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Jack J. Katzfey
and
Kathleen L. Mcinnes

Abstract

The ability of the CSIRO-9 General Circulation Model (GCM) to capture surface cutoff lows over eastern Australia is investigated by comparing composites of ten GCM cases with ten observed lows. The lows are also studied individually to compare their development and movement, as well as synoptic features, which may have been smoothed out in the compositing process. Finally, the incidence of all such lows in the 1 × CO2 and 2 × CO2 simulations are examined to determine the possible effects a doubling of CO2 will have on their occurrence.

The GCM surface lows were found to develop from an upper-level cutoff low in a manner similar to the observed lows. In both sets, this development took place between the upper-level subtropical and polar jets in all seasons except summer, where only one jet was evident. Latent heat release appeared to play an important role in the intensification of the surface lows. The main difference between the two sets of cutoff lows was that the GCM surface lows tended to develop farther to the east of the upper-level cutoff, the upper-level features were less intense and occlusion did not take place. As a result, the GCM lows had a greater eastward translation compared to the observed lows, which often meander along the east coast for several days while they intensify. These features appear to be related to the low resolution of the GCM.

The frequency of east Australian cutoff lows was underpredicted in the WM by about 45% in the 1 × CO2 simulation, with the greatest underprediction occurring in autumn and winter. Analysis of upper-level jet structure indicated that the GCM produced a poor simulation of the dual jet structure aloft, which may account for this problem. The 2 × CO2 simulation produced even fewer cutoff lows over eastern Australia. This was probably caused by the reduced baroclinicity due to increased warming of polar regions, which resulted in an even weaker dual jet structure. The cast Australian cutoff lows were found to be more intense in the 2 × CO2 Simulation, suggesting the greater role played by latent heat effects once development has been initiated.

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Kevin J. E. Walsh
and
Jack J. Katzfey

Abstract

A regional climate model (DARLAM) is implemented over the Australian region and a 20-yr seasonally varying simulation is examined for the presence of tropical cyclone–like vortices (TCLVs). The horizontal resolution of the model is 125 km with nine vertical levels and is forced at its boundaries by the output of the Commonwealth Scientific and Industrial Research Organisation GCM using a mixed layer (or “slab”) ocean. Additional simulations are performed with a horizontal resolution of 30 km and with 18 vertical levels to examine the impact of increasing resolution on storm intensity. A sample of TCLVs from the 125-km resolution simulation is simulated at 30-km resolution to determine whether they reach observed tropical storm intensity at the finer resolution. It is found that stronger vortices in the 125-km resolution simulation are more likely to intensify when simulated at finer resolution than weaker vortices. In this way, a detection threshold for vortices generated in the 125-km resolution simulation is established and then used to detect TCLVs in that simulation. The regional climate model DARLAM provides a good simulation of both cyclogenesis and its seasonal variation under the current climate. The response of the model under enhanced greenhouse conditions is studied. Under 2 × CO2 conditions, there is no statistically significant change in regions of formation of TCLVs, with only a slight southward shift simulated. Nevertheless, there are statistically significant effects on the poleward movement of TCLVs, with storms generally tending to travel farther poleward in a warmer climate once they have formed. An analysis is undertaken to determine the reasons for this behavior. While the dynamical constraints on the maintenance of TCLV intensity under 2 × CO2 conditions (e.g., vertical wind shear) are similar to those in the current climate, thermodynamic conditions (e.g., sea surface temperatures) are quite different and are likely to be at least partly the cause of this effect. Other causes include the combination of the slight southward shift in formation and a tendency for TCLV tracks to be more southward in enhanced greenhouse conditions, a consequence of more southward steering winds.

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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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