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Amy Solomon
,
Lisa Goddard
,
Arun Kumar
,
James Carton
,
Clara Deser
,
Ichiro Fukumori
,
Arthur M. Greene
,
Gabriele Hegerl
,
Ben Kirtman
,
Yochanan Kushnir
,
Matthew Newman
,
Doug Smith
,
Dan Vimont
,
Tom Delworth
,
Gerald A. Meehl
, and
Timothy Stockdale

Abstract

Given that over the course of the next 10–30 years the magnitude of natural decadal variations may rival that of anthropogenically forced climate change on regional scales, it is envisioned that initialized decadal predictions will provide important information for climate-related management and adaptation decisions. Such predictions are presently one of the grand challenges for the climate community. This requires identifying those physical phenomena—and their model equivalents—that may provide additional predictability on decadal time scales, including an assessment of the physical processes through which anthropogenic forcing may interact with or project upon natural variability. Such a physical framework is necessary to provide a consistent assessment (and insight into potential improvement) of the decadal prediction experiments planned to be assessed as part of the IPCC's Fifth Assessment Report.

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Siegfried Schubert
,
David Gutzler
,
Hailan Wang
,
Aiguo Dai
,
Tom Delworth
,
Clara Deser
,
Kirsten Findell
,
Rong Fu
,
Wayne Higgins
,
Martin Hoerling
,
Ben Kirtman
,
Randal Koster
,
Arun Kumar
,
David Legler
,
Dennis Lettenmaier
,
Bradfield Lyon
,
Victor Magana
,
Kingtse Mo
,
Sumant Nigam
,
Philip Pegion
,
Adam Phillips
,
Roger Pulwarty
,
David Rind
,
Alfredo Ruiz-Barradas
,
Jae Schemm
,
Richard Seager
,
Ronald Stewart
,
Max Suarez
,
Jozef Syktus
,
Mingfang Ting
,
Chunzai Wang
,
Scott Weaver
, and
Ning Zeng

Abstract

The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land–atmosphere feedbacks on regional drought. The runs were carried out with five different atmospheric general circulation models (AGCMs) and one coupled atmosphere–ocean model in which the model was continuously nudged to the imposed SST forcing. This paper provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Niño–Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic multidecadal oscillation (AMO), and a global trend pattern.

One of the key findings is that all of the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific forcing pattern, with a cold Pacific leading to reduced precipitation and a warm Pacific leading to enhanced precipitation over most of the United States. While the response to the Atlantic pattern is less robust, there is general agreement among the models that the largest precipitation response over the United States tends to occur when the two oceans have anomalies of opposite signs. Further highlights of the response over the United States to the Pacific forcing include precipitation signal-to-noise ratios that peak in spring, and surface temperature signal-to-noise ratios that are both lower and show less agreement among the models than those found for the precipitation response. The response to the positive SST trend forcing pattern is an overall surface warming over the world’s land areas, with substantial regional variations that are in part reproduced in runs forced with a globally uniform SST trend forcing. The precipitation response to the trend forcing is weak in all of the models.

It is hoped that these early results, as well as those reported in the other contributions to this special issue on drought, will serve to stimulate further analysis of these simulations, as well as suggest new research on the physical mechanisms contributing to hydroclimatic variability and change throughout the world.

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