Monday, September 25, 2006

Convection Creeping East - 25 September 2006 (to be, or not to be, an MJO)

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 following is a guest contribution from Klaus Weickmann of ESRL/PSD. I added some additional input such as the week 1-3 outlooks and the “parenthetical title above”.


Sea surface temperatures anomalies over the Indo-Pacific continue to resemble the mature stage of an El Nino. The total SSTs also reflect the phase of the seasonal cycle with 29C water extending from northeast of the Philippines and then southeast to a broad region around the date line. Another large band of anomalously warm water extends from the Gulf of Mexico southeast toward the tropical Atlantic. Smaller areas are present to the west of Central America and in the West Indian Ocean.


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/


Since early August 2006, equatorial convection has been more or less enhanced over the western Indian Ocean near 60E, suppressed along 80-100E and enhanced over the western equatorial Pacific. This pattern is producing the “spread out” convective regime typical of El Nino events. Kelvin and Rossby wave activity has contributed to transient fluctuations of the convective pattern with the most recent intensifications occurring over the Indian Ocean in the first 10 days of September and the west Pacific (120-140E) in the most recent seven days. Thus since our last update more than a week ago, the center of tropical convective forcing has moved east and now extends in a broad band from the Arabian Sea to near the equator at 160E where it splits into two bands, one southeast along the SPCZ and the other northeast along the ITCZ. There is a projection onto a MJO, with the Wheeler index (greater than 1 sigma) suggesting a convective center north of Indonesia.

Current full-disk satellite imagery has the centroid of the forcing ~10N/140E, and rough calculation gives an eastward movement of 6m/s (~ 4-5 degrees of longitude/day) during the last 17 days. The daily mean surface vector wind anomalies from September 24th indicated equatorial westerly wind anomalies of ~5-10m/s accompanying this tropical forcing. The latest 5-day averaged TAO buoy array data also supports the presence of these surface westerlies, and may be a westerly wind event accompanying possibly a moderate MJO. This evolution needs to be CAREFULLY monitored particularly for impacts onto our warm event (understanding seasonal cycle issues).

Upper level easterly wind anomalies cover most of the equatorial region to the west of Indonesia and continue west toward South America. Westerly flow anomalies have developed from east of the date line to South America. Between these two bands of equatorial wind anomalies, cross equatorial flow over the western Pacific appears as the root of the strong subtropical jet that extends across the subtropical Pacific and feeds into the trough over the eastern USA. The trough is the result of an anticyclonic wave break over the northeast Pacific that can be linked to the convection over the west Pacific via a deep trough near the date line about 4 days ago (~20 September).

Despite these tropical connections, the mid-latitude wave pattern has been mostly progressive over the Pacific Ocean for the last 10-15 days and over the next few days baroclinic wave activity (including the remnants of Super Typhoon Yagi) is expected to amplify a ridge along the USA west coast and produce a deep trough over the east USA by the middle of this week. The models are all consistent with this development and maintain the trough for several days. Tropical convection should stay active over the western Indian Ocean and the west Pacific with some eastward movement possible.

Several tropical cyclones have developed in the broad convective band across the eastern hemisphere and activity should continue to be favored there, especially over the western Pacific. Currently a Kelvin wave is moving east and exciting convection along the ITCZ east of the dateline so this area also needs to be watched. Over the North Atlantic Ocean the easterly upper level wind anomalies are contributing to a low shear environment but subsidence is strengthening in the region. If convection over the western Pacific moves farther east the shear environment may become less favorable for tropical cyclone development over the eastern tropical Atlantic. The progressive wave pattern in mid-latitudes combined with stronger than normal westerly flow aloft over the southern USA has helped steer tropical cyclones away from the US mainland.


To summarize, I think we have 1) a weak warm event whose future evolution is still unclear, 2) a strengthening MJO signal, 3) at least one convectively coupled Kelvin wave moving into the Western Hemisphere as I type (which could “speed things up a lot”), 4) tropical convective forcing from the Indian Ocean Dipole (enhancement (suppression) to the west (east)), which is another SST boundary forced component, 5) a sub-monthly component that has contributed to mixed global AAM signals especially from the East Asian Mountain torque, and 6) lots and lots of white noise. Of course, the global circulation is well on its way to boreal autumn, only adding more uncertainty.


So, where is the global circulation within the GSDM framework? Global AAM signals are relatively weak and mixed. There are still zonal mean easterly wind anomalies throughout much of the subtropical atmosphere contributing to tropospheric AAM anomalies of at least minus ½ sigma based on the reanalysis data through 9/22 (zonal mean easterly anomalies roughly 2-5m/s at 200mb). However, there are robust regional and zonal mean signals (especially from the Southern Hemisphere) including a recent positive AAM tendency just north of the equator, consistent with the development of the upper tropospheric westerly wind anomalies discussed above. I think we are in a GSDM Stage 2 situation, and GSDM Stage 3 (warm El-Nino response) may be probable for weeks 2-3 particularly if the MJO signal remains in tact. Uncertainty remains very high for any predictions.


Week 1 (27 September – 02 October 2006): GSDM Stage 2 transitioning to Stage 3 is the best I can offer. Much of this period will be characterized by a ridge along the USA West Coast with an anomalously deep trough just east of the Mississippi River. This means essentially warm/dry for the western states, cool with light precipitation possible for the North Central States then wet from mainly the Mid-Atlantic-New England. There may also be a period or two of rainfall across the Deep South linked to jet streaks along the along the subtropical jet. Record low temperatures for this time of year are probable particularly for locations around the Great Lakes while portions of the West may experience record highs. This regime may deamplify toward the end of this period.


The Tropical North Atlantic looks to remain suppressed for tropical cyclone activity while the East Pacific becomes more active. Please see http://www.nhc.noaa.gov/ for the latest tropical cyclone information.


Week 2 (03 – 09 October 2006): GSDM Stage 3 may be most probable. 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 North Pacific 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. One or two respectable mobile synoptic-scale troughs may move from west-east spreading precipitation with them. Moisture return from the Gulf of Mexico will be an issue for the central part of the country, especially for concerns about severe local storms. The East Pacific (along with the West and perhaps Central Pacific) may become quite active with tropical cyclone activity, possibly spreading into the Southwest Caribbean.


Week 3 (10 – 16 October 2006): Unclear. We may see an evolution to GSDM Stage 4 should a decent dynamical signal projecting onto an MJO move into the Western Hemisphere.

Week 1 does not look wet for Southwest Kansas, which is no surprise given climatology. With possible daily variations, temperatures should average to around normal. Week 2 seems probable to warm to above average with maybe better chances for rain later during the period. If GSDM Stage 4 appears during week 3, rain chances may improve at that time.

It is unlikely for me (Ed Berry) to do regular postings for at least the next 2 weeks, but please keep checking. Hopefully during week 3 I can start to maintain at least one posting/week, especially given the current weather-climate situation.


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), has been posted on the ESRL/PSD MJO web site at


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

Klaus Weickmann and Ed Berry

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