moonlightcowboy's WunderBlog

Is the MJO working?

By: moonlightcowboy, 4:55 PM GMT on April 25, 2008

(NOTE: Work in progess)

What is the MJO? - The "MJO" of the "Madden-Julian Oscillation" is a naturally occurring component of our coupled ocean-atmosphere system. It significantly affects the atmospheric circulation throughout the global Tropics and subtropics, and also strongly affects the wintertime jet stream and atmospheric circulation features over the North Pacific and western North America. As a result, it has an important impact on storminess and temperatures over the U.S. During the summer the MJO has a modulating effect on hurricane activity in both the Pacific and Atlantic basins. Thus, it is very important to monitor and predict MJO activity, since this activity has profound implications for weather and short-term climate variability through the year.

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Figure 1: Equatorial vertical cross section of the MJO as it propagates from the Indian Ocean to the western
Pacific. Red arrows indicate direction of wind and red (blue) SST labels indicate positive (negative) SST
anomalies respectively.


The MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall then generally becomes very nondescript as it moves over the cooler ocean waters of the eastern Pacific but reappears over the tropical Atlantic and Indian Ocean. Each cycle lasts approximately 30-60 days.

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There is strong year-to-year variability in MJO activity, with long periods of strong activity followed by periods in which the oscillation is weak or absent. This interannual variability of the MJO is partly linked to the ENSO cycle. Strong MJO activity is often observed during weak La Niña years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Niño episodes.

200-hPa Velocity Potential Anomalies & Hurricanes/Typhoons



(ABOVE) Composite evolution of MJO events during the summer months together with points of origin of tropical cyclones that developed into hurricanes / typhoons (open circles). The green (brown) shading roughly corresponds to regions where convection is favored (suppressed) as represented by 200-hPa velocity potential anomalies. Composites are based on 21 events over a 35 day period. Hurricane track data is for the period JAS 1979-1997. Points of origin in each panel are for different storms.

So, what's happening NOW with the MJO?
The MJO signal has weakened somewhat. But, what activity there is has also propagated eastwards into the western Pacific. It will be interesting to see how the MJO will contribute more to the patterns of tropical rainfall during in the next weeks.

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(ABOVE) The current velocity potential anomalies indicate large-scale divergence over parts of Africa and the Indian Ocean with areas of generally large-scale subsidence across much of the western hemisphere. Positive anomalies (brown contours) indicate unfavorable conditions for precipitation
Negative anomalies (green contours) indicate favorable conditions for precipitation

Although tropical cyclones occur throughout the NH warm season (typically May-November) in both the Pacific and the Atlantic basins, in any given year there are periods of enhanced / suppressed activity within the season. There is evidence that the MJO modulates this activity (particularly for the strongest storms) by providing a large-scale environment that is favorable (unfavorable) for development. The strongest tropical cyclones tend to develop when the MJO favors enhanced precipitation. As the MJO progresses eastward, the favored region for tropical cyclone activity also shifts eastward from the western Pacific to the eastern Pacific and finally to the Atlantic basin.

Where's the MJO and what's the MJO FORECAST:



Phase diagrams for the last 40 (above) days illustrates the phase and amplitude of the MJO. Counter-clockwise movement around the diagram indicates an eastward propagating signal across eight phases from the Indian Ocean to the Pacific and later the western hemisphere. Color of lines distinguish different months and dates are annotated. The farther away from the center of the circle the stronger the MJO signal.

So, the MJO appears to be in Phase 6, or the western Pacific, and is, now, again showing signs of weaker status at it moves eastwards.



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Other BLOGS and updates: Got an ITCZ? Scratch it!
Weather456's blog on the ITCZ anomalies

Other good tropical blogs on WU here at "TROPICAL LAGNIAPPE"

(some of this info is from NOAA's CPC)

Got an ITCZ? Scratch it!

By: moonlightcowboy, 4:40 AM GMT on April 04, 2008

For me, the tropical season begins with NOAA beginning its official monitoring of the Itcz in April. That means season is fast approaching and another year of anticipation and blob watching begins. In the last several days, it's been fun to see how many have been visiting Doctor Master's blog and talking tropics - SST's, ENSO, LA NINA, etc, etc.! Some of the avatars haven't been seen much since last season and it's good to seem checking in and getting ready. So, in the spirit of getting ready, this blog gives us some insight on the Itcz, what it means and what we can expect. PLEASE, feel free to comment, dissent or add any discussion on the topic! Thanks!!!



So, what's the ITCZ? It's the Inter-Tropical Convergence Zone (ITCZ), and it appears as a band of clouds, usually thunderstorms, that circle the globe near the equator. The solid band of clouds may extend for many hundreds of miles and is sometimes broken into smaller line segments. The ITCZ follows the sun in that the position varies seasonally. It moves north in the northern summer and south in the northern winter. The ITCZ (pronounced "itch") is what is responsible for the wet and dry seasons in the tropics.

It exists because of the convergence of the trade winds. In the northern hemisphere the trade winds move in a southwesterly direction, while in the southern hemisphere they move northwesterly. The point at which the trade winds converge, forces the air up into the atmosphere, forming the ITCZ.

The tendency for thunderstorms in the tropics is to be short in their duration, usually on a small scale but can produce intense rainfall. It is estimated that 40 percent of all tropical rainfall rates exceed one inch per hour. Greatest rainfall typically occurs when the midday Sun is overhead. On the equator this occurs twice a year in March and September, and consequently there are two wet and two dry seasons.

Further away from the equator, the two rainy seasons merge into one, and the climate becomes more monsoonal, with one wet season and one dry season. In the Northern Hemisphere, the wet season occurs from May to July, in the Southern Hemisphere from November to February.



SURFACE MAP (above) In the above map the location of the Itcz is shown by the red line that runs across the width of the map. If you'll notice, the Itcz is running lower than 10n in the cAtl.
The ITCZ has been fairly active, but it's running behind its climatological mean average for its (African land) position for this time of year which is approximately 11,12n. It's been a little high, around 6 or 7 north near the African coast; but, it's pretty much flat-lined across the Atlantic to sAmerica at or near the equator.



MEAN POSITION OF THE ITCZ According to the chart, the mean position for the first 10 days in April would have the ITCZ running near 11n (which seems high to me). It is actually running somewhat closer to 6n off the African coastline (imo) and near the equator at South America. In 2007 the Itcz dropped considerably below the mean during the months of June and July, and only rising above the mean in late August and early September, before dropping off considerably in the middle of that same month and towards the end of the season.

Mean ITCZ

LATEST (Itcz) TEXT SUMMARY
During the period from April 1-10, 2008, the African portion of the Intertropical Front (ITF) was located at around 10.5 degrees north latitude, compared with the normal position of 8.6 degrees north. The position is similar to the same dekad of 2007 when the ITF was around 10.6N. Figure 1 shows the current position compared to normal, and it is apparent from this diagram that the positive northward bias is due to a northward surge over in the east, meanwhile the ITF lags slightly in the west. From Figures 2 and 3 it is clear that the ITF is flat with it resting in an almost straight line constantly at 10.5 degrees north. Compared with the 1979-2006 mean of around 11.5N in the west, and a normal position of around 8.6N the ITF is lagging behind west of Nigeria, and advancing faster than normal for points east.


2008 10-Day Mean ITCZ Position
April 1-10
15W - 10W - 5W - 0 - 5E - 10E - 15E - 20E - 25E - 30E - 35E
10.1 10.6 10.3 10.5 10.3 10.7 10.7 10.6 10.3 10.3 10.7


http://earthobservatory.nasa.gov/Newsroom/NewImages/Images/itcz_goes11_lrg.jpg

GOES PROJECT IMAGE from NASA
This image clearly shows the Itcz running at a considerably higher latitude than it's presently running.


MYSTERY of the ITCZ
What Keeps the ITCZ North of the Equator?

It is a long-standing mystery that the ITCZ stays north of the equator over the Atlantic and eastern Pacific Oceans despite that the annual-mean solar radiation at the top of the atmosphere is symmetric with respect to the equator. This article reviews recent progress that has shed new light on this old puzzle.

(excerpt in part)...The ITCZ problem thus involves a circular chicken-and-egg argument. The ITCZ stays north of the equator because SST is higher; and the SST is higher north because the ITCZ stays there. The positive WES feedback is at the center of this circular argument. In a coupled ocean-atmosphere model, the WES feedback destabilizes the symmetric climate, leading to an asymmetric steady state with a single ITCZ on only one side of the equator (Xie and Philander 1994). A condition for this spontaneous development of latitudinal asymmetry is the equatorial upwelling that prevents the ITCZ from forming at the equator. This necessary condition thus explain why climatic asymmetry only develops over the eastern Pacific and Atlantic where the equatorial upwelling is observed.
(Complete article here.)

Heavier precipitation occurring over warmer waters


ITCZ SUMMARY

The Itcz is beginning its climatilogical climb further north, despite it's day to day flucuations n and s. Presently, the ITCZ appears to be quite active, but it's running quite low in the western side of the cAtl near the equator. Climatologically, I believe it's running lower than average, especially on the wAtl side. This may allow for a longer period of SAL intrusion into the Atlantic as well, but that remains to be seen. As it moves further north, activity will eventually pick up and create opportunities for tropical waves to organize and produce spinning storms, but that is more likely to occur a little further into the season. A more northerly Itcz makes it easier for developing storms to coriolis. And, more Itcz moisture further north also weakens SAL and dry/stable air, making development conditions more favorable.

Look here on the Latest Surface Map for the Itcz location, waves, and other surface features. The Meteosat Infrared Color satellite picture (below) shows the Itcz location. Notice how far south it is near South America.



Don't forget to check out Tropical Lagniappe for some great links to other blogs and websites from fellow WU members! There's some great info here and I always learn something each time I visit them!


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

moonlightcowboy's WunderBlog

About moonlightcowboy

"There is no heavier burden than a great potential." - Charles Schultz, in the Peanut's character of Linus.