GSDM Stage 1

This discussion will be “short and sweet”. The distribution of global tropical SSTs remains similar to that discussed on my posting last Friday (3/16/07). The horseshoe pattern of warmth persists across the west central Pacific with anomalies of ~1-2C and 29-31C actual SSTs while similar cool SST anomalies exist along the equator from 150W to South America. The latter have actually cooled a bit during the last few days while colder anomalies ~2-4C persist in the subsurface down to roughly 200m.

Tropical convective forcing is getting better organized across the Eastern Hemisphere centered at about 0/120E. Latest 3-day averaged OLRA are ~minus 50-70W/m**2. Other pockets of enhancement still remain across the South Pacific, Brasil and equatorial Africa. The dynamical MJO signal discussed in my last posting has lost coherence. The future evolution of the tropical convection is unclear. Based on its current position around 120-130E and the existing low AAM circulation anomalies, we speculate that convection will remain quasi-stationary or shift slightly west over the next one to two weeks. Kelvin waves excited by the convection should also help excite transient convective episodes over the west Pacific. This complicated picture may all be part of a slow process of the atmosphere transitioning to a more pronounced La-Nina base state.

GSDM Stage 1 (most probable during La-Nina) still best describes the current weather-climate situation. It is probable this situation will persist for at least the next 2-3 weeks. Relative atmospheric angular momentum (AAM) remains well below the 1968-1997 reanalysis data climatology at ~1.5 standard deviations. For the Pacific-North American sector, more strong troughs are expected for the western USA with a southwest flow storm track across the Plains probable at least into early April. This pattern is favorable for late season snowstorms across portions of the Rockies and northern/central High Plains with a greater than climatology risk for severe local storms across the Plains and Mississippi Valley.

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