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As Winter 2001-02 approaches, NOAA scientists say the leading climate patterns expected to impact winter weather are: the El Niño Oscillation (ENSO), the Arctic Oscillation (AO), the Madden-Julian Oscillation (MJO) and other long-term climate trends. Scientists at NOAA's National Weather Service say these patterns are the physical basis for this season's winter outlook.

El Niño Southern Oscillation (ENSO)

Since early 2001, water temperatures in the central-equatorial Pacific Ocean have steadily increased, rising to their highest levels since the 1997-1998 El Niño warm episode. While there is no clear consensus among recent sea-surface temperature forecasts that El Niño will return soon, several predictions indicate warmer-than-normal oceanic conditions will continue through the first half of 2002. Considering the sea surface temperature predictions, the time of year and the observed oceanic and atmospheric circulation patterns, the intensity of any warming will be weak, or moderate. As a result, it's unlikely the ENSO phenomena will be dominant factors influencing this winter season.

Weak ENSO and ENSO-neutral years, scientists say, tend to feature increased variability and increased occurrence of weather extremes in both temperature and precipitation for many areas of the country, compared with moderate or strong ENSO years.

Madden-Julian Oscillation (MJO)

The MJO phenomenon is one factor likely to contribute to increased variability this winter. The MJO influences the pattern of tropical rainfall, and produces 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 influences the occurrence of extreme weather events, such as floods, along the Pacific Northwest Coast.

Arctic Oscillation (AO)

The AO is a major source of intra-seasonal variability over the United States, North Atlantic and Europe during winter. It 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 strong polar vortex, with the mid-latitude jet stream shifted to the north of its normal position. Associated with this phase is an increase in the occurrence of extreme warm days over much of the contiguous United States.

The negative phase features high-latitude blocking, frequently in the vicinity of Greenland and/or Alaska. Associated with this phase, there is an increase in the occurrence of extreme cold days, especially from the Great Plains to the Southeast.

Long-Term Climate Trends

In the absence of significant forcing by the leading patterns of natural climate variability (i.e. ENSO, AO), estimates of decadal trends provide the basis for the forecast. One tool that is used at NOAA's Climate Prediction Center is the average conditions during the last 10 years compared to the long-term average for 1971-2000. Average winter temperature departures from normal for ENSO-neutral years in the 1990's are considerably cooler than those for the 10-year average (especially along the northern tier-of-states). This fact is an especially important consideration for the current seasonal outlook.

Implications for the U.S. in Winter 2001-2002

  • The ENSO phenomena are unlikely to be dominant factors influencing the winter weather in the United States;
  • The relatively weak ENSO forcing indicates that the coming winter will not be as warm as those of the late 1990's, when strong El Niño and La Niña events occurred;
  • Due to the weak ENSO forcing, increased variability and increased occurrence of weather extremes is anticipated this winter;
  • There is an increase in the occurrence of heavy precipitation events in the Pacific Northwest during weak-ENSO and ENSO neutral winters when the MJO is active. However, these events are relatively rare and are strongly dependent on the intensity and period of the MJO activity;
  • The unpredictability of the seasonal phase of the AO introduces uncertainty in the seasonal outlook;
  • The negative phase of the AO is associated with relatively frequent Nor'easters and heavy lake effect snows in the Northeast and Midwest, and also cold air outbreaks into the deep south. Currently, researchers can only forecast the phase of the AO out to 5 to 10 days, which introduces some uncertainty in the seasonal outlook.

New this year, the forecast of the MJO and the AO/NAO will be included—when applicable—in the Weekly Threats Assessment, which is published online every Tuesday at NOAA's Climate Prediction Center.

The Climate Prediction Center is one of nine National Centers for Environmental Prediction located in Camp Springs, Md. The Climate Prediction Center monitors and forecasts short-term climate fluctuations and provides guidance information on the long-term global effects climate patterns can have on the nation.

October 2001