In this study, the intraseasonal variations in storm-track activity, surface air temperature, and precipitation over North America associated with the Madden–Julian oscillation (MJO) in boreal winter (November–April) are investigated. A lag composite strategy that considers different MJO phases and different lag days is developed. The results highlight regions over which the MJO has significant impacts on surface weather on intraseasonal time scales. A north–south shift of storm-track activity associated with the MJO is found over North America. The shift is consistent with the MJO-related surface air temperature anomaly over the eastern United States. In many regions over the western, central, and southeastern United States, the MJO-related precipitation signal is also consistent with nearby storm-track activity. An MJO-related north–south shift of precipitation is also found near the west coast of North America, with the precipitation over California being consistent with the MJO-related storm-track activity over the eastern Pacific. MJO-related temperature and storm-track anomalies are also found near Alaska. Further analyses of streamfunction anomalies and wave activity flux show clear signatures of Rossby wave trains excited by convection anomalies related to MJO phases 3 and 8. These wave trains propagate across the Pacific and North America, bringing an anticyclonic (cyclonic) anomaly to the eastern part of North America, shifting the westerly jet to the north (south), thereby modulating the surface air temperature and storm-track activity over the continent. Rossby waves associated with phases 2 and 6 are also found to impact the U.S. West Coast.

Extratropical cyclones give rise to much of the precipitation over California. Observed California winter precipitation is highly correlated to a metric of extratropical cyclone activity over the Eastern Pacific. The lack of precipitation over the recent winters is coincident with consecutive winters of much below average cyclone activity. Analysis of variability in cyclone activity and California precipitation simulated by models participating in Coupled Model Intercomparison Project Phase 5 indicates that most models can simulate the relationship between cyclone activity and precipitation well. Examination of projected change suggests (1) no evidence of a long‐term downward trend in California region cyclone activity within the examined scenarios and (2) that the intermodel spread in California precipitation projection can be largely explained by the spread in the projection of extratropical cyclone activity. This highlights the need to further understand physical mechanisms for the variation in projection of cyclone activity in this region.

Extratropical cyclones cause much of the high‐impact weather over the midlatitudes. With increasing greenhouse gases, enhanced high‐latitude warming will lead to weaker cyclone activity. Here we show that between 1979 and 2014, the number of strong cyclones in Northern Hemisphere in summer has decreased at a rate of 4% per decade, with even larger decrease found near northeastern North America. Climate models project a decrease in summer cyclone activity, but the observed decreasing rate is near the fastest projected. Decrease in summer cyclone activity will lead to decrease in cloud cover, giving rise to higher maximum temperature, potentially enhancing the increase in maximum temperature by 0.5 K or more over some regions. We also show that climate models may have biases in simulating the positive relationship between cyclone activity and cloud cover, potentially underestimating the impacts of cyclone decrease on accentuating the future increase in maximum temperature.

Understanding the change in intensity and frequency of extreme precipitation plays an important role in flood risk mitigation and water resource management in China. In this study, we analyzed the abrupt changes and long‐term trends in extreme precipitation intensity and frequency over China from 1960 to 2015 based on daily precipitation from stations. The possible teleconnection with large‐scale climate index was also been investigated. The major results are as follows: (1) 14.72% and 23.51% of all the stations over China have a change point in intensity and frequency, respectively. Moreover, most of change points occurred after 1975. (2) Extreme precipitation intensity and frequency show similar significant change trends, with a decreasing trend along the strip extending the northeast to southwest direction and an increasing trend in the two sides of the strip; 56.44% and 66.23% of all the stations shows the increasing trends in intensity and frequency. (3) Large‐scale climate indices have more influences on frequency rather than intensity. Especially, Dipole Mode Index with 1 year ahead has significantly positive correlation with extreme precipitation frequency in most areas of China, except for South China.