Wednesday, April 12, 2006

Slopping Around, Seasonal Transition and Discipline

Important weather-climate changes have continued since my April 8th posting. I will have to be brief.

Equatorial cold tongue SSTs have continued warm with the readings no lower ~27C from about 120-140W with overall very weak anomalies. SSTs of 29C and greater continue from the South Pacific (anomalies ~1.5C over the subtropics) into the IO. See the following links for global SST details:

The most recent of our ~30 day modes of tropical convective variability reached Indonesia about a week ago, before losing its coherence. Since early January, there have been 3 such situations, with the first and most recent (number 3) having at least weak MJO components. During the past week a reasonably strong convectively coupled Kelvin wave moved into the western hemisphere (WH). Per time-filtered Hovmoller plots of OLRA, the convective signal with this Kelvin wave is currently in the region of northern South America. Other areas of tropical thunderstorm clusters continued across Indonesia, the South Pacific and central Africa. The point is at this time any coherent convective signal is likely in the WH, with the other more regionalized forcing linked to SSTs and even the seaonal cycle.

Global relative AAM remains ~2 standard deviations below the 1979-1998 climatology. However, there was some increase in the anomalous zonal mean westerly flow across the subtropical and tropical atmospheres during the past 2-3 weeks, tied to the most recent eastward shift of the tropical convection. This westerly flow did propagate into the lower mid latitudes, and contributed to the recent increase of the east Asian/North Pacific jet at ~50N. Zonal mean anomalous easterly flow is again returning to much of the tropical/subtropical belt while peristing across the north polar latitudes (see link below). In terms of the SDM, the circulation loosely transitioned from Stage 1 to 2.

To summarize thus far, while eastward shifting tropical convective forcing events numbers 1 and 3 had some similarities (see previous posting for details), the circulation response was quite different. Event number 1 lead to SDM Stage 3 whereas number 3 barely got the atmosphere into Stage 2. Reasons for this are tremendously complex, and do include issues such as the La Nina base state, non-linear feedbacks and the seasonal cycle. Additionally, the stationary circulation regime we have seen since mid-late February linked to La-Nina (SDM Stage 1) is breaking down as La-Nina itself is currently weaking (loosely stated). My thought is that we may start to see more oscillatory behaviors, which will require serious disciplined daily weather-climate monitoring. This includes the notion that is not unreasonable to expect enhanced tropical convective forcing to re-emerge into the IO by ~late week 2, and see the circulation (and tropical convection) go into SDM Stage 4 by then (by passing Stage 3).

I like the general idea of most models bringing at least a couple of more troughs into the western and central part of the country for week 1. This will all be in the presence of increasing split flow across North America. During week 2, many models are starting to show a retrogression of the trough position that has been just off the USA west coast for at least the past 6 weeks to around 150W or so. That notion is also not unreasonable. However, given all considerations, as the North Pacific jet starts to retract, a more full-latitude ridge may develop in the region of the western states (with still split flow and perhaps a closed low or two across roughly the Southern Plains). Once tropical convection becomes enhanced across the IO into Indonesia by ~ late week 2 into weeks 3-4, retrogression may continue across the PNA leading to more western USA troughs by early May (back to SDM Stage 1).

Punch line is that after about another week of relatively active weather for much of the country, there should be roughly a week or so period of quiet weather. However, a very active storm track from the Rockies into the Plains may become established by early May, which would also consistent with climatology, but perhaps enhanced. My confidence in this predictive insight is definitely low.

For southwest Kansas, again I have been getting a bit too optimistic about rainfall recently. The weather for at least the next week looks very warm with lots of wind and little precipitation. For week 2, it is possible a southern closed low(s) could bring us some needed precipitation. However, temperatures are likely to stay above normal and the odds would still tilt toward continued dryness.

I will try to do an update this upcoming weekend.
Ed Berry

No comments: