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The Antarctic surface mass balance (SMB) has global climatic impacts through its effects on global sea-level rise. The forced increase in Antarctic SMB over the second half of the 20th century was argued to stem from multiple forcing agents, including ozone and ozone-depleting substances (ODSs). Here we use ensembles of fixed-forcing model simulations to quantify and contrast the contributions of stratospheric ozone, tropospheric ozone and ODSs to increases in the Antarctic SMB. We show that ODSs and stratospheric ozone make comparable contributions and together account for 44 % of the increase in the annual mean Antarctic SMB over the second half of the 20th century. In contrast, tropospheric ozone has an insignificant impact on the SMB increase. A large portion of the annual mean SMB increase occurs during austral summer, when stratospheric ozone is found to account for 63 % of the increase. Furthermore, we demonstrate that stratospheric ozone increases the SMB by enhancing the meridional mean and eddy flows towards the continent, thus converging more water vapor over the Antarctic.

Previous studies found many climate properties such as northern hemisphere (NH) surface temperature and precipitation respond non-monotonically when CO _2 is increased from 1 × to 8 × CO _2 relative to pre-industrial levels. Here, we explore the robustness of the non-monotonicity in the NH precipitation response in 11 coupled climate models. Eight models show a decrease in NH precipitation under repeated CO _2 doubling, indicating that the non-monotonic response is a common but not universal result. Although common, the critical CO _2 level where the NH precipitation decrease first occurs differs widely across models, ranging from 2×CO _2 to 8×CO _2 . These models also show a prominent weakening in the Atlantic meridional overturning circulation (AMOC) at the same critical CO _2 level, with the AMOC weakening leading the precipitation decrease. The sensitivities of NH precipitation and the AMOC to CO _2 doublings are positively correlated, especially when the AMOC weakens beyond 10 Sv. This suggests that the differences in models’ AMOC response can explain their contrasting NH precipitation responses, where models with a large AMOC weakening have decreased NH precipitation. Regionally, this decrease in NH precipitation is the most prominent over the North Atlantic, Europe and the tropical Pacific. Our results suggest that special care must be taken with the use of pattern scaling to inform regional climate decision-making.

Recently established North Atlantic ocean observing arrays, such as RAPID/MOCHA, have revealed a large degree of high-frequency variability in the Atlantic Meridional Overturning Circulation (AMOC). Climate modeling studies of the AMOC, however, have traditionally focused on the low-frequency variability of the annual mean AMOC, with an emphasis on multi-decadal and longer time-scale variability. Thus, little is known about the sources of interannual wintertime, wind-driven AMOC variability. Analyzing the Community Earth System Model, we here show the existence of a robust leading mode of interannual variability in the wintertime AMOC that is distinct from the leading mode of the annual mean. We further show that this mode of variability is significantly linked to the El Niño-Southern Oscillation via the North Atlantic Oscillation.

Over the coming century, both Arctic and Antarctic sea ice cover are projected to substantially decline. While many studies have documented the potential impacts of projected Arctic sea ice loss on the climate of the mid-latitudes and the tropics, little attention has been paid to the impacts of Antarctic sea ice loss. Here, using comprehensive climate model simulations, we show that the effects of end-of-the-century projected Antarctic sea ice loss extend much further than the tropics, and are able to produce considerable impacts on Arctic climate. Specifically, our model indicates that the Arctic surface will warm by 1 °C and Arctic sea ice extent will decline by 0.5 × 10 ^6 km ^2 in response to future Antarctic sea ice loss. Furthermore, with the aid of additional atmosphere-only simulations, we show that this pole-to-pole effect is mediated by the response of the tropical SSTs to Antarctic sea ice loss: these simulations reveal that Rossby waves originating in the tropical Pacific cause the Aleutian Low to deepen in the boreal winter, bringing warm air into the Arctic, and leading to sea ice loss in the Bering Sea. This pole-to-pole signal highlights the importance of understanding the climate impacts of the projected sea ice loss in the Antarctic, which could be as important as those associated with projected sea ice loss in the Arctic.

