There are two signals that are currently suggesting cold weather for the eastern US during the month of February and they include the Madden-Julian Oscillation (MJO) index and the recent major Sudden Stratospheric Warming (SSW) event.This blog will focus on the MJO.
The MJO is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days.It is a large-scale coupling between atmospheric circulation and tropical deep convection.The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics.Rather than a standing pattern like ENSO, the MJO is a traveling pattern, propagating eastwards at about 5 m/s through the portion of the Indian and Pacific oceans where the sea surface is warm.There is evidence that the MJO influences the ENSO cycle.While it does not cause El Nino or La Nina, it can contribute to the speed of development and intensity of El Nino and La Nina episodes.
The MJO influences both precipitation and surface temperature patterns across the US.The two most significant impacts over the US during the northern hemisphere winter are an increase in the frequency and intensity of heavy precipitation events along the US west coast, and an increase in the frequency and intensity of cold air outbreaks across the eastern US.The MJO also influences tropical cyclone activity in both the eastern Pacific and Atlantic basins during the northern hemisphere summer.
Research has found that the location of the MJO, or phase, is linked with certain temperature and precipitation patterns around the world.The MJO phase diagram (below) illustrates the progression of the MJO through different phases, which generally coincide with locations along the equator around the globe.When the index is within the center circle, the MJO is considered weak, meaning it is difficult to discern.Outside of this circle, the index is stronger and will usually move in a counter-clockwise direction as the MJO moves from west to east.
The very latest MJO forecast propagates the MJO into phase 8 come early February (see MJO figure).Phase 8 generally results in a colder-than-normal pattern for the eastern US (see temperature composites figure for JFM at bottom).