Friday, October 27, 2006

Truce Over With???

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 still resembles a mature warm event, with positive values across the western Indian Ocean and along the equator from ~170E-South America and cool readings around Indonesia particularly south of the equator. Magnitudes as low as ~minus 2C and lower were observed north of Australia on 26 October while ~plus 2C just east of the equatorial date line and west of South America (keep in mind the seasonal cycle of SSTs). The SST horse shoe pattern of cool surrounding warm anomalies (would be reversed for a cold event) across the tropical Indo-Pacific remains, and is best defined across the Southern Hemisphere with values ~minus 1-2C.

SSTs of 30C and warmer cover much of the equatorial date line region while ~29-29.5C waters are present across the central Indian Ocean and around both sides of Central America. Weak positive SST anomalies cover most of the tropical Atlantic with actual temperatures ~28-29C.

There is some evidence that the warm-cool-warm SSTs discussed above have shifted ~10 degrees east during the last week or so. The recent surface westerly wind event (WWE) with anomalies in excess of 10m/s (westerly anomalies still remain along and east of the date line) has apparently excited another oceanic Kelvin wave which is propagating toward the east Pacific. Anomalies as high as plus 4C down ~150m at 170W were observed per recent TAO array buoy data. This oceanic Kelvin wave along with an advective component from the anomalous westerly flow may be helping to shift the SST distribution to the east.

The WWE tied to the recent MJO helped to strengthen our warm ENSO. At this point I see 2 ocean basins with warm SSTs, the central equatorial Indian Ocean and the date line. There are folks that will call the former a positive phase of the Indian Ocean Dipole (IOD) and others which feel this SST distribution is consistent with a warm event (I feel the latter). Whatever the case, I try to understand the weather-climate situation from a dynamical forcing-response-feedback perspective, and I think I can see physically how this whole SST situation evolved starting around March 2006 (too long to discuss here). The point is both of these warm ocean basins (and other oceanic regions) are currently impacting the circulation and will continue to do so.

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.

Full disk satellite imagery and other monitoring tools (coherent modes Hovmollers, velocity potential/OLRA animations, etc.) suggest the dynamical signal with the MJO has returned to the Eastern Hemisphere and was located ~60E. The phase speed of this signal has slowed considerably and its magnitude has significantly weakened. Tropical convective forcing has been increasing across much of the central Indian Ocean and in a band from the South Pacific toward the Philippines (with Tropical Cyclone Cimaron). Enhanced convection has recently been propagating east-southeast along the South Pacific Convergence Zone (SPCZ) (weakly projecting onto a convectively coupled Kelvin wave) while also shifting back toward the Philippines. In fact, the recent pattern of Indian Ocean enhancement-Indonesian suppression- west Pacific enhancement is less robust than a week ago.

My own thought is we may see some consolidation of the tropical convective forcing ~ 10-15N/90-120E during the next 1-2 weeks (north of the cool SSTs). However, flare-ups across both the Indian Ocean and especially the date line/South Pacific region may occur “any time”. Illustrations of this kind of behavior are the weather-climate events during past 4 weeks or so which we hope to detail in another weather-climate discussion for the ESRL/PSD MJO web page.

Briefly, as the dynamical signal with the MJO interacted with the warm date line SSTs during late September and early October, a Rossby wave energy dispersion (RWD) occurred across the PNA sector leading to a deep trough across the Great Lakes and North Atlantic blocking by around 12-14 October (recall the Buffalo, NY snow event). There were subsequent RWDs from the North Atlantic blocking which interacted with other wave packets moving across Asia. These helped to re-invigorate the date line tropical convection through their enhanced divergent upper tropospheric outflows all while the MJO signal was moving through the Western Hemisphere, by about 15 October. Among other responses, there was a central/east tropical Pacific basin wide surface westerly wind event with anomalies well in excess of 10m/s which helped to strengthen the warm ENSO. To me, this is a situation of tropics forcing extratropics (including from the MJO and ENSO in this case) and then extratropics forcing back (with properly phased wave trains) to maintain an “equilibrium weather-climate situation” through the interactions of multiple time scales.

A “repeat” of the above recently occurred about 5 days ago with another flare-up of date line tropical forcing (~every 20 days for a time-scale?) which led to intense baroclinic storm development from the central/southern Rockies to the eastern part of the USA. A difference is the MJO signal had re-emerged into the Eastern Hemisphere (and weakened). The models did struggle with the synoptic details of this storm (which was responsible for a blizzard across eastern Colorado and possibly more than 2 dozen tornadoes across Southwest Kansas).

