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Tropical Cyclone sensitivities to CO2 doubling: Roles of atmospheric resolution and background climate changes.

August 16th, 2019

Key Findings

  • Higher resolution coupled models (comparing 2 degree, 0.5 degree and 0.25 degree ) have slower surface warming due to greater Southern Ocean heat uptake.
  • Increased CO2 and warming drive a substantial decrease in global tropical cyclone (TC) frequency in the 0.5 degree model, but either no change or an increase in the 0.25 degree model (depending on whether or not that model’s climatological SST biases are corrected).
  • The 0.25 degree model shows a substantial increase in TC intensity and Category 3-5 TC frequency. This model is unique in projecting an increase in global TC frequency in a warming climate, whereas most of the climate models in the world show a decrease.
  • Projected changes in small scale disturbances (i.e., TC seeds) are an important driver for TC frequency changes in these models.

G. Vecchi, T. Delworth, H. Murakami, S. Underwood, A. Wittenberg, F. Zeng, W. Zhang, K. Bhatia, W. Cooke, J. He, A. Rosati, K. van der Weil, W. Anderson, V. Balaji, J. Baldwin, J-H Chen, K. Dixon, R. Gudgel, L. Harris, L. Jia, N. Johnson, S. Kapnick, T. Knutson, S-J Lin, M. Liu, J. Ng, J. Smith, G. Villarini, X. Yang. Climate Dynamics. DOI: 10.1007/s00382-019-04913-y

This research explored the sensitivity of large-scale surface climate and tropical cyclone activity to a doubling of CO2, using three coupled global climate models that span a range of horizontal atmospheric and land resolutions. The authors investigated the impact of resolution changes in the atmosphere within a family of coupled global climate models with identical ocean and sea ice components, and whose atmospheric configurations differ only in their horizontal resolution (~200km, ~50km, and ~25km).

The models used for these experiments are derived from GFDL’s coupled models CM 2.1 and CM 2.5: the Low Ocean Atmosphere Resolution version of CM2.5 (LOAR); the Forecast-oriented Low Ocean Resolution version of CM2.5 (FLOR); and the high atmospheric resolution version of FLOR (HiFLOR), with, respectively, resolutions of ~200km, ~50km, and ~25km.

In these experiments, the models exhibited similar changes in background climate fields thought to regulate TC activity, such as relative sea surface temperature (SST), potential intensity, and wind shear. However, global TC frequency decreased substantially in the 50km model, while the 25km model showed no significant change. The inconsistent changes in TC frequency is explained by different changes in frequency of synoptic disturbances (i.e., TC seeds). The 25km model also had a substantial increase of Category 3-4-5 hurricanes.

Idealized perturbation experiments were performed to understand the TC response. Each model’s response to SST forcing depends on each model’s background climatological SST biases: removing these biases leads to a global TC intensity increase in the ~50km model, and a global TC frequency increase in the ~25km model (HiFLOR), in response to CO2-induced warming patterns and CO2 doubling. The highly unusual response of HiFLOR for global TC frequency is somewhat surprising and appears unique among climate models. This remains an uncertain finding and will be subject to further studies and follow-on studies with other high resolution models.

TCs and surface climatological changes can cause large societal and ecological impacts. It is important to increase our understanding of how these will change in the future, with higher atmospheric CO2 levels caused by further anthropogenic emissions.

Figure 11. Response of global-mean TC frequency in the idealized forcing experiments; leftmost bars are for the FLOR model, the second set of bars for the HiFLOR model, and rightmost bars show the difference between the response of HiFLOR and FLOR. In each group, the blue bar/symbols show the response to a combined uniform 2K warming and a CO2 doubling ( ObC+2K+2 CO2), the gray bar/symbols show the response to CO2 doubling with fixed SST ( ObC+2 CO2), and the red bar/symbols show the response to uniform 2K warming ( ObC+2K). Bars show the percent change in TC frequency averaged over 50 years (relative to the ObC control). Black lines show the 95% confidence interval on the change (computed as in Fig. 10).


Figure 17. Response of TC density in HiFLOR. Shading indicates the change between the perturbation and reference climate experiments for TC density (number of TC days per season in a 10° latitude by 10° longitude box centered at each 1° interval, as in Vecchi et al. (2014)). TC days are defined for FLOR as times when the maximum zonal wind speed of the TC exceeds 15.3 m/s, and a warm core is identified by the Harris et al. (2016) TC tracker. Blue and green shading indicates decreases in TC density, red/yellow/orange shading indicates increases in TC density. Differences are averaged over years 201-250 for the transient 2 x CO2 response (panel a), and over 50 years of model simulation for the various nudged SST experiments (panels b – f). Panels d and e highlight that HiFLOR projects significant increases in global TC frequency in case of 2K warming only and 2K warming plus 2xCO2 as seen in Figure 11.