Friday, March 30, 2007

GSDM Stage 1 Perturbed

The west Pacific SST horseshoe of warmth remains; however, anomalies are weakening due to recent days of intense tropical rainfall. Magnitudes are generally 0.5-1C with SSTs ~29-30C, with the warmest waters now around 10S/160W as of March 29th. Conversely, the equatorial Indian Ocean continues to slowly warm due to suppression (with the exception of a tropical cyclone in the South Indian Ocean) with SST totals and anomalies comparable to the west Pacific warm pool. Cooling along the equatorial east Pacific cold tongue has again intensified this past week with negative anomalies ~minus 1-3C east of 140W and at least minus 4C in the subsurface. These colder than normal waters extend down to roughly 200m at the date line per latest TAO buoy data. Whether or not La-Nina is evolving is an open question (recall May 2003); however, it is not unusual to see “pulses” of cooling if this is the case.

Hard times continue trying to understand the tropical convective forcing, and there is a seasonal cycle consideration to remember. There is no coherent MJO dynamical signal. Most significant behavior during the last 10 days has been an eastward shift of intense (OLRA ~ minus 50-70 W/m**2) forcing from ~120E to 160E just south of the equator. I maintain this is a transient event linked to very complicated tropical-extratropical interactions. These can only be reasonably understood observationally from disciplined detailed daily monitoring of fields such as animations of upper and lower tropospheric wind fields utilizing the GSDM framework. Some of these interactions include last week’s extension of the North Pacific jet shifting this forcing to the east, possibly reinforced by feedbacks from the extratropics including a strong positive East Asian mountain torque.

Full disk satellite imagery already shows the west Pacific tropical forcing drifting west and northwest, and the coherent modes Hovmollers suggest a weak projection on the convectively coupled Rossby mode. In fact, a tropical cyclone may develop out of this region north of the equator during the next few days per JTWC. Weaker forcing (diurnal cycle understood) also persists across northwest South America, west central Africa and ~0/120E.

Our best weather-climate signal remains global relative AAM including the zonal mean contributions. This quantity is a good diagnostic measure of the character and evolution of the global and zonal mean circulation anomalies. Relative AAM continues at around 1.5 standard deviations below the 1968-1997 reanalysis data climatology, and its tendency has been weak for the last couple of weeks. Furthermore, zonal mean upper tropospheric easterly wind anomalies remain at around 5m/s throughout much of the tropical and subtropical atmospheres. These easterlies are contributing to a zonal mean sink of AAM ~20N while flux convergence of its transports occur ~35N. All of these observations are consistent with a GSDM Stage 1 base state (most probable during a La-Nina) that has been around since at least late February.

Given the warm Indian Ocean SSTs, feedbacks from our low AAM regime and the slow westward shift of tropical forcing back to 120E discussed on 3/27, we need to monitor for the development of intense tropical convection across the Indian Ocean into Indonesia sometime weeks 2-3. In the meantime, the transient SPCZ convection discussed above has generated enough anomalous (~20-30m/s at 150mb on 3/29) upper tropospheric equatorial westerly wind flow over the east Pacific to perturb our GSDM Stage 1 state toward Stage 2 for week 1. In fact, animations of daily mean upper tropospheric vector wind anomalies show well defined twin tropical anticyclones at 160E as of 3/29. These anticyclones are already interacting with Rossby wave energy dispersion (RWD) processes from Asia to North America, including the ridge building into Alaska and the developing downstream trough across western Canada. It is possible the models may be underestimating the impacts of this trough onto the Rockies and Plains early-mid next week given the large circulation anomalies tied to this RWD and the complex dynamics. Overall, as shown by most ensemble numerical models, a colder regime is probable for the eastern two-thirds of the USA for at least week 1.

My thoughts for weeks 2-3 remain the same. I speculate whatever “renegade” North Pacific jet that does come out may lead to a similar evolution of synoptic events that was observed during the last 7-10 days. If the tropical convective scenario discussed above does occur, a shift back to a GSDM Stage 4-1 would be most probable by sometime week 3 (April 14-20). Although models do not predict the evolution of tropical convection very well after week 1, the NCEP week 2 ensemble mean total precipitation forecast lends loose support to renewed Indian Ocean convection. The latter is likely a SST response. This would lead to a renewed active pattern for much of the western and central USA, including severe local storms across the Plains. Careful monitoring will be needed to see if Eastern Hemisphere tropical forcing eventually develops into a MJO.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I am unclear when I will be able to another discussion next week due to travel. We are also working on a weather-climate discussion for the ESRL/PSD MJO web site, which will hopefully be posted during the next couple of weeks. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry

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