CliMAS Colloquia

Kaylie Bonifer:

Sarah Henry:

Anthropogenic Forcing of Historical and Projected Tropical Cyclone Activity

Limited observations have made it challenging to attribute changes in tropical cyclone (TC) activity to anthropogenic influences. Here, we use NeuralGCM, a hybrid machine learning general circulation model to perform ensemble attribution experiments from 2004 to 2023. The factual simulations are prescribed with ERA5 reanalysis sea surface temperature (SST) and sea ice concentration (SIC), while the counterfactual simulations are driven by SST and SIC with the estimated anthropogenic influence removed, representing a climate state (including climate variability) without anthropogenic influences. Compared to the counterfactual experiment, the factual experiment shows a significant decrease in TC frequency and lifetime in the North Atlantic, eastern North Pacific, and western North Pacific basins. To evaluate whether anthropogenic signals in TC activity are detectable in the observational record, the factual simulations are extended back to 1940. TC activity in the historical simulations does not reveal a trend consistent with the changes between the counterfactual and factual experiments. The advantages and limitations of the NeuralGCM are also discussed.

To better understand the effects of anthropogenic forcings on tropical cyclone (TC) activity, we investigate the individual impacts of anthropogenic aerosols (AAER), greenhouse gases (GHG), and biomass burning aerosols (BMB) on TC frequency using CESM2 large ensemble simulations. Changes in TC genesis are estimated using a dynamic genesis potential index (DGPI), which reveals distinct impacts from AAER and GHG. Increasing GHG emissions result in a decrease and southward shift in DGPI over the North Atlantic and the eastern North Pacific since 1940, whereas the time-dependent AAER emissions lead to a decrease and southward shift in DGPI before 1980s and an increase and northward shift thereafter. In addition, BMB emission increases lead to a slight increase in TC frequency in the North Atlantic. Various dynamic and thermodynamic parameters act cooperatively to change TC frequency associated with GHG and AAER over the North Atlantic, while the increase in DGPI associated with BMB is mainly due to the changes in dynamic variables. AAER effects largely dominate DGPI changes in earlier decades, but GHG effects become increasingly dominant in recent decades. Near-future projections, characterized by continued increases in GHG emissions and steady or declined AAER emissions, suggest decreased DGPI over the North Atlantic and eastern North Pacific driven primarily by GHG forcing. A comparison between DGPI and a traditional GPI shows similar increases or decreases in TC genesis potential with quantitative differences. However, comparisons between CESM2, CESM1, and six CMIP6 models show considerable inter-model spread, highlighting model uncertainties and the need for caution when applying and interpreting multi-model ensemble means.