S. Gualdi, E. Scoccimarro, and A. Navarra
Abstract This study investigates the possible changes that the greenhouse global warming might generate in the characteristics of the tropical cyclones (TCs). The analysis has been performed using scenario climate simulations carried out with a fully coupled high-resolution global general circulation model. The capability of the model to reproduce a reasonably realistic TC climatology has been assessed by comparing the model results from a simulation of the 20th Century with observations. The model appears to be able to simulate tropical cyclone-like vortices with many features similar to the observed TCs. The simulated TC activity exhibits realistic geographical distribution, seasonal modulation and interannual variability, suggesting that the model is able to reproduce the major basic mechanisms that link the TC occurrence with the large scale circulation.
The results from the climate scenarios reveal a substantial general reduction of the TC frequency when the atmospheric CO2 concentration is doubled and quadrupled. The reduction appears particularly evident for the tropical North West Pacific and North Atlantic (NA). In the NWP the weaker TC activity seems to be associated with a reduced amount of convective instabilities. In the ATL region the weaker TC activity seems to be due to both the increased stability of the atmosphere and a stronger vertical wind shear. Despite the generally reduced TC activity, there is evidence of increased rainfall associated with the simulated cyclones. Despite the overall warming of the tropical upper ocean and the expansion of warm SSTs to the subtropics and mid-latitudes, the action of the TCs remains well confined to the tropical region and the peak of TC number remains equatorward of 20° latitude in both Hemispheres.
An extended version of this work is published in Journal of Climate (Gualdi et al. 2008).
Tropical cyclones (TCs) are non-frontal synoptic scale low-pressure systems, which develop over warm pools of the tropical or sub-tropical oceans, with organized convection and definite cyclonic surface wind circulation (Holland 1993). A severe
J.B. Elsner and T.H. Jagger (eds.), Hurricanes and Climate Change, 287
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tropical cyclone is also known as "hurricane" in North Atlantic and North-East Pacific and "typhoon" in West Pacific. TCs are one of the most devastating natural phenomena, which often cause severe human and economic losses. Therefore, the understanding of the mechanisms that underlie their formation and evolution is a high-priority issue from both the scientific, social and economic point of view.
The increased frequency and intensity of observed hurricanes since 1995 (Goldenberg et al. 2001, Webster et al. 2005) and the extraordinary nature of the North Atlantic hurricane season that occurred in 2005 have triggered a lively discussion about the possible changes of TC frequency and intensity due to global climate change (e.g., Emanuel 2005, Trenberth 2005, Pielke et al. 2005, Anthes et al. 2006, Pielke et al. 2006., Landsea et al. 2006, among other).
A number of studies have shown that TC activity varies substantially from interannual to decadal timescales. For example, the sensitivity of the TC activity to the phase of El Nino/Southern Oscillation (ENSO) has been documented in several studies (e.g., Gray 1984, Chan 2000, Chia and Ropelewski 2002). Similarly, low-frequency modulations of the North Atlantic Oscillation (NAO) exert significant influences on the behaviour of TCs (e.g., Elsner and Kocher 2000).
The wide interannual and decadal changes, associated with natural modes of climate variability, make difficult the identification of changes in the TC features that could be unambiguously attributed to the global warming (Walsh 2004). The detection of possible trends becomes even harder when observational data-sets only are used. The destructive nature of TCs, in fact, makes the collection of observed data extremely difficult and expensive. For this reason, databases of observed TCs are available only for a few regions (particularly North Atlantic) and are generally limited in length. Furthermore, due to subjective measurements and variable procedures, reservations have been raised about the reliability of the existing tropical cyclone data-bases for estimating climatological trends (Landsea et al. 2006, Landsea 2007).
To overcome the limitations of the observational data-sets, the possible influences of global warming on TC activity also have been explored using numerical models. Since the early work of Broccoli and Manabe (1990), a number of studies have been performed both with global and regional models, reaching conflicting conclusions. Haarsma et al. (1993), for instance, found a significant increase of the number of simulated TCs in greenhouse warming experiments. However, similar simulations, but performed with higher resolution models, showed a significant reduction of the global TC activity in a warmer earth (Bengtsson et al. 1996, Sugi et al. 2002, McDonald et al. 2005, Yoshimura et al. 2006, Bengtsson et al. 2007). Royer (1998), on the other hand, found increased (decreased) TC activity in the Northern (Southern) Hemisphere, whereas Chauvin et al. (2006) showed that possible changes in the frequency of TC occurrence in the North Atlantic strongly depend on the characteristics of the sea-surface temperature (SST) spatial distribution produced by the scenario simulations.
While the issue of the TC frequency response to greenhouse warming remains arguable, some consensus has been achieved about the effects on the TC intensity. Consistent with the theoretical findings of Emanuel (1987) and Holland (1997), numerous model studies, have found that the intensity of simulated TCs tends to increase in a warmer earth (e.g., Walsh and Ryan 2000, Sugi et al. 2002, Knutson and Tuleya 2004, Chauvin et al. 2006, Yoshimura et al. 2006, Oouchi et al. 2006, Bengtsson et al. 2007). In particular, these works have shown that in a warmer climate TCs might be characterized by stronger winds and more intense precipitations. These results appear to be rather robust, as they have been obtained using a variety of models (global and regional), different resolutions and convective para-metrizations. However, it is important to note that most of these studies have been conducted analyzing experiments performed with atmospheric only models forced with prescribed SSTs, thus the large majority of these experiments do not include air-sea interactions. Moreover, the SST patterns used to force the atmosphere were based on (generally low-resolution) climate scenario simulations performed with other models. This procedure, therefore, might be affected by possible inconsistencies between the simulations from which the SST patterns were taken and the atmospheric runs used to analyze the TC behaviour.
Though the air-sea feedbacks are known to be important for TC intensity (Emanuel 2003), there are only a few analysis of the TC response to global warming performed with coupled models, which, moreover, have been carried out using limited area models with simplified experimental setting (Knutson et al. 2001). On the other hand, in-depth investigations of TCs and their simulation conducted with fully coupled global models, the same as those used to perform the climate scenarios, would provide further insight into these phenomena and into our ability to reproduce and predict their behaviour.
In this study, we document the ability of a high-resolution coupled atmosphere-ocean general circulation model to simulate tropical cyclone-like vortices and explore how the features of these phenomena are possibly altered by greenhouse warming. The analysis is performed on idealized greenhouse gas forcing scenarios and a simulation of the 20° Century climate. The difference with respect to most of the previous works published on he same subject is that we use a fully coupled model, where air-sea feedbacks are accounted for.
In Section 2, a description of the model, scenario simulations and of the methodological approach used in the present paper is provided. In Section 3, we examine the ability of the model to simulate TCs. Section 4 presents an assessment of the possible changes of the TC characteristics as a consequence of global warming. In Section 5, the main findings of this work will be discussed, and the summary in Section 6 close the paper.
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