Wherefore El Niño:The impact of extra-tropical atmospheric circulations upon the tropical Pacific

Bruce T. Anderson, Boston University (brucea@bu.edu)

Introduction

Previous research has shown that the northern hemisphere wintertime climate is strongly forced by large-scale boundary conditions associated with the concurrent El Niño/Southern Oscillation (ENSO)

Impacts local, regional and global atmospheric dynamics Influences various climatological fields such as temperature, precipitation, and “storminess”

As such, researchers have spent considerable time investigating forcing mechanisms for the ENSO as well as the evolution of the atmospheric and oceanic components of the ENSO system

Introduction con’t.

One intriguing set of results involves fields in the extra-tropics that may be related to the initiation of ENSO eventsIn this talk, we will summarize results from the last few years that tell a compelling story of how the extra-tropics can influence the tropical Pacific :

Influence of upper-tropospheric circulations upon the global and regional Hadley cellSignature of these changes in the surface pressure and wind patterns Impact upon surface and subsurface temperature fieldsSubsequent influence upon the upper-tropospheric circulations (?)

Global-Scale Hadley Cell CirculationNon-ENSOENSO

Caballero, 2007 (GRL)

Role of Stationary Wave ActivityOverturning Circulation Stationary Wave Stress

Caballero, 2007 (GRL)

Role of Stationary Wave Activity

High Overturning

Weak Overturning

Caballero and Anderson, 2009 (GRL)

Upper-air Stationary Waves

Anderson, 2004 (J.Clim)

Upper-tropospheric Circulations

Anderson, 2004 (J.Clim)

Surface Signature

Anderson, 2004 (J.Clim)

Intraseasonal Variability and Seasonal Mean

Anderson, 2007 (J.Clim)

Intraseasonal Variability

Anderson, 2007 (J.Clim)

Intraseasonal Variability and SLP

Anderson, 2007 (J.Clim)

Intraseasonal Variability and Winds

Anderson, 2007 (J.Clim)

Surface and Subsurface Temperatures

Anderson, 2004 (J.Clim)

Recharge/Discharge Paradigm for ENSO

(Meinen and McPhaden, 2000)

Surface and Subsurface Temperatures

Anderson, 2004 (J.Clim)

Evolution of Thermocline Depth and SSTs

Obs. 1993-99

Model

(Meinen and McPhaden, 2001)

(Anderson and Maloney, 2006)

Evolution of ENSO System

Anderson, 2004 (J.Clim)

Midlatitude forcing and Rossby Wave Breaking

(Anderson, 2004)

(Croci-Maspoli et al., 2007)

(Woollings et al, 2008)

Conclusions

Equatorial Pacific SST anomalies appear to be partly related to changes in large-scale, extra-tropical atmospheric circulations 12-15 months prior to boreal winter ENSO eventsRelated to changes in hemispheric-scale upper-air circulations Associated with anomalous equatorward propagation of stationary wave activity over the North Pacific

Generates vertical circulation patterns over the sub-tropical and extratropical Pacific, which are co-located with variations in surface pressure and wind stress fields over the central sub-tropical and tropical North PacificSurface circulations can initiate the recharge/discharge mechanism that serves as the onset (or “trigger”) for ENSO events

? Role of tropical, sub-tropical, and/or extratropical processes in forcing/maintaining large-scale mode of variability

Central Pacific SST anomaliesInternal atmospheric variability associated with Rossby wave breaking

El Nino/Southern Oscillation

El Nino/Southern Oscillation Impacts

El Nino/Southern Oscillation Impacts