Large-scale control on the frequency of tropical cyclones and seeds: a consistent relationship across a hierarchy of glo
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Large‑scale control on the frequency of tropical cyclones and seeds: a consistent relationship across a hierarchy of global atmospheric models Tsung‑Lin Hsieh1 · Gabriel A. Vecchi1,2,3 · Wenchang Yang2 · Isaac M. Held1 · Stephen T. Garner4 Received: 1 May 2020 / Accepted: 23 August 2020 © The Author(s) 2020
Abstract A diagnostic framework is developed to explain the response of tropical cyclones (TCs) to climate in high-resolution global atmospheric models having different complexity of boundary conditions. The framework uses vortex dynamics to identify the large-scale control on the evolution of TC precursors—first non-rotating convective clusters and then weakly rotating seeds. In experiments with perturbed sea surface temperature (SST) and CO2 concentration from the historical values, the response of TCs follows the response of seeds. The distribution of seeds is explained by the distribution of the non-rotating convective clusters multiplied by a probability that they transition to seeds. The distribution of convective clusters is constrained by the large-scale vertical velocity and is verified in aquaplanet experiments with shifting Inter tropical Convergence Zones. The probability of transition to seeds is constrained by the large-scale vorticity via an analytical function, representing the relative importance between vortex stretching and vorticity advection, and is verified in aquaplanet experiments with uniform SST. The consistency between seed and TC responses breaks down substantially when the realistic SST is perturbed such that the spatial gradient is significantly enhanced or reduced. In such cases, the difference between the responses is explained by a change in the ventilation index, which influences the fraction of seeds that develop into TCs. The proposed TC-climate relationship serves as a framework to explain the diversity of TC projection across models and forcing scenarios.
1 Introduction High-resolution global models ( 𝛥x ≤ 50 km in the atmosphere) are indispensable tools to project the frequency of tropical cyclones (TCs) in future climates. The explicitly resolved vortices are often used as proxies for real-world TCs because of the realistic geographic distribution, seasonal cycles and interannual variability (see Walsh et al. 2016 for a review). However, large uncertainties remain in the projections of TC frequency (Walsh et al. 2016; Knutson * Tsung‑Lin Hsieh [email protected] 1
Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA
2
Department of Geosciences, Princeton University, Princeton, NJ, USA
3
Princeton Environmental Institute, Princeton University, Princeton, NJ, USA
4
NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA
et al. 2019), due to inconsistency across models in the projected mean climate as well as the TC-climate relationship (Vecchi et al. 2019; Camargo et al. 2020). The inconsistency of modeled TC-climate relationships arises in part from the fact that the TCs are not wellresolved, so they are much m
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