Abstract. Warm-season precipitation over the Canadian Prairies plays a crucial role in activities in environment and society and has particular importance to agricultural production over the region. This research investigates how a warm season precipitation deficit over the Canadian Prairies is related to tropical Pacific forcing in the early summer 2015 drought. The significant deficit of precipitation in May and June of 2015 were coincident with a warm phase of El Nino–Southern Oscillation (ENSO) and a negative phase of Madden–Julian Oscillation (MJO)-4 index as they both favor a positive geopotential height anomaly in western Canada. Further investigation during the instrumental record period (1979–2015) shows that the warm-season precipitation in the Canadian Prairies and the corresponding atmospheric circulation anomalies over western Canada teleconnected with the lower boundary conditions in the tropical western Pacific. MJO may play a crucial role in determining the summer precipitation anomaly in the western Canadian Prairie when equatorial central Pacific is warmer than normal (NINO4 > 0) and MJO is more active. The mechanism of this teleconnection may be due to the propagation of stationary Rossby wave that is generated in the MJO-4 index region. When the tropical convection around MJO-4 index regions (western tropical Pacific, centered over 140 E) is more active than normal when NINO4 > 0, a Rossby wave train originates from western Pacific and propagates into the midlatitude North America causing an anomalous ridge in the upper level over western Canada.

Abstract. Warm-season precipitation on the Canadian Prairies plays a crucial role in agricultural production. This research investigates how the early summer 2015 drought across the Canadian Prairies is related to the tropical Pacific forcing. The significant deficit of precipitation in May and June 2015 coincided with a warm phase of the El Niño–Southern Oscillation (ENSO) and a negative phase of Madden–Julian Oscillation (MJO)-4 index, which favour a positive geopotential height (GPH) anomaly in western Canada. Our further investigation during the instrumental record (1979–2016) shows that warm-season precipitation in the Canadian Prairies and the corresponding atmospheric circulation anomalies over western Canada teleconnected with the lower boundary conditions in the tropical western Pacific. Our results indicate that MJO can play a crucial role in determining the summer precipitation anomaly in the western Canadian Prairies when the equatorial central Pacific is warmer than normal (NINO4 > 0) and MJO is more active. This teleconnection is due to the propagation of a stationary Rossby wave that is generated in the MJO-4 index region. When the tropical convection around MJO-4 index region (western tropical Pacific, centred over 140∘ E) is more active than normal (NINO4 > 0), Rossby wave trains originate from the western Pacific with wavenumbers determined by the background mean wind and meridional absolute vorticity gradient. Under warm NINO4 conditions waves are generated with smaller wavenumbers compared to cold NINO4 conditions. These waves under warm NINO4 can propagate into the mid-latitudes over North America, causing a persistent anomalous ridge in the upper level over western Canada, which favours dry conditions over the region.

The circulation patterns of persistent cold weather spells with durations longer than 10 days in central–eastern North America (United States and Canada; 32°–52°N, 95°–65°W) are investigated by using NCEP reanalysis data from 1948 to 2014. The criteria for the persistent cold spells are: (1) three-day averaged temperature anomalies for the regional average over the central–eastern United States and Canada must be below the 10th percentile, and (2) such extreme cold spells must last at least 10 days. The circulation patterns associated with these cold spells are examined to find the common signals of these events. The circulation anomaly patterns of these cold spells are categorized based on the El Nino–Southern Oscillation, Arctic Oscillation (AO), and other climate indices. The atmospheric circulation patterns that favor the cold spells are identified through composites of geopotential height maps for the cold spells. Negative AO phases favor persistent cold spells. Phases of sea surface temperature (SST) modes that are associated with warm SSTs in the eastern extratropical Pacific also favor persistent cold events in the study region. Stratospheric polar vortex breakdown alone is not a good predictor for the regional extreme cold spells in central–eastern North America. The meridional dispersions of quasi-stationary Rossby waves in the Pacific–North America sector in terms of cut-off zonal wavenumber modulated by background flow are analyzed to provide insight into the difference in evolution of the cold spells under different mean AO phases. The waveguide for AO > 1 is in a narrow latitudinal band centered on 40°N, whereas the waveguide for AO <–1 is in a broader latitudinal band from 40° to 65°N. The circulation patterns and lower boundary conditions favorable for persistent cold spells identified by this study can be a stepping-stone for improving winter subseasonal forecasting in North America.