Wednesday, February 19, 2014

Kelvin Wave Provoking El Nino; Atmosphere in Disagreement

A Kelvin Wave currently moving across the eastern Pacific appears to be trying to provoke an El Nino, but the rest of the atmosphere is in disagreement.

The graphic above shows water temperature anomalies across the Equatorial Pacific, with longitude values on the bottom legend and depth values in meters on the left legend. The Kelvin wave is clearly shown by the strong positive water temperature anomalies from the 100 to 200 meter depth in the central/western ENSO monitoring area in the Equatorial Pacific. With a Kelvin Wave, you have a mass of warm water pushing east under the surface, and pushing east across most (if not all) of the Equatorial Pacific. This wave warms the waters below the surface, and if those subsurface waters are mixed to the surface, we can see an El Nino develop. Due to that process, Kelvin Waves are typically associated with El Nino formations, and that is why we are monitoring this event for possible El Nino provocation down the line in spring.

However, the rest of the atmosphere does not agree that we will see an El Nino anytime soon.

One feature we see over the Eastern Pacific is a swath of anomalously strong upper level winds, as the purples and yellows on the attached image denote. Thanks to the Earth System Research Laboratory (ESRL), we can compare these uppr level winds to see what ENSO state the atmosphere is currently in- a La Nina, El Nino, or neutral-ENSO state.

If we look at the ESRL's 200mb wind anomaly composite for February and March over the Equatorial Pacific, we see that El Nino's are typically recognized in those time periods by strong upper level winds due west of Mexico, with anomalously weak upper level winds over the ENSO monitoring area. Taking a look back at the latest observed upper level winds, it is clear that the atmosphere is in more of a La Nina state, as we usually see strong upper level winds over the eastern Equatorial Pacific during an El Nino instead of weak upper level winds we are currently seeing.

Another item we can use to judge what state the El Nino-Southern Oscillation phenomenon is in is the sea level anomaly. In other words, we can see if the waters over the eastern Equatorial Pacific are higher or lower than normal, and use that to help tell us what phase the ENSO is in. If we check out the composite graphic above, it is clear that the El Nino phenomenon is identified by higher than normal sea level anomalies, with the La Nina producing lower than normal sea level anomalies. The November 1997 sea level composite on the top of this graphic shows the El Nino sea level anomaly, while the February 1999 composite below that depicts sea level anomalies for a La Nina.
Taking a glance at the latest observed sea level anomalies, it is more apparent that while we aren't in an El Nino, which would show up as a swath of yellows and oranges to indicate higher than normal sea level anomalies, we also aren't in a defined La Nina, which would appear as a swath of deep blues on the right hand side of this chart. We do see some blues in the Equatorial Pacific, which could argue for a weak La Nina if you really want to be picky about it, but I'm not really seeing any defined ENSO state going one way or the other.

This chart, showing the anomalous depth of the 20 degree (C) isotherm over time, gives you a hint at what the progression of a Kelvin Wave looks like, with two clear examples of Kelvin Waves observed in November 2013 into January 2014, and from September 2013 into October 2013. The latest observation of this 20 degree isotherm confirms that we are looking at a Kelvin Wave. with this wave looking to be the strongest out of the last three events. It's possible that this sends us into an El Nino pattern when spring and summer come around, but for now, an El Nino does not look to be on the way, at least for the near future.


No comments: