Reply on RC2

Edgar Dolores Tesillos

doi:10.5194/wcd-2021-73-ac2

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Strong low-level winds associated with extratropical cyclones can have substantial impacts on society. The wind intensity and the spatial distribution of wind maxima may change in a warming climate; however, the involved changes in cyclone structure and dynamics are not entirely clear. Here, such structural changes of strong North Atlantic cyclones in a warmer climate close to the end of the current century are investigated with storm-relative composites based on Community Earth System Model Large Ensemble (CESM-LE) simulations. Furthermore, a piecewise potential vorticity inversion is applied to associate such changes in low-level winds to changes in potential vorticity (PV) anomalies at different levels. Projected changes in cyclone intensity are generally rather small. However, using cyclone-relative composites, we identify an extended wind footprint southeast of the center of strong cyclones, where the wind speed tends to intensify in a warmer climate. Both an amplified low-level PV anomaly driven by enhanced diabatic heating and a dipole change in upper-level PV anomalies contribute to this wind intensification. On the contrary, wind changes associated with lower- and upper-level PV anomalies mostly compensate for each other upstream of the cyclone center. Wind changes at upper levels are dominated by changes in upper-level PV anomalies and the background flow. Altogether, our results indicate that a complex interaction of enhanced diabatic heating and altered non-linear upper-tropospheric wave dynamics shape future changes in near-surface winds in North Atlantic cyclones.

Highlights

Extratropical or mid-latiude cyclones strongly modulate weather and climate in the North Atlantic region
Our results indicate that a complex interaction of enhanced diabatic heating and altered non-linear upper-tropospheric wave dynamics shape future changes in near-surface winds in North Atlantic cyclones
In order to provide an overview of the simulated cyclone climatology and put the following, more specific results in context, this section describes the climatology of cyclones in the CESM-LENS simulations in comparison with ERA-Interim data

Summary

Introduction

Extratropical or mid-latiude cyclones strongly modulate weather and climate in the North Atlantic region. While future changes in the North Pacific and, in particular, the Southern Ocean, are, to first order, characterized by a poleward shift of storm tracks (O’Gorman, 2010), the pattern of projected changes in the North Atlantic region is more complex (Zappa et al, 2013) Such changes in the spatial distribution and intensity of cyclones are thought to result from a “tug of war” (Shaw et al, 2016) between different processes, such as changes in horizontal temperature gradients and baroclinicity, vertical stability, tropopause height and latent heat release (O’Gorman and Schneider, 2008; Pfahl et al, 2015). Tamarin and Kaspi (2017) and Tamarin-Brodsky and Kaspi (2017), using PV inversion, showed that such diabatically induced low-level PV increases contribute to an enhanced poleward motion of cyclones as the climate warms It is not clear how (diabatic) PV changes link to structural changes in other 60 impact-relevant cyclone properties such as low-level wind velocity. To assess the ability of CESM-LENS to simulate observed cyclone track density and statistics, the historical simulations (1990–2000) are compared with the ERA-Interim reanalysis (Dee et al, 2011) for the period 1979-2010

Cyclone identification

Composite analysis

Piecewise PV inversion

Storm tracks

Composites of intense extratropical cyclones

PPVI to extratropical cyclone extremes

Conclusions