Abstract
Tropical and extratropical interactions on the intraseasonal time scale are studied in the context of the Arctic Oscillation (AO) and the Madden–Julian oscillation (MJO). To simplify the discussion, a high (low) MJO phase is defined as strong (suppressed) convective activity over the Indian Ocean. In the Northern Hemisphere (NH) winter season, a high (low) AO phase is found more likely coupled with a high (low) MJO phase. Based on the regressed patterns and composites of various dynamical fields and quantities, possible mechanisms linking the AO and the MJO are examined. The analysis indicates that the MJO influence on extratropical circulations seems more evident than the AO influence on tropical circulations. The MJO interacts with the AO through meridional dispersion of Rossby waves in the Pacific sector. The geopotential height anomaly center over the North Pacific associated with the MJO can either reinforce or offset the AO Pacific action center. As a result, the AO pattern can be greatly affected by the MJO. When the AO and the MJO are in the same (opposite) phase, the Pacific action center becomes much stronger (weaker) than the Atlantic action center. The eddy momentum transports associated with the MJO in the Pacific sector are closely related to the retraction and extension of tropical Pacific easterlies and the subtropical Asian–Pacific jet. Because of its large scale, this regional effect is also reflected in the zonal mean state of wave transport and wave forcing on zonal wind, which in turn affects the phase of the AO.
Corresponding author address: Dr. Shuntai Zhou, NOAA/NWS/NCEP Climate Prediction Center, W/NP53, Room 808, 5200 Auth Road, Camp Springs, MD 20746. Email: shuntain.zhou@noaa.gov
