Sunday, October 22, 2006

Update on What Goes Around Gets Messy -- written 10/21/06

This posting will be very brief. I hope to do a more complete effort late this upcoming week after I have completed covering overnight shifts (~10/26-27/06). Please see other postings for all web links.

The spatial distribution of global tropical SSTs are consistent with a mature warm ENSO. Strong westerly wind anomalies (~10m/s) and actual westerlies remain across the equatorial date line region. In fact, TAO buoy data suggest a downwelling oceanic Kelvin wave has developed east of the date line and is moving east along the equatorial cold tongue. We will see how robust this warm event gets (there is always uncertainty in many different aspects). At this point odds would favor above normal surface temperatures for especially the north central states during DJF and JFM (see CPC outlooks). However, please remember these are seasonal mean indications and this does not at all rule out possibilities of extreme cold episodes along with other forms of high impact severe winter weather for the northern or any portions of the USA.

The dynamical signal with the MJO is moving into the Eastern Hemisphere while SST boundary forced tropical convection slowly increases across the central Indian Ocean and west Pacific (per full disk satellite imagery). Three-day mean charts of OLRA indicate enhanced rainfall is present just south of the equator ~60E, 160E-160W including the SPCZ and from much of northern South America into the Atlantic ITCZ. Anomalous upper tropospheric twin tropical/subtropical anticyclones (with lower level westerly anomalies – baroclinic response) accompany these 3 regions, particularly for the MJO dynamical signal.

Re-analysis data plots from ESRL/PSD shows global relative AAM tendency has become strongly negative (as of 10/18) with a magnitude of ~ minus 25 Hadleys. Both the global mountain and frictional torques are contributing. Zonal mean easterly wind anomalies are replacing westerly anomalies throughout the tropical and subtropical atmospheres while existing westerly anomalies propagate into the midlatitudes of both hemispheres. In this type of situation a synoptician is going to see a tendency for troughs and ridges to become northeast-southwest oriented across the Northern Hemisphere along with features such as anticyclonically wave breaking (AWB) “upper level lows”. These tilts will contribute to AAM being fluxed poleward from the tropics into eventually the midlatitudes.

GSDM Stage 4 best describes the current weather-climate situation. The future evolution of the tropical convective forcing is unclear, including if there will be another MJO. My own thought is we may see some consolidation of the tropical forcing across the Eastern Hemisphere during the next 1-3 weeks, perhaps around 10N/90-120E to the north of the cool Indonesian SSTs. While this may be occurring, the date line region and even the SPCZ may continue experience tropical convective flare-ups (there are still the ~20-30 day tropical convective variability and convectively coupled Kelvin waves going on). Locations such as the South Indian Ocean-Bay of Bengal and west central and South Pacific may be vulnerable to tropical cyclone development (and we need to watch the Caribbean and even the North Atlantic for especially hybrids). During weeks 2-3 GSDM Stage 1 with subtropical jets may be most probable particularly for the Asia-PNA sector. This would suggest western North America to become increasingly at risk to strong synoptic-scale troughs while an active storm track develops across the Plains possibly going well into November. Most readers should be familiar what all this means in terms of weather particularly if a cold air source evolves during November.

Ed Berry

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