Papers on Arctic Oscillation and global warming

This is a list of papers on the global warming effects to the Arctic Oscillation – both observed changes and future projections are included. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

Arctic Oscillation responses to greenhouse warming and role of synoptic eddy feedback – Choi et al. (2010) “This study investigates possible changes in the leading mode over the Northern Hemisphere, representing the Arctic Oscillation (AO), in response to the projected increases in greenhouse gas concentrations. This is performed by comparing present-day and future patterns simulated by a relatively high-resolution atmospheric general circulation model. It is demonstrated that the dipole pattern associated with the AO distinctively shifts poleward in the future climate. The poleward shift is more pronounced over the Pacific region than over the Atlantic region. This change in the AO pattern is consistent with the change in the synoptic eddy feedback, estimated from the divergence of the eddy-vorticity flux, indicating a close linkage between the AO change and the change in the synoptic eddy feedback. Further analysis of changes in eddy feedback strength suggests a possible hypothesis that the poleward shift of the jet stream and storm tracks can make synoptic eddy feedback more effective over the higher latitudes, which in turn enhances the poleward shift of the AO mode.” Choi, D.-H., J.-S. Kug, W.-T. Kwon, F.-F. Jin, H.-J. Baek, and S.-K. Min (2010), Arctic Oscillation responses to greenhouse warming and role of synoptic eddy feedback, J. Geophys. Res., 115, D17103, doi:10.1029/2010JD014160.

Changes in the Arctic Oscillation under increased atmospheric greenhouse gases – Wu et al. (2007) “The Arctic Oscillation (AO) under increased atmospheric concentration of greenhouse gases (GHG) was studied by comparing an ensemble of simulations from 13 coupled general circulation models with GHG at the pre-industrial level and at the late 20th century level, for November to March. The change in the linear AO pattern as GHG increased reveals positive sea level pressure (SLP) anomalies centered over the Gulf of Alaska, and weaker negative SLP anomalies over eastern Canada and North Atlantic – a pattern resembling the nonlinear AO pattern arising from a quadratic relation to the AO index. This quadratic AO pattern itself has positive SLP anomalies receding from Europe but strengthening over the Gulf of Alaska and surrounding areas as GHG increased. This study points to the importance of the nonlinear structure in determining how the linear oscillatory pattern changes when there is a change in the mean climate.” Wu, A., W. W. Hsieh, G. J. Boer, and F. W. Zwiers (2007), Geophys. Res. Lett., 34, L12701, doi:10.1029/2007GL029344. [Full text]

Influence of Arctic Oscillation towards the Northern Hemisphere Surface Temperature Variability under the Global Warming Scenario – Hori et al. (2007) “Future projection of the Arctic Oscillation (AO) signature and its significance towards the northern hemispheric surface temperature trend have been examined using 20 state-of-the-art Atmosphere-Ocean General Circulation Model (AOGCM) outputs forced under the IPCC SRES-A1B and 20C3M emission scenario. Models are mostly successful in simulating the observed AO structure and the corresponding surface temperature variability. It is found that while AO exhibits a large positive trend, especially during the autumn season with a relatively smaller trend during the winter and spring seasons. In all seasons the interannual variance in AO remains the same for both scenarios. These features in the timeseries leads to two distinct patterns of temperature variability. One is the “polar amplification” pattern due to the long-term anthropogenic forcing, which is much larger in its amplitude. Another is related to the natural variability of the AO, which is confined over the land surface and is marked by a dipole pattern of temperature between the Eurasian continent and Greenland. It is argued that the gradual trend in the AO is not a result of enhanced natural variability of the AO dynamics itself, but rather a result of the large anthropogenic forced linear trend projected onto the mean climatological state of the Arctic region. Distinguishing these two patterns of warming is crucial for detecting the signal of future global warming trend over the Eurasian continent and other regions.” Masatake E. Hori, Daisuke Nohara and Hiroshi L. Tanaka, Journal of the Meteorological Society of Japan, Vol. 85 (2007) , No. 6 pp.847-859. [Full text]

