ENSO and Complexity Making a Comeback

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 SST anomalies continues to resemble a mature warm event, with positive values across the western Indian Ocean and along the equator from ~140E-South America with cool readings around Indonesia particularly south of the equator. In fact, the SST horse shoe pattern of cool surrounding warm anomalies (would be reversed for a cold event) across the tropical Indo-Pacific is quite well defined. Anomalies are generally from 1-2C with magnitudes around 3C along the East Pacific cold tongue, extending to depths of ~200m around the date line. SSTs of 29C and warmer cover much of the equatorial date line region, the central Indian Ocean and both sides of Central America. Weak positive SST anomalies cover most of the tropical Atlantic with actual temperatures ~28-29C.

SST and SST anomaly tendencies during the past week or so have reinforced the spatial distribution discussed above. Magnitudes have been ~plus .5-1C, with the warming along the equatorial cold tongue the result of a basin-wide surface westerly wind event whose anomalies have exceeded 10m/s (more said below). At this point our warm event has become better defined during the past 1-2 weeks. I have expressed concern in past writings this event may have already peaked. Whether or not this is the case, is unclear to me, as is the future for this warm ENSO (there are seasonal cycle issues). Whatever the case, the SSTs linked to this ENSO evolution have already impacted synoptic variability across North America since at least June 2006, and are doing so as I type.

The following are links to ENSO discussions.

http://www.cdc.noaa.gov/people/klaus.wolter/MEI

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.html

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.

http://www.cpc.ncep.noaa.gov/products/predictions/90day/

Per coherent modes and velocity potential Hovmollers (and other monitoring tools), the dynamical signal with the MJO is in the Western Hemisphere ~0/60W having a phase speed of ~10-15m/s. The MJO signal typically propagates eastward much faster across the Western Hemisphere due to less air/sea coupling with cooler SSTs (loosely acting like a Kelvin wave enhancing upper tropospheric divergence). Full disk satellite imagery shows quite a bit of convective enhancement centered on northern South America with OLRAs ~minus 50-90W/m**2. The recent heavy rainfall across the USA Deep South had a contribution from this MJO signal as the result of a local enhancement of the subtropical jet (STJ) via complex interactions with the extratropics.

Well defined twin upper tropospheric subtropical anticyclones are present around 90-120W (including the Gulf of Mexico) with wind speed anomalies ~20-30m/s per animations of 150mb daily mean vector wind anomalies. Additionally, Rossby wave energy is dispersing into the extratropics of both hemispheres leading to, for example, a negative projection onto the North Atlantic Oscillation (NAO).

Full disk satellite imagery also shows tropical convection increasing across the west central Pacific centered ~0/150-160E as well as the equatorial Indian Ocean ~60E, while strong suppression remains across Indonesia with positive values of OLRA in excess of 70 W/m**2 ~0/100E. This spatial pattern of enhancement-suppression-enhancement across the Eastern Hemisphere would be expected from the SSTs, with the former termed by some folks as a positive phase of the Indian Ocean Dipole (IOD) and the latter due to ENSO. It is typical to see a warm western Indian Ocean during a mature warm ENSO, and I like to understand climate variability as dynamical processes involving forcing-response-feedbacks (and so on) globally interacting with regional behaviors, etc.

After reaching a positive maximum of ~20 Hadleys (reanalysis data climatology from 1968-1997) just before October 1st, the global tendency of relative AAM has dropped to ~minus 10 Hadleys as of October 14th. As expected with a MJO, this positive AAM tendency started along/just south of the equator ~ September 18th, and has coherently propagated poleward into both hemispheres, particularly across the Northern Hemisphere (currently around 40N). A time-latitude section of 200mb zonal mean zonal wind anomalies loosely shows this poleward propagation, with westerly anomalies currently ~5-10m/s centered at about 30N (one level as opposed to a vertical integral).

After roughly October 1st there was relative AAM flux divergence from about 45N (sink) leading to flux convergence near 30N (source; we are looking at divergence of the AAM transports), again consistent with the dynamical signal of the MJO moving into the Western Hemisphere. This redistribution of AAM helped to weaken the midlatitude flow while increasing the westerly flow across the subtropics (including the STJ). A forecaster looking at a polar stereographic projection map of 250mb total vector winds (for example) would have observed a split flow pattern along the North American West Coast at times during this period as a response.

The point is using monitoring tools such as AAM tendencies and transports allows some understanding of the recent increases of the zonal mean westerly flow throughout the subtropical and midlatitude atmospheres. Through tropical-extratropical interactions involving the midlatitude synoptic eddies (reader needs to think about this) not only have we seen the central and east Pacific basin wide surface westerly wind event (discussed above), there has also been a strengthening of the North Pacific jet over the last 1-2 weeks, the latter having seven-day mean anomalies of ~20m/s. For the USA the result has been recent PNA responses (from Rossby wave energy dispersions -- index sign depends on location of features; one weakness of it) and an enhanced STJ all contributing to recent cool outbreaks for the central states and heavy rain for the Deep South. Please see our recent weather-climate discussion dated August 18th as an example to interpret some of these behaviors (link below).

The enhanced surface westerlies across the north tropical Pacific Ocean basin for at least the past 1-2 weeks has brought the global frictional torque down to around minus 20 Hadleys, and the global mountain torque is also starting to decline (see ESRL/PSD plots for details). GSDM Stage 2 would best describe the weather-climate situation from roughly October 1-14 (at times tilting toward Stage 3), with a loose GSDM Stage 4 the last few days. Animations of 150mb and 250mb daily mean vector wind anomalies once again present a strong signal of anomalous twin tropical/subtropical anticyclones around 160E tied to the tropical convective flare-up in that region (with well defined twin cyclones ~90E tied to the suppression; this is a baroclinic process). A Rossby wave energy dispersion is in progress and will lead to another PNA. This has some similarities to what occurred around October 8th, except the MJO dynamical signal is now in the Western Hemisphere, as opposed to the west Pacific. In fact, upper tropospheric easterly wind anomalies of ~20-30m/s at 150mb have been appearing across the equatorial east Pacific during the last few days, having more consistency with Stage 4 as opposed to Stage 2. This all suggests a retrogression of existing circulation anomalies during the next few days or so across the northern extratropics particularly from Asia-North America (“bottom line”).

To summarize, I think we have 1) a warm event (with the understanding of Indian Ocean-Pacific SSTs) whose future evolution still remains unclear (but likely to persist into 2007), 2) at least a moderate MJO signal moving across the Western Hemisphere, 3) intensifying tropical convective forcing from the Indian Ocean, 4) a mountain-frictional torque index cycle, 5) baroclinic wave packets/fast Rossby wave energy dispersions interacting with the west central Pacific tropical forcing, and 6) seasonal cycle issues.

There are other matters I am unclear about such as any sub-monthly component involving the mountain torque and other faster modes of tropical convective variability. The future of the MJO even after week 1 is also unclear. I do think the dominant forcing from the tropics is coming from the west central Pacific, and it may not just be a warm ENSO signal. The SSTs are also cool across the tropical northwest Pacific (opposite of last year) and the evolution of this whole “SST situation” is anything but trivial to understand. It would be interesting to see how well coupled GCMs would simulate these SST events since the start of this year.

In any case, we must also remember the suppression from Indonesia also has circulation impacts, and I can easily see Indian Ocean tropical forcing disrupting events across the west Pacific. The statistical models (Wheeler phase space technique, for example) bring the dynamical signal well into the Eastern Hemisphere (~60-80E) by about day 10, and I think that is reasonable. My own thoughts is there may be 2 well defined regions of enhanced tropical convection during week 2, the Indian Ocean and west central Pacific, while the Western Hemisphere becomes suppressed. During week 3 there may be some consolidation of the 2 regions north of the cool SSTs across Indonesia. Confidence in any scientifically defensible statistically useful prediction particularly beyond week 1 is as low as it gets. This is the real world!

Week 1 (18-24 October 2006): I think a loose rendition of GSDM Stage 2 is most probable, but roughly 5-10 degrees farther west (for the USA) than about a week ago. This would mean a trough ~100W with a large amplitude ridge just off the USA west coast possibly into Alaska. Nearly all models are predicting some version of this response since the Rossby wave energy dispersion discussed above should be in the initial conditions. This means a much cooler than normal regime for a good part of the country especially the central and north central states. The warm locations may be the West Coast and Florida. One surge of cool air is occurring as I type, with another due by this weekend per operational NCEP GFS (and other models).

I have a bit of concern for this second surge of cool air. The jet streak that will contribute to it (interacting with “everything else”) may involve the remnants of Typhoon Soulik. The baroclinic development which ensures on the Plains by this weekend may begin with amplification of the trough a bit farther west into the central Rockies. That may suggest more robust cyclogenesis across the Mid/Upper Mississippi Valley. Even though moisture will be relatively limited, a “decent” snow event is not out of the realm of possibilities for locations such as Iowa into the Great Lakes (latter including lake effect). Other effects on the weather should be apparent.

The Tropical North Atlantic may remain suppressed for tropical cyclone activity. There is always the concern for hybrids this time of year and I could also see a period of upper tropospheric easterlies across the tropical Atlantic, latter perhaps allowing a window of opportunity for development. Please see http://www.nhc.noaa.gov/ for the latest tropical cyclone information. Active regions for tropical cyclones may include the west central Pacific and even the Bay of Bengal, particularly weeks 2-3.

Week 2 (25-31 October 2006): If the west Pacific is “calling the shots”, then similar to week 1 except for the usual synoptic variations of amplitude.

Week 3 (1-7 November 2006): I can envision possibilities (like anyone else), but let us just say “unclear” for now.

Climatologically this is the dry time of the year for Southwest Kansas. Other than one or two minor episodes of precipitation, week 1 looks generally cool and dry. There is a small possibility of a “surprise” this weekend. I would expect week 2 to remain generally dry with roughly seasonable temperatures (on average!). Even though not discussed for week 3, opportunities for precipitation may increase for this part of the world should “STJ activity” become more robust.

Since I continue to have to cover lots of shifts (including mids), I may not be able to update this Blog until late next week. We do have plans to write another weather-climate discussion for the ESRL/PSD MJO web page to hopefully post by the end of November. Please see the Appendix.

Appendix

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

http://www.cdc.noaa.gov/MJO/Predictions/wb2006.pdf

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), along with all the other issuances, has been posted on the ESRL/PSD MJO web site at

http://www.cdc.noaa.gov/MJO/Forecasts/climate_discussions.html

Ed Berry