Sunday, December 30, 2007

Getting What was "Expected"???

Continuing data issues limit a complete discussion of the current global weather-climate situation. I will only be able to speculate about components involving the earth-atmosphere AAM budget (torques, transports, etc.) as well as the GWO.

Global tropical SSTs have been relatively “steady state” since my posting nearly 10 days ago. Very warm ocean waters (high heat content) extend from north of Australia into the South Pacific, having anomaly magnitudes in excess of 2C and totals greater than 30C especially ~10S. The latter have been shifting slowly south with the seasonal/annual cycle. SST anomalies remain well below normal in all Nino regions (~160E-South America), with magnitudes less than minus 3C extending to depths ~100m east of 140W. The weak oceanic Kelvin wave continues to propagate east at around 150m depth per latest 5-day averaged TAO buoy data having reached about 150W. Finally, monitoring suggests the convectively suppressed Indian Ocean is slowly warming while much of the tropical and North Atlantic Ocean remains warmer than climatology.

So what is the point to giving a quick overview of the global tropical SSTs, at least for this posting (see and other relevant links for details)? Although I can just offer speculation here not having objective data sets, the on-going circulation response cannot be attributed to El-Viejo alone. In fact, this is often (always?) the case where the ENSO variability is only a component. Simple monitoring of the global circulation for the last several years in itself demonstrates that Eastern Hemisphere/west Pacific Ocean warm pool SSTAs have had significant impacts onto the global circulation (including right now).

Strong-severe tropical convection currently extends from the central equatorial Indian Ocean east-southeast to just north of Australia into a southwestward shifted South Pacific Convergence Zone (SPCZ). The core of this tropical convective forcing is ~10S/130E, also shifting south with the seasonal cycle. Suppression exists across the region of the equatorial date line. OLR anomaly magnitudes are in excess of 50 W/m**2.

There is a significant MJO component to this tropical forcing, located in phase 5 per WH(2004) phase space plots updated through 29 December. However, as suggested in my last posting, atmosphere-ocean coupling involving the very warm SSTs north of Australia appears to have been in progress for at least the last week. Not only has the WH(2004) MJO signal slowed down, other plots such as Hovmollers of OLR/A and velocity potential suggest a recent stationary signal. However, I do think a relatively fast MJO dynamical signal will emerge into the Western Hemisphere during the next few weeks. Again, there is HUGE uncertainty about timing and other details.

Animations of upper tropospheric daily mean vector wind anomalies show the classic expected response given the spatial distribution of the tropical forcing discussed above. Twin subtropical anticyclones (cyclones) are centered ~110E (~date line) with anomaly magnitudes in excess of 40m/s for the equatorial Pacific westerlies. There have been several episodes of meridionally oriented Rossby wave energy dispersions (RWDs) into the extratropics tied to circulation features such as these for the last several weeks. For the PNA sector these RWDs have led to robust cyclonic circulation anomalies around Alaska poleward of a central Pacific Ocean anticyclone. Several strong troughs have impacted the USA west coast on into the Plains as a result, accompanied by high impact weather events including ice and snowstorms.

For the Asian-North American extratropics, this response fits ~phases 3-4 for the WB(200?) DJF MJO composite (a component of the GWO; publication is planned), but it is somewhat different for the tropics. The point is there are highly complex non-linear dynamical feedback processes involving the extratropics and particularly the synoptic eddies going on. For instance, during the last ~5 days a fast baroclinic wave packet tied to presumably a positive East Asian mountain torque (explained by the GWO) has been aimed at North America. This will lead to the west coast ridge amplification and anomalous cold for the central/eastern USA the models have been predicting. However, this is a transient event and when updated, all that is probable to be observed is at least one circuit around GWO phase 3 (old GSDM Stage 1).

The East Asian jet extension as a response to the positive mountain torque will strongly impact the USA west coast by the end of this upcoming week per all models. As shown by DJF GWO and MJO composites WB are working on, the MJO itself does not generate enough extratropical westerly wind flow to allow the East Asian jet to impact the USA west coast. A strong positive East Asian mountain torque needs to be involved, on average.

Uncertainty is huge where the global circulation goes from here, especially when lack of resources denies me much needed data sets (latter speaking for myself). I think GWO phase 3 best describes the current global weather-climate situation. As long as the tropical convective forcing stays coupled north of Australia, I do not think this subseasonal base state (La-Nina understood) will change. However, these coupling events have recently lasted ~2 weeks, and several tropical cyclones are developing/occurring with the convection. As mentioned above, I do think a MJO component will continue into the Western Hemisphere during the next several weeks. Support for this notion are an intense westerly wind burst (WWB) north of Australia heading into the South Pacific, as well as the strong upper tropospheric equatorial westerly wind flow anomalies also mentioned above. Whether or not this WWB will be close enough to the equator (may be too far south) to generate another downwelling oceanic Kelvin wave is unclear.

Similar to about the first half of December 2007, I suspect any larger GWO orbit through phases 5-8 that may occur ~mid/late January 2008 will be relatively fast. Hence, for the lower 48 states, weeks 2-3 may have similar weather to that observed during much of December. Delivery of Arctic airmasses will be limited as long as large cyclonic circulation anomalies persist near Alaska. Nevertheless, particularly given January climatology, extreme winter weather may occur from the west coast-central/southern Rockies into the Plains. Heavy rainfall and even severe local storms may be a concern for especially the lower Mississippi and Ohio Valleys. Should phases 5-6 of the GWO occur, say by ~ week 3, Arctic air may spill into the USA. Predicting the latter involves daily monitoring; accept it!!! Finally, on more of a seasonal time scale, “bucking” the La-Nina composite signal, my subseasonal monitoring tells me not to be so pessimistic in regard to dryness across the southwestern states into the Central/Southern Plains for JFM. In fact, precipitation anomalies for portions of California through Desert Southwest and Central/Southern Plains may end up being positive for JFM if on-going feedback processes do not change.

Internationally, heavy-severe rainfall/thunderstorms will be a concern for Eastern Hemisphere areas already discussed above for weeks 1-2. These impacts may spread into the South Pacific Islands and eventually tropical South America ~ weeks 2-4 (could be sooner). Yes, the sprit of “Gabrielle” is currently festering west of Africa.


An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at

We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the legacy GSDM is to extend current thinking beyond the MJO, the GWO quantifies variations used to derive the original GSDM in a manner that is “user friendly” analogous the WH(2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.

Links to CPC and PSD ENSO discussions:

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us (soon) to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will attempt another discussion next weekend.

Ed Berry

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