El Niño Southern Oscillation [ENSO] NORMAL: - Easterly trade winds between ± 30° latitude (Coriolis Force) - Sea Surface Height slant to west - Warm basin in W. Pacific - Cool surface in E. (upwelling H 2 O) - Convective loop reinforces this state (+ feedback) - Darwin, AU, Indonesia – low press - Tahiti, Peru, high press - SOI =Southern Oscillation Index - SOI = (P Tahiti – P Darwin ) - Upwelling at Peru – nutrients - anchovies - Zimbabwe – normal precip/corn

NORMAL Condition in Pacific

Hadley Cells (± 30° Latitude) F Coriolis = - ω x v where ω = the rotational angular velocity of the Earth and v = wind velocity. ω v

Easterly trade winds Ocean current & upwelling Warm water Cool water Australia Peru

El Niño Phase ENSO as an Integrating Concept…McPhaden, Zebiak, Glantz, Science, 15Dec2006. Sea Surface Temperature (SST) Wind stress vectors indicate direction and intensity La Niña Phase

El Niño Phase

1997-8

Global Mean Surface Air Temperature to 2012 “Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.” – IPCC – “WGI – September 23, 2013

Recent global-warming hiatus tied to equatorial Pacific surface cooling & deeper ocean warming Kosaka, Xie, Nature 2013, doi: /nature12534 La Nina: An extended ENSO cool period has reduced the Pacific Ocean surface temperature during this period Ocean Storage: A transfer of heat from the atmosphere to the cool (La Nina) sea surface in the equatorial Pacific has held down the average air temperature and warmed the deeper ocean. Question: Will average surface warming return during warm ENSO phase?

Feedback in Climate [ Or why we need models ] Positive feedback example: warmer A warmer surface temperature reduces the size of the highly reflective polar ice fields. The less reflective soil or open ocean absorbs more solar radiation increasing the surface temperature even further. Negative feedback example: T  T warmer A warmer surface temperature evaporates additional water forming more cloud. The cloud layer reflects incoming solar energy, reducing the surface temperature. T  T Feedback  nonlinear behavior  need for climate models

General Circulation Models (GCMs)

Oscillations and Teleconnections Q: Debate about what causes trade winds to relax at onset of El Nino Dynamical I.: The weakening of trades at onset of warm phase results from large-scale deterministic processes operating during previous phases. Example: A warm layer that propagate across Pacific at onset of the warm phase (El Nino) reflect off South America and enhance the probability of the onset of the following cool phase (La Nina). That is, the seeds of destruction of a phase are planted by that phase itself. Dynamical II.: Ocean/Atmosphere coupling where ocean lags behind atmosphere and sets the time scale of the oscillation. Sea Surface Temp (SST) Atmospheric Circulation Ocean Circulation and Wind position of thermocline

Oscillations and Teleconnections Stochastic: Stable preferred state (warm in West, cool in East) is interrupted occasionally by a high energy meteorological forcing event. Teleconnection: Coupled high/low pressure pairs North Atlantic Oscillation example. H L Iceland Azores Prevailing Westerly Winds