Since the last discussion on 18 December 2003, considerable subseasonal variability has occurred, including a well-defined Madden-Julian Oscillation (MJO). Section 1 gives a brief presentation of a pattern transition associated with the MJO that contributed to the recent cold temperatures across the northeastern portion of the USA. Section 2 gives an overview of the current climate situation, while Section 3 presents some predictive insights for weeks 1-3.
Figure 1 is time-longitude (hovmoller) plot of outgoing longwave radiation anomalies (OLRA, a proxy for deep tropical convection) with superimposed contours to illustrate time-space filtered modes of deep tropical convection such as the MJO and Kelvin wave (KW). The moist phase of an MJO is evident in the shading and contours from ~ late November 2003 to early January 2004. Within the MJO envelope there were several convective flare-ups. We focus on one that occurred near the dateline around 3 January (highlighted on the fig.), over anomalously warm SSTs.
Figure 2 is a hovmoller plot of 250mb geopotential height anomalies for the period of 1 December 2003 - 25 January 2004. Our specific period of interest is from about 25 December to 18 January across the Pacific/North American (PNA) sector. From roughly 25 December - 4 January there were negative height anomalies around 120W with positive anomalies near 70W. Starting about 10 January 2004, this pattern reversed, with the negative anomalies persisting near 60W until the end of the record. The pattern transition occurred from about 4- 10 January 2004. In terms of the PNA, the transition was from a negative (trough across the western USA) to a positive phase (trough over the eastern USA) and was associated with a shift of anomalous cold temperatures from the western to the eastern part of the country. The source for the cold air is unknown although stratosphere-troposphere interactions related to a sudden warming, which occurred in mid December 2003, may have played a role.
Figure 2
Recall from Fig. 1 the flare-up of tropical convection near the dateline around 3 January, associated with the MJO. The sudden onset and isolated nature of this flareup suggests forcing of a Rossby wavetrain over the east Pacific and North America with a response time of about 5-10 days. Figure 3 presents a 7- day mean of 150mb vector wind anomalies centered on 10 January. A Rossby wave train is highlighted on the figure extending from an anticyclone near 15N/160W to a cyclonic gyre just east of Washington, DC. An animation of daily 150 mb winds (click here) shows nicely the downstream energy propagation from the dateline convective flareup. This pattern led to the southward transport of bitterly cold air across especially the northeastern part of the country. The MJO appears to have contributed to this change in the anomalous circulation.
Figure 4 depicts SSTAs for the period of 18-24 January 2004. Positive anomalies ~1.0C are present in the central and southern Indian Ocean (IO) and around the equatorial dateline. As discussed in Section 1, the latter region is where the strong convection associated with the MJO occurred around 3 January. Thus, the effects of the anomalously warm SSTs near the dateline contributed to the circulation response described there.
Additional affects of the MJO can be seen in plots available on the TAO/TRITON web site (click here) . For example, a depth-longitude section shows a strong signature of an oceanic Kelvin wave forced by the December 2003 MJO.
The next logical question is the future evolution of the MJO. A phase space diagram of the MJO (Fig. 5) and Fig. 1 indicate a new MJO is developing across the south IO and Africa. Figure 6 shows the zonal and global integral of relative atmospheric angular momentum tendency. After about 15 January 2004, the tendency becomes negative, and continues downward. Regionally this results from the collapse of the equatorial and subtropical upper level westerly flow anomalies (see Fig. 3). This result is consistent with the forcing of mountain and frictional torques by the MJO when convection anomalies are over the western IO and Africa.
Figures 7a and 7b are recent full disk satellite images of the western Pacific and IO basins. The large complex of tropical thunderstorm clusters over the Indian Ocean, including tropical cyclone Elita ~ 17S/44E, is being preceded by a strong convectively driven atmospheric KW approaching the Maritime Continent. These features represent the somewhat amorphous convective envelope of the new MJO. Over the Pacific Ocean, convection is suppressed as the subsiding branch of the MJO dominates the region. However, some weak convection is still occurring over the positive SST anomalies.
Figure 8 shows the 150 mb vector wind anomalies for 20-26 January 2004. The negative AAM tendency is illustrated by the transition to equatorial easterlies from 120W to Africa and the weaker westerly anomalies over the Indian Ocean when compared to Fig. 3. Farther downstream, a wavetrain-like feature appears to emanate from the west Pacific warm pool region giving an anomaly pattern of opposite polarity to the Rossby wave train shown in Fig. 3. This reflects a retrogression of anomalies (see Fig. 2), partly due to the atmosphere adjusting to a new pattern of tropical forcing. The pattern has contributed to the recent cold and stormy situation across most of the lower 48 states.
Finally, Fig. 9 is the most recent daily map of the 150mb vector wind anomalies. Westerly wind anomalies have continued to weaken over Indonesia and are now concentrated in the subtropics of the eastern hemisphere contributing to strong but retracted jet streams in both the northern and southern hemispheres.
As discussed above, it seems plausible that a new MJO is developing and is now centered at ~ 10S, 70E. The expected atmospheric response to this convection could provide predictive insight for weeks 1-3. If the MJO propagates east at 4 m/s (~ 3 deg long/day), it should be approaching 120E by the end of week 2.
Fig. 10 is a week 2 forecast of MJO OLRA by Matt Wheeler (BMRC) using 27 January 2004 initial conditions. His prediction is consistent with the phase speed considerations given above. The MJO 200mb streamfunction anomaly forecast produced by Wheeler (Fig. 11) shows twin subtropical anticyclones over the Indian Ocean and twin subtropical cyclones near the dateline. This pattern of anomalies seems reasonable and already appears to be in a development stage. Further development may be associated with wave energy sources from the anomalous convection over the Indian Ocean. These tend to be trapped south of the mean jet stream and then amplify as they reach the mid-latitude Pacific Ocean. This process can act to amplify existing circulation anomalies in the PNA sector (see Fig. 9), eventually leading to a pattern that projects onto the negative phase of the PNA.
The ensemble forecasts systems (NCEP and CDC) appear to lose the MJO after about the first week although both predict a tendency for persistent ridging in the North Pacific during the 2 week integration. click here to see recent forecasts.
For the shorter term, Fig. 12 is the CDC 500mb ensemble mean forecast valid 0000 UTC 03 February 2004 from 0000 UTC 28 January initial conditions. The forecast suggests two storms will affect the U.S. during the next 7-9 days. The timing and amplitude of these systems is uncertain and will be affected by energy dispersion from the Pacific and by the pattern transition currently underway.
Figure 12
Within the tropics there is also considerable uncertainty regarding the tropical convective forcing, independent of the MJO. The atmospheric Kelvin wave currently moving over Indonesia is likely to help rejuvenate convection over the positive SSTA near the dateline. The extent of this flareup, its interaction with the extratropics and the possible excitation of an equatorial Rossby wave all add uncertainty to the evolution of circulation anomalies in the coming weeks. Continuous daily monitoring may provide some short-term lead time to these developments.