Reanalysis-1 (R1) data woes still continue meaning ESRL/PSD AAM plots have not updated since May 11th (as well as the GWO phase space plot).
There has been no significant change to the distribution of global tropical SSTs discussed in my May 18th posting (please review for details). La-Nina development during the next 3-6 months still remains a possibility.
The tropical convective forcing dynamical signal is well into the Western Hemisphere (WH). It is centered ~0/80W and is moving east at around 10m/s or roughly 7-8 deg long/day. Enhanced convection extends from the East Pacific ITCZ all the way into west central Africa. This signal has some projection onto a convectively coupled Kelvin wave. However, it is only a component of strong tropical-extratropical interactions that have occurred during the past few weeks. These interactions included the global mountain-frictional torque index cycle earlier this month within a general envelope of 20-30 day atmospheric variability.
Full disk satellite imagery not only shows the enhanced tropical convection across the WH, but also a loose consolidation of forcing centered ~5-10N/140E. The latter extends from eastern Indonesia into portions of the central Pacific. The Indian Ocean remains suppressed. However, strong enhancement is spreading across equatorial Africa as I type, and the Indian Ocean could become active much sooner than I think given the warm SSTs in that region.
It has been a real struggle trying to monitor these fast modes of tropical/extratropical variability and interactions. There has been no recent MJO signal, and as shown by the GWO phase space plot, our base state has been orbiting around GSDM Stage 1 since mid February. These orbits have come from competing tropical forcing from the East Indian and west Pacific Oceans, as well as mountain-frictional torque index cycles. Interactions with the mid-latitude synoptic eddies have maintained a loose AAM transport signal of fluxing momentum out of the subtropics into the mid-latitudes hence persisting a GSDM Stage 1 base state. Some of the past week 2-3 predictability “successes” for the USA have come from daily monitoring the GWO and AAM transport signals.
Most recently there has been the issue of whether or not an early onset to the Indian Monsoon would occur. In past discussions I have felt that was probable. However, per yearly ESRL/PSD time-latitude sections of OLRA, there was already enhanced convection across the Indian Ocean and portions of India at this time. Currently we have suppression there. My feeling is the current WH dynamical signal will be the catalyst needed to lead to the onset of the Indian Monsoon. Extrapolation would be in ~2 weeks, possibly much sooner given current satellite trends. However, my concerns about the warm west Pacific remain and a resumption of forcing from ~0/80-150E extending into the Southeast Asian monsoon region may occur by ~week 3 instead. Whatever does happen is unclear to me.
Animations of upper tropospheric vector wind anomalies shows twin tropical anticyclones centered ~80W as part of the WH dynamical signal. Even larger twin subtropical anticyclones are present ~150-160E (anomalies ~40m/s at 150mb) directly linked to the west Pacific convection locally forced by the warm SSTs. A strong Rossby wave energy dispersion across the North Pacific from these anticyclones is contributing to the present digging western USA trough. Not only have I struggled with the degree of amplification of this trough, but so have the models.
200mb zonal mean zonal wind anomalies (a good proxy for vertically integrated tropospheric AAM) remain above average throughout both the subtropical (~5m/s) and midlatitude (~10m/s) atmospheres of both hemispheres. Anomalous upper tropospheric subtropical easterly flow is returning to the Eastern Hemisphere and the recent extended North Pacific jet (for this time of year) is collapsing. Just how much easterly flow returns to the tropics and subtropics may have some dependency on the magnitude of the Indian Monsoon. If the west Pacific forcing continues to be an important component to the weather-climate system going into boreal summer, above average zonal mean tropical/subtropical westerly flow may be probable. My own feeling is the latter will have an influence. If so, speculation suggests a possible negative factor for Atlantic tropical cyclone development.
I think the weather-climate system is in a GSDM Stage 3-4 situation. I think Stage 4 is probable by week 2 and then Stage 1 afterwards. However, unlike much this past winter and spring, uncertainty is much higher than average. After an active week 1 for especially the Rockies and Plains, there should be a break by around the middle of next week. I like the solution offered by the ESRL/PSD ensemble suggesting more anomalous troughs particularly across the southwestern states starting ~day 10. Afterwards, a general western states trough and southeast USA ridge pattern is the most probable solution I can offer, especially by week 3. This would target the Northern Plains and Upper Mississippi Valley with rounds of MCS activity including severe storms and heavy rainfall, while intense heat occurs across the Deep South.
An experimental phase space plot of the GSDM utilizing normalized relative AAM 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. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post an update this Friday.