Genesis, Evolution, and Termination Michael McPhaden,Yukari Takayabu, Toshio Iguchi, Misako Kachi, Akira Shibata and Hiroshi Kanzawa Ian Bailey Atmospheric and Climatic Studies

Quick Review: What is the El Nino Southern Oscillation? ( a.k.a who was paying attention in 520?)  Quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years.  Associated with warming; accompanies high surface pressure in the western Pacific.  Can cause extreme weather (such as floods and droughts) in many regions of the world.

 Can last 9 months – 2 years  The first signs of an El Niño are: 1. Rise in surface pressure over the Western Pacific. 2. Fall in air pressure over Eastern Pacific. 3. Weakening trade winds heading east 4. Warm water spreads from the west Pacific to the east Pacific, taking moisture with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific.

 The El Nino, aside from being one of the fastest onsets of the ENSO on record, broke SST records on a monthly basis throughout its genesis.  Let’s take a look an exactly what happened in terms of it’s Evolution:

Month by Month Analysis:  The normal seasonal cycle is represented by the magenta lines.  The actual resulting temperatures and wind speeds are the blue lines.  Notice the massive difference in what the models expected and what this particular ENSO produced!  What could have caused this? What were the mechanisms that drove this?

Delayed Oscillator Theory:  “Evolution of the climate system in the tropical Pacific on interannual time scales is governed by the interplay between large-scale equatorial ocean wave processes and ocean-atmosphere feedbacks.”  Predicts that a build-up of heat in the Western Pacific is a precursor of El Nino

 large build- up of heat content in the West due to strong trade winds from weak La Nina

 1996: unusually high SST occur in Western Pacific, adding to heat content build up.

 The largest element of the intraseasonal variability in the tropical atmosphere (30–90 days).  Associated with deep convection and low level westerly winds.  Observed over warm surface water.

 Late 1996: MJO westerly winds intensify over massive heat content and advect warm water East.  Result: Warm pool expands East, allowing for continued MJO fueling.  Westerly wind forcing generates downwelling intraseasonal equatorial Kelvin waves. These waves expand eastward, depressing the thermocline in the East by more than 90m, allowing for further development of warm surface temperature.  Process is very cyclical. The situational intensity feeds itself.

 1997: Rossby waves propagate West in response to weakened trade winds. SST “cool”.  Net Result: Flattening of thermocline and disappearance of SST gradient.  Equatorial Cold Tongue fails to develop degrees Celsius fills equatorial basin.

 The models predicted warming of SST, but predictions were much too weak or developed too slowly. The best available ENSO model didn’t even predict the event!  Conflicting model predictions lead to uncertainty in the forecasting community.  Limited by: model imperfections, CHAOS, error in initial conditions, unable to simulate intraseasonal variations (seasonally and decadally only)

 The ENSO event ended as abruptly and rashly as it occured. The processes involved have been hypothesized by Yukari Takayabu and his team.  Let’s take a look first at what exactly happened:

 Thermocline in the Eastern Pacific shallows, allowing for strong upwelling of subsurface water to cool SST induced by intensifying Easterly winds.  May 18-20: sudden westward expansion of the cold tongue occurs. What caused the intensification of the Easterly winds that allowed for this to happen?

 The Madden-Julian Oscillation.  The same process that started the entire ENSO event, now completing its cycle, has reverse effects on it.

 MJO intensifies Easterly winds, leading to propagation of precipitation systems.  First system: denoted by the dashed line. The weaker of the two systems, allows for cold water to appear on the surface around 13 May.  Second System: denoted by the solid line. Further intensifies the winds, enhancing the precipitation system, causing the westward expansion of the cold tongue.

 A. Shows well developed precipitation system at the center. Note the primary precipitation system that initiated the cold tongue to the East.  B. Depiction of Rossby Wave (West) and Kelvin Wave (East) aloft. This pattern is a response to the intense imposed heating on the equator, known as the Matsuno-Gill Pattern.  C. Notice how closer to the surface, the Rossby Wave Pattern diminishes and the Kelvin Wave Pattern dominates. This signifies the existence of West directed Wave Bursts trapping the Rossby Waves aloft, extending the cold tongue.  D. Shows developed Easterly winds with convergence aligned east and toward the Equator. This is known as the Ekman convergence.

 “As a result, belts rich in water vapor [were] produced ahead of the precipitation system, which helps to maintain its eastward propagation. Thus, interaction between the precipitation processes and the dynamical waves maintained this MJO.”  With the large scale subsidence in the west relaxed and established in the east, the MJO could complete its cycle and complete its unusual precipitation propagation, expanding the cold tongue and diminishing the ENSO.

 The rapid onset of the ENSO event was brought about by weakened trade winds from La Nina, intense heat content pooling in the west, and the eastward push of this heat pool by the MJO, shutting down the equatorial cooling tongue and distributing abnormally warm SST across the Pacific Basin.  The rapid destruction of the event, in turn, was directly related to the MJO push of precipitation systems forcing subsurface upwelling of cool water, along with precipitation from the systems, east establishing and expanding the equatorial cooling tongue back across the Pacific.

 The two papers complimented each other well. McPhaden provided hard data to prove the cause of the ENSO event, while Takayabu provided the necessary data and explanation for the destruction of the event.  Other researches may have other theories to explain this, but these two provided complimentary data and logical explanations for exactly what happened. I believe future research on this event will continue, however it will be diminished since it is likely to be based on the work these two teams have done.

 Everyone else, Questions?