The NAO, the AO, and Global Warming: How Closely Related? – Cohen & Barlow (2005) “The North Atlantic Oscillation (NAO) and the closely related Arctic Oscillation (AO) strongly affect Northern Hemisphere (NH) surface temperatures with patterns reported similar to the global warming trend. The NAO and AO were in a positive trend for much of the 1970s and 1980s with historic highs in the early 1990s, and it has been suggested that they contributed significantly to the global warming signal. The trends in standard indices of the AO, NAO, and NH average surface temperature for December–February, 1950–2004, and the associated patterns in surface temperature anomalies are examined. Also analyzed are factors previously identified as relating to the NAO, AO, and their positive trend: North Atlantic sea surface temperatures (SSTs), Indo–Pacific warm pool SSTs, stratospheric circulation, and Eurasian snow cover. Recently, the NAO and AO indices have been decreasing; when these data are included, the overall trends for the past 30 years are weak to nonexistent and are strongly dependent on the choice of start and end date. In clear distinction, the wintertime hemispheric warming trend has been vigorous and consistent throughout the entire period. When considered for the whole hemisphere, the NAO/AO patterns can also be distinguished from the trend pattern. Thus the December–February warming trend may be distinguished from the AO and NAO in terms of the strength, consistency, and pattern of the trend. These results are insensitive to choice of index or dataset. While the NAO and AO may contribute to hemispheric and regional warming for multiyear periods, these differences suggest that the large-scale features of the global warming trend over the last 30 years are unrelated to the AO and NAO. The related factors may also be clearly distinguished, with warm pool SSTs linked to the warming trend, while the others are linked to the NAO and AO.” Cohen, Judah, Mathew Barlow, 2005, J. Climate, 18, 4498–4513. [Full text]

The Arctic climate paradox: The recent decrease of the Arctic Oscillation – Overland & Wang (2005) “A current paradox is that many physical and biological indicators of Arctic change—summer sea-ice extent, spring surface air temperature and cloud cover, and shifts in vegetation and other ecosystems—show nearly linear trends over the previous two and a half decades, while the Arctic Oscillation, a representative atmospheric circulation index often associated with Arctic change, has had a different, more episodic behavior, with a near-neutral or negative phase for 6 of the last 9 years (1996–2004) following a positive phase (1989–1995). Stratospheric temperature anomalies over the Arctic, which serve as an index of the strength of the polar vortex, also show this episodic character. Model projections of Arctic temperature for 2010–2029 show model-to-model and region-to-region differences suggesting large variability in the future response of atmospheric circulation to external forcing. Thus internal processes in the western Arctic may have a larger role in shaping the present persistence of Arctic change than has been previously recognized.” Overland, J. E., and M. Wang (2005), Geophys. Res. Lett., 32, L06701, doi:10.1029/2004GL021752. [Full text]

Interdecadal Arctic Oscillation in twentieth century climate simulations viewed as internal variability and response to external forcing – Yukimoto & Kodera (2005) “Interdecadal variations similar to the Arctic Oscillation (AO) are investigated for internal variability (INTV) and response to external forcing (REXT) in an ensemble simulation of twentieth century climate. The significant trend in REXT implies that a sizeable part of the observed AO trend in recent decades can also be attributed to anthropogenic forcing. INTV is characterized by a barotropic dipole of zonal wind anomalies and associated wave propagation, suggesting a mechanism similar to the month-to-month AO. Its thermal structure can be attributed to dynamic processes. REXT exhibits a thermal structure that can be explained by responses to the forcing due to increased greenhouse gases. A westerly wind anomaly in the stratosphere as a thermal response corresponds to anomalies in wave propagation and meridional circulation that are similar to INTV, which may induce the AO-like annular pattern.” Yukimoto, S., and K. Kodera (2005), Geophys. Res. Lett., 32, L03707, doi:10.1029/2004GL021870.

The Recent Trend and Variance Increase of the Annular Mode – Feldstein (2002) “This study examines whether both the trend and the increase in variance of the Northern Hemisphere winter annular mode during the past 30 years arise from atmospheric internal variability. To address this question, a synthetic time series is generated that has the same intraseasonal stochastic properties as the annular mode. By generating a distribution of linear trend values for the synthetic time series, and through a chi-square statistical analysis, it is shown that this trend and variance increase are well in excess of the level expected from internal variability of the atmosphere. This implies that both the trend and the variance increase of the annular mode are due either to coupling with the hydrosphere and/or cryosphere or to driving external to the climate system. This behavior contrasts that of the first 60 years of the twentieth century, for which it is shown that all of the interannual variability of the annular mode can be explained by atmospheric internal variability.” Feldstein, Steven B., 2002, J. Climate, 15, 88–94. [Full text]

How linear is the Arctic Oscillation response to greenhouse gases? – Gillett et al. (2002) “We examine the sensitivity of the Arctic Oscillation (AO) index to increases in greenhouse gas concentrations in integrations of five climate models (the Hadley Centre coupled models (HadCM2 and HadCM3), the European Centre/Hamburg models (ECHAM3 and ECHAM4), and the Goddard Institute for Space Studies stratosphere-resolving (GISS-S) model) and in the National Centers for Environmental Prediction reanalysis. With the exception of HadCM2 all the models show a significant positive AO response to greenhouse gas forcing, but in the models lacking a well-resolved stratosphere that response is smaller than observed. In these models the AO index is linearly dependent on the radiative forcing, even up to ~20 times current CO₂ levels. By contrast, the GISS-S stratosphere-resolving model shows an AO response comparable to that observed, but the sensitivity of the model to further increases in forcing is reduced when CO2 levels exceed ~1.5 times preindustrial values. It has been suggested that greenhouse gas forcing results in the equatorward deflection of planetary waves, which leads to a cooling and strengthening of the polar vortex and hence an increase in the surface Arctic Oscillation. In the observations the number of sudden warmings has reduced dramatically, consistent with this planetary wave effect, leading to a large mean cooling of the vortex. However, neither the GISS-S nor the HadCM3 models are able to reproduce the observed temperature changes, suggesting that this explanation for the impact of the inclusion of a stratosphere in the model may be incomplete.” Gillett, N. P., M. R. Allen, R. E. McDonald, C. A. Senior, D. T. Shindell, and G. A. Schmidt (2002), J. Geophys. Res., 107(D3), 4022, doi:10.1029/2001JD000589. [Full text]

Annular Modes in the Extratropical Circulation. Part II: Trends – Thompson et al. (2000) “The authors exploit the remarkable similarity between recent climate trends and the structure of the “annular modes” in the month-to-month variability (as described in a companion paper) to partition the trends into components linearly congruent with and linearly independent of the annular modes. The index of the Northern Hemisphere (NH) annular mode, referred to as the Arctic Oscillation (AO), has exhibited a trend toward the high index polarity over the past few decades. The largest and most significant trends are observed during the “active season” for stratospheric planetary wave–mean flow interaction, January–March (JFM), when fluctuations in the AO amplify with height into the lower stratosphere. For the periods of record considered, virtually all of the JFM geopotential height falls over the polar cap region and the strengthening of the subpolar westerlies from the surface to the lower stratosphere, 50% of the JFM warming over the Eurasian continent, 30% of the JFM warming over the NH as a whole, 40% of the JFM stratospheric cooling over the polar cap region, and 40% of the March total column ozone losses poleward of 40°N are linearly congruent with month-to-month variations in the AO index. Summertime sea level pressure falls over the Arctic basin are suggestive of a year-round drift toward the positive polarity of the AO, but the evidence is less conclusive. Owing to the photochemical memory inherent in the ozone distribution, roughly half the ozone depletion during the NH summer months is linearly dependent on AO-related ozone losses incurred during the previous active season.” Thompson, David W. J., John M. Wallace, Gabriele C. Hegerl, 2000, J. Climate, 13, 1018–1036. [Full text, Paper 1 full text]

The Arctic and Antarctic oscillations and their projected changes under global warming – Fyfe et al. (1999) “The Arctic Oscillation (AO) and the Antarctic Oscillation (AAO) are the leading modes of high‐latitude variability in each hemisphere as characterized by the first EOF of mean sea‐level pressure. Observations suggest a recent positive trend in the AO and it is speculated that this may be related to global warming. The CCCma coupled general circulation model control simulation exhibits a robust and realistic AO and AAO. Climate change simulations for the period 1900–2100, with forcing due to greenhouse gases and aerosols, exhibit positive trends in both the AO and the AAO. The model simulates essentially unchanged AO/AAO variations superimposed on a forced climate change pattern. The results do not suggest that a simulated trend in the AO/AAO necessarily depends on stratospheric involvement nor that forced climate change will be expressed as a change in the occurence of one phase of the AO/AAO over another. This pattern of climate change projects exclusively on the AAO pattern in the southern hemisphere but not in the northern hemisphere where other EOFs are involved. The extent to which this forced climate change pattern and the unforced modes of variation are determined by the same mechanisms and feedbacks remains an open question.” Fyfe, J. C., G. J. Boer, and G. M. Flato (1999), Geophys. Res. Lett., 26(11), 1601–1604, doi:10.1029/1999GL900317. [Full text]