Scalability of future climate changes across Japan examined with large-ensemble simulations at + 1.5 K, +2 K, and + 4 K global warming levels

Abