The Madden-Julian Oscillation (MJO) is a coupling between atmospheric circulation and deep tropical convection. In other words, it is an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Oceans. Variations in wind and temperature produced by the MJO can have a significant influence on global atmospheric and oceanic circulations. The MJO is also known as the 30-60 Day Tropical Wave since it typically circles the globe along the equator in about 30-60 days, or 45 days on average.

MJO activity in the pacific is greatest during weak La Ninas and ENSO-neutral conditions (such as we are currently in). During moderate to strong El Ninos, MJO activity is minimal or even absent. The MJO plays an important role on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics, but for the purposes of this blog, I am only going to tell you how it affects the weather across the western US.

The greatest affects from the MJO in the United States are felt during the winter months along the west coast. As the MJO propagates eastward across the Pacific Ocean, subduction in advance of this oceanic tropical wave (Kelvin wave) anomalously warms the sea waters.

As this Kelvin wave, and its associated deep convection, nears the central Pacific Ocean, it encounters the sub-tropical jet. This sub-tropical jet will then transport this moisture northeastward into the mid-latitudes. This is also known as the Pineapple Express. A perfect example of this occurred in December of 2010 (when parts of the Sierra's received over 10 feet of snow and valley locations received more than a foot of rain), and a similar set up is currently in the developing stages.

However, with warmer temperatures aloft with the ongoing event, snowfall totals will be significantly less than what they were in 2010. Snowfall amounts over the next several days will range from 1.5-3 feet above 7,000 feet in the Northern Sierra's with some valley locations picking up at least a foot of rain.

When the MJO is affecting the western United States, effects typically seen are:

- Unusually heavy precipitation - Greater cloud cover - Warmer than normal nighttime temperatures

The MJO peaked back in late November and early December of 2011. Perhaps you can recall, during this time, multiple upper level mid-latitude troughs (Rossby waves) formed or passed over the western US, and often remained over this area for several days at a time. This resulted in periods of moderate to heavy rain/snow over portions of the southwest during this time.

The pattern in early December of 2011 was more like what we see during an El Nino as opposed to what we typically see during a La Nina. This was due to the MJO peaking during that time. After the MJO passed, the southwestern US saw dry conditions, which is what we expect to see during La Nina winters.

The MJO also influences the frequency and intensity of cold air outbreaks across the eastern US, along with tropical cyclone activity in both the eastern Pacific and Atlantic basins during the Northern Hemisphere summer. I will save this for another time.

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