Eastern Hemisphere equatorial SSTs remain above average across the Indian and west central Pacific Oceans with magnitudes ~1-2C and totals around 29-30C. There has been cooling along the Pacific Ocean cool tongue east of ~160W during the last week. Anomalies vary from ~minus 0.5 to at least minus 2C with the coldest west of South America. At depth negative anomalies lower than minus 4C is present ~140W/100m per latest 5-day averaged TAO buoy data through 16 July. These at depth negative anomalies were generated when the East Pacific oceanic thermocline was raised by the strong trade wind surge nearly a month ago. In response to increasing tropical convection across the Eastern Hemisphere (discussed below), surface easterlies are increasing west of the equatorial date line which may contribute to another trade wind surge farther east. This dynamic process may upwell the colder water to the surface and intensify La-Nina.
Whatever the case, and as shown by most numerical and statistical forecast tools, La-Nina conditions (ocean forcing-circulation response, etc.) are probable through at least boreal fall. My own feelings are the coolest anomalies will remain across the East Pacific and this will not be a basin wide situation. The positive SST anomalies across the west Pacific warm pool are deep and have been persistent for at least the past 5-7 years. The latter is one of my reasons for the possibility of a decent El-Nino “sometime in the future”.
There is still the horseshoe spatial pattern of anomalous warmth from the west central Pacific Ocean into the extratropics. Per latest CPC data through 14 July totals in excess of 30C were observed at 20N. Warmth also continues from the tropical East Pacific (10-20N) Ocean into the Caribbean while slightly below normal SSTs are generally the rule across the equatorial Atlantic. In fact, totals are only around 27-28C (still warm enough for tropical cyclone development) northeast of South America in contrast to above 30C across the Gulf of Mexico.
Now that we have some understanding of recent SST trends, discussion can move onto tropical convective forcing. Again, it is fundamental to remember that tropical ocean forcing onto the atmosphere is bridged through tropical convection. After the June weak-moderate MJO, a dynamical signal best observed in the upper tropospheric velocity potential field has been propagating through the Western Hemisphere at ~15-20m/s. Even though this signal decoupled from the MJO convection, it remained relatively coherent through dynamic interactions with various feedback anomalies including extratropical processes such as Rossby wave trains. About a week ago there was an intensification of convection (with tropical cyclones) across the East Pacific. Currently the dynamical signal is centered on Africa, and is about to emerge into the India Ocean.
Full disk satellite imagery shows intense equatorial convection centered ~70E having 3-day averaged OLRA ~minus 50-70 W/m**2. In the wake Typhoon Man-Yi, tropical forcing is suppressed from the west central into the northwest Pacific. Hence we are already starting to observe a spatial pattern of equatorial Indian Ocean enhanced rainfall with west central Pacific suppression. There are still large clusters of strong/severe thunderstorms from northern India into Southeast Asia, with another region of suppression across the portions of central India into the Bay of Bengal. In addition to the unusually strong South Asian monsoon system, the current China convection also had a contribution due to northward propagation from the June MJO.
Given how far south the Indian Ocean convection is, I think it is probable another MJO signal is evolving across this region. I feel I can defend this notion based on some of the circulation responses I am seeing. Per animations of ESRL/PSD operational data upper tropospheric daily mean vector wind anomalies there is increasing divergence with developing twin tropical/subtropical anticyclones ~60E with some hint of twin cyclones near the date line. Additionally, weak easterlies are starting to appear locally around Africa and west of South America and even in the zonal mean particularly north of the equator. The R1 data AAM tendency plot (only through 13 July) also supports a weak negative zonal mean tendency around the equator of roughly 1-2 Hadleys.
The global circulation remains painfully complicated to write about particularly in terms of attribution of events during the past month or so. Per Global Wind Oscillation (GWO) phase space plot updated through 11 July (5-day average), 2 circuits have been completed during the past 40 days tilted toward GSDM Stage 1. The physical processes responsible were not only circulation responses from the MJO, but perhaps more importantly from extratropical processes such mountain-frictional torque index cycle variations. Adding to the complexity is there has not only been poleward propagation of zonal mean zonal wind anomalies (already discussed in previous posts), but also equatorward across the Northern Hemisphere.
In a valiant attempt to be brief and understanding the relevant terms of the AAM budget, I think the earlier June orbit had an important contribution from the East Asian mountain torque. I can see evidence of this from the animations (recall the past 2 pairs of twin anticyclones and cyclones), and I think this orographic forcing played a role in initiating equatorward propagation of anomalous zonal mean westerly flow (~10 m/s at 200mb) from the higher latitudes (~60N) into eventually the midlatitudes (~30-40N). In fact, there were also other anomalous zonal mean bands of westerlies and easterlies moving south. I think the second more recent circuit was tied to intensification of the west central Pacific Ocean tropical convection that eventually spun-off Typhoon May-Yi. My point is this equatorward propagating zonal mean anomalous westerly wind flow contributed to the recently observed anomalous North Pacific Jet (~25 m/s anomalies at 250mb per recent weekly mean) that led to the current positive projection onto the Pacific-North American teleconnection (PNA).
Working with shorter wavelengths due to boreal summer, this is actually a regional response to complicated global and zonal mean dynamical tropical-extratropical forcing. What I termed as a “Stage 3 circulation with Stage 1 convection” in my 13 July posting was in terms of this regional response. As I type there is another very strong Rossby wave energy dispersion emanating from the west central Pacific that will amplify the current western states “wicked” ridge. Finally, the synoptic pattern seen across the USA during the last week-10 days has been roughly 10-15 degrees farther east than I thought to be most probable in postings a few weeks ago.
Like a broken record, I once again make it clear that uncertainty is huge for any week 1-4 prediction. I am going with the notion there is another MJO signal developing across the Indian Ocean, and this will augment the South Asian monsoon system during the next few weeks. In fact, there is some statistical support for a MJO coming not only from the Wheeler phase space plot, but also from the plot produced by United Kingdom Meteorological Office. Again, tied to our weak La-Nina, there is a stationary component dynamically forcing the global circulation and the enhanced Eastern Hemisphere monsoon system is a response to that. MJO and other variability that is probable to occur will be superimposed upon this base state. In other words, there will be additional circuits in GWO phase space tilted toward GSDM Stage 1 but whose amplitudes are unclear. As this MJO organizes it should propagate east and northeast similar to the June event.
International ramifications include an intensification of rainfall from the equatorial Indian Ocean into India and much of Indo-China particularly ~week 2. Intense frontal precipitation including severe thunderstorms may extend from this tropical convection into portions of China. Meanwhile the tropical northwest Pacific Ocean should be relatively suppressed at least week 1 only to get quite active by ~week 3. Tropical cyclone concerns are unclear after the week 1 East Pacific activity. Given the warm SSTs across the Gulf of Mexico and the Caribbean, I do have some concerns for a “hybrid system” weeks 2-3 in that region. One source may come from anticyclonically wave breaking lows as the eastern USA trough weakens. Locations such as the Philippines and Japan may see an increasing threat from tropical cyclones weeks 3-4.
Finally, Rossby wave energy dispersion into the austral winter Southern Hemisphere may favor another strong trough for western and southwestern South America later week 1 into week 2. This is the Southern Hemisphere analog to a western North American trough during boreal winter GSDM Stage 1 situations. Hence another possibility exists for an exceptionally cold and wet pattern for locations such as Argentina by week 2.
For the USA, we have to deal with predictability issues caused by the shorter wavelengths and existing zonal mean and circulation anomalies from past “events”. I think the models are reasonable with the slow eastward shift of the anomalous large amplitude trough-ridge-trough pattern week 1. The Pacific Northwest is cooling down and the Great Lakes and eastern states should do the same by this weekend. Afterwards, very warm air is probable to spread back into particularly the Northern Plains and Upper Mississippi Valley. Mainly diurnal thunderstorm activity should persist from the Deep South into the Rockies.
It is during weeks 2-4 (going into mid August) I think our part of the world will feel impacts from the possible MJO and other complicated circulation variations linked to the tropics, etc., as described by the GSDM. Most numerical ensembles show deamplification and progression of the current pattern during week 2. I think this idea is reasonable. However, I am holding the line that whatever synoptic evolution occurs, it is probable there will be a net retrogression of the sick USA synoptic pattern that has featured the “wicked ridge of the west” for about the past week. In fact, more consistent with GSDM Stage 1 and possibly Stage 2 working with the seasonal cycle heading into August, the ridge may shift into the east Pacific Ocean (~140W) leading to trough development across the Rockies and western Plains with a Southeast states ridge. This may allow some reversal of the temperature and precipitation anomalies seen for the past several weeks.
An experimental phase space plot of the GSDM (which we call a Global Wind Oscillation (GWO)) utilizing normalized relative AAM time tendency anomaly (Y-axis) and normalized relative AAM anomaly (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, with rigor and thoroughness. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to update this Blog by around Friday. In general, due to covering shifts and travel, my postings on this Blog will be irregular through at least August.