Saturday, September 16, 2006

War Update

This is a short update to the previous posting issued on September 13th, 2006.

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

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.

Tropical convective forcing continues centered around 10N/100E with a weaker region near 0/160E. As suggested by the coherent modes Hovmollers and observed on full disk satellite imagery, convectively coupled Kelvin and Rossby waves have been emanating from the Indian Ocean and date line convection, respectively. A weak consolidation of these features is possible within the next few days. However, as far as I am concerned, there remain two distinct regions of tropical forcing across the Eastern Hemisphere, with the Indian Ocean dominating. Monitoring tools such as the Wheeler phase space plot and 5-day averaged 200mb velocity potential suggest that the weak Indian Ocean MJO signal may be stalling.

Over the last several days the trades have been intensifying from the central Pacific Ocean to near Indonesia, with a separate region of above average easterlies across the central equatorial Indian Ocean ~60-90E. Anomalies have generally been from 2-5m/s. While global tropical SSTs have not changed much during the past week, I want to make the point that the anomalous warming around the equatorial date line has been slowed, at least for now. Positive anomalies of ~plus 1-1.5C remain. There has also been a reduction of positive SST anomalies along the equatorial Pacific cold tongue from ~120-160W, with magnitudes ~0.5C. However, subsurface anomalies at depths of ~100m are still ~plus 3-4C in this region. These depth anomalies represent greater than normal deepening of the oceanic thermocline along the cold tongue, and not actual SSTs. In other words, upwelling these positive depth anomalies to the surface via enhanced trades does not necessarily mean warming of the ocean surface.

Animations such as 150mb and 250mb daily mean vector wind anomalies show a relatively clean signal of twin Indian Ocean subtropical anticyclones with down stream twin subtropical cyclones across Indonesia. This is the expected baroclinic response to the anomalous divergent outflow from the enhanced convection in that region. These features have been nicely linking up with baroclinic extratropical wavetrains, including Rossby wave energy dispersions across the Pacific Rim leading to the large anomalous anticyclonic gyre around and north of Alaska (and the downstream western USA trough). The latter has had daily mean wind anomalies of around 30-40m/s at 250mb. There are also loose twin anticyclones over the central and east Pacific, linked to the date line forcing (representing the warm ENSO signal).

While global AAM signals remain somewhat mixed, I think I can see the possibility of a response signal emerging. As of September 13th the global mountain torque was roughly 20 Hadleys (leading to a slight positive AAM tendency), with most of that coming from East Asia. Within the GSDM framework attribution may be to both the submonthly component and the tropical convective forcing from the west central Pacific. However, for about the past week-10 days (reanalysis data) there is some evidence from the AAM transports that fluxing out of the northern midlatitudes centered ~40N has been occurring. Overall, global relative AAM remains about 1 sigma below normal per reanalysis data climatology (1968-1997). This has translated to a split flow across the Pacific-North American sector, with weakly mean (September 10-16) anomalous westerlies of roughly 25m/s (operational data climatology) across the North Pacific at about 45N.

The point is I think I can see how the 2 regions of Indian Ocean dominated tropical forcing (with subsequent feedbacks and forcing from the extratropics) are currently impacting the global circulation, giving loosely a GSDM Stage 1-2 behavior. The role of the seasonal cycle and other nonlinear dynamical processes that are not well understood will determine where the coupled earth-atmosphere-ocean system goes from here. Included is the evolution of our weak warm ENSO; for example, “how big it will get and how long it will last”. Right now I feel the latter is unclear. For weeks 1-3, I think variations in the GSDM Stage 1-2 response is the only statistically useful outlook I think I can offer. Please see previous posting for the weekly outlooks.

I will try to do another posting on about the middle of next week (roughly Tuesday-Wednesday 9/19-20).

Ed Berry

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