The Opposite Trend of Summer Stationary–Transient Wave Interference in the Eastern and Western Hemispheres and Its Relationship with Heat Waves and Anomalous Tropical Diabatic Heating

Recent studies have found that the increasing trend of boreal summer temperature extremes is closely linked to a particular upper level circulation pattern. This circulation, referred to here as extreme temperature (ET) pattern, consists of anomalous high pressures co-located at centers of temperature extremes, mainly over Europe, Asia and Greenland. Given the quasi-stationarity of this circulation system, this research examines the possible interference between this circulation pattern and the stationary wave. Previous studies showed that the summer stationary wave is largely driven by diabatic heating from various tropical regions, mainly Indian-South East Asian and Central American regions. The response to this heating field is baroclinic over the Eastern Hemisphere (EH) and, with the additional effect of orography, barotropic over the Western Hemisphere (WH). Because anomalous heating can generate atmospheric wave trains, we hypothesize that the upward trend of the ET pattern is caused by decadal time-scale heating trends over Indian-South East Asia and Central America. The former (latter) heating is referred to as EH (WH) heating.

To test the hypothesis, the interference between transient anomalous waves and the climatological stationary wave is examined separately for the EH and WH. For each hemisphere, an index was constructed which is designed to measure the sign (constructive or destructive) and strength of daily interference. Using this index, composites of 300-hPa streamfunction, surface temperature, and latent heating are calculated and analyzed. The ERA-Interim and JRA-55 reanalysis are used for the analysis and the time period examined is 1979-2017.

The result shows that the ET pattern resembles the constructive interference pattern in the EH, and the destructive interference pattern in the WH. This rather surprising result can be explained by the fact that (1) the EH stationary wave pattern is driven by EH heating, while the WH stationary wave pattern is driven by both EH and WH heating: (2) The two heating centers drive stationary wave that tend to oppose each other in the WH, with the circulation driven by EH dominating: (3) Over 1979-2017 period, the EH heating had been expanding poleward, while the WH heating had significantly intensified. As a result, in the WH, the circulation trend pattern is dominated by WH-heating-driven circulation which destructively interferes with the climatological stationary wave. This result is supported by initial-value calculations forced by observed heating. The result of this study indicates that changes in the tropical diabatic heating field has played an important role in driving the ET pattern, hence the temperature extremes.