Identification

Title

Attribution of the late-twentieth-century rainfall decline in southwest Australia

Abstract

There was a dramatic decrease in rainfall in the southwest of Australia (SWA) in the mid-1960s. A statistical method, based on the idea of analogous synoptic situations, is used to help clarify the cause of the drying. The method is designed to circumvent error in the rainfall simulated directly by a climate model, and to exploit the ability of the model to simulate large-scale fields reasonably well. The method uses relationships between patterns of various atmospheric fields with station records of rainfall to improve the simulation of the local rainfall spatial variability. The original technique was developed in a previous study. It is modified here for application to two four-member ensembles of simulations of the climate from 1870 to 1999 performed with the Parallel Climate Model (PCM). The first ensemble, called natural, is forced with natural variations in both volcanic activity and solar forcing. The second ensemble, called full forcing, also includes three types of human-induced forcing resulting from changes in greenhouse gases, ozone, and aerosols. The full-forcing runs provide a better match to observational changes in sea surface temperature in the vicinity of SWA. The observed rainfall decline is not well captured by rainfall changes simulated directly by the model in either ensemble. There is a hint that the fully forced ensemble is more realistic, but it is nothing more than a hint. The downscaling approach, on the other hand, provides a much more accurate reproduction of the day-to-day variability of rainfall in SWA than the rainfall simulated directly by the model. The downscaled ensemble mean rainfall in full forcing declines over the region with a spatial pattern that is similar to the observed decline. This contrasts with an increase of rainfall in the downscaled rainfall in the natural ensemble. These results give the clearest indication yet that anthropogenic forcing played a role in the drying of SWA. Note, however, that ambiguities remain. For example, although the observed decline fits within the range of downscaled model simulation, the ensemble mean rainfall decline is only about half of the observed estimate, the timing differs from the observations, drying did not occur in the downscaling of one of the four full-forced ensemble members, and not all potential forcing mechanisms are included in full forcing (e.g., land surface changes). Furthermore, while the observed rainfall decline was a sharp reduction in the 1960s, followed by a near-constant rainfall regime, the full-forcing ensemble suggests a more gradual rainfall decline over 40 yr from 1960.

Resource type

document

Resource locator

Unique resource identifier

code

http://n2t.net/ark:/85065/d7j966m1

codeSpace

Dataset language

eng

Spatial reference system

code identifying the spatial reference system

Classification of spatial data and services

Topic category

geoscientificInformation

Keywords

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keyword value

Text

originating controlled vocabulary

title

Resource Type

reference date

date type

publication

effective date

2016-01-01T00:00:00Z

Geographic location

West bounding longitude

East bounding longitude

North bounding latitude

South bounding latitude

Temporal reference

Temporal extent

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End position

Dataset reference date

date type

publication

effective date

2006-05-15T00:00:00Z

Frequency of update

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Conformity

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Use constraints

Copyright 2006 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be "fair use" under Section 107 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Law (17 USC, as revised by P.L. 94-553) does not require the Society's permission. Republication, systematic reproduction, posting in electronic form on servers, or other uses of this material, except as exempted by the above statements, requires written permission or license from the AMS. Additional details are provided in the AMS Copyright Policies, available from the AMS at 617-227-2425 or amspubs@ametsoc.org. Permission to place a copy of this work on this server has been provided by the AMS. The AMS does not guarantee that the copy provided here is an accurate copy of the published work.

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None

Responsible organisations

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contact position

OpenSky Support

organisation name

UCAR/NCAR - Library

full postal address

PO Box 3000

Boulder

80307-3000

email address

opensky@ucar.edu

web address

http://opensky.ucar.edu/

name: homepage

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pointOfContact

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