Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 33

2.5 The Madden-Julian Oscillation (MJO)
The MJO is a low-latitude intraseasonal oscillation, meaning that it passes
through an identified cycle in a period of 60 to 90 days, quite unlike the annual,
biennial or decadal cycles of other oscillations. It is so named for Roland
Madden and Paul Julian of NCAR who discovered the wave in the early
1970s. Its identification and possible forecasting is of considerable importance
in long-range predictability of tropical and subtropical weather as well as short-
term climate variability (see Background Box 2.1 ).
The MJO is a feature of the tropical atmosphere-ocean system that plays a
significant role in precipitation variability. It is characterized by anomalous
rainfall conditions, that can be either enhanced or suppressed. The beginning
of the cycle, the anomalous rainfall event, usually appears first over the Indian
Ocean and Pacific Ocean. It remains identifiable as it moves over the very warm
water of the western and central parts of the Pacific Ocean. On meeting the
cooler waters of the eastern Pacific Ocean it usually becomes less defined, only
to reappear over the tropical Atlantic Ocean and Indian Ocean. As noted this
cycle can last two to three months.
Clearly, the higher or lower than normal precipitation of the event is asso-
ciated with both surface and upper air conditions as they relate to ascending and
descending air. A knowledge of the cycle characteristics enables a clearer
understanding of tropical rainfall variability and of Pacific Ocean and Atlantic
Ocean tropical cyclones. Additionally, such information can be related to the
potential impact upon middle-latitude precipitation. In their study of the MJO's
role in hurricanes in the Caribbean Sea and Gulf of Mexico, Maloney and
Hartmann ( 2000 ) found a distinct relationship. They noted that hurricanes are
four times more likely to occur when, in the ascending phase, the rising air and
surface westerly winds are conducive to formation of the storm.
The Climate Prediction Center (CPC) of NOAA (www.cpc.ncep.noaa.gov/)
provides a fascinating account of how a tropical oscillation may impact rainfall
in the Pacific northwest of North America and their analytical description is
given in Background Box 2.2 .
The CPC further notes that there is a coherent simultaneous relationship
between the longitudinal position of maximum MJO-related rainfall and the
location of extreme west-coast precipitation events. Extreme events in the
Pacific northwest are accompanied by enhanced precipitation over the western
tropical Pacific and Indonesia (typically centered near 1208 E) with suppressed
precipitation over the Indian Ocean and the central Pacific.
The MJO is thought to play a significant role in the formation and frequency
of hurricanes. If an easterly wave, the initial formation feature of Atlantic
hurricanes, meets the cloudy sky phase of the MJO the conditions for hurricane
formation improve. The MJO is, of course, moving in the opposite direction to
the easterly wave. Unfortunately, although hurricane researchers of the National