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Abstract We investigate a scaling relationship between global tropical cyclone (TC) frequency and the latitude of the intertropical convergence zone (ITCZ) in simulations performed with a 50‐km‐resolution aquaplanet version of the Geophysical Fluid Dynamics Laboratory Atmosphere Model 4.0. The simulations use fixed, zonally symmetric sea surface temperature distributions, including some with uniform warming and cooling perturbations. We find that TC frequency per unit area is proportional to the Coriolis parameter at the ITCZ, following the same scaling introduced in a previous study. We hypothesize that TCs in these simulations originate as precursor disturbances at the ITCZ and intensify into TCs upon reaching sufficiently warm SSTs. We test this interpretation by tracking TC precursors, with different methods based on precipitation and vorticity, and comparing TC precursor frequency with TC frequency and ITCZ latitude. Both tracking methods show that precursors predominantly originate around the poleward edge of the ITCZ, consistent with our hypothesized TC genesis pathway. We also verify that most TC genesis events are immediately preceded by the occurrence of a precursor in the same area. However, precursor frequency is only weakly correlated with the Coriolis parameter at the ITCZ and precursor frequency. The correlation is stronger for vorticity‐based precursors than for precipitation‐based precursors. These mixed results provide partial, but not complete, support for our hypothesized interpretation. They also illustrate how results can depend on the choice of precursor tracking scheme, underlining a need for improved understanding of how best to define and track TC precursors.
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Idealized Aquaplanet Simulations of Tropical Cyclone Activity: Significance of Temperature Gradients, Hadley Circulation, and Zonal Asymmetry
Abstract Earlier studies have proposed many semiempirical relations between climate and tropical cyclone (TC) activity. To explore these relations, this study conducts idealized aquaplanet experiments using both symmetric and asymmetric sea surface temperature (SST) forcings. With zonally symmetric SST forcings that have a maximum at 10°N, reducing meridional SST gradients around an Earth-like reference state leads to a weakening and southward displacement of the intertropical convergence zone. With nearly flat meridional gradients, warm-hemisphere TC numbers increase by nearly 100 times due particularly to elevated high-latitude TC activity. Reduced meridional SST gradients contribute to a poleward expansion of the tropics, which is associated with a poleward migration of the latitudes where TCs form or reach their lifetime maximum intensity. However, these changes cannot be simply attributed to the poleward expansion of Hadley circulation. Introducing zonally asymmetric SST forcings tends to decrease the global TC number. Regional SST warming—prescribed with or without SST cooling at other longitudes—affects local TC activity but does not necessarily increase TC genesis. While regional warming generally suppresses TC activity in remote regions with relatively cold SSTs, one experiment shows a surprisingly large increase of TC genesis. This increase of TC genesis over relatively cold SSTs is related to local tropospheric cooling that reduces static stability near 15°N and vertical wind shear around 25°N. Modeling results are discussed with scaling analyses and have implications for the application of the “convective quasi-equilibrium and weak temperature gradient” framework.
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- Award ID(s):
- 1830729
- PAR ID:
- 10288957
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 78
- Issue:
- 3
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- 877 to 902
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Global tropical cyclone (TC) frequency is investigated in a 50‐km‐resolution aquaplanet model forced by zonally symmetric sea surface temperature (SST). TC frequency per unit area is found to be proportional to the Coriolis parameter at the intertropical convergence zone (ITCZ), as defined by the latitude of maximum precipitation. As the latitude of maximum SST is shifted northward from the equator, the precipitation maximum moves northward and TC frequency increases. When the SST maximum is shifted northward past 25°N, the precipitation maximum remains between 15°N and 20°N, and TC frequency per unit area is approximately constant. When applied to observed precipitation and SST data, the same scaling captures a substantial fraction of observed TCs. Results suggest that future changes in TC activity will be modulated by changes in the large‐scale circulation, and in particular that the ITCZ location is an important determinant of the number of TCs.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JAS-D-20-0079.1
