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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 Canada 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. 1. Introduction 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. 2. 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. 1 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, respectively. 3. 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 2 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. 3