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Combined impacts of ENSO and MJO on the 2015 growing season drought on the Canadian Prairies

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.
