CURRENT ISSUE: June 2006
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MJO's MojoBy Laura Edwards
Western Regional Climate Center
Who are Madden and Julian, and why are they oscillating?
The Madden-Julian Oscillation, or MJO, was first defined in 1971 by Madden and Julian in a paper that presented a 40-50 day oscillation in the tropical Pacific region. Nowadays, this can mean the improved ability to forecast weather here in California several days in advance.
Figure 1, right. Development of an MJO event. http://www.cpc.noaa.gov/products/precip/CWlink/MJO/MJO_summary.pdf
At the core of the MJO is the zonal wind. This is the wind that moves west to east in the tropical region, and brings us our weather from the ocean. The zonal wind can move, or propagate, storm systems across the Pacific Ocean where they can make an impact on th western North American coast. Generally, these storm systems develop in the Indian Ocean, with warm sea surface temperatures (SSTs) to the east. The warm SSTs drive warmer air to move upward in the atmosphere, helping to generate convection, similar to the type of afternoon thunderstorms we experience. The warm SSTs and rising air to the east encourage convection to move toward the east, leaving behind cool SSTs on the west side with cooler, sinking air. (see http://www.cpc.noaa.gov/products/precip/CWlink/mjo_iso.html)
This eastward movement can be detected around the globe. Often, convective activity is suppressed in cooler ocean waters, such as the eastern Pacific Ocean, but reappears in the tropical Atlantic Ocean and Africa.
Scientists are continuing to learn about the MJO and its characteristics. Many find it difficult to predict with modern meteorological forecasting models, but satellites have been useful in assisting this effort. Recent studies of the MJO and its eastward propagation use Outgoing Longwave Radiation (OLR), derived from satellite observations. OLR is measured by how reflective the clouds are. The ocean surface mostly absorbs light, and appears dark, but clouds reflect light and appear lighter or white. Using satellite images that are taken over a series of days, climatologists and forecasters can look for an MJO pattern developing in the Indian Ocean. Convective clouds are tall, and reflect a lot of light back to space, and can be monitored for eastward progression across the Pacific Ocean.
Figure 2, left. Plots like these are used to monitor for MJO conditions. http://www.cpc.noaa.gov
What the MJO means for California can be different things. Sometimes the MJO has no clear effect on the weather or seasonal climate. At other times, the moisture carried by the zonal winds through these convective storms can be brought up to southern California and end up with some heavy precipitation and flooding. This phenomenon is nicknamed Pineapple Express, as the moisture plume often passes over Hawaii. This kind of event was experienced in December 2004 and January 2005, where heavy rain and snow fell over southern California.(see http://www.ncdc.noaa.gov/oa/climate/research/2005/california-storms2005.html for more details) In scientific journals there have been some articles written on the effect of MJO and other tropical phenomenon on California (1,2). Jones (2) found that the frequency of extreme precipitation events in California are more common when the MJO is active, but there are other factors involved from large-scale to local geographical influences.
MJO events typically have a period of about 40-50 days. Some events are stronger than others, and some research has shown some interannual variability in the intensity of the MJO. Other than OLR from satellites, forecasters can use tools such as upper air winds, radiosonde measurements, and mid-atmospheric conditions. Each of these tools serves a purpose in monitoring for MJO-like characteristics. Despite this, prediction skill is limited to about 15-20 days lead time, with large ranges of intensity from year to year.
1. Mo, K. C. and R. W. Higgins, 1998. Tropical influences on California precipitation. Journal of Climate, 11, pp. 412-430.
2. Jones, C., 2000. Occurrence of extreme precipitation events in California and relationships with the Madden-Julian Oscillation. Journal of Climate, 13, pp. 3576-3587.
Last updated 7/25/06.