Atmosphere Remains Slow

Note: The ESRL/PSD NCEP/NCAR Reanalysis-1 data (R1) AAM plots now present the “complete” budget (updated through 24 August as of this writing), and will be referred to.

Per NOAA/CPC and NOAA/PMEL/TAO buoy data; the overall spatial distribution of global tropical SSTs has not changed much for at least several weeks. The equatorial Indian Ocean and particularly the west central and northwest Pacific Ocean basins (with the horseshoe into the subtropics/midlatitudes) remain well above climatology. Anomaly magnitudes are ~1-1.5 deg C with the former and up to 2 deg C for the latter, with totals in excess of 30 deg C along the equator near 150E. The west Pacific Ocean anomalies are deep extending to at least 400m, suggesting an anomalously warm west Pacific Ocean warm pool.

The negative SST anomalies across the equatorial East Pacific cold tongue have weakened slightly during the past week, with recent 5-day averaged values varying from ~.5C just east of the date line to around 2C near 120W (~Nino 3). The cool anomalies still extend to around 150-200m deep in the region of 140-160W, but with values ~minus 2-2.5C at 150m/150W. It will be interesting to monitor the persistence of these anomalies going into boreal fall since the seasonal cycle favors cooling. The equatorial Atlantic Ocean and the Caribbean Sea have slightly positive SST anomalies with cool pockets inter-dispersed. Totals range from 27-28C across the former to in excess of 29C for the latter.

Again, while the global circulation is strongly La-Nina like (GSDM Stage 1), the SSTs only weakly support that response. To me, the seasonal transition to boreal fall may give us some insight whether or not the SSTs become more La-Nina like, or if a different evolution occurs. For example, the South Asian monsoon system will shut down and the Eastern Hemisphere tropical convective forcing will shift south. How will a possibly coupled system respond should, for example, MJO activity increase?

The strongest tropical convective forcing is still across the Eastern Hemisphere, centered ~10-15N/90-100E per full disk satellite imagery. From time-latitude sections of OLR/A and daily monitoring, this activity has been shifting northeast off the equator during the past week. Currently enhanced rainfall extends from northern India east-southeast into the far west Pacific Ocean. This is the third (but weak) event of this nature since about June, and is typical during boreal summer. As discussed below, the region of tropical thunderstorm clusters actually represents a weak consolidation. Other enhanced activity is occurring around the Americas and north equatorial Africa. The Wheeler and Hendon (2004) multivariate RMM phase space plots from several international weather centers generally suggest a ~1 standard deviation MJO projection across the Indian Ocean (more said below).

Since at least mid-August, relatively “well-orchestrated” Rossby wave energy dispersions (RWDs) linked to the Indian Ocean/South Asian monsoon system tropical forcing have been propagating into the Tropical Northwest Pacific Ocean (TNWP). A tropical convective flare-up over the TNWP occurred early last week, moving west and linking with the clusters farther west. However, to be brief, monitoring tells me this chain of events allowed circulation anomalies to impinge upon global north-south mountain massifs driving the mountain torque to ~plus 20 Hadleys on about 18 August. Along with a positive global frictional torque, the global relative atmospheric angular momentum (AAM ) tendency spiked to slightly greater than 20 Hadleys. Since then, the global mountain torque and relative AAM tendency have collapsed to less than minus 20 Hadleys.

The point is I think a weak mountain-frictional torque index cycle variation has occurred during the last 7-10 days within our low AAM base state. When updated, the ESRL/PSD Global Wind Oscillation (GWO) quasi-phase space plot should present a weak circuit in the direction GSDM Stage 2, before heading “down”. This notion was offered as a possibility in my posting a week ago. Some westerly wind flow has been added to both the northern subtropical and midlatitude atmospheres, and will contribute to a Stage-2 like response across the PNA sector this upcoming week.

Global total and relative AAM remain extremely low, less than 3 atmospheric momentum units (AMUs) below the R1 data climatology (at least 2.5 standard deviations) through 24 August. Deep zonal mean easterly wind anomalies still dominate much of the tropical atmosphere, with the largest magnitudes now just south of the equator near 5S (~10 m/s at 200mb). Worth repeating, the cause of this extremely low AAM, GSDM Stage 1 regime is unclear. I can speculate this all goes back to dynamical forcing-response feedback processes linked to AAM transports and large GWO circuits that started ~ 1 December 2006, when El-Nino was “stopped in its tracks”. In any event, the SSTs themselves do not support this global circulation. The intense zonal mean easterly wind anomalies have been broadening in latitude into the subtropical atmospheres of both hemispheres during the last several weeks. There is still little evidence for these zonal mean easterly wind anomalies to propagate off the equator. Daily monitoring of if/when these zonal mean easterlies do propagate poleward off the equator is critical to determining the most probable future global circulation state. I make a plea to having a diagnostic tool such as the baroclinic CHI problem to see what is going on vertically.

Animations of upper tropospheric daily mean vector wind total/anomaly fields give a loose signal of twin anticyclones ~90E with downstream very weak cyclones just east of the date line. Cross equatorial flow from the Southern Hemisphere remains robust in the region of the date line. In fact, I speculate that the intense RWDs across the Southern Hemisphere leading to this cross equatorial flow may be forcing the zonal mean anomalous easterlies closer to the equator as a feedback.

In any case, there has also been episodic (~every 3-5 days) meridional-zonal-meriodional northeast-southwest (northwest-southeast for the Southern Hemisphere) vacillations of the midlatitude synoptic eddies over the past couple of weeks. It is not unusual to observe this type of behavior during strong GSDM Stage 1 situations. Most importantly, for the first time in at least a couple of months a relatively strong inter-hemispheric symmetric AAM transport signal has appeared. The signal is most robust ~50N and 40S. Hence there is the possibility of a stronger positive mountain-frictional torque index cycle than recently observed over the next couple of weeks.

If there is any truth to the RMM predictions of a weak MJO also during the next couple of weeks, perhaps a larger circuit in GWO quasi-phase space toward Stage 2 will occur. However, this is all speculation, and I do expect an overall GSDM Stage 1 regime to persist until further notice. Only those who do rigorous daily weather-climate monitoring utilizing the GSDM will be the first to catch any kind of a rapid change. It is unlikely any multi-model ensemble prediction scheme will catch an abrupt subseasonal change “until it happens (like ~1 December 2006)”. We also need to remember that predictability with any technique/forecast process is generally quite low during seasonal transitions, which we are about to experience.

There is some model agreement for progressive troughs and ridges across the USA during the next few weeks as seasonal strengthening of the westerlies occur. After this upcoming week’s central/eastern USA trough, I offer a low probability of a stronger event having a Rockies-Plains trough later week 2 into week 3 (~ 7-16 September?). Weather ramifications should be apparent, including the first possible snowfall across the Northern Rockies and an increased likelihood of storminess across the Plains.

In the longer term, should this low AAM regime persist through boreal winter, much of the USA focusing on the western USA into the Plains may have the odds tilted toward a cold and stormy pattern (western trough/southeast states ridge), if there is an Arctic cold air source. Per above, stay tuned.

It is obviously unclear to say where any possible land falling tropical cyclone will go. However, please see the latest statements from the NOAA/NWS/Tropical Prediction Center. At least for week 1, once again relatively stable air is penetrating the deep Tropical North Atlantic Ocean (TNA) basin. Thus tropical cyclone activity for at least the TNA may be suppressed this upcoming week (understanding hybrids and what can occur across the Caribbean).

Locations from the central and eastern equatorial Indian Ocean into China and Southeast Asia are likely to continue with the poundings of intense monsoon rainfall for at least this week into possibly week 2. Perhaps the recent northward shift discussed above may be the last for boreal summer. The likelihood of severe thunderstorms including tropical cyclone activity across the TNWP including the Philippines may increase by week 2. Equatorial North Africa may remain active for at least week 1.

Appendix

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

http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg

http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg

http://www.cdc.noaa.gov/map/clim/gsdm.shtml

We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory component of the GSDM.

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps and signal to noise ratio plots). We hope that an opportunity will arise for us 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 try to post an update on about Friday, 31 August.

Ed Berry