11:45 AM | *Stratospheric warming, MJO, and other signals that suggest a return to an extended period of colder-than-normal weather for the the eastern US*
We are now experiencing warmer-than-normal weather conditions in the eastern US and there will be mild spells right into the first week of February, but the signs are increasing for a return to a cold pattern next month and it may very well last for an extended period of time. The MJO is a tropical disturbance that propagates around the global tropics and it will be transitioning into “phases” during the month of February that are conducive to colder-than-normal weather in the eastern US. In addition, stratospheric warming will unfold over the next week or so in the polar region of the Northern Hemisphere and this can set off a chain of events in the atmosphere that ultimately results in colder-than-normal weather for the eastern US. There are other signals as well that support the notion of a return to colder-than-normal in the eastern US and it very well could stick around for awhile.
Madden Julian Oscillation (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 (thunderstorms). The movement of the MJO not only has wide ranging impacts on the patterns of precipitation, surface temperatures, and atmospheric circulation in the tropics, but it also influences precipitation and temperature patterns across the globe. One significant impact of the MJO over the U.S., for example, can be an increase in the frequency and intensity of cold air outbreaks across central and eastern sections of the nation during the northern hemisphere winter.
Research and empirical observations have found that the location or “phase” of the MJO is linked with certain temperature and precipitation patterns around the world. The MJO phase diagram (above) illustrates the progression of the MJO index 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 European model MJO index forecast propagates the MJO through phases 5 and 6 over coming days and then well into phase 7 during the first week of February (follow green line between arrows in a counter-clockwise fashion). This time of year, phases 5, 6 and 7 of the MJO index signal warmer-than-normal temperatures in the central and eastern U.S., but beyond this warmer-than-normal period, the MJO is likely to propagate into phases 8, 1 and 2 which usually are associated with colder-than-normal weather in the central and eastern US during this time of year. In fact, the MJO index is forecasted by the Euro to increase in amplitude when it enters phase 7 (i.e., move farther away from the inner circle), and an increase in amplitude may actually increase its influence on the overall pattern in February.
Another way to monitor the potential for wintertime Arctic air outbreaks in the eastern U.S. is to track the temperature pattern in the stratosphere over the polar region of the Northern Hemisphere. Sudden stratospheric warming (SSW) events are large, rapid temperature rises in the winter polar stratosphere and they can set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from polar regions into the central and eastern U.S. Indeed, there is strong evidence for stratospheric warming to take place in the Northern Hemisphere over the next five-to-ten days (top) and this increases the chances for colder-than-normal air masses to drop southward from high latitudes into middle latitudes.
During the winter months in the lower polar stratosphere, temperatures are typically lower than minus 70° Celsius (purple area). The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex. The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes. This dominant structure is sometimes disrupted in some winters or even reversed. Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50°K in just a few days. This sets off a reversal in the west-to-east winds and the collapse of the polar vortex or a shifting of it from its usual position over the North Pole to another part of the polar region. In response to the stratospheric warming (expansion) at the high latitudes, the troposphere in turn cools down (contraction) dramatically and this cold air displacement is then often transported in the troposphere from high latitudes to the middle latitudes.
Other supporting signals for February cold (AO index, snow cover)
Two other supporting signals for cold weather in the eastern US following this current warmer-than-normal stretch include the forecast of the Arctic Oscillation (AO) index in coming days and the current snow cover extent across the Northern Hemisphere. The AO is defined by surface atmospheric pressure patterns and it is tracked closely by meteorologists during winter seasons as it can provide clues as to whether Arctic air has the potential to be transported from the northern latitudes to the middle latitudes. When the AO index is negative for a sustained period, there tends to be high pressure in the polar regions, weaker zonal winds, and greater movement of polar air into the middle latitudes. The forecast by most computer models have the AO index dropping deeper into negative territory as we transition into the early part of February.
Another source of inspiration for those that who would like to see more cold weather this season in the eastern US is the current snow cover across the Northern Hemisphere. In fact, the snow cover is quite extensive at the current time across the Northern Hemisphere and this certainly should help in the generation of additional deep, cold air masses in coming weeks.
Meteorologist Paul Dorian
Extended video discussion on the multiple signs of February cold for the eastern US: