1.  Weather-Climate Overview

Since our last report dated 16 August 2004, a convectively active phase of the Madden Julian Oscillation (MJO) emerged from the equatorial western Pacific Ocean region, centered at roughly 160E, and propagated eastward in a coherent manner through the western hemisphere.  The MJO signal has been largest just north of the equator, and at this time has worked its way back into the Indian Ocean.  Additionally, periodic convective flare-ups have continued across the equatorial region from 150E-160W in response to anomalously warm sea-surface temperatures.  A link to the Climate Prediction Center's (CPC) latest advisory on ENSO is avaliable below:

 Latest CPC ENSO Advisory

Figure 1 shows Hovmoller plots of outgoing longwave radiation anomalies(OLRA) and contours that isolate certain coherent OLR modes, including the MJO.  The northern tropics (top panel) shows unusually well-defined eastward progression of negative OLRA (positive anomalies of tropical convection) across the western hemisphere.  It starts ~17 August 2004 at 160E and comes around to 80E by 28 September.  In addition to the tropical signal, wave energy dispersion from the northern mid-latitudes helped initiate the event around mid September near 60E.  The Indian Ocean convection is likely MJO # 7 (since fall 2003 -- see 16 August discussion).    There is also a weak area of convection centered at roughly 170E beginning on ~14 September and represents a separate area of tropical convective forcing.  .

Eastward movement of MJO #6 across the western hemisphere is a little less clear in the equatorial plot of OLR anomalies (bottom panel of Fig. 1).  OLR anomalies over the South American-Atlantic region seem to fit in.  Also, the blue contours identify a second MJO confined apparently to equatorial regions.  While the accompanying convection anomalies clearly impacted the circulation, their identification as an MJO is questionable.  The time-space filter used to produce the contours keys primarily on OLR anomalies to identify an MJO.  An alternate tool (discussed below)  includes both OLR and upper/lower level winds to identify an MJO, and it does not classify the flareup sequence as an MJO.  Finally, there is a signature of a convectively coupled Kelvin wave (green contours; KW) moving eastward from the Indian Ocean convection , which indicates an eastward shift of the MJO may be underway.

Figure 2 is a phase space diagram that tracks the progress of the MJO using an "EOF-based" technique, the alternate tool referred to above.  The trajectory shows a very slow eastward propagation of moderate MJO anomalies from the western Pacific in mid-August to the eastern Indian Ocean at present.  There is no sign of the "MJO "identified along the equator with the space-time filter.  On the other hand, the space-time filter captures the slow movement seen in Fig. 2 as an MJO (see Fig. 1).  Note that MJO #7 increased in strength after ~10 September 2004, and is now approaching Indonesia.

Figures 3 and 4 are full-disk satellite imagery for the Indian and western Pacific Oceans, respectively.  The Indian Ocean has a large concentration of tropical thunderstorm activity centered at about 10N/100E, which has been slowly shifting east toward the Maritime Continent region.  Meanwhile, the equatorial convection over the Pacific Ocean is centered around 160E, and represents yet another flare-up in the region due to the warm SSTs.  The satellite pictures nicely illustrate the two areas of tropical convective forcing.

Figure 5 is a time-latitude plot of 150 mb zonal mean zonal wind anomalies for the last 60 days ending 26 September 2004.  In the Northern Hemisphere, a transition in the phase of zonal wind anomalies is seen, centered on about 1 September 2004. Stronger westerly flow in the subtropics in August 2004 gives way to stronger westerly flow at high northern latitudes in September 2004.  As seen from the coherent mode Hovmollers above, there were negative OLRA (positive convection anomalies) in the region ~150E -170W from late July to about mid August.  Convection at this location relates fairly well to westerly wind anomalies centered at 30N and easterly anomalies near 50N.  Since then, tropical convective
forcing has been shifting back into the Indian Ocean and hence a reversal of the wind anomalies has occurred.  Flare-ups over the relatively warm SSTs near the dateline have contributed to enhanced westerly flow around 25N, especially over the North Pacific.

Some characteristics of the zonal mean flow anomalies are:

• the August (September) anomalies favor cyclones (anticyclones) across the midlatitiudes,
• the August (September) anomalies support anomalously high (low) AAM, meaning more (less) westerly flow when averaged over the globe,
• the westerly flow anomalies at 25N starting September 10 are linked with strong cross-equatorial flow at 150 mb into the northern hemisphere over the west Pacific,
• since 9/20, westerly wind anomalies have appeared along and just north of the equator, and are shifting poleward.

Figure 5

Figure 6 shows a 30-60N average of the 250 mb meridional wind anomaly.  Up to this point the focus has been on the tropics, especially with respect to the MJO.  Now attention is shifted to the northern extratropics in the presence of the tropical convective forcing.  Fig. 6 is used to illustrate the behavior of baroclinic waves including baroclinic wave packets, ,as well as persistent mid-latitude troughs and ridges. Four features are highlighted:
 

• the early August 2004 return of the cool, wet summer pattern for the central-eastern USA is characterized by a trough at ~105W. This feature was described in the August 16 discussion,
• relatively weak tropical forcing during the last half of August was associated with phase propagation of baroclinic waves across the Pacific North American region; energy propagation becomes more prominent as tropical forcing increases, particularly after September  7, 2004,
• two energy propagation episodes are highlighted. One is "moderate and fast (35 m/s) ", and starts over the North Atlantic on about 6 September.  The other is "strong and slow (18 m/s)" and starts near 120E around 14 September 2004,
• the stronger event - a baroclinic wave packet - was initiated through tropical-extratropical interaction in the region around 120E.

Figure 6

The following animation links and sequence of figures provide additional background about the recent evolution of the atmospheric circulation.  Key points are made about each figure and the animation links are provided to provide a more complete picture of how the atmospheric variability.  The animations are for various averages and lengths, and are updated on a daily basis.

150mb Daily Mean Animation

150mb 7-Day Mean Animation

250mb Daily Mean Animation

250mb 7-Day Mean Animation
 

Figure 7 shows 150 mb circulation features associated with the "moderate and fast" wave energy dispersion event discussed previously.  A wavetrain is evident from Europe to southeast Asia.  The twin subtropical anticyclones near 60E suggest the wave dispersion is helping to initiate Indian Ocean convection.

Figure 8 shows the OLR anomalies centered on September 10, 2004.  Convection is beginning over the west Indian Ocean as part of a NE-SW band of negative OLR anomalies ahead of the wavetrain depicted previously in Fig. 7.   Convection is also active over the west Pacific and a tropical cyclone has moved from there to north of the Phillipines.

Figure 9 shows the daily OLR for September 14, 2004.  The two active convection regions have been highlighted, including extensions into high latitudes.  The meridional extension along 120E of negative OLR from north of the Phillipines to eastern China reflects a tropical-extratropical interaction that helps initiate the "strong/slow" energy dispersion event.

Figure 10 shows that the convection over southeast Asia along 20N see in Fig. 9 interacts with an extratropical wave and amplifies a ridge-trough couplet over the mid-latitude west Pacific.  The result is downstream propagation of a baroclinic wave packet.  A deep trough across the western USA occurred a few days later.

Figure 11 is a 7-day mean of OLR anomalies that shows the 2 areas of enhanced tropical convection ~ 80E and 160E, which played a role in the above sequence of events.

Figure 12 is a 7-day mean of 150mb vector wind anomalies that follows the above selected daily maps.  Some wind and wavetrain features are highlighted.  There is north-south symmetry in the wavetrains apparently emanating from the west-central Pacific region, suggestive of tropical-extratropical interaction.  The center of the "strong/slow" baroclinic wave packet would be located over the USA in this picture. It contributes to the appearance of a single wavetrain, despite being excited over the west Pacific around 16 September.

Fig. 13 is the daily mean wind anomaly at 250 mb from 27 September 2004.  It shows the energy of the strong/slow baroclinic wave packet has split into two paths. One goes slowly southeast into the Indian Ocean and the other goes quickly eastward into the North Pacific.  Satellite evidence for an interaction between west Pacific tropical convection, the baroclinic wave packet and the ridge near the dateline was observed over the western Pacific Ocean. On the short term the question is how much the ridge will amplify and help drive a deep and westward displaced trough over the USA. Meanwhile the anomalously strong subtropical flow (25N) discussed earlier has now shifted eastward and poleward, and is coming into the southwestern USA. The signature of twin anticyclones represent recent and ongoing responses to tropical convection anomalies over the Indian Ocean and the west-central Pacific Ocean.

2.  Predictive Insights 

Before proceeding on to the predictions for weeks 1-3,  Figures 14a-d shpw the four stages of  a Synoptic-Dynamic Model (SDM) of subseasonal variability, which is under development by the authors.  In this and subsequent discussions the SDM will be referred to when making forecasts.

Figure 15 and 16 are predictions of MJO anomalies for week 2 utilizing statistical  techniques (details can be found on the respective web sites).   Both techniques predict that the convectively active phase of the MJO will be over the western Pacific in the week 2 time frame.  This seems reasonable, although the warm SSTs just west of the dateline and the seasonal cycle could effect the MJO's behavior.

The predictive information given below depends on the evolution of the tropical convection over the eastern hemisphere, particularly the MJO currently over the eastern Indian Ocean.  There is considerable uncertainty both about the MJO and the convective area west of the dateline.  The MJO convection already seems to be moving east.  The Kelvin wave moving into Indonesia may help enhance the western Pacific convection over the anomalously warm SSTs.  The possibility exists that the western Pacific convection could become dominant sooner than anticipated.  For another interpretation of the MJO, see the discussion from CPC under the link "Expert Discussions".  Consistent with the predictions given in Figs. 15 and 16, CPC feels MJO activity will propagate from the Indian Ocean sector into the western Pacific over the next 10-15 days.
 

Predictions From The CDC MJO Experimental Web Site

Predictions From The CPC MJO Experimental Web Site

Ensemble Numerical Model Output

       To Summarize:

• For week 1 (October 1-7): We are in stage 4 of the Synoptic-Dynamic Model (SDM), which has lead to a relatively moist STJ into the southwestern USA.   Superimposed upon this stage is the baroclinic wave packet discussed above, which has worked around back into the North Pacific and will lead to a strong but transient trough for the central part of the country this week.  Convection across the western Pacific contributed to this event.  The forecast implications would be for below normal temperatures for much of the central USA, and a perhaps a significant rainfall event and a severe local storm concern from the Plains on to the east.  Given that the cold air source is likely to be from Alaska and northwest Canada, a killing freeze for sensitive crops is a concern for the northern Plains states this weekend into early next week.
• For week 2 (October 8-14): The atmosphere may transition from stage 4 to stage 1 of the SDM.  However, the uncertainty of the transition is high, leading to a low confidence forecast for week 2.  Should the convection develop more rapidly across the western Pacific (~ 150E), the atmosphere may go to stage 2 of the SDM even before the end of week 2.  Thus a low confidence week 2 forecast is for the currently evolving wave train to deamplify during week 2, and then through a discontinous retrogression, go into stage 1 of the SDM.  That would suggest more cold and wet conditions for the Rockies and Plains states, with warmer than normal weather across the eastern USA.
• For weeks 3-4 (October 15-28): We expect stage 2 of our SDM as the MJO comes out into the western Pacific, and  persists over the relatively warm SSTs.