A comprehensive stratosphere-resolving atmospheric model, with interactive stratospheric ozone chemistry, coupled to ocean, sea ice and land components is used to explore the tropospheric and surface impacts of large springtime ozone anomalies in the Arctic stratosphere. Coupling between the Antarctic ozone hole and Southern Hemisphere climate has been identified in numerous studies, but connections of Arctic ozone loss to surface climate have been more difficult to elucidate. Analyzing an ensemble of historical integrations with all known natural and anthropogenic forcings specified over the period 1955–2005, we find that extremely low stratospheric ozone changes are able to produce large and robust anomalies in tropospheric wind, temperature and precipitation in April and May over large portions of the Northern Hemisphere (most notably over the North Atlantic and Eurasia). Further, these ozone-induced surface anomalies are obtained only in the last two decades of the 20th century, when high concentrations of ozone depleting substances generate sufficiently strong stratospheric temperature anomalies to impact the surface climate. Our findings suggest that coupling between chemistry and dynamics is essential for a complete representation of surface climate variability and climate change not only in Antarctica but also in the Arctic.

In the Arctic stratosphere, total column ozone in the spring can vary, from year to year, by as much as 30%. This large interannual variability, however, is absent from many present-generation climate models, in which the prescribed seasonal cycle of stratospheric ozone includes, at best, smooth multi-decadal trends. We here investigate the extent to which interannual variability in Arctic stratospheric ozone is able to affect the surface climate of the Northern Hemisphere extratropics. We do this by contrasting pairs of model integrations with positive and negative springtime ozone anomalies, using a simple yet widely used climate model. For ozone anomaly amplitudes somewhat larger than the recent observed variability, we find a significant influence on the tropospheric circulation, and the surface temperatures and precipitation patterns. More interestingly, these impacts have very clear regional patterns—they are largest over the North Atlantic sector—even though the prescribed ozone anomalies are zonally symmetric. However, confirming other studies, for ozone anomaly amplitudes within the observed range of the last three decades, our model experiments do not show statistically significant impacts at the surface.

In order to quantify the impact of recent efforts to abate surface ozone (O _3 ) pollution, we analyze changes in the frequency and return level of summertime (JJA) high surface O _3 events over the eastern United States (US) from 1988–1998 to 1999–2009. We apply methods from extreme value theory (EVT) to maximum daily 8-hour average ozone (MDA8 O _3 ) observed by the Clean Air Status and Trends Network (CASTNet) and define O _3 extremes as days on which MDA8 O _3 exceeds a threshold of 75 ppb (MDA8 O _3 >75). Over the eastern US, we find that the number of summer days with MDA8 O _3 >75 declined on average by about a factor of two from 1988–1998 to 1999–2009. The applied generalized Pareto distribution (GPD) fits the high tail of MDA8 O _3 much better than a Gaussian distribution and enables the derivation of probabilistic return levels (describing the probability of exceeding a value x within a time window T) for high O _3 pollution events. This new approach confirms the significant decline in both frequency and magnitude of high O _3 pollution events over the eastern US during recent years reported in prior studies. Our analysis of 1-yr and 5-yr return levels at each station demonstrates the strong impact of changes in air quality regulations and subsequent control measures (e.g., the ‘NO _x SIP Call’), as the 5-yr return levels of the period 1999–2009 correspond roughly to the 1-yr return levels of the earlier time period (1988–1998). Regionally, the return levels dropped between 1988–1998 and 1999–2009 by about 8 ppb in the Mid-Atlantic (MA) and Great Lakes (GL) regions, while the strongest decline, about 13 ppb, is observed in the Northeast (NE) region. Nearly all stations (21 out of 23) have 1-yr return levels well below 100 ppb and 5-yr return levels well below 110 ppb in 1999–2009. Decreases in eastern US O _3 pollution are largest after full implementation of the nitrogen oxide (NO _x ) reductions under the ‘NO _x SIP Call’. We conclude that the application of EVT methods provides a useful approach for quantifying return levels of high O _3 pollution in probabilistic terms, which may help to guide long-term air quality planning.