Edward Berry, NWS and Klaus Weickmann, CDC
At the time of our last update (23 November 2004) the MJO signal was very weak. Convection was flaring up episodically over the west Pacific and Indian Ocean regions, and was suppressed over Indonesia. This activity continued during most of December, especially near 160E in association with anomalously warm SSTs. The MJO also continued to be weak.
For information on the status of El Nino and the MJO see the following links:
1. Weather-Climate Overview
Figure 1 (below) shows Hovmoller plots of outgoing longwave radiation anomalies (OLRA) for the northern, equatorial and southern tropics from top to bottom, respectively. The contours isolate three coherent OLR modes, including the MJO, the Kelvin wave and the equatorial Rossby wave. The two regions of enhanced convective activity along 80E and 160-170E are clearly visible, as is the suppressed region near Indonesia. The episodic flare-ups of enhanced convection over the western Pacific have occurred in association with equatorial Rossby waves with the most recent having an intense convective signal. The dark dashed lines in the top two panels highlight a westward movement by the suppressed convection around Indonesia. The cause for this behavior is unclear but it has a counterpart in the northern hemisphere mid-latitude circulation as discussed below.
For the southern tropics, the contours actually depict a weak MJO starting near 40E during mid-October, and ending near the date line in early December. However, the (Wheeler Phase Space Diagram) indicates that the MJO has been nonexistent since roughly the middle of October.
At the present time (22 December 2004) thunderstorm activity is weak across
the western Pacific warm pool region. Convection has intensified just
west of Indonesia over the last few days in a broad region centered along
the equator near 100E. Other convection is present across south central
Africa and the Amazon region of South America. Overall, there is a
lack of coherent tropical convective forcing at this time. For the
latest satellite imagery see: Latest Indian Ocean Satellite
Picture; Latest
Western Pacific Satellite Picture
Fig. 1 latest images
Figure 2 (below)
is a time-longitude plot of 250mb height anomalies averaged between 30-60N
for the period 10 October-16 December 2004. Three periods are highlighted.
The first from about 15-25 October featured large positive height anomalies
along ~160W and negative height anomalies along 120W. The two "L"s mark
the times when California and parts of the southwest USA received unprecedented
precipitation amounts.
The second period from about 4 November-3 December is highlighted by the dark dashed lines. They depict a slow westward movement of 250mb height anomalies having a zonal scale of approximately wavenmber 4. The timing and speed of movement is comparable to the retrogression of convection anomalies seen in Fig. 1 (i.e., 1-3 m/s). The physical mechanism(s) responsible for any linkage is unclear.
The final period starts on about 8 December and shows a nearly stationary
pattern of negative (positive) height anomalies around 160W (110-120W).
There is a hint of slow retrogression to these anomalies, which may have ramifications
for week 1-3 predictions (see Section 2).
Figure 2
Figure 3 (below) presents a 30-day average of 250mb vector wind (top) and OLR (bottom) anomalies for the period of 4 November-3 December 2004. This is during the period of the retrogression of the 250mb height anomalies discussed in Fig. 2. In the top panel, a well-defined wavetrain has been highlighted that appears to emanate from the Indian Ocean toward North America. In the bottom panel, the OLR anomalies imply enhanced convection over the Indian Ocean, suppressed convection to the east over Indonesia and spotty enhanced convection over the west Pacific. The wavetrain originates from Indian Ocean/Indonesian region where twin anticyclones and a trough over southeast Asia are prominent. Figures 1 and 2 suggest that this whole pattern shifted westward during the 30 day period. For example, the trough on the date line in Fig. 3 is the residual in Fig. 2 of a trough that shifts from east of the date line to west of the date line. Of course, considerable synoptic variability is also evident in Fig. 2.
Figure 3
2. Predictive Insights
In the 23 November 2004 discussion, we suggested an MJO was redeveloping over the Indian Ocean, and that the circulation states depicted in Stages 1 and 2 of our Synoptic Dynamic Model (SDM) were probable across the Pacific/North American sector for weeks 1-3 (link to discussion with SDM). Instead, OLR anomalies generally shifted toward the west across the eastern hemisphere, and no MJO developed. The Pacific/North American circulation anomalies also exhibited a retrogressive tendency (see discussion above) and by 10 December had evolved into a positive phase of the Pacific/North American teleconnection pattern (see PNA index ). These events may not have been predictable.
Currently convection continues to slowly intensify across Indonesia with another concentrated area across southcentral Africa. Convection across the western Pacific has weakened and moved northwest. Latest animations of 150mb vector wind anomalies show there is wave energy dispersing along 25-30N interacting with the Indonesian convection and the remnants of the west Pacific convection. All available MJO indices suggest a very weak signal, thus statistical and numerical predictions of a MJO cannot be considered useful at this time (see lastest MJO forecasts ).
Confidence in the upcoming week 1 outlook is fair, due to generally good numerical model agreement. For weeks 2 and 3 confidence is low due to the lack of an MJO signal at the present time. The recent still on-going retrogression, and the slow increase of convection around Indonesia suggests Stage 1 of the SDM may be present during week 2. Our predictive insights are given below.
Week 1 (23-29 December 2004): The retrogression should lead to a
large amplitude ridge ~140W with an anomalously deep trough ~100W by about
23 December. Nearly all of the available operational numerical models
including their ensembles are in agreement with this solution (see links below).
A severe Arctic outbreak is expected in the Northern Plains and Upper Mississippi
Valley by Christmas Eve. Additionally, baroclinic cyclonic development
is expected across the south central states, and the mean-sea level low response
should rapidly deepen and track almost due north into southern Canada by
the end of the work week. Although the exact storm track is still unclear,
this will be a major winter storm, with blizzard conditions possible across
portions of the Ohio Valley and Great Lakes States, and strong to severe
thunderstorms across the east and southeast USA. The western states
should be relatively quiet with valley fog and strong offshore coastal wind
events being a concern through much of the weekend. Precipitation
may increase along the west coast early next week.
Week 2 (30 December-05 January 2005): This is the period when the
seasonal Pacific jet stream may intensify, meaning a rather active circulation
regime globally as the tropical convection shifts south of the equator and
interhemispheric temperature contrasts become relatively large. Tropical
convective forcing may also become intense near 150E. Ensemble model
output and synoptic experience indicate a discontinuous retrogression, leading
to a trough along the west coast and Stage 1 of the SDM. For week 2
this would suggest a slight probability for a major winter storm across the
Rockies and Plains, meaning severe winter weather in the cold sector with
heavy rainfall and strong to locally severe thunderstorms in the warm sector.
Anomalously cold air would dominate the western states while the southeastern
USA is relatively warm.
Week 3 (06-12 January 2005): Since the evolution of tropical convective
forcing is very uncertain, any prediction for week 3 is expected to have
little skill.
Latest
CDC Ensemble Forecasts
Latest NCEP Ensemble Forecasts
Additional NCEP Ensemble Output
Latest Canadian Ensemble Output
