El Nino is trying but.....too little too late?
The strong MJO that came out of the Indian Ocean in early January has weakened considerably. As expected. it excited convection near the dateline 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 Dec 2006. 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 dateline 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, more in line with the current anomalous circulation regime. We expect this perhaps by the end of week 2 and its implications for the east Pacific would be a retrogression of the east Pacific-North American ridge and a better shot at the undercut scenario people have been talking about.
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 surface in the equatorial Pacific Ocean. This cold signal may continue to deepen and eventually reach the surface putting an end to the basinwide aspects of this El Nino.
(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. This then is the time scale on which a reversal of the transports could occur.)