MJO Modulation on 2009/10 winter snowstorms in the United States

During the winter of 2009/10, a number of record-breaking snowfall events registered in the eastern United States are shown to have been modulated by the pulsation of tropical MJO through an atmospheric teleconnection pattern. The intraseasonal variability over the eastern subtropical Pacific near Mexico (the equatorial central Pacific) had reached themaximum(second largest) strength since 1979/80. Fromlate December to mid-February, the convection over these two regions experienced a remarkable wet-dry-wet cycle; correspondingly, the daily snowfall over the eastern United States also exhibited a wet-dry-wet cycle. As the MJO convection reached the central Pacific, a teleconnection pattern extended to North America, resulting in a westward-tilted deep anomalous trough anchored over the eastern United States, producing a low-level pressure dipole anomaly with an anticyclone (cyclone) centered at the U.S. West (East) Coast. The convection over the Indian Ocean varied in phase with the central Pacific convection, reinforcing the extratropical atmospheric teleconnection pattern. As a result, the enhanced high-latitude cold air penetrated southward, affecting the central and eastern United States. Meanwhile, warmer moist air was transported from the tropical central Pacific by the existing El Niño through Mexico to the southernUnited States along with the upper-level subtropical westerly jet, which extended from the subtropical Pacific to theAtlanticOcean. As such, the eastern United States was located in a convergence zone between the enhanced cold air from the high latitude and the warm, moist air supplied from the subtropics, resulting in favorable conditions for extremely heavy snowfall.