Changing Windstorm Conditions Over the US Northeast (NE) from a Large Ensemble
A large ensemble generated using the Max-Planck Institute (MPI) Earth System Model is used to generate projections of NE windstorms under different Shared Socioeconomic Pathways (SSPs) and to attribute changes to projected land use land cover (LULC) change, externally forced changes, and internal climate variability. To reduce the influence of coarse grid cell resolution and uncertainties in surface roughness lengths, windstorms are identified using simultaneous widespread exceedance of local 99th percentile 10-meter wind speeds (U99). Projected declines in forest cover in the NE and the resulting reductions in surface roughness length under SSP3-7.0 lead to projections of large increases in U99 and derived windstorm intensity and scale. However, these projected changes in regional LULC under SSP3-7.0 are unprecedented in a historical context and may not be realistic. After corrections are applied to remove the influence of LULC on wind speeds, regionally averaged U99 exhibit declines for most of the single model initial-condition large ensemble (SMILE) members which are broadly proportional to the radiative forcing and global air temperature increase in the SSPs, with a median value of -0.15 ms-1°C-1. While weak cyclones are projected to decline in frequency in the NE, intense cyclones and the resulting windstorms and indices of socioeconomic loss do not. Where present, significant trends in these loss indices are positive, and some MPI SMILE members generate future windstorms that are unprecedented in the historical period.
Extreme windstorms pose a significant hazard to infrastructure and public safety, particularly in the highly populated US Northeast (NE). However, the influence climate change and changing land use will have on these events remains unclear. This work seeks to fill this knowledge gap.
The Northeastern U.S. exhibits a high frequency of damaging windstorms in the contemporary climate and has a high density of population and assets. This research explores future windstorms using a large ensemble and finds:
- Projected changes in NE windstorms are critically determined by assumptions regarding LULC. SSP3-7.0 is widely used in climate science but includes LULC projections for the NE that differ substantially from current landcover and the projected deforestation rate greatly exceeds historical values. After applying corrections for LULC-induced declines in surface roughness, the regionally averaged 99th percentile wind speeds (U99) exhibits declines for most of ensemble members.
- Intense cyclones (those with low central low pressure < 980 hPa) associated with NE windstorms during the historical period are not projected to decline in frequency.
- The loss index (LI) from NE windstorms exhibits a large amount of variability over time and between ensemble members, but the majority of members from the SSP5-8.5 experiment exhibit increases in LI through time, even in the absence of population change.
- Some individual ensemble members yield windstorms that are historically unprecedented in terms of either intensity (maximum wind speed) or spatial scale.
- Analyses of this large ensemble does not support the hypothesis of an increasing role for transitioning tropical cyclones in the generation of NE windstorms.