Renegade North Pacific Jet

Above average SSTs still persist across the tropical west central Pacific with the warmest centered ~ 5S/165E having magnitudes of 1-2C and actual SSTs ~30C. Weak-moderate warm anomalies are also present across much of the equatorial Indian and north tropical Atlantic Oceans. The equatorial east Pacific remains moderately cool east of 120W with negative anomalies as low as -4C down to roughly 200m.

The tropical forcing discussed on my March 20th posting has loosely separated into 2 regions. One area remains quasi-stationary ~0/120E while another region has moved into the warm waters of the southwest Pacific centered at roughly 5-10S/170E. The former region continues to have the strongest interactions with the extratropics while the latter may be a response to the transient strong Pacific jet stream embedded within the low relative AAM, GSDM Stage 1.

We speculate that a Rossby wave energy dispersion from west Pacific forcing last week contributed to the snow storm for the USA northeast. Animations of 150mb and 250mb daily mean vector wind anomalies suggest that both tropical forcing present across Indonesia and a strong east Asian mountain torque of roughly 20 Hadleys (30 Hadleys globally) combined to add more westerly flow to the Pacific subtropics. Now there is an intense jet across the North Pacific having anomalies of at least 30-40 m/s. This feature is “out running” the tropical forcing back at 120E, and perhaps shifting the convection east across the southwest Pacific. In fact, according to the coherent modes Hovmollers, the convection across the southwest Pacific projects onto a Kelvin wave.

The future evolution of the tropical forcing still remains unclear. However, for at least the next 1-3 weeks, the most important convection to impact the extratropics should remain quasi-stationary centered in the region of 120E (consistent with a low AAM regime) while the southwest Pacific has episodic flare-ups. The strong North Pacific jet is transient. As most models show, this jet should amplify a strong trough across the western USA by early next week. For weeks 2-3, given persistence of GSDM Stage 1, ridge amplification across the central and east Pacific into Alaska with more troughs impacting the western and central states is probable. This is an active pattern for the Rockies and Plains favorable for late season snowstorms and outbreaks of severe thunderstorms with individual synoptic baroclinic cyclones. Careful monitoring will be needed to see if the Eastern Hemisphere tropical forcing develops into a MJO, and if the current cooling of the tropical east Pacific develops into a La-Nina.

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 next Tuesday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann