Friday, October 06, 2006

MJO is for Real; Large Uncertainty Remains for the Future of our Warm Event

Please see past postings for web site links. I am going to discontinue inserting most of them in an effort for brevity. I also need to do the same with these postings.

The spatial distribution of global tropical sea surface temperature (SST) anomalies has not changed significantly since our last posting on September 26th. Positive anomalies persist (keep in mind the seasonal cycle) across the central Indian Ocean and from the date line to the west coast of South America, with weekly mean values ~plus .5-1.0C (around plus 2C west of South America). Above average SSTs also remain from the Caribbean into much of the Tropical North Atlantic Ocean. Below normal SSTs are still present from about 90-150E within 25-30 degrees of the equator. Actual SSTs of 29C and greater continue over the central Indian Ocean, from east of the Philippines to the South Pacific centered ~160E and on “both sides” of Central America. A recent development since the last posting has been a decided cooling (~minus .5-1.0C) of equatorial SSTs from around the date line to near the west coast of South America, as well as portions of the Tropical Northwest Pacific.

The following are links to ENSO discussions.

Please also see the following CPC link (and others therein) for further ENSO, etc., insights, and
remember that official USA information on anything related to ENSO comes from CPC.

Since about September 1st, tropical convective forcing has been coherently propagating eastward west of the date line, generally along and north of the equator. This forcing “originated” around Africa, and while undergoing some complex variations, its propagation speed has remained around 4-5 m/s (3-4 deg long/day). Monitoring tools such as the Wheeler phase diagram and coherent modes Hovmollers indicate this region of enhanced tropical rainfall projects onto a Madden-Julian Oscillation (MJO), and I agree. In fact, I would consider it at least a moderate MJO for this time of year.

On about September 28th this MJO consolidated with a Rossby mode over the warm SSTs near 10-15N/155E, leading to an intense convective flare-up with negative OLR anomalies of at least minus 90W/m**2. Centered near 155E, there was an upstream (downstream) westerly wind burst (trade wind surge) on the equator with wind speed anomalies of at least 5m/s (leading to anomalous convergence). The anomalous westerlies did shift off the equator into both hemispheres, and “led” to some enhancement along the SPCZ as well. A reinvigoration of both convection and anomalous surface westerlies occurred near the equatorial date line a couple of days ago.

Current full disk satellite imagery shows the tropical convective forcing with the MJO extending from equatorial date line to near 15N/140E. There is also other enhancement across the North Tropical East Pacific ITCZ due to at least one convectively coupled Kelvin wave emanating from the MJO, as well as a flare-up around the Bay of Bengal. Strong suppression exists from far east Africa into the most of the Indian Ocean.

Anomalous upper tropospheric westerlies across the Western Hemisphere have been one of the responses to the MJO. Per animations of daily mean 150mb vector wind anomalies, there have been at least 2 “wavy” bands, with one across the equatorial atmosphere (moving into Africa) and the other extending from ~10N/180 into the western portion of the USA (leading to the current heavy precipitation across portions of the Southwest). Anomalies have been ~15-25m/s. Per time-latitude section of 200mb zonal mean zonal wind anomalies, westerly anomalies ~5m/s have been present since about September 30th from 0-30N (with other zonal mean interhemispheric symmetry of anomalies).

As would be expected, based on the reanalysis data (and its 1968-1997 climatology), the tendency of global relative AAM is strongly positive with a magnitude of ~25 Hadleys. Much of that contribution (in the zonal mean) started just north of the equator about a week ago, and is currently shifting poleward. Both the global mountain and frictional torques remain positive (latter recently increasing due to the trades). The global AAM based on both the reanalysis and operational data sets is near average (but obviously increasing). The interested reader needs to look at the actual plots (from the ESRL/PSD web site) to grasp the important details of the distribution of zonal mean AAM anomalies as well as the torques and transports.

To summarize, I think we have 1) a weak warm event whose future evolution is unclear (perhaps in doubt), 2) at least a moderate MJO signal, 3) at least one convectively coupled Kelvin wave moving into the East Pacific as I type (which could “speed things up a lot”), 4) tropical convective forcing from the Indian Ocean Dipole, which is another SST boundary forced component that could become important once again, 5) a sub-monthly component involving the global mountain torque, 6) a baroclinic wave packet interacting with the west central/northwest Pacific tropical forcing (with distorted twin subtropical anticyclones) which is part of the GSDM Stage 2 process (not discussed) and 7) the usual lots and lots of white noise.

There are two points I want make having given the above very brief background on the current weather-climate situation. First, the anomalously cool SSTs around Indonesia have been expanding eastward for about the past week-10 days while the SSTA tendency has been negative along the equatorial cold tongue. There has also been cooling across the Tropical Northwest Pacific particularly around the Philippines. This needs to be monitored! There are numerous scenarios that can be envisioned including the reflection of our recent oceanic Kelvin wave back to the west as an upwelling oceanic Rossby mode. This all means the future evolution of our current warm El-Nino is unclear, including the possibility it could weaken or even dissipate during the next few months.

The second point is I think we are transitioning into GSDM Stage 2 given the MJO and earth –atmosphere angular momentum budget situation. Most numerical models and their ensembles have generally captured this change. How long GSDM Stage 2 will persist is unclear. My thoughts for the MJO is that the dynamical signal will continue its movement into the Western Hemisphere (as currently shown by Hovmoller plots of 200mb velocity potential), increasing to around 10 deg long/day. The Western Hemisphere ITCZs are likely to become enhanced (including perhaps the SACZ). Tropical convection may persist around the date line and increase across Africa into the Indian Ocean. Thus a transition to GSDM Stage 3 and even Stage 4 may occur during weeks 2-3. My predictability confidence is about average for week 1 (everything considered!), then very low for weeks 2-3.

Week 1 (07 – 13 October 2006): GSDM Stage 2 is most probable. This will manifest itself as a large amplitude mid/upper tropospheric ridge along the west coast of North America into Alaska/western Canada and an anomalously deep trough around the Mississippi Valley. In fact, the NCEP GFS ensemble predicts ~minus 3 sigma 500mb height anomalies at tau 168hrs from 0000 UTC 6 October 2006 initial conditions. Thus below to well below normal temperatures are probable from the east slopes of the Rockies into much of the central and eastern parts of the country while the Pacific Northwest has very warm temperatures. In fact, widespread record low temperatures (at least) may occur across much of the north/central Plains into the Great Lakes states after areas such as the Front Range into the Great Lakes see their first “flakes of snow”.

Above average precipitation is probable generally east of the Mississippi River while the western third of the country is dry (after this weekend). Depending on the magnitude of the baroclinic development roughly the middle of next week, high winds are possible across much of the north central states along with a possibility of severe local storms along the cold front centered on the Ohio and Tennessee Valleys.

The Tropical North Atlantic looks to remain suppressed for tropical cyclone activity while the Central and East Pacific becomes more active. Please see for the latest tropical cyclone information.

Week 2 (14 – 20 October 2006): The circulation may transition to GSDM Stage 3. For this time of year, we may see a relatively strong North Pacific Jet lead to a split flow pattern across much of North America (not the strong jet that may be seen during January, for example, during a warm event). This flow pattern will probably include a subtropical jet. Much of the lower 48 states should experience near-above normal temperatures, meaning a warming trend for the central and east (“Indian Summer”). Most of the country would also be dry. The East Pacific (along with the West and perhaps Central Pacific) may become (remain?) quite active with tropical cyclone activity, possibly spreading into the Southwest Caribbean (sooner?).

There is only one problem I have with the above. I have concern that the cool SSTs across the Tropical Northwest Pacific could lead to some persistence of the GSDM Stage 2 regime which evolves during week 1 (perhaps even retrograde it). There are a few numerical and statistical tools which imply this (without having much clue of the tropical convective forcing during week 2; it is coming from initial condition information).

Week 3 (21 – 27 October 2006): Unclear.

Overall, I do not see a wet period for Southwest Kansas through at least week 2. Some light precipitation is probable around Sunday Night into Monday and perhaps around the middle of next week. Cooler than normal temperatures look like a good bet during week 1, especially starting Monday. Temperatures may moderate to about normal week 2 (understanding daily variability). Monitoring will be needed to get a better sense for week 2 and beyond. For instance, if a 20-30 degree retrogression of the week 1 positive PNA pattern were to occur with a subtropical jet entering the Desert Southwest, a different outcome to the weather experienced than implied above is likely.

I will try to do an update to this blog next week. Please see the Appendix.


The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006).

From taking into consideration the interactions of 4 different subseasonal time scales, a sequence of maps depicting a coherent set of repeatable events has been derived for the Northern Hemisphere cold season from November-March. This set is broken up into 4 stages, referred to as GSDM (for Global Synoptic-Dynamic Model) Stages 1-4 in the text of my Blog. Figure 13 in our paper presents a schematic of the GSDM. Ideally it would be advantageous to post our weather-climate discussions with greater frequency to provide additional detail while having a more complete weather-climate record of attribution and prediction. In these discussions I adapt the GSDM for the warm season. Our list of work includes a seasonally adjusted rendition of the GSDM.

Our latest weather-climate discussion dated August 18th, 2006 (and updated September 9th), has been posted on the ESRL/PSD MJO web site at

Ed Berry

No comments: