified from *Bromwich et al. *[2000]

## Annual Mean (May-April) correlation of ERA-40 MSLP and the Southern Oscillation Index (SOI / ENSO) 1980s 

- From *Fogt and Bromwich *[2005]

## 1990s 

- Annual Mean (May-April) correlation of ERA-40 MSLP and the Southern Oscillation Index (SOI / ENSO)
**1990s **

- From *Fogt and Bromwich *[2005]

## Upper Air Observations Showing Decadal        ENSO Variability

- With the correlation isolines interpreted on the spatial plots as isobars, changes in the circulation cause a more significant correlation in the zonal (west-east) wind / SOI over Southern South America and in the meridional (north-south) wind  / SOI over the Antarctic Peninsula
- These decadal variations in ENSO teleconnection to the South Pacific are evidenced in the upper air observations at Punta Arenas and Bellingshausen, respectively

- From *Fogt and Bromwich *[2005]

## The Antarctic Oscillation (AAO)

- The AAO (calculated from the standardized zonal (latitude-averaged) MSLP difference from 45o and 65o S) has been shown to have a trend towards its positive polarity [i.e., *Thompson and Solomon* 2002; *Marshall* 2003], implying a strengthening of the meridional (north-south) pressure gradient and the circumpolar zonal (west-east) winds.  The trend is most pronounced during the 1990s (red regression line).

- -- 0.56 decade –1 ± 0.24
- -- 1.69 decade –1 ± 1.09
- Significant at 99.9% level

## Antarctic Peninsula Warming

- Annual mean surface temperatures have risen upwards of 5K along the Antarctic Peninsula in the last 50 years.
- Both ENSO and the AAO have been shown to affect temperatures in the Antarctic Peninsula [i.e., *van den Broeke* *et al.* 2004; *Turner *2004].

- From *Vaughan et al.* [2001]

- ERA-40 2m temperature
- correlation with AAO, 1990s

## Forcing of the Antarctic Oscillation

- The forcing of the AAO is still a large debate:
- greenhouse gases [e.g., *Cai et al.* 2003]
- ozone [e.g., *Thompson and Solomon* 2002]
- Tropical SSTs in the Pacific [e.g.,*Fogt and Bromwich* 2005]
- Of these, the Tropical SSTs suggest that ENSO and the AAO are related.
- In fact, *Fogt and Bromwich* [2005] show that the decadal variability of the ENSO teleconnection is governed by the coupling with the AAO.

## Difficulties in grasping high southern latitude climate change: data problems

- The NCEP-NCAR Reanalysis (NNR) has proven to have many shortcomings in Antarctica, including erroneous trends in the MSLP fields [e.g., *Hines et al.* 2000]
- ERA-40 also has many shortcomings and is strongly guided by the assimilation of satellite data.
- During the data sparse polar winter before the assimilation of satellite data, ERA-40 performs with uncharacteristically low skill [correlations with observed MSLP at the coastal stations average ~0.27 from 1958-1972; *Bromwich and Fogt *2004]
- Further, ERA-40 and NNR produce different analyses over the data sparse Southern Ocean.

## ERA-40 and NNR Differences

- For the 1958-1978 period, 500 hPa geopotential height differences in the South Pacific average ~50 gpm.
- However, there are a few isolated events when the difference is greater than 200 gpm.
- With limited observations to validate either reanalysis, it is difficult to determine which is more accurate.

## Summary

- There is strong decadal variability of the ENSO signal in the Bellingshausen Sea / Drake Passage region
- The AAO has been progressing towards its positive polarity, especially in the 1990s, thus strengthening the meridional (north-south) pressure gradients.
- The annual mean surface temperatures in the Antarctic Peninsula have risen upwards of 5K over the last 50 yr, one of the strongest warming rates seen globally.
- Both the AAO and ENSO have been shown to influence the temperatures of the Antarctic Peninsula region.
- The forcing of the AAO is still debatable; however, it is likely to be in part forced by the Tropical Pacific SSTs, and thus coupled with ENSO.
- The relatively new ERA-40 reanalysis also has shown disparities with observations (as does NNR), thus making high southern latitude climate change studies difficult in perhaps the world’s largest meteorological data void.