Tropical Cyclone World: a Fine-Resolution Tropical Aquaplanet Climate Model Simulation

A climate theory that links tropical cyclone formation rate to large scale climate conditions remains elusive. The present study aims to investigate the sensitivity of tropical cyclone formation rate to specific climate conditions using an idealized modeling framework. The use of aquaplanet simulations as idealized experiments to test climate hypotheses has been widespread since the work of Hayashi and Sumi (1986). Here, an aquaplanet simulation is performed of a fine-resolution (~40 km) global climate model (GCM), the ACCESS atmospheric model (Bi et al. 2013; Hardiman et al. 2015). We use this model to investigate whether it is possible to create a global climate that has the maximum possible tropical cyclone formation rate for a specified sea surface temperature (SST). This concept is known as “tropical cyclone world” (e.g. Khairoutdinov and Emanuel 2013; Reed and Chavas 2015). A constant global SST of 30^oC is imposed and the model is started from an atmospheric restart file corresponding to SST fields at September 1 1988, with normal Earth topography.

Usually in perturbation GCM experiments of this kind that impose a large climate perturbation on a Earth-like climate restart file, the first few months of the experiment are discarded as a spinup period and the climatology is then analysed. Instead, here we examine the spinup period as giving insights into the development of the tropical cyclone world from an initial state of an Earth-like climate. Upon the removal of all topography and the imposition of a tropical SST globally, mid-latitude cyclones provide a source of convection that initiates the development of tropical cyclones in the mid-latitudes and polar regions of the aquaplanet. Within two weeks, mid-latitude cyclones are completely converted into tropical cyclones. This is easily recognized by the generation in mid-latitude and polar regions of symmetric storms of considerably smaller size than the asymmetric mid-latitude cyclones, and with central pressures as low as 940 hPa. In this experiment, tropical cyclones continue to form in tropical latitudes and drift poleward but their numbers are dwarfed by the mid-latitude and polar “tropical” cyclones.

This aquaplanet simulation is then run for a period of 10 years and the simulated climatology of tropical cyclones is examined.  Results from this simulation will be presented at the conference, along with an examination of how these model results can be used to inform a climate theory of tropical cyclone formation.

References

Bi, D., Dix, M., Marsland, S. J., O’Farrell, S., Rashid, H., Uotila, P., ... & Yan, H. (2013). The ACCESS coupled model: description, control climate and evaluation. Aust. Meteorol. Oceanogr. J, 63(1), 41-64.

Hayashi, Y. Y., & Sumi, A. (1986). The 30-40 day oscillations simulated in an "aqua planet" model. J. Meteor. Soc. Japan, 64(4), 451-467.

Hardiman, S. C., Boutle, I. A., Bushell, A. C., Butchart, N., Cullen, M. J., Field, P. R., ... & O’Connor, F. M. (2015). Processes controlling tropical tropopause temperature and stratospheric water vapor in climate models. J. Climate, 28(16), 6516-6535.

Khairoutdinov, M., & Emanuel, K. (2013). Rotating radiative‐convective equilibrium simulated by a cloud‐resolving model. J. Adv. Model. Earth Sys., 5(4), 816-825.

Reed, K. A., & Chavas, D. R. (2015). Uniformly rotating global radiative‐convective equilibrium in the Community Atmosphere Model, version 5. J. Adv. Model. Earth Sys, 7(4), 1938-1955.