Is the recent intensification of the Indian Ocean convection going to “disrupt this feedback loop?” Has this latest MJO caused our warm event to (finally) peak? Are we going to see 2 (or more) regions of tropical forcing with the Indian Ocean convection (eventually spreading east inconsideration of the SSTs?) becoming dominate at some point? Will there be another coherent MJO signal? These are just some of the issues that are unclear.

Evidence of circulation impacts from the variability of the date line tropical forcing can also be seen from plots of the earth-atmosphere AAM budget available from ESRL/PSD (using the re-analysis data and its 1968-97 climatology). Loosely, around 1 October global relative AAM tendency was positive with positive global mountain and frictional torques. That all reversed by about the middle of this month only to recently become positive (magnitudes ~10-20 Hadleys). Global relative AAM has lagged by roughly 10 days.

There has also been poleward propagation of zonal mean westerly wind anomalies into both the midlatitude and subtropical atmospheres of the Northern Hemispheres with anomalies ~5-10m/s at 200mb linked to the tropical forcing. Animations of both 150mb and 250mb daily mean vector wind anomalies show the presence of anomalous twin subtropical anticyclones across the Western Hemisphere coming back into the Indian Ocean while a separate pair persists near the date line. The former have been associated with the MJO signal while the latter is at least partly from ENSO. There is a well defined subtropical jet (STJ) extending from the date line pair (as also seen from full-disk satellite water vapor imagery) with additional enhancement from the anticyclones downstream. Anomalies with this STJ have been in excess of 25 m/s at 150mb at times.

Overall we have a weather-climate situation I would offer is best described by GSDM Stage 4 but with additional complexity due to the date line tropical forcing. I have been terming the latter as a signal due to the current warm phase of ENSO throughout this discussion. However, in reality I think there may be even slower time scales involved with this along with “other processes”, in addition to ENSO.

To summarize, I think we have 1) a warm event (plus a global warming signal whose cause is unclear?), 2) possibly an increasing role of the Indian Ocean SSTs, 3) the MJO signal which may become very weak, 4) 20-30 day tropical convective variability, 5) some evidence of mountain-frictional torque index cycle variations (not discussed – may be linked to (4)), fast RWDs, baroclinic wave packets and all sorts of other white noise, and 6) seasonal cycle issues.

The latter (6) may involve just how sensitive the response across the Asia-PNA sector will become to tropical convective forcing (or lack of) as the climatological strengthening of the East Asian Jet (EAJ) occurs. For instance, if we do get into a situation of 2 regions of tropical forcing (with suppression, say around the Philippines), the response may become GSDM Stage 1 with STJs through late December. Afterwards, as both the convection and EAJ shift south, enhanced convection across the central Pacific may become dominate and lead to GSDM Stage 3 by sometime in January. We will see.

In what follows, I am leaning toward my notion of consolidation of the tropical forcing during weeks 1-2 per above. Afterwards, I think it is unclear to offer anything statistically useful without additional tools. Confidence remains as low as it can ever get making week 1-3 predictions in this weather-climate situation. This is why detailed disciplined daily monitoring is critical.

Week 1 (28 October – 3 November 2006): GSDM Stage 4 with additional forcing from the central Pacific appears probable. The models are generally consistent with mid/upper tropospheric trough development across the east Pacific (~140W) with a downstream ridge along the west coast and subsequent eastern states trough by early next week. I see no reason to differ given the central Pacific twin subtropical anticyclones. At some point I would expect the westerly flow to increase across East Asia as trough development occurs there. This should allow the east Pacific trough to approach the west coast going into week 2.

This situation is generally dry for most of the country with warm conditions across the western states and cool for the east (not all the unusual for this time of year). The Pacific Northwest should have increasing opportunities for rainfall. There may also be opportunities for rainfall across the Deep South at times tied to the STJ.

Week 2 (4-10 November 2006): GSDM Stage 1 with an above average STJ may be most probable. At this point particularly the Pacific Northwest may receive substantial precipitation, which could spread slowly south. Depending on how far inland the trough gets, cooler and wetter weather may become more likely for the portions of the Rockies. The rest of the country may see above normal temperatures.

Week 3 (11-17November 2006): Unclear; however, an eastward progression of the situation discussed for week 2 would be reasonable if there is any truth to it.

Climatologically this is still the dry time of the year for Southwest Kansas. The recent storm/rain event was a pleasant “surprise (other than the tornadoes depending on your perspective)”. Week 1 looks generally dry with near-below normal temperatures (on average). Dryness may continue well into week 2 while temperatures may warm to well above average. Jet streaks along the STJ could give us at least a minor precipitation surprise by then. An active regime may return for this part of the world week 3 (for this time of year).

I will be on travel to ESRL/PSD next week so I do not know when there will be another posting on this Blog (perhaps a short one ~ middle of 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.


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

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