PGW projections of the returned 1960s U.S. Northeast drought and sensitivity examinations of PGW methods

As the most severe drought over the Northeastern United States (NEUS) in the past century, the 1960s drought had pronounced socioeconomic impacts. Although a persistent wet period followed, the conditions driving the 1960s extreme drought could return in the future, along with its challenges to water management. To project the potential consequences of such a future drought, pseudo-global warming (PGW) simulations using the Weather Research and Forecasting Model are performed to simulate the dynamical conditions of the historical 1960s drought, but with modified thermodynamic conditions under the shared socioeconomic pathway SSP585 scenario in the early (2021-2027), middle (2041-2047) and late (2091-2097) 21st century. Our analysis focuses on essential hydroclimatic variables including temperature, precipitation, evapotranspiration, soil moisture, snowpack and surface runoff. In contrast to the historical 1960s drought, similar dynamical conditions will generally produce more precipitation, increased soil moisture and evapotranspiration, and reduced snowpack. However, we also find that although wet months do become much wetter, dry months also may become drier, meaning that wetting trends that are significant in wet months can be essentially negligible for extremely dry months. For these months, the trend towards wetter conditions provides little relief from drying. These conditions may even aggravate water shortages due to an increasingly rapid transition from wet to dry conditions. Other challenges emerge for residents and stakeholders in this region, including more extreme hot days, record-low snow pack, frozen ground degradation and subsequent decreases in surface runoff.

Although the PGW approach pursued in this study is analogous to other recent studies, there is also a pressing need to ascertain confidence in projections using the PGW method.  Most PGW studies only modify the temperature forcing since it is the most significant for driving impacts on climate, but other meteorological forcings may also impact regional climate trends. For example, the large geopotential height increments at higher atmosphere levels tend to increase the stability and weaken the precipitation events associated with typhoons.  PGW studies usually only consider the changes at the regional mean scale but ignore spatially-dependent contributions from climate change. Therefore, in order to investigate the sensitivity of PGW-based projections, additional simulations were conducted under the RCP8.5 emission scenario but with different forcing modification methods.  We thus answer three questions: Are PGW simulations sensitive to the spatial scale of climate perturbations? Besides temperature, which climatological variables are crucial to PGW simulations? And finally, how should researchers design and conduct their PGW simulations?