Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 35

Background Box 2.2
Evolution of a precipitation event
The typical scenario linking the pattern of tropical rainfall associated with the MJO to
extreme precipitation events in the Pacific Northwest features a progressive eastward
moving circulation pattern in the tropics and a westward moving circulation pattern
in the mid-latitudes of the North Pacific. Typical wintertime weather anomalies
preceding heavy precipitation events in the Pacific Northwest are as follows.
7-10 days prior to the heavy precipitation event
Initially, the heavy tropical rainfall associated with the MJO shifts eastward to the
western tropical Pacific. A moisture plume extends northeastward from the western
tropical Pacific towards the general vicinity of the Hawaiian Islands. A blocking
anticyclone is located in the Gulf of Alaska with a strong polar jet stream to its north.
3-5 days prior to the heavy precipitation event
The heavy tropical rainfall shifts eastward towards 1808 longitude and begins to
diminish. An associated moisture plume extends further to the northeast, often
traversing the Hawaiian Islands. The strong blocking high weakens and shifts
westward. A split in the North Pacific jet stream develops, characterized by an
increase in the amplitude and areal extent of the upper tropospheric westerly zonal
winds on the southern flank of the block and a decrease on its northern flank. The
tropical and extratropical circulation patterns begin to ''phase'', allowing a devel-
oping mid-latitude trough to tap the moisture plume extending from the deep tropics.
The heavy precipitation event
As the pattern of enhanced tropical rainfall continues to shift further to the east and
weaken, the deep tropical moisture plume extends from the subtropical central
Pacific into the mid-latitude trough now located off the west coast of North America.
The jet stream at upper levels extends across the North Pacific with the mean jet
position entering North America in the northwestern United States. Deep low
pressure located near the Pacific Northwest coast can bring up to several days of
heavy rain and possible flooding. These events are often referred to as ''pineapple
express'' events, so named because a significant amount of the deep tropical moist-
ure traverses the Hawaiian Islands on its way towards western North America.
Source: Climate Predictions Center of NOAA, www.cpc.ncep.noaa.gov/
(Maruyama 1997 ). Associated with fluctuations of the ITCZ, it is a periodic
reversal of winds in the lower stratosphere at elevations between 20 and 30 km.
The alternation of easterly and westerly equatorial upper-air winds has been
systematically observed, with increasing precision, since the famous eruption of