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Wenju Cai
,
Jozef Syktus
,
Hal B. Gordon
, and
Siobhan O’Farrell

Abstract

The response of a coupled oceanic–atmospheric–sea ice climate model to an imposed North Atlantic high-latitude freshening is examined. The imposed freshening lasts for 5 yr with a total salt deficit equivalent to about eight times the observed Great Salinity Anomaly during the late 1960s and early 1970s.

The thermohaline circulation associated with North Atlantic Deep Water Formation (NADWF) initially weakens, but it recovers within 20 yr of the imposed freshening being removed. The effect of the weakened NADWF is gradually transmitted from high latitudes to the entire Atlantic Ocean. The response at the equator lags that at 62°N by about 10 yr. In the midlatitude (from 30° to 58°N) region, the lag causes a warming during the initial weakening and a cooling during the recovery. Changes in the thermohaline circulation significantly modify the large-scale North Atlantic circulation. In particular, the barotropic Gulf Stream weakens by about 18%.

An interesting feature is the dipole structure of the initial response in sea surface temperature, with cooling in the sinking region and warming south of it. This dipole structure plays an important role for the recovery of the NADWF once the imposed freshening is removed. It increases the surface density in the sinking region and increases the north–south pressure gradient. Thus, the conditions set up during the initial weakening contribute to the recovery process.

Modifications of the thermal structure of the ocean surface lead to changes in the atmospheric circulation, in particular, a weakening of the westerlies over the midlatitude North Atlantic and a southward shift over Western Europe. The North Atlantic oscillation (NAO) index under the imposed freshening is negative, consistent with findings from observational studies. The associated climate changes are similar to those observed with negative NAO values.

Effects of various oceanic and atmospheric feedbacks are discussed. The results are also compared with those from ocean-only models, where the atmosphere–ocean interactions and some of the oceanic feedbacks are excluded.

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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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Jonathan Spinoni
,
Paulo Barbosa
,
Edoardo Bucchignani
,
John Cassano
,
Tereza Cavazos
,
Jens H. Christensen
,
Ole B. Christensen
,
Erika Coppola
,
Jason Evans
,
Beate Geyer
,
Filippo Giorgi
,
Panos Hadjinicolaou
,
Daniela Jacob
,
Jack Katzfey
,
Torben Koenigk
,
René Laprise
,
Christopher J. Lennard
,
M. Levent Kurnaz
,
Delei Li
,
Marta Llopart
,
Niall McCormick
,
Gustavo Naumann
,
Grigory Nikulin
,
Tugba Ozturk
,
Hans-Juergen Panitz
,
Rosmeri Porfirio da Rocha
,
Burkhardt Rockel
,
Silvina A. Solman
,
Jozef Syktus
,
Fredolin Tangang
,
Claas Teichmann
,
Robert Vautard
,
Jürgen V. Vogt
,
Katja Winger
,
George Zittis
, and
Alessandro Dosio

Abstract

Two questions motivated this study: 1) Will meteorological droughts become more frequent and severe during the twenty-first century? 2) Given the projected global temperature rise, to what extent does the inclusion of temperature (in addition to precipitation) in drought indicators play a role in future meteorological droughts? To answer, we analyzed the changes in drought frequency, severity, and historically undocumented extreme droughts over 1981–2100, using the standardized precipitation index (SPI; including precipitation only) and standardized precipitation-evapotranspiration index (SPEI; indirectly including temperature), and under two representative concentration pathways (RCP4.5 and RCP8.5). As input data, we employed 103 high-resolution (0.44°) simulations from the Coordinated Regional Climate Downscaling Experiment (CORDEX), based on a combination of 16 global circulation models (GCMs) and 20 regional circulation models (RCMs). This is the first study on global drought projections including RCMs based on such a large ensemble of RCMs. Based on precipitation only, ~15% of the global land is likely to experience more frequent and severe droughts during 2071–2100 versus 1981–2010 for both scenarios. This increase is larger (~47% under RCP4.5, ~49% under RCP8.5) when precipitation and temperature are used. Both SPI and SPEI project more frequent and severe droughts, especially under RCP8.5, over southern South America, the Mediterranean region, southern Africa, southeastern China, Japan, and southern Australia. A decrease in drought is projected for high latitudes in Northern Hemisphere and Southeast Asia. If temperature is included, drought characteristics are projected to increase over North America, Amazonia, central Europe and Asia, the Horn of Africa, India, and central Australia; if only precipitation is considered, they are found to decrease over those areas.

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