Friday, January 19, 2007

El- Nino is trying, but too little too late?

The following is a version of a posting to the HMT forecast at

Figures to go along with the discussion are on that Blog.

The strong MJO that came out of the Indian Ocean in early January has weakened considerably. As expected, it excited convection near the date line and attempted to generate a strong combined Pacific Ocean jet stream. This MJO also helped force the cold regime over the USA that we discussed on 29 December 2006 (on the HMT conference call). However, two factors indicate our scenario of a strong combined jet stream over the Pacific for this winter is now less likely.

First, since about 1 December the atmospheric momentum transports have been moving momentum out of the subtropics and into mid-higher latitudes. This pattern has been so strong that the forcing produced by the MJO over the warm El Nino waters appears only as a small perturbation in a persistent flow regime. Regionally this zonal mean regime is characterized by split flow patterns, especially over the oceans. Convection is currently increasing over the Indian Ocean and Indonesia while a portion of the convection at the date line is moving westward at ~4 m/s as an equatorial Rossby wave. This combination suggests convection will become centered somewhat to the west of its current position during the next 2-3 weeks, possibly ~0/140E. Implications would be for a retrogression of the east Pacific-North American ridge (from week 1) to perhaps 140-150W (weeks 2-3) and a better shot at the subtropical jet undercut scenario favoring precipitation along the USA west coast, especially California.

The second factor is more relevant for the atmospheric circulation beyond week 3. The onset of the Southern Hemisphere monsoon (also associated with the strong MJO) has produced strong anomalous northerly flow across the equator. This appears to be linked with an amplification and deepening of the cold water (negative anomalies) below the equator in the Pacific Ocean. This cold signal may continue to deepen and eventually reach the surface putting an end to the basin wide aspects of this El Nino.

For the USA, after the storm system and surge of cold air this weekend into early next week, much of the western two-thirds should have a moderating temperature trend. There may be storm development close enough to the east coast that may lead to significant winter weather for inland locations. During weeks 2-3 (~late January-mid February), a return to a cold regime, similar to what has already been experienced for the last 7 days or so, appears probable. However, this time there may also be a source of Arctic air from Siberia (where temperatures are currently lower than minus 50C, but anomalies are still slightly positive) as well as Alaska, meaning possibly more severe cold than observed last week. Additionally, significant winter weather hazards would again be possible from the Rockies into the Plains with heavy rain and thunderstorms (possibly severe) across the Deep South. As discussed above, precipitation may also increase along the California coast especially week 3.

Please note: These are probabilistic statements, which we will try to quantify in future posts. The decay time scale for momentum transport anomalies is on the order of 1-2 days. Thus this is the time scale which a reversal of the transports could occur.

Klaus Weickmann and Ed Berry

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