October 12, 2005 – As Winter 2005-2006 approaches, NOAA scientists say the leading climate patterns expected to impact winter weather are: the long-term climate trends, the North Atlantic Oscillation (NAO), and the Madden-Julian Oscillation (MJO).
Long-Term Climate Trends
In the absence of a significant El Niño and La Niña event estimates of long-term trends along with a variety of dynamic and statistical tools provide the foundation for the forecast. One tool that is used at NOAA's Climate Prediction Center is the average conditions during the last 10 years compared with the long-term average for 1971-2000. Average winter temperature departures from normal for the period 1971-2000 are considerably cooler than those for the most recent 10-year average over much of the nation (figure).
North Atlantic Oscillation (NAO)
The NAO is a major source of intra-seasonal variability over the United States, North Atlantic and Europe during the winter. The NAO modulates the circulation pattern over the middle and high latitudes, thereby regulating the number and intensity of significant weather events affecting the U.S.
The positive phase features a northward shift in the jet stream relative to its normal position. Associated with this phase is an increase in the occurrence of relatively warm days over the eastern two-thirds of the U.S. (See Powerpoint presentation attached.)
The negative phase features a southward shift of the jet stream. It is associated with an increase in more frequent cold air outbreaks and Nor’easters along the East Coast.
Madden-Julian Oscillation (MJO)
The MJO phenomenon is another factor likely to contribute to increased variability during the winter. The MJO influences the pattern of tropical rainfall, and produces El Niño/Southern Oscillation (ENSO)-like features on time scales of approximately 30-60 days (intra-seasonal). The MJO is most active during ENSO-neutral and weak-ENSO winters, and can influence the occurrence of extreme weather events, such as multiple day rain events and flooding along the Pacific Northwest Coast.
El Niño Southern Oscillation (ENSO)
Since early 2005 water temperatures in the central-equatorial Pacific Ocean have been near normal (ENSO-neutral). These conditions are expected to continue during this winter. As a result, it is unlikely that El Niño or La Niña will be a factor influencing the winter weather patterns. ENSO-neutral years often feature increased variability and increased occurrence of weather extremes in both temperature and precipitation for many areas of the country.
Implications for the U.S. in Winter 2005-2006