Saturday, March 29, 2008

Bear Market Atmosphere

Equatorial SST anomalies from the Indian Ocean into the Pacific loosely exhibit a spatial pattern of cool-warm-cool with the warmth centered on Indonesia. Interestingly this pattern is roughly opposite to that seen during boreal fall 2006 (Indian Ocean dipole matters understood). Anomaly magnitudes are ~1-3C. The horseshoe distribution of warm SSTs into the extratropical North and South Pacific Oceans is as robust as “ever”, while positive SST anomalies continue just west of South America (local El-Nino conditions) and portions of the tropical Atlantic Ocean.

The warmest SSTs globally extend from Indonesia into the Southwest Pacific, with totals ~29-30C. The Indian Ocean is roughly 1-2C degrees cooler. TAO buoy array 5-day averaged data present a strong signal of deep (at least 200m) anomalous SST warmth of roughly 5C west of the equatorial date line while the cool La-Nina anomalies to the east are ~100-150m deep. As discussed before, this tells us that the equatorial Pacific thermocline is steeper than normal, typical of a cold event.

The cool SSTs with La-Nina have warmed respectably during the last several weeks, having anomalies ~1-2C east of the date line and totals ~25-26C. One of many important subseasonal monitoring issues will be to see if these cool SSTs survive the boreal spring portion of the seasonal cycle. There are several forecast tools suggesting this El-Viejo will persist the remainder of 2008 (see IRI link below).

In any case, we will see what happens. For readers who simply want to know when to “expect” the next severe local storms/tornado outbreak, an analogy to the above is if overnight thunderstorms will survive the morning diurnal minimum.

The following are additional links for SST information. (note the initial projection) (link 18).

Moist tropical convective forcing has propagated east from the Indian Ocean into the region of Indonesia and the South Pacific Ocean Convergence Zone (SPCZ) during the last 2-3 weeks. Recent full disk satellite imagery and the many other plots I look at suggest this forcing is getting a bit more concentrated in the region of 120-140E on the equator. Loosely the enhanced to severe tropical rainfall extends from the southeast Bay of Bengal into the South Pacific Ocean. Strong suppression exists across both the Indian Ocean and the equatorial date line.

There was a MJO component to the eastward shift (#4 for this boreal cold season). The WH (2004) phase space plots caught on to some of the reality of the tropical forcing, and through 28 March indicate a less than 1 sigma projection in phase 6. There was a greater than 1 standard deviation projection onto this index until 26 March. However, I think the MJO signal has become unclear. Furthermore, I am suspicious there may an attempt of air-sea coupling involving the moist tropical convective forcing discussed above. These subseasonal variations involving the tropical forcing working with other dynamical processes (a few discussed below) have been impacting the global weather for the last few months, including the USA. The latter is true regardless of whether or not there has been a MJO, and operational forecast centers need to start understanding this.

The eastward shift of the moist tropical forcing into Indonesia and the Southwest Pacific Ocean interacting with extratropical processes has added decent westerly wind flow to the global atmosphere. This is the largest contribution of such wind flow since late January-early February. Now there is the issue of understanding the complicated dynamics responsible and where that westerly wind flow has been distributed, at least in the zonal mean sense.

In several past discussions I have mentioned the possibility (and reality) of non-linear forcing-response-feedback processes tied to variable tropical convective forcing and higher latitude blocking structures. Rossby wave energy dispersions (RWDs) have linked the tropics and extratropics in at times seemingly simple patterns. However, an astute reader needs to make some effort to monitor daily (and longer averaging periods) mean animations of various wind fields (upper and lower tropospheric) working with the WB (2007) GSDM framework to see these.

RWDs linked to the recent eastward movement of moist tropical forcing contributed to a strong positive global mountain torque event peaking to ~30 Hadleys (ESRL/PSD plots using R1 data/climatology) roughly 26 March. Blocking structures located near Kamchatka and Greenland also contributed by raising MSLPs well above any climatology. Recall that only north-south global mountain massifs can cause these torques. Positive events occur as the earth rotates against anomalously high pressure thus adding momentum (and westerly wind flow) to the atmosphere. The opposite is true for negative mountain torques. Hence the global AAM computed tendency also spiked ~plus 30 Hadleys on 26 March. The latter is why the GWO nearly orbited to phase 5 on 25 March with ~2 sigma projection before collapsing.

There are a lot of other observations that I could share if the proper resources were available. For instance, I think I can argue that the positive global mountain torque event was also related to the recent poleward propagation of zonal mean easterly wind flow anomalies into the subtropical atmospheres. However, to save space, let it be typed that the positive zonal mean AAM tendency (~2 Hadleys) which started on the equator about 2 weeks ago has already shifted into the midlatitude northern hemisphere atmosphere. The RWDs tied to the convection and the positive global mountain torque may be the culprits.

