HR: 11:20h
AN: PP52A-05 INVITED    [Abstracts]
TI: Tropical Signature of Global Warming: El Niño or a La Niña?
AU: * Vecchi, G A
EM: Gabriel.A.Vecchi@noaa.gov
AF: GFDL-NOAA, NOAA/Geophysical Fluid Dynamics Laboratory Princeton Forrestal Campus Rte. 1 P.O. Box 308, Princeton, NJ 08542-0308, United States
AU: Soden, B J
AF: RSMAS-U. Miami, University of Miami Rosenstiel School of Marine and Atmospheric Science, Miami, FL 33149, United States
AU: Clement, A
AF: RSMAS-U. Miami, University of Miami Rosenstiel School of Marine and Atmospheric Science, Miami, FL 33149, United States
AB: Understanding the response of tropical atmospheric and oceanic circulation to increasing greenhouse gases is a central question in climate change research, since changes in tropical climate conditions can have far-reaching effects. There is a long-standing debate in the climate community on how the tropical Pacific will respond to an increase of greenhouse gases: will the structure of changes in the ocean surface temperature more closely resemble an El Niño or a La Niña? This dispute extends beyond global warming and has been a topic of intense interest within the paleo-climate community: El Niño and La Niña-like responses have been invoked as dynamical frameworks for interpreting past climate changes which occurred on timescales of centuries to millions of years. In this talk some of the theoretical, modeling understanding, and observational evidence for long-term changes to the tropical Pacific climate system will be highlighted. The dynamical arguments for an El Niño-like and a La Niña-like response will be described, contrasted and illustrated using climate model integrations of different levels of complexity. In models with a simplified representation of atmospheric physics, feedbacks originating in the ocean drive the system to a La Niña-like state. In models that include atmospheric general circulation components, atmospheric energy and mass balance constraints result in a reduction of the strength of the atmospheric overturning circulation – which is manifest primarily in the zonally-asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component. In these models changes over the tropical Pacific Ocean roughly resemble El Niño-like conditions; however, the mechanisms controlling these change are fundamentally different from those of El Niño. Even though modeling studies can help reconcile aspects of the diverging theoretical and model-based understanding, a true reconciliation requires observational evidence. Instrumental observations of sea level pressure indicate that over the 20th Century the Pacific Walker circulation appears to have weakened; however, it will be shown that in the instrumental record the differing reconstructions of historical sea surface temperature (SST) are inadequate to distinguish between an increase or decrease in East-West SST gradient across the Pacific. We outline what we view as a possible way forward, with paleo-proxy observations, to reconcile these diverging views. The problem of the Pacific response to global warming represents an important intersection of theory, modeling, direct observations and paleoclimate reconstructions. Theory and modeling have begun to converge in that robust mechanisms appear in a consistent manner in models of different complexity. However, the test of how these mechanisms operate in reality is in the hands of the observationalists, and the consequences for our understanding of the climate not only in the tropical Pacific, but in all the regions affected by ENSO, are great.
DE: 1600 GLOBAL CHANGE
DE: 1620 Climate dynamics (0429, 3309)
DE: 1626 Global climate models (3337, 4928)
DE: 3374 Tropical meteorology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting