Abstract
In 2012, extreme anomalous climate conditions occurred around the globe. Large areas of North America experienced an anomalously hot summer, with large precipitation deficits inducing severe drought. Over Europe, the summer of 2012 was marked by strong precipitation anomalies with the UK experiencing its wettest summer since 1912 while Spain suffered severe drought. What caused these extreme climate conditions in various regions in 2012? This study compares attribution conclusions for 2012 climate anomalies relative to a baseline period (1964–1981) based on two sets of parallel experiments with different model configurations (with coupling to an ocean mixed layer model or with prescribed sea surface temperatures) to assess whether attribution conclusions concerning the climate anomalies in 2012 are sensitive to the representation of air-sea interaction. Modelling results indicate that attribution conclusions for large scale surface air temperature (SAT) changes in both boreal winter and summer are generally robust and not very sensitive to air-sea interaction. This is especially true over southern Europe, Eurasia, North America, South America, and North Africa. Some other responses also appear to be insensitive to air-sea interaction: for example, forced increases in precipitation over northern Europe and Sahel, and reduced precipitation over North America and the Amazon in boreal summer. However, the attribution of circulation and precipitation changes for some other regions exhibits a sensitivity to air-sea interaction. Results from the experiments including coupling to an ocean mixed layer model show a positive NAO-like circulation response in the Atlantic sector in boreal winter and weak changes in the East Asian summer monsoon and precipitation over East Asia. With prescribed sea surface temperatures, some different responses arise over these two regions. Comparison with observed changes indicates that the coupled simulations generally agree better with observations, demonstrating that attribution methods based on atmospheric general circulation models have limitations and may lead to erroneous attribution conclusions for regional anomalies in circulation, precipitation and surface air temperature.
Highlights
Weather and climate extreme events can have devastating impacts on human society, the economy, and the infrastructure
When this warming is prescribed in the uncoupled model simulations it leads to enhanced convection and increased precipitation over the Maritime Continent (MC), leading to negative anomalies in SW cloud radiative effect (CRE) associated with the increase in cloud cover (Fig. 12e)
These results suggest that air–sea interaction over the MC is fundamental for the attribution of East Asian circulation and precipitation responses due to anthropogenic forcing changes seen in 2012 relative the baseline period of 1964–1981
Summary
Weather and climate extreme events can have devastating impacts on human society, the economy, and the infrastructure. Most of the above attribution studies on extreme events in 2012 have used atmospheric general circulation models (AGCMs) forced by prescribed sea surface temperatures (SSTs), with and without anthropogenic influences. Dong et al (2017b) demonstrated that attribution conclusions for forced decadal surface air temperature changes derived from AGCM experiments are generally robust and not very sensitive to air-sea coupling. The evidence that air–sea coupling is important for the simulation of climate, climate variability, and the attribution of externally forced decadal changes and temperature extremes motivates us to investigate the implications for attribution studies of some events in a particular year. 3. Section 4 presents the global responses of surface air temperature, circulation and precipitation to changes in anthropogenic forcing in 2012 in the coupled and uncoupled model simulations; An assessment of whether coupling improves the comparison between the model simulations and observations, and the effect this has on attribution statements are in Sect.
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