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A Short Note on Pineapple Express*
Ruping Mo
National Laboratory for Coastal and Mountain Meteorology / Pacific Storm Prediction Centre,
Environment Canada, Vancouver, BC, Canada
Corresponding author’s address:
Ruping Mo
Pacific Storm Prediction Centre
Environment Canada
201-401 Burrard Street
Vancouver, BC V6C 3S5
E-mail: [email protected]
Technical Report 2010-001
National Laboratory for Coastal and Mountain Meteorology
25 January 2010
This note is an invited contribution to a textbook entitled “Meteorology Today: An Introduction to
Weather, Climate, and the Environment”, by C. D. Ahrens, P. L. Jackson, and C. E. J. Jackson.
The “Pineapple Express” is a meteorological phenomenon that is a warm, moist
southwesterly flow bringing heavy precipitation and sometimes extensive flooding to the
North American West Coast. Typically in the cold season from October to March, an area
of low pressure over the Gulf of Alaska maintains a stream of very wet and warm air
flowing northeastward from near the Hawaiian Islands (where pineapples are grown) onto
the Pacific coast of North America (see Fig. 1). Such an “atmospheric river” is usually
marked by a quasi-stationary surface front along which mobile cyclones develop and
track toward the coast. Periods of heavy precipitation occur when the warm moist air is
forced over the coastal mountain ranges. The warm air can also melt the snow pack
rapidly in the mountains, further aggravating flooding in some areas.
An example
Figure 1 shows a Pineapple Express heading toward British Columbia (BC), in mid
January 2005. It spread periods of heavy rainfall across the southern portions of the
province. At Vancouver Harbour weather station, 103.4 mm of rain was recorded within
24 hours on 17 January, which was the 7th wettest day in the 80-year period between
1925 and 2005, and another 73.8 mm of rain was recorded on the following day. The
heavy and sustained rainfall across the BC South Coast wiped out the snowpack over the
mountains, and caused mudslides and flooding with roads being closed and families
evacuated. Further inland and to the north, the precipitation fell as freezing rain or heavy
snow, creating severe avalanche hazards along the Trans-Canada Highway and most
other mountain passes.
Figure 1: The Pineapple Express phenomenon, as illustrated by the satellite (GOESWest) water vapour image with derived winds at 1200 UTC, 17 January 2005. The water
vapour image detects an infra-red wavelength that is sensitive to water vapour emissions.
Major low and high sea level pressure centers are marked by the black letters L and H,
Synoptic features and possible connection with MJO
Studies suggest that the Pineapple Express is usually associated with a distinct weather
pattern in the mid-troposphere. At 500 hPa there are higher than normal heights (i.e high
pressure at about 5.5 km above sea level) over the Bering Sea preceding the event; lower
than normal 500 hPa heights (and therefore pressure at 5.5 km above sea level) over the
Gulf of Alaska throughout the event; and higher than normal 500 hPa heights over the
southwestern United States and adjacent eastern Pacific Ocean during and after the event.
This configuration in the mid-troposphere provides a persistent storm track from the
southwest toward the northeast, while the transport of moisture is largely associated with
mobile cyclones that track from the subtropical Pacific toward the Pacific coast of North
America. There is evidence to suggesting that the tropical Madden-Julian Oscillation
(MJO – a cycle that enhances and suppresses rainfall in the tropics) exerts a strong impact
on the Pineapple Express. It is thought that tropical temperature patterns associated with
the MJO helps to create the mid-tropospheric (500 hPa) pattern needed for the Pineapple
Express to develop. In addition, heavy tropical rainfall of the MJO as it shifts eastward
from the eastern Indian Ocean to the western tropical Pacific may feed moisture directly
into the Pineapple Express system; such a system is sometimes called a “Tropical Punch”
by meteorologists.