SAM – Watts Up With That?

By Andy May

The Antarctic Oscillation (AAO) is also called the Southern Annular Mode or SAM. It is defined as the difference between the zonal (meaning east-west or circumpolar) sea level air pressure between 40°S and 65°S. That is the sea level pressure at 65°S is subtracted from the sea level pressure at 40°S (Gong & Wang, 1999). As the difference increases and SAM becomes more positive, the Southern Hemisphere circumpolar westerly (clockwise as viewed from above the South Pole) winds move closer to Antarctica and generally increase in intensity.

SAM has the third strongest 1950-2021correlation with HadCRUT5, after the AMO and WHWP, of all the oscillations discussed in this series. The R² that SAM, by itself, has with HadCRUT5 is 0.55. Only the AMO (R² = 0.58) and WHWP (R² = 0.56) have higher correlations, all three together achieve an R² of 0.77 as shown in post 1, figure 3.

When the SAM is positive in winter the persistent westerly winds and the Southern Ocean current that surround Antarctica intensify and allow less Antarctic cold air to escape to the mid-latitudes. The mid-latitude areas, mainly Patagonia, New Zealand, and Southern Australia warm as a result and often become drier. However, in the summer a positive SAM can also draw tropical moisture south causing northern and eastern Australia to get more precipitation. The effects of SAM on the southern mid-latitudes are dependent upon the season and the effects are often opposite in summer and winter. A negative SAM, during the summer, can cause heatwaves in Southern Australia.

When SAM is negative, the belt of circumpolar winds move toward the equator and weaken. Figure 1 illustrates the difference between a positive and negative SAM or AAO. Also, this video from the Bureau of Meteorology of Australia is informative.

Antarctic temperatures are always cold and don’t vary much (see figure 2), but when SAM is negative it has a big effect on the Southern Hemisphere mid-latitude temperatures and precipitation. The positive SAM occurs more frequently during La Niña events and the negative is often associated with El Niño events.

The Southern Annular Mode is a function of air circulation around Antarctica and not ocean sea surface temperatures (SSTs) or SST distribution, thus it is not directly connected to global mean surface temperature (GMST). However, through its outsized influence on Southern Hemispheric weather it does seem to influence the HadCRUT5 GMST or at least correlate with it as shown in figure 3.

As noted in Turner (2019), the phase of the SAM “exerts the greatest control” over Antarctic temperatures. Compare the pre-2000 to post-2000 trends in figure 2 to the shift from a negative to a positive SAM in figures 3 and 4 to observe this. The western Antarctic Peninsula shows more of a change in temperature trends than East Antarctica.

The GMST is not a very good measure of global climate, the real climate state on Earth is too complicated to describe with any one number but none-the-less, it has become the climate indicator that nearly everyone uses to attempt to measure the impact humans have on climate. As figure 3 shows, the changes in SAM track the changes in GMST as measured by the Hadley Centre.

Figure 3 suggests that Southern Hemisphere wind circulation patterns affect global climate or that both are influenced by some common force. The common forcing could be changes in the Sun, Earth’s orbit, or additional atmospheric CO₂. Thus, it could be that global circulation patterns are not just moving heat around in a random fashion, it could be that as they change, regardless of the cause, they affect global climate over the long term.

Unlike most climate oscillations, the CMIP6 climate models do a fair job modeling SAM and collectively they also reproduce the positive trend since 1925 (see figure 3) pretty well (IPCC, 2021, pp. 71, 113, 115, 491). The IPCC believes that the recently increasingly positive SAM is due to CO₂-fueled warming (IPCC, 2021, p. 114) & (Lee, Petersen, & Lin, 2019). But it is hard to determine cause and effect in this case.

AR6 admits there is a “lack of consistency in links between increasing greenhouse gases,” the Southern Annular Mode and drought in Cape Town as an example (IPCC, 2021, p. 110). Lee, et al. did a comprehensive model study (using E3SM) of the annual and seasonal SAM and found that while historical and standard model test data (AMIP) created a statistically significant positive trend in recent years, their CO₂-based model experiments did not, in fact their 4 times CO2 (4xCO2) model trends opposite (negative) to observations. This casts considerable doubt on the idea that CO₂-based warming is affecting the SAM as claimed by the IPCC. Neither their 4xCO2 nor their 1pctCO2 model runs were statistically significant, and neither compared well to observations annually or by season (Lee, Petersen, & Lin, 2019).

The SAM trends positive with time since the early 20^th century as shown in figure 3. This suggests that the southern polar vortex is increasing in strength on average. It also suggests more frequent La Niña conditions in the tropical Pacific. The SAM, the polar vortex strength, and the ENSO are all affected by stratospheric circulation patterns, which, in turn are influenced by the solar cycle (Haigh, 2011). Haigh also reports that solar activity appears to affect SAM, this can be seen, in a rough way, in figure 4.

The general correspondence in positive SAM trends and solar activity is fair. SAM shows a strong positive trend from 1925-1945 in the first part of the Modern Solar Maximum and from 1970 to 2000 in the second part. But the correspondence from 1945 to 1975 is not very good. After the end of the Modern Solar Maximum in 2005, the SAM trend levels out. The pentadecadal decrease in solar activity from about 1963-1977 in solar cycle 20 and the solar cycle minimum between solar cycles 18 and 19 (1952-1956) may account for some of the interruption in the positive SAM trend from 1945 to 1975, but some other unknown factors are needed to fully explain it.

Discussion

The Southern Annular Mode or Antarctic Oscillation is defined as the sea level air pressure difference between 40°S and 65°S. It is positive when the SLP in higher at 40°S than at 65°S. The climate response varies by season and location, but generally a positive trend means warming in the mid-latitudes and a movement of the strongest surface circumpolar winds poleward (Lee, Petersen, & Lin, 2019). This accounts for much of the good correlation between SAM and HadCRUT5 shown in figure 3.

As Joanna Haigh and others have pointed out there does appear to be a connection between solar activity and SAM, but it is not perfect since solar activity was high during most of the mid-20^th century cooling period from 1945 to 1975. Specifically, the 1945 to 1960 portion during solar cycles 18 and 19. Thus, if solar activity influences SAM, something else contributed to the mid-20^th century cooling. That period of cooling was global and can be clearly seen in Southern Hemisphere temperature reconstructions (Brönnimann, Brugnara, & Wilkinson, 2024).

The introduction of more CO₂ in the E3SM model studies by Lee, et al. produce a negative trend or no trend in SAM which is contrary to observations, making CO₂ an unlikely cause of the recent positive SAM trend. Another possible influence on SAM may be trends in other oscillations or changes in the stratosphere as proposed by Wallace (2006). There does appear to be a strong connection between SAM and the stratospheric polar vortex.

The SAM has a powerful influence on global climate and can affect weather in the Northern Hemisphere (Lin, Yu, & Hall, 2025), in particular the Warm Arctic-Cold Eurasian weather pattern that causes a lot of extreme winter weather. The SAM also affects the Indian summer monsoon and other eastern Asia weather phenomena.

The next post will discuss the Atlantic Meridional Mode or AMM.

Brönnimann, S., Brugnara, Y., & Wilkinson, C. (2024). Early 20th century Southern Hemisphere cooling. Climate Past, 20. doi:10.5194/cp-20-757-2024

Gong, D., & Wang, S. (1999). Definition of Antarctic Oscillation Index. Geophysical Research Letters, 26(4), 459-462. doi:10.1029/1999GL900003

Haigh, J. (2011). Solar Influences on Climate. Imperial College, London. Retrieved from https://www.imperial.ac.uk/media/imperial-college/grantham-institute/public/publications/briefing-papers/Solar-Influences-on-Climate—Grantham-BP-5.pdf

IPCC. (2021). Climate Change 2021: The Physical Science Basis. In V. Masson-Delmotte, P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, . . . B. Zhou (Ed.)., WG1. Retrieved from https://www.ipcc.ch/report/ar6/wg1/

Lee, D. Y., Petersen, M. R., & Lin, W. (2019). The Southern Annular Mode and Southern Ocean Surface Westerly Winds in E3SM. Earth and Space Science, 6(12), 2624-2643. doi:10.1029/2019EA000663

Lin, H., Yu, B., & Hall, N. (2025). Link of the Warm Arctic Cold Eurasian pattern to the Southern Annular Mode variability. npj Climate & Atmospheric Science, 8. doi:10.1038/s41612-025-01102-z

Turner, J., Marshall, G. J., Clem, K., Colwell, S., Phillips, T., & Lu, H. (2019). Antarctic temperature variability and change from station data. International Journal of Climatology, 2986-3007. doi:10.1002/joc.6378

Wallace, J. (2006). On the Role of the Arctic and Antarctic Oscillations in Polar Climate. ECMWF Seminar on Polar Meteorology (pp. 75-88). ECMWF.

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