Madden-Julian Oscillation seen as key weather factor this winter
By Bob Berwyn
SUMMIT COUNTY — Colorado-based meteorologists say they think they know why this winter’s La Niña has misbehaved so far. According to the January – March outlook from the the National Weather Service in Boulder, an active phase of the Madden-Julian Oscillation has masked La Niña up to now, sending strong pulses of tropical moisture across the Pacific and into the southwestern Quadrant of the U.S.
The resulting weather pattern has looked much more like an El Niño phase of the Pacific cycle, but the good news (for Colorado) is that the Madden-Julian phase has weakened. That means that a more typical La Niña pattern — with a storm track out of the northwest — may once again begin to dominate Colorado weather.
In fact, a series of small and fast-moving storms is starting to line up in the Pacific Northwest, with a chance of snow in the northern mountains Wednesday night and the potential for a few more disturbances to cross the area on and off through the end of the week.
The Madden-Julian oscillation is a 30- to 60-day cycle featuring an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Ocean.
There is an ongoing research project in the Indian Ocean to get a better handle on this so-called intraseasonal seasonal cycle, believed to be one of the most significant factors that affects winter precipitation in the western U.S.
According to the Boulder-based forecasts, the Madden-Julian oscillation is most noticeable during weak La Niña conditons, or during neutral years, when the Pacific is between La Niña and El Niño phases.
Essentially, during the active phase of an MJO, the westward-moving waves of enhanced tropical energy simply overpower what we’ve come to expect from a classic La Niña pattern.
Madden-Julian oscillations are expressed as Kelvin waves that circle the entire globe along the Equator, normally with 30 to 60 days. Impacts include large-scale variations in temperature, wind and precipitation similar to those observed during moderate to strong El Niños, but of much shorter duration.
Here’s the NWS explanation of the latest MJO:
“The Madden-Julian Oscillation Index peaked once again in late November and early December of 2011. During this five-week period, several upper- level mid‐latitude troughs (Rossby waves) formed and passed over the western U.S., and in many instances remained over this region for several days at a time. This large-scale upper air pattern often resulted in periods of moderate to heavy precipitation/snowfall across the southwest U.S., including southern and eastern Colorado; conditions more often seen during El Niños.”
The MJO disturbance originates in the equatorial Indian Ocean about every 30 to 90 days. It’s part of the Asian and Australian monsoons and can enhance hurricane activity in the northeast Pacific and Gulf of Mexico, trigger torrential rainfall along the west coast of North America and affect the onset of El Niño.
Scientists believe that the Madden-Julian is the world’s greatest source of atmospheric variability in the one- to three-month time frame.
“The Madden-Julian Oscillation has a huge impact all over the globe,” said Chidong Zhang of the University of Miami, the research project’s chief scientist. “It connects weather and climate, and it is important to forecasting.”
“The MJO drives weather in both hemispheres even though it sits along the equator,” said NCAR’s Jim Moore, director of the DYNAMO project office. “Its origins have never been measured in such a systematic fashion before.”
The main observation sites will be based in the Maldives, Diego Garcia and Manus Island, as well as aboard research ships and aircraft in the Indian Ocean. The major radar array and land-based observation “Super Site” will be located on Addu Atoll.
The MJO plays a key role in driving tropical weather and climate variations during all seasons of the year. It also interacts with other atmospheric patterns, such as the El Niño/Southern Oscillation and the North Atlantic Oscillation, that can shape weather and climate patterns across much of the globe.
Scientists need to better understand the MJO, both to improve long-range weather forecasts and seasonal outlooks worldwide, and perhaps make the leap to longer-term forecasts of climate that may extend years into the future.
In winter, for example, the onset of an MJO can set off atmospheric waves that travel across the globe and, about 10 days later, influence the location and severity of major storms on the west coast of North America, some of which cause significant flooding.
“If you can find out how an MJO event starts, you may get a couple of weeks’ warning about wintertime storms in the United States,” says NCAR scientist Mitchell Moncrieff, a member of the DYNAMO Science Steering Committee.