Elsevier

Icarus

Volume 299, 1 January 2018, Pages 364-369
Icarus

Low probability of tropical cyclones on ocean planets in the habitable zones of M dwarfs

https://doi.org/10.1016/j.icarus.2017.08.007Get rights and content

Highlights

  • Genesis potential of tropical cyclones (TC) on ocean planets is lower than in TC basins on the Earth.

  • The main reason is the much smaller rotation rate of the tidally locked habitable zone planets.

  • Our analysis suggests that the wind speed of potential TC on planets with eccentric orbits could be similar to that on Earth but future study is needed to confirm it.

Abstract

The genesis potential index (GPI) of tropical cyclones (TC) on ocean planets in the habitable zones of M dwarfs is analyzed based on 3D GCM simulations. We found that GPI on these planets are smaller than those in TC basins on the Earth mainly because of slow rotation of such planets. GPI's on exoplanets with eccentric orbits are strong function of time with values generally greater than those on circular orbits. Future high resolution models are needed to better understand whether TCs could form on ocean exoplanets, and what their potential intensities and distributions might be.

Introduction

Most studies on the climate of rocky exoplanets focus on large-scale atmospheric circulation patterns (Pierrehumbert, 2010, Yang et al., 2013, Hu and Yang, 2014, Wang et al., 2014, Wang et al., 2016, Wolf and Toon, 2015, Kopparapu et al., 2016). On the Earth the distribution of life is influenced by meteorological phenomena. Tropical cyclones (TC), one of the most destructive weather on the Earth, induce heavy precipitation, strong winds, and storm surges (Rappaport, 2000). On the other hand, TC can cause upwelling of nutrient-rich water from deep ocean, which supplies the growth of phytoplankton (Shibano et al., 2011). Thus it is useful to study the frequency and distribution of TC on potentially habitable exoplanets.

Gray (1979) developed an index to describe the seasonal and spatial variability of observed TC, which includes parameters specific to modern climate of the Earth. Inspired by Gray's work, Emanuel and Nolan (2004) developed a more general genesis potential index (GPI). Based on monthly NCEP Reanalysis data from 1950 to 2005 (Kalnay et al., 1996), GPI calculated between 60°S and 60°N (Camago et al., 2007) are in good agreement with observed spatial distribution and seasonal variation of tropical cyclones. In addition, the properties of TC on the Earth in the past and future climate can be projected using general circulation models (GCMs) (Knutson et al., 2010, Lin et al., 2015, Yan et al., 2016).

Although applying a cyclone genesis index developed for the Earth to exoplanets may not reflect realistic cyclone activities on exoplanets, exoplanet climate might be influenced by cyclones through modifications of ocean heat transport (Emanuel, 2001, Jansen et al., 2010). Thus, a theoretical analysis of cyclone genesis could be helpful to better understand exoplanet climate. In this work we apply the GPI analysis to evaluate the probability of TC on ocean exoplanets in the HZ of M dwarfs based on GCM results.

Section snippets

Evaluation of TC genesis

According to Emanuel and Nolan (2004), GPI=|105η|3/2(H50)3(Vpot70)3(1+0.1Vshear)2where η is the absolute vorticity (s−1) at 850 hPa (in this work we use its vertical component), H is the relative humidity at 600 hPa (%), Vshear is the vertical wind shear (m/s) between 850 and 200 hPa, and Vpot is the potential intensity (m/s), a measure of the maximum surface wind which can be sustained by a TC within a certain environment. Vpot can be calculated based on sea surface temperature (SST), sea

Results and discussions

The distributions of maximum GPI in both cases are shown in Fig. 2. The white areas have SST <5 °C (Fig. 1). TC does not occur in cold regions on the Earth and thus there is no calculation at these locations. The zonal symmetry reflects the zero obliquity assumption in the GCM simulations. The regions with high GPI in Fig. 2 indicate the locations with highest possibility of TC genesis, in analog to TC basins on the Earth. The maximum GPI in case 1 is ∼6 in regions near 180° longitude (Fig. 2

Conclusions

In this study, we analyzed the genesis potential index (GPI) on hypothetical ocean planets in the habitable zones of M dwarfs based on 3D GCM simulation results. Planets on both circular and eccentric orbits are studied. In both cases the exoplanet GPI are smaller than that in the South Pacific TC basin on the Earth because of slow planet rotation. Exoplanets on eccentric orbits have much greater maximum GPI than those on circular orbits.

The combination of colder and drier middle atmospheres

Acknowledgements

We thank K. A. Emanuel for providing programs to calculate potential intensity. We thank D. Koll and the other anonymous reviewer for providing constructive comments which improved the quality of this work. J.B. and F.T. are supported by the Tsinghua University Initiative Science Research Program (523001028).

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