Tropical cyclones and their precursors
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.
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 90th 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.
Anthropogenic Change in Tropical Cyclones
There is no consensus on whether climate change has yet affected the statistics of tropical cyclones, owing to their large natural variability and the limited period of consistent observations. In addition, projections of future tropical cyclone activity are uncertain, because they often rely on coarse-resolution climate models that parameterize convection and hence have difficulty in directly representing tropical cyclones. Here we used convection-permitting regional climate model simulations to investigate whether and how recent destructive tropical cyclones would change if these events had occurred in pre-industrial and in future climates. We found that, relative to pre-industrial conditions, climate change so far has enhanced the average and extreme rainfall of hurricanes Katrina, Irma and Maria, but did not change tropical cyclone wind-speed intensity. In addition, future anthropogenic warming would robustly increase the wind speed and rainfall of 11 of 13 intense tropical cyclones (of 15 events sampled globally). Additional regional climate model simulations suggest that convective parameterization introduces minimal uncertainty into the sign of projected changes in tropical cyclone intensity and rainfall, which allows us to have confidence in projections from global models with parameterized convection and resolution fine enough to include tropical cyclones.
Patricola CM, Wehner MF (2018) Anthropogenic Influences on Major Tropical Cyclone Events. Nature, 563, 339-346.
This research was supported by the U.S. Department of Energy Office of Science (BER RGCM program). High-performance computing resources provided by the National Energy Research Scientific Computing Center (NERSC).