[Show abstract][Hide abstract] ABSTRACT: The recent decade has seen an exceptional number of high-impact summer extremes in the Northern Hemisphere midlatitudes. Many of these events were associated with anomalous jet stream circulation patterns characterized by persistent high-amplitude quasi-stationary Rossby waves. Two mechanisms have recently been proposed that could provoke such patterns: (i) a weakening of the zonal mean jets and (ii) an amplification of quasi-stationary waves by resonance between free and forced waves in midlati-tude waveguides. Based upon spectral analysis of the midtropo-sphere wind field, we show that the persistent jet stream patterns were, in the first place, due to an amplification of quasi-stationary waves with zonal wave numbers 6–8. However, we also detect a weakening of the zonal mean jet during these events; thus both mechanisms appear to be important. Furthermore, we demon-strate that the anomalous circulation regimes lead to persistent surface weather conditions and therefore to midlatitude synchro-nization of extreme heat and rainfall events on monthly time-scales. The recent cluster of resonance events has resulted in a statistically significant increase in the frequency of high-ampli-tude quasi-stationary waves of wave numbers 7 and 8 in July and August. We show that this is a robust finding that holds for dif-ferent pressure levels and reanalysis products. We argue that re-cent rapid warming in the Arctic and associated changes in the zonal mean zonal wind have created favorable conditions for dou-ble jet formation in the extratropics, which promotes the develop-ment of resonant flow regimes. climate change | Arctic amplification | climate impact | planetary waves | midlatitude weather C limatic warming over the 20th century has increased the frequency of extreme heat and heavy rainfall events (1–7). On a global scale, the magnitude of this gradual increase can largely be explained by a slowly warming atmosphere, i.e., by thermodynamic arguments only. Thus, the rise in the number of heat extremes can largely be explained by a shift in the mean to warmer values (4, 5, 8). Likewise, upward trends in annual maximum daily rainfall are consistent with the increase in at-mospheric moisture associated with warmer air (1, 2). Global warming is also likely to affect large-scale atmospheric circulation patterns, which potentially could alter the frequency of heat and precipitation extremes on seasonal to subseasonal timescales (9–11). In principle, changes in atmospheric dynamics could cause a disproportionate change in the number and/or intensity of extreme weather events (12–14), beyond what is expected from thermodynamics. Moreover, the magnitude of several recent summer extreme weather events in the Northern Hemisphere midlatitudes cannot be explained by a simple shift in the mean (12, 15, 16). These events, which include high-impact extremes like the European heat wave of 2003 (15), the Russian heat wave and the Pakistan flooding in 2010 (17), and heat waves in the United States in recent years (18), were associated with anomalous circulation patterns characterized by persistent, blocking weather conditions (10, 19–22). Atmosphere Dynamical Mechanisms To explain the persistent weather conditions during recent ex-treme summers, several atmosphere dynamics mechanisms
Proceedings of the National Academy of Sciences 08/2014; · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Eurasian Arctic contains some of the largest rivers on Earth. Our synthesis of river monitoring data reveals that the average annual discharge of freshwater from the six largest Eurasian rivers (Yenisey, Lena, Ob', Kolyma, Pechora, S. Dvina) to the Arctic Ocean increased about 7% from 1936 through 1999. Correspondence between discharge from these Eurasian arctic rivers and the North Atlantic Oscillation (NAO) suggests that variations in discharge are coupled to hemispheric climate patterns. Increases in discharge also correspond to increases in global, pan-Arctic, and Eurasian arctic temperatures. If the increasing river discharge is a response to global warming, the quantity of extra water delivered to the Arctic Ocean within the next century could approach that predicted by climate models to significantly impact the Atlantic thermohaline circulation.
[Show abstract][Hide abstract] ABSTRACT: Large uncertainty surrounds projections of global sea-level rise, resulting from uncertainty about future warming and an incomplete understanding of the complex processes and feedback mechanisms that cause sea level to rise. Consequently, existing models produce widely differing predictions of sea-level rise even for the same temperature scenario. Here we present results of a broad survey of 90 experts who were amongst the most active scientific publishers on the topic of sea level in recent years. They provided a probabilistic assessment of sea-level rise by AD 2100 and AD 2300 under two contrasting temperature scenarios. For the low scenario, which limits warming to <2 °C above pre-industrial temperature and has slowly falling temperature after AD 2050, the median ‘likely’ range provided by the experts is 0.4–0.6 m by AD 2100 and 0.6–1.0 m by AD 2300, suggesting a good chance to limit future sea-level rise to <1.0 m if climate mitigation measures are successfully implemented. In contrast, for the high warming scenario (4.5 °C by AD 2100 and 8 °C in AD 2300) the median likely ranges are 0.7–1.2 m by AD 2100 and 2.0–3.0 m by AD 2300, calling into question the future survival of some coastal cities and low-lying island nations.
[Show abstract][Hide abstract] ABSTRACT: Maps of global temperature trends over the 20th Century show a
conspicuous region of cooling near the southern tip of Greenland. It has
long been speculated whether this is related to a slowdown in the
Atlantic meridional overturning circulation (AMOC), since a cooling
patch in this area is a prime response to an AMOC slowdown in climate
models. Moreover, Thompson et al. (Nature 2010) reported a sudden drop
in Northern Hemisphere sea surface temperatures around 1970. We discuss
multiple lines of evidence suggesting that both the cooling near
Greenland and the drop in Northern Hemisphere SST are due to a sudden
reduction in the AMOC starting around 1970, linked to the Great Salinity
Anomaly. Since ~ 1990 the AMOC appears to be recovering. This time
evolution is consistently suggested by an AMOC-index based on surface
temperatures, by the hemispheric temperature difference, by coral-based
proxies and by oceanic measurements. We relate this sudden slowdown to
the melting history of the Greenland Ice Sheet. Using multi-proxy
temperatures in the AMOC-index suggests that the sudden AMOC decline
from 1970-1990 is an unprecedented event in the past millennium.
[Show abstract][Hide abstract] ABSTRACT: In Earth's present-day climate, the annually-averaged surface air
temperature in the Northern Hemisphere (NH) is ? 1.5°C higher than
in the Southern Hemisphere (SH). This interhemispheric temperature
difference has been known for a long time, and scientists have pondered
over its origin for centuries. Frequently suggested causes include
differences in seasonal insolation, the larger area of tropical land in
the NH, albedo differences between the Earth's polar regions, and
northward heat transport by the ocean circulation. Here we
systematically assess the origin of the interhemispheric temperature
difference. To this end we combine an analysis of climatological data as
well as observations of the Earth's energy budget with simulations using
a coupled climate model. We find that the interhemispheric temperature
difference is predominantly caused by meridional heat transport in the
oceans, with an additional contribution from the albedo difference
between Antarctica and the Arctic.
[Show abstract][Hide abstract] ABSTRACT: The prediction of global sea-level rise is one of the major challenges of climate science. While process-based models are still being improved to capture the complexity of the processes involved, semi-empirical models, exploiting the observed connection between global-mean sea level and global temperature and calibrated with data, have been developed as a complementary approach. Here we investigate whether twentieth century sea-level rise could have been predicted with such models given a knowledge of twentieth century global temperature increase. We find that either proxy or early tide gauge data do not hold enough information to constrain the model parameters well. However, in combination, the use of proxy and tide gauge sea-level data up to 1900 AD allows a good prediction of twentieth century sea-level rise, despite this rise being well outside the rates experienced in previous centuries during the calibration period of the model. The 90% confidence range for the linear twentieth century rise predicted by the semi-empirical model is 13–30 cm, whereas the observed interval (using two tide gauge data sets) is 14–26 cm.
Environmental Research Letters 03/2013; 8(1):014013. · 3.58 Impact Factor