Tropical Oceanic Influences on Observed Global Tropical Cyclone Frequency

The global tropical cyclone (TC) number has historically been relatively constant from year-to-year, however, the reason remains unknown. Furthermore, climate projections are inconclusive regarding future global TC frequency changes. Here, we investigated years in which observed global TC activity deviated from the mean and potential links to ocean drivers from 1980 to 2021. We found that the annual global number of named storm days and accumulated cyclone energy (ACE) were significantly linked with El Niño–Southern Oscillation (ENSO) and the Atlantic Meridional Mode (AMM). La Niña and positive AMM are associated with the bottom percentiles of both TC metrics, and vice versa for El Niño and negative AMM. The ENSO Longitude Index explains variability in annual global named storm days and ACE as well as the Niño 3.4 index. This research reveals that reliable future projections of ENSO are necessary, but not sufficient, to understand future changes in global TC frequency.

• Patricola CM, Cassidy DJ, Klotzbach PJ (2022) Tropical Oceanic Influences on Observed Global Tropical Cyclone Frequency. Geophysical Research Letters, 49(13), e2022GL099354.

This research was supported by the U.S. Department of Energy Office of Science (BER RGCM program) under Early Career Research Program Award Number DE-SC0021109.

Tropical cyclones and their precursors or "seeds"

Atlantic tropical cyclone (TC) genesis is strongly linked with African easterly waves (AEWs) on the weather time scale. However, the TC-AEW relationship is unclear on interannual to climate time scales, and it is unknown whether AEWs are necessary to maintain climatological TC frequency, that is, whether TCs are limited by AEWs. We investigated the impact of AEW suppression on seasonal Atlantic TC activity using a 10-member ensemble of regional climate model simulations in which AEWs were either prescribed or removed through the lateral boundary condition. The climate model experiments produced no significant change in seasonal Atlantic TC number, indicating that AEWs are not necessary to maintain climatological basin-wide TC frequency even though TCs readily originate from these types of disturbances. This suggests that the specific type of “seedling” disturbance is unimportant for determining basin-wide seasonal Atlantic TC number, and that in the absence of AEWs, TCs will generate by other mechanisms. The results imply that changes in the presence of AEWs may not be reliable predictors of seasonal variability and future change in Atlantic TC frequency.

• Patricola CM, Saravanan R, Chang P (2018) The Response of Atlantic Tropical Cyclones to Suppression of African Easterly Waves. Geophysical Research Letters, 45, 471-479.

This research was supported by the U.S. National Science Foundation and the U.S. Department of Energy Office of Science (BER RGCM program).  High-performance computing resources provided by the Texas Advanced Computing Center (TACC) at The University of Texas at Austin through the Extreme Science and Engineering Discovery Environment (XSEDE) and by the Texas A&M Supercomputing Facility.

We investigated the impact of suppressing AEWs using a 3-member ensemble of convection-permitting regional model simulations, in which AEWs were either retained or removed through the lateral boundary conditions. Suppressing AEWs did not substantially change seasonal TC number, but did influence TC intensity and genesis location. Suppressing AEWs produced stronger TCs and reduced (increased) TC genesis in the eastern Atlantic (Gulf of Mexico). These results provide robust evidence that AEWs may not be reliable predictors of basin-wide seasonal TC frequency.  However, this research suggests that AEWs may influence TC tracks and landfall location.

• Danso, D. K., Patricola, C. M., & Bercos-Hickey, E. (2022). The Influence of African Easterly Wave Suppression on Atlantic Tropical Cyclone Activity in a Convection-Permitting Model.  Geophysical Research Letters, 49, e2022GL100590.

This research was supported by the U.S. Department of Energy Office of Science (BER RGCM program) under Early Career Research Program Award Number DE-SC0021109 and under Award Number DE-AC02-05CH11231.  High-performance computing resources provided by the National Energy Research Scientific Computing Center (NERSC).

ENSO diversity and tropical cyclone variability

El Niño, the occurrence of unusually warm sea-surface temperature over the equatorial East Pacific Cold Tongue, is an important predictor of seasonal Atlantic tropical cyclone activity.  In recent decades El Niño has been characterized more often by Central Pacific Ocean warming, and there is no consensus regarding how this shift in location of ocean warming impacts Atlantic tropical cyclones due to a short data record.  In addition, an increasing trend in the intensity of Central Pacific, or “Warm Pool,” El Niño has been observed recently and projected in the future.  It is unknown how this potential change will impact Atlantic tropical cyclones.  We find, using climate model simulations, that for observed warming intensities characteristic of the top 90^th percentile, Warm Pool El Niño is 50% less effective at suppressing Atlantic tropical cyclones than Cold Tongue El Niño.  However, for the same absolute warming intensity (~2.25°C), Warm Pool El Niño is 50% more effective than Cold Tongue El Niño.  Atlantic tropical cyclones are suppressed regardless of El Niño type, since both are characterized by sufficient warming east of the Pacific warm pool, which satisfies the sea-surface temperature threshold for an eastward migration of deep convection leading to tropical cyclone suppression via tropical Atlantic vertical wind shear enhancements.  This work highlights the necessity to understand how the frequency, location, and intensity of El Niño are expected to change in the future in order to make the best-informed forecasts and projections of Atlantic tropical cyclone activity.

• Patricola CM, Chang P, Saravanan R (2016) Degree of simulated suppression of Atlantic tropical cyclones modulated by flavour of El Niño. Nature Geoscience, 9, 155–160.
• Patricola CM, Camargo SJ, Klotzbach P, Saravanan R, Chang P (2018) The Influence of ENSO Flavors on Western North Pacific Tropical Cyclones. Journal of Climate, 31(14), 5395-5416.

This research was supported by the U.S. National Science Foundation (Grant AGS-1347808).  High-performance computing resources provided by the Texas Advanced Computing Center (TACC) at The University of Texas at Austin through the Extreme Science and Engineering Discovery Environment (XSEDE) and by the Texas A&M Supercomputing Facility.

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