NCEP/Climate Prediction Center ATLAS No. 8

Relationships Between El Nino-Southern Oscillation and the Arctic Oscillation: A Climate-Weather Link

1. Introduction

        Seasonal forecasters at CPC constantly have to choose one phase of ENSO as base of the forecast for the future seasons. During the late autumn of 2000, a 3rd straight winter of cold episode (La Nina) conditions was prospected for the upcoming winter. This after several straight months of forcasts suggested that ENSO neutral conditions would predominate in the tropical Pacific throught the period. This raised a number of questions about the forecast, and in particular about the potential impacts on the weather in the United States. In addition to the status of ENSO, a major source of uncertainty in the forecast was the phase of the AO/NAO, which remains largely unpredictable, and which has considerable variability on intraseasonal time scales. Major flips in the AO/NAO often occur on time scales of the order of a week or two, and the associated impacts on weather in eastern North America are large. A complicating factor during the autumn of 2000 was the very strong tropical intraseasonal activity (MJO), which was responsible for increased variability in the tropics and subtropics (30S-30N) and which complicated the interpretation the patterns of tropical convection.

        These uncertainties about the impacts of ENSO-neutral versus La Nina conditions and the phase of the AO/NAO naturally raised the questions:

        Is there any relationship between the phase of ENSO and the phase of the AO, and if so, how might this influence the weather?

        ENSO is the leading pattern of interannual climate variability in the tropical Pacific, while the AO has considerable variability on intraseasonal time scales in the middle and high latitudes. Thus, relationships between the phase of ENSO and the phase of the AO are at the heart of the climate-weather link.

        In this Atlas, we examine the ENSO and AO influence on the Unitelinkage between the phase of ENSO and the phase of the AO and its influence on several meterological fields of interest to forecasters:

         1.  500-hPa height and anomalies
         2.  500-hPa height tendency  (magnitude and seasonal average)
         3.  1000-5000-hPa thickness  (mean and anomaly)
         4.  300-hPa wind
         5.  surface air temperature
         6.  precipitation

        Our purpose is to provide a useful diagnostic tool to support the 6-10 day, monthly and seasonal forecast process at CPC. Given the current phase of ENSO, the Atlas provides a measure of the variability that might be expected for extreme phases of the AO.

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