My point to all this confusion is even though there was an “effort” to add westerly wind flow to the global atmosphere, much of it has gone into the midlatitude Northern Hemisphere and Southern Hemisphere equatorial/subtropical atmospheres. This is one reason for the lack of a decent subtropical jet across the southern USA typical of late March. Global AAM still remains low, ~minus 1 sigma, and is probable to decrease again.

Starting roughly 10 March, zonal mean easterly wind flow anomalies propagated off the equator into the subtropical Northern Hemisphere and midlatitude Southern Hemisphere atmospheres. However, for the Northern Hemisphere, through processes involving the dynamics of the baroclinic eddies (with the RWDs) there was an abrupt shift into the higher latitudes which fed back as yet another contribution to the positive global mountain torque discussed above (confused?).

One bottom line is that locations across the USA currently experiencing drought, floods, etc., are not going to see any real change to the patterns responsible for that for at least the next 1-3 weeks. This is another analogy to the global financial markets, where rallies (like those seen during the past couple weeks) do occur in bear markets. Hence the term, “bear market atmosphere”, where there was a "rally" for increasing global westerly wind flow.

Animations of various tropospheric daily mean vector wind total and anomaly fields show a nice baroclinic response to the Eastern Hemisphere tropical forcing with RWDs linking into the extratropics. That includes a great circle arc to the large anomalous upper tropospheric anticyclonic gyre from northeast Asia into the central Pacific Ocean and trough digging into the western USA (wind speed anomalies ~30m/s at 250mb 28 March). The seemingly zonal flow across the CONUS, resulting in a northward displaced storm track for late March, is a regional-scale response to large-scale circulation anomalies. The consolidation of the extratropical ridges (Kamchatka, central Pacific Ocean) was expected per blog discussions at least 2 weeks ago and differed from nearly all week-2 ensemble mean predictions.

Most numerical ensemble prediction schemes loosely maintain the current circulation structure across the PNA sector for at least the next 2 weeks. There will be variations in amplitude and synoptic details are white noise after day 3. The GWO is probable to orbit around phase 3 (legacy GSDM Stage 1) after its recent excursion nearly into phase 5 (GSDM Stage 2).

Persistence of a northward shifted storm track across the USA having more western states troughs progressing into the Plains is probable for at least the next 2-3 weeks. I can argue some variations of this pattern which may occur; however, those are synoptic details. Weather ramifications should be more than apparent by now.

Repeating, this will increase the risk of high impact weather to above climatology centered on the middle of the country. Locations such as the Upper Mississippi Valley hit hard by severe winter weather may not only experience more of it, but also have “spring” delayed. Violent outbreaks of severe local storms including tornadoes may occur from the lower-mid Mississippi Valley into Iowa and the Ohio Valley, along with more flooding. The latter is probable to shift northwest into the northern and central Plains/Upper Mississippi Valley going into May and June.

Drought must be a concern for the Southern and Central High Plains, particularly southwestern Kansas into western Texas. In fact, these conditions may significantly worsen during the next several weeks. Hence there is an increased risk of high fire dangers, dust storms, crop failures and water shortages for these areas. I can even see a situation where this dryness may expand into the western Corn Belt (ex, Iowa) by mid-summer (not to contradict the above).

Severe weather internationally has decreased during the last week. I continue to leave it to the expertise of the appropriate weather centers internationally to alert the public of these risks.

Severe thunderstorms and flooding rainfall are probable for particularly Indonesia into the Philippines weeks 1-3, impacting locations east of Australia and the South Pacific Islands at times. That does include the risk of tropical cyclones. Intense clusters of thunderstorms are also probable to continue across tropical South America at least week-1. The suppressed Indian Ocean may become active by week-3. Concerns for additional tropical cyclones remain across both the North and South Indian Ocean, eventually all shifting north of the equator. Climatologically, the Bay of Bengal has one peak period of tropical cyclones during May.


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 to the WH (2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.

Please see the revised description of the GSDM per above link. Also, I encourage the readers to study the annotated MJO and GWO phase space plots to help relate the global variations explained by those techniques to “weather”.

Links to CPC and PSD ENSO discussions:

The following is a link to information about the stratosphere:

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. In addition, a paper is in preparation by WB that will formally introduce the GWO. Given shift work and upcoming travel, updates remain very difficult. I may not be able to post another discussion until the weekend of 12-13 April.

Ed Berry

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