Friday, June 22, 2007

Now Comes the Details and Timing, with lots of Stress and Uncertainty

There is not a significant amount of new information (which does not preclude daily monitoring) I can add today from my posting last Tuesday (19 June), and I will attempt to keep this discussion short. The spatial distribution of global tropical SSTs remains similar to earlier this week and the most intense tropical convective forcing is across the Eastern Hemisphere centered ~10N/110E.

We have a slowly evolving weather-climate situation. A development that I have been monitoring during the week is a possible separation of the Eastern Hemisphere tropical forcing, into perhaps stationary and propagating components. From various monitoring tools including full disk satellite imagery, Hovmoller plots of OLR and their anomalies, animations of upper tropospheric winds, etc., very intense tropical rainfall remains across much of the Arabian Sea into the Bay of Bengal (BB). In fact, a weak tropical cyclone did develop (in spite of high vertical wind shear) across the BB during the week and the remnants are now over India. Meanwhile, there has been eastward propagation north of the equator (~3-4 m/s) projecting onto a MJO which is currently beginning to excite convection from the warm west central into the Tropical Northwest Pacific (TNWP). I am concerned that our old situation (since 2002) of 2 regions of Eastern Hemisphere tropical convective forcing may return during the next few weeks. Any circulation response to this possibility will be complicated. The history has been a GSDM Stage 4-1 behavior, and that is consistent dynamically when the Indian Ocean forcing dominates. Stay tuned.

Upper tropospheric Western Hemisphere equatorial westerly wind anomalies remain well above average having magnitudes ~30m/s at 150mb just west of South America while at 250mb zonal mean anomalies are ~ plus 5m/s (as of 21 June per operational data (FNL)). I can see some evidence from the animations that this anomalous upper tropospheric westerly wind flow is interacting with extratropical Rossby wave trains of both hemispheres, particularly the Southern Hemisphere. This may be the start of the process of poleward and downward propagation, and may eventually (~weeks 1-4) lead to an intensification of the North Pacific Jet (for this time of year). In fact, Rossby wave energy dispersions interacting with the renewed Eastern Hemisphere tropical forcing has led to the recent negative phase of the NAO. Meanwhile the trades have become quite strong from the west central into the eastern Pacific Ocean with anomalies ~5-10m/s around the equatorial date line. The latter is largely a response to the convection to the west. Yes, if the tropical convective forcing remains well entrenched across the Eastern Hemisphere that would help persist these strong trades perhaps reinvigorating La-Nina development. Again, stay tuned.

Global relative AAM tendency per R1 data was ~plus 25 Hadleys 19 June with contributions coming from a strong positive global frictional torque of ~20 Haldeys and an increasing positive global mountain torque (see ESRL/PSD plots for details). Actual global AAM is still more than 1 standard deviation below the 1968-1997 R1 climatology. I think there is a transition from GSDM Stage 1 to Stage 2 going on as I type, and (when updated) the Global Wind Oscillation (GWO) should present a nice orbit in phase space toward this stage.

Huge uncertainty exists on the details how the circulation across the PNA will get to the state discussed on 19 June leading to the central USA cool/wet regime weeks 2-3. Upper tropospheric animations of daily mean vector wind anomalies through 21 June show stationary twin subtropical anticyclones ~40-60E (for at least the last 7-10 days) tied to the Indian Ocean convection with twin cyclones downstream, and another “anticyclone-cyclone couplet” from ~140E to just east of the date line. The latter is tied to the MJO. Even with a possible stationary Indian Ocean signal, I am holding my thought of intense tropical convective forcing across the west central and TNWP leading to the most probable PNA circulation response to GSDM Stage 2 by week 2 (as discussed 19 June). How long this situation persists afterwards is now unclear to me.

As part of catching up, I can see how the models arrive at their solution of an anomalously strong trough “slamming” the USA west coast by late next week. In reality, there may be a stronger North Pacific Jet tied to this trough than shown by the models by the start of week 2, and a split flow situation may develop across west and central North America for a few days. Who knows, might we bypass GSDM Stage 2 and go directly into Stage 3??? However, a more probable offering would suggest circulation anomalies to then consolidate leading to a ridge west of North America and a trough across the central part of the USA by week 3. Thus, still stated with relatively high confidence, my outlook for anomalously cool/wet across the central part of the country for ~weeks 2-3 remains unchanged. Other weather concerns were discussed on the 19 June posting.


An experimental phase space plot of the GSDM (which we call a Global Wind Oscillation (GWO)) utilizing normalized relative AAM time tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

These are probabilistic statements, and work is ongoing to quantify in future posts. We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion early next week.

Ed Berry

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