Local weather, particularly extreme local weather, is often determined by fluctuations in large patterns of regional atmospheric pressure and sea surface temperatures, such as the Arctic Oscillation (and its close relative, the North Atlantic Oscillation) and other patterns associated with El Niño-Southern Oscillation (ENSO). These recognizable patterns come and go over a period of months to years. And these patterns may be altered by global warming.
Circulation variations, such as the ones associated with ENSO, largely determine where stormy, wet weather is favored and where dry weather prevails. In regions where the effects of these circulation variations are similar to global warming effects, new extremes are observed. While ENSO and other sources of natural variability can determine the location of extremes,1 the intensity and duration of the associated extremes such as droughts, and the associated heat waves, have increased with climate change. Similarly, in the wet areas, the intensity of rains and risk of flooding is greater.
Regional circulation patterns have significantly changed in recent years.2 For example, changes in the Arctic Oscillation cannot be explained by natural variation and it has been suggested that they are broadly consistent with the expected influence of human-induced climate change.3 The signature of global warming has also been identified in recent changes in the Pacific Decadal Oscillation, a pattern of variability in sea surface temperatures in the northern Pacific Ocean.4
El Niños have become more common and their intensity has nearly doubled in recent years, a statistically rare event. This recent shift towards more intense and frequent El Niños is related to the recent increase in dry areas around the world.5 However, past observations and reconstructions of El Niño events from non-instrumental records such as corals show that El Niño events naturally fluctuate in magnitude and frequency over time, and this has been demonstrated in long climate model simulations of past and future climate as well.6
El Niño’s center of action appears to be shifting from the eastern to the central Pacific, which in turn is affecting the distribution and frequency of weather events.7 However, due to the wide natural fluctuations within circulation patterns, it is difficult to attribute recent changes solely to human activity. Further research is needed to determine how global warming impacts these circulation patterns.
Recent extreme weather events partially driven by circulation changes:
“Snowmageddon,” United States, 2010
A combination of changes in El Niño and the Arctic Oscillation (which is closely related to the North Atlantic Oscillation) has been identified as the immediate driver of the heavy snowfall experienced by the mid-Atlantic states in the United States during the winter of 2010.8 The warm tropical Atlantic contributed extra moisture to the record snows. Global warming may have contributed to the record negative phase of the Arctic Oscillation that helped to drive the extreme weather south to the U.S. (See Winter Storms section for more detail.)
Pakistan floods, 2010
The weather pattern over northern India, the Himalayas, and Pakistan is usually dominated by a circulation pattern that prevents mid-latitude weather systems from penetrating very far south. However, in 2010, active weather systems spread southward into Pakistan. Global warming had increased the amount of atmospheric moisture available to condense into rain, and La Niña, a circulation pattern that can produce heavy rains in Pakistan, was in progress. These systems combined with the monsoon to produce record rainfall. Record-breaking high temperatures in Moscow were another consequence of the mid-latitude weather moving southward, as was the excessive rain and flooding over China.9 Several studies are currently exploring how climate change may have contributed to these events, but scientists at the World Meteorological Organization are already convinced that higher Atlantic Ocean temperatures from global warming were a contributing factor.10
Southeastern U.S. floods and drought, early 2000s
The recent dramatic swings between drought and flooding in the southeastern United States have been found to be linked to changes in the North Atlantic Subtropical High (also known as the Bermuda High), and these changes may be linked to global warming.11
The precise connections between global warming and circulation changes are still being investigated and our understanding will evolve accordingly. However, we now know that climate change is already affecting regional circulation patterns and by extension helping to shape local extreme weather.