Not What I Expected

Tropical sea surface temperatures continue to cool across the equatorial Pacific Ocean while weak-moderate positive anomalies remain from the west central to the South Pacific. In fact, negative temperature anomalies as low as -2C have recently been observed at around 120W per TAO buoy data, extending to depths of at least 200m. An evolution to La-Nina is looking more probable.

The dynamical signal with the MJO continues to propagate east at roughly 5m/s (4 deg long/day) toward the west central and South Pacific per diagnostic monitoring tools and satellite imagery. For instance, as of March 5th the Wheeler index suggests this signal to be at ~160E. We prefer to describe the recent eastward shift as part of a weather-climate or subseasonal event. These typically consist of multiple time scales, one of which is the MJO but others of which are less oscillatory.

Tropical convective forcing has also been getting better organized from the central Indian Ocean to Indonesia during the last few days, centered at about 10S/120E. Hovmoller plots of both total and anomalous OLR suggest this forcing may become a stationary possibly linked to a developing cold event. The global circulation has responded accordingly with anomalous zonal mean easterly flow propagating off the equator into the subtropical atmosphere particularly for the Northern Hemisphere (magnitudes ~minus 10m/s for the latter). Global relative AAM remains very low at about 2 standard deviations below the 1968-1997 reanalysis climatology with its tendency at roughly minus 10 Hadleys. The recent peak of AAM tendency to plus 10 Hadleys was the result of the subseasonal signal discussed above.

As predicted, GSDM Stage 1 best describes the current global circulation. However, regionally from the Asian to North American sector the distribution of circulation anomalies has become very complicated. Animations of daily mean 150mb and 250mb vector wind anomalies show twin anticyclones centered around 115E with downstream troughs near the date line. However, there are also residual anomalies linked to past forcing from the tropical west Pacific including those across the Arctic. From a synoptic view point the ridge that is most probable to be centered ~140-150W in this situation has been split in two; one portion is at the date line with the other over western North America. Most ensemble means (not details) suggested this possibility 7-10 days ago. There is some hint from recent model runs for current Gulf of Alaska trough to slowly move east and southeast into western North America during the next couple of weeks. My own thought a week ago was for an enhanced probability for this process to happen sooner and farther south than the models were showing, allowing a resumption of an active regime for much of the country. This appears less likely in the immediate future.

Whatever the case over the next 7-10 days, I think forecast uncertainty for a week 2-3 prediction for the USA is very high, despite what appears to be good model agreement of a low amplitude trough across western North America and a northward displaced storm track. In fact, I think it probable that a strong synoptic scale trough will impact the western and central USA with the extreme weather typical of GSDM Stage 1 during weeks 2-3. This would mean at least one prominent “southwest flow cyclonic baroclinic event” across the Plains similar to the 2 recent events. Until then, a rather benign weather pattern with warm and dry across the southern states and a bit unsettled across the north can be expected.

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 will try to post another discussion this Friday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann