Saturday, January 26, 2008

El-Veijo Getting Juiced

There is relatively little change in the spatial distribution of global tropical SSTs (I am not forgetting the Atlantic Ocean) since the 17 January posting. Mature basin-wide cold event SST conditions prevail across the Indo-Pacific sector, with anomaly magnitudes as low as ~minus 3C (totals ~24C) around 0/150-160W per TAO array buoy data. Tied to moist tropical forcing returning to the Indian Ocean (discussed below), a recent trade wind surge has led to renewed cooling along the equatorial central and eastern Pacific Ocean. The cold anomalies extend to roughly 150-200m deep, with magnitudes ~minus 5C (100m) at 120W.

Warmer than normal waters having similar anomalies as their La-Nina counterpart prevail generally along and south of the equator from the Indian Ocean-northwest Australia. SST totals in this region are ~29-30C. There is still the well-pronounced warm SST anomaly horseshoe extending from the far equatorial west Pacific Ocean into especially the Northern Hemisphere. Finally, responding to a recent westerly wind burst south of the equator, positive anomalies (5-day mean) in excess of 4C have developed ~200m at 165E. Together with the cold anomalies farther east, this is a strong signal of a steeper than normal equatorial oceanic thermocline, typical of La-Nina. Whether or not a coherent oceanic Kelvin wave gets (or already is) generated is unclear.

Full-disk satellite imagery and other diagnostic monitoring tools indicate renewed strong-severe tropical convective forcing from South Africa-central equatorial Indian Ocean (as expected per past discussions). Sporadic thunderstorm activity has been recently occurring across Indonesia while a remnant cluster attempts to develop into a South Pacific Ocean tropical cyclone. WH(2004) phase space plots support the notion that the strong MJO signal since early-mid December 2007 has returned to the Indian Ocean. Again, it is dynamical interactions with the extratropics such as Rossby wave energy dispersion (RWD) processes that allow this kind of behavior to occur. Model predictive schemes of the MJO utilizing the WH(2004) methodology were generally mixed about the signal returning to the Indian Ocean.

MJO tropical variability thus far has been the strongest since the boreal 2004-05 cold season, and this is unusual for a strong La-Nina such as the one currently in progress. Whether or not the start of MJO#3 for the boreal 2007-08 is in progress remains unclear. However, the west central-South Pacific Ocean tropical forcing finally weakening suggests that it is, as do some other tools.

As was the case for MJOs #1-2 constructively and destructively interfering with our La-Nina base state, the redevelopment of Indian Ocean tropical forcing is probable to enhance/amplify it. Keep in mind that dynamical processes in the atmosphere cannot be “shoe-horned” into simple "recipes". For example, it is not accurate to attribute the intense USA west coast winter storm ~3-4 January 2008 simply to the MJO. In reality the moist MJO tropical convective forcing centered on Indonesia amplified an already existing La-Nina base state allowing a cold trough to slam the west coast. There is no short cut to understanding the dynamics of physical atmospheric processes involving forcing-response-feedback-subsequent interactions impacting multiple time and space scales. The latter is important if we are to improve to making predictions of especially, for instance, high impact weather events weeks 1-3. Numerical models will struggle when there are rapid and complex changes involving tropical convective forcing with subsequent extratropical interactions, no matter how much bias correction is employed.

Since early-mid December 2007, responding to MJO#2, well defined poleward propagation of zonal mean easterly wind flow anomalies occurred from the equatorial into the subtropical/mid-latitude atmospheres of both hemispheres. Currently there are westerly wind flow anomalies shifting off the equator into the subtropics, particularly the Northern Hemisphere (~10m/s at 200mb). That is why there are several low-latitude closed lows with subtropical jets, globally.

With the added subtropical westerly wind flow from the last MJO, global relative AAM continues to hang around ~minus 1 standard deviation below the R1 data climatology through 23 January. Also helping to keep global AAM from plunging like the financial markets have done so far this year is a ~10-15 Hadley positive frictional torque. The latter is a direct response to the poleward and downward propagation of the zonal mean easterly wind flow anomalies discussed above, especially to around 20-40N (weaker ~15-30S).

A strong zonal mean poleward AAM transport signal has appeared ~50N while weakly equatorward ~20N, meaning midlatitude split flows that have regionally been across the oceans and roughly central Asia. Finally, also consistent is a strong positive zonal mean earth AAM signal of ~1 AMU centered ~45N suggesting a lot of surface high pressure across the Northern Hemisphere midlatitudes (weaker Southern Hemisphere). The point is that in the presence of the strong MJO variability, typical of a mature La-Nina, a loose equilibrium now exists between processes wanting to remove and add angular momentum to the atmosphere that are meridionally symmetric about the equator.

Also updated through 23 January, the GWO signal has orbited weakly to phases 4-5 in the daily mean. I suspect the orbit will shift a bit toward the left into the region of phase 3 (old GSDM Stage 1 – “La-Nina attractor”) during the next week or so. In fact, supportive of this notion, animations of upper tropospheric daily mean vector wind anomalies from ESRL/PSD already show twin tropical anticyclones developing across the Indian Ocean with cyclones shifting back toward the date line. Zonally and meridionally oriented RWDs forced by both the GWO (which considers the MJO) and MJO respectively have been leading to the expected retrogression (3-week lead time, before the numerical models caught on) of the midlatitude ridge back into the central Pacific Ocean.

Summarizing, it is still unclear whether or not MJO #3 is developing across the South Indian Ocean. However, given the warm SSTs it is probable to see the enhanced tropical forcing shift eastward into the region Indonesia-Australia (~80-140E south of the equator) by roughly week-2. Perhaps this will be another MJO with several “stops along the way (coupling with SSTs, etc.)” similar to the last one. El-Viejo is probable to get pumped once again, possibly similar to late December 2007 into earlier this month. Most numerical ensemble prediction schemes now show a PNA response typical of GWO/MJO phases 3-4 (GSDM Stage 1); that is, central Pacific Ocean ridge, trough from Alaska (good block never did develop per past posts) to the USA west coast-deep southeast states ridge. I speculate this whole pattern will drift east weeks 3-4 especially if there is another MJO (timing is always white noise). Weather ramifications of this pattern for the lower 48 states should be apparent by now (see 17 January discussion).

In the longer term, poleward shifting zonal mean easterly wind flow anomalies do increase the probability of anomalous midlatitude ridges. Where these ridges become established obviously has a lot to do where drought may occur say, for this upcoming boreal summer. Right now, that is unclear. Hence statements about possible drought centered on Iowa this upcoming warm season are very much premature. In fact, I can easily speculate an opposite scenario.

Internationally, a significant tropical cyclone risk exists from Madagascar into the South Indian Ocean week 1 likely shifting to the north coast of Australia weeks 2-3. The west central-South Pacific Ocean is going to be a bit of a “wild card” from now on (not discussed above) due to warm SSTs and other issues (stay tuned). One or two tropical cyclones may impact the paradise islands week 1 with hopefully some suppression weeks 2-3. The severe flooding rainfall events for South Africa should slowly diminish during the next couple of weeks. At least diurnal/frontal seasonal thunderstorm activity is probable week 1 for tropical South America including Brasil, with possible suppression starting weeks 2-3 should there be another MJO.


An experimental quasi-phase space plot of the GSDM utilizing time series of normalized global relative AAM time tendency (Y-axis) and normalized global relative AAM anomaly (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 t0 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. Work is on-going by Weickmann and Berry to submit a paper that will formally introduce the GWO. I will attempt another posting the weekend of 2-3 February.

Ed Berry

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