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This study investigates 1) the eastward propagation of the Madden–Julian oscillation (MJO) and global occurrences of extreme precipitation, 2) the degree to which a general circulation model with a relatively realistic representation of the MJO simulates its influence on extremes, and 3) a possible modulation of the MJO on potential predictability of extreme precipitation events. The observational analysis shows increased frequency of extremes during active MJO phases in many locations. On a global scale, extreme events during active MJO periods are about 40% higher than in quiescent phases of the oscillation in locations of statistically significant signals. A 10-yr National Aeronautics and Space Administration (NASA) Goddard Laboratory for the Atmospheres (GLA) GCM simulation with fixed climatological SSTs is used to generate a control run and predictability experiments. Overall, the GLA model has a realistic representation of extremes in tropical convective regions associated with the MJO, although some shortcomings also seem to be present. The GLA model shows a robust signal in the frequency of extremes in the North Pacific and on the west coast of North America, which somewhat agrees with observational studies. The analysis of predictability experiments indicates higher success in the prediction of extremes during an active MJO than in quiescent situations. Overall, the predictability experiments indicate the mean number of correct forecasts of extremes during active MJO periods to be nearly twice the correct number of extremes during quiescent phases of the oscillation in locations of statistically significant signals. [ABSTRACT FROM AUTHOR]

The role of the North Pacific as a regulator of boreal summer climate over Eurasia and North America is investigated using observational data. Two summertime interannual climate modes associated with sea surface temperature (SST) variability in the North Pacific are identified. The first mode shows an elongated zone of warm (cold) SST anomalies in the central North Pacific along 40°N, with temporal variability significantly correlated with El Niño during the preceding spring, but its subsequent evolution is quite different from El Niño. The second mode exhibits a seesaw SST variation between the northern and southern North Pacific and is independent of El Niño. Both modes are linked to coherent SST anomalies over the North Atlantic, suggesting the presence of an “atmospheric bridge” linking the two extratropical oceans. Using the principal component of the most dominant mode as the North Pacific index (NPI), composite analyses show that the positive (negative) phase of NPI features a warm (cold) North Pacific associated with the formation of contemporaneous low-level stationary anticyclones (cyclones) over the North Pacific and North Atlantic, respectively. The anticyclones (cyclones) are linked by quasi-zonally symmetric circulation anomalies in the middle to upper troposphere spanning Eurasia and North America, accompanied by a poleward (equatorward) shift of the subtropical jet and storm tracks. Associated with the positive (negative) phase of NPI, are hot/dry (cool/wet) summers over Japan, Korea, and eastern-central China, which are linked to hot/dry (cool/wet) conditions in the Pacific Northwest, western Canada, the U.S. northern Great Plains, and the Midwest. Cumulative probability computed from pentad temperature and rainfall data show that the odds of occurrence of extreme events are impacted consistently with the mean climate shift during opposite phases of the NPI. The possible roles of air–sea interaction and transient-mean flow interaction in exciting and sustaining the climate modes are discussed. [ABSTRACT FROM AUTHOR]