There have been tremendous changes in the Pacific Ocean over the past two months which continue to favor a moderate to strong El Niño event later this spring and summer. Since my previous post on February 21, NOAA's Climate Prediction Center (CPC) has issued an El Niño watch.

To begin, we are currently observing what looks to be the strongest downwelling oceanic Kelvin wave event since satellite records began in the 1970s. This still needs to be verified in reanalysis, but a large swath of 6°C (11°F) ocean temperature anomalies at a depth of 100 - 200 meters (Figure 1) clearly illustrates the significance of this event. To review, oceanic Kelvin waves travel only from west to east at extremely slow speeds (2-3 m/s). These waves have been alluded to as the facilitators of El Niño. There are two phases of an oceanic Kelvin wave, the “Upwelling” phase and the “Downwelling” phase. The Upwelling phase of an oceanic Kelvin wave pushes colder water from the sub-surface towards the surface, resulting in cooling at the surface. The Downwelling phase is the opposite, where warmer waters at the surface of the West Pacific warm pool are forced to sink, resulting a deepening of the thermocline and net warming in the sub-surface.

In the West Pacific, the thermocline is rising in response to strong upwelling (cold ocean temperature anomalies near the surface). In the central and eastern Pacific, the thermocline is deepening as the warm pool has begun to rapidly shift towards the Date Line. An enlightening time lapse can be found on the NOAA/CPC webpage. Note the lens of colder than average ocean temperature anomalies at the surface in the far eastern Pacific. This can be attributed to a surge in the Easterly trade winds over the eastern equatorial Pacific, which pushes water away from the coast, resulting in some upwelling off the west coast of South America. The surge in the trade winds is just an expression of atmospheric processes occurring in the tropics at intra-seasonal (weekly) timescales. Nevertheless, it is evident that the entire West Pacific Warm Pool has begun to shift eastward, and there is a large adjustment in the Pacific Ocean currently underway.

That being said, we still need to see some favorable atmospheric forcing this month to continue the forward advancement of a full-basin El Niño. In particular, west-to-east blowing winds along the Equator are needed to keep pushing warm water eastwards towards South America. Keeping this in mind, there are some signs of an upcoming period of westerly wind bursts along the equatorial Central Pacific in the next few weeks.

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2658


Figure 1. Departure of ocean temperature from average along the Equator in the Pacific Ocean on March 29, 2014 (top), shows a large area of 6°C (11°F) ocean temperature anomalies at a depth of 100 - 200 meters. A time lapse is available here. Image credit: NOAA/CPC.