Intraseasonal Modulation of the North Pacific Storm Track by Tropical Convection in Boreal Winter

The modulation of the North Pacific storm track by tropical convection on intraseasonal time scales (30– 90 days) in boreal winter (December–March) is investigated using the NCEP–NCAR reanalysis and NOAA satellite outgoing longwave radiation (OLR) data. Multivariate empirical orthogonal function (MEOF) analysis and case compositing based upon the principal components (PCs) of the EOFs reveal substantial changes in the structure and intensity of the Pacific storm track quantified by vertically (925–200 mb) averaged synoptic eddy kinetic energy (SEKE) during the course of a typical Madden–Julian oscillation (MJO) event. The storm-track response is characterized by an amplitude-varying dipole propagating northeastward as the center of the anomalous tropical convection moves eastward across the eastern Indian Ocean and the western-central Pacific. A diagnosis of the SEKE budget indicates that the storm-track anomaly is induced primarily by changes in the convergence of energy flux, baroclinic conversion, and energy generation due to the interaction between synoptic eddies and intraseasonal flow anomalies. This demonstrates the important roles played by eddy–mean flow interaction and eddy–eddy interaction in the development of the extratropical response to MJO variability. The feedback of synoptic eddy to intraseasonal flow anomalies is pronounced: when the center of the enhanced tropical convection is located over the Maritime Continent (western Pacific), the anomalous synoptic eddy forcing partly drives an upper-tropospheric anticyclonic (cyclonic) and, to its south, a cyclonic (anticyclonic) circulation anomaly over the North Pacific. Associated with the storm-track anomaly, a three-band (dry–wet–dry) anomaly in both precipitable water and surface precipitation propagates poleward over the eastern North Pacific and induces intraseasonal variations in the winter hydroclimate over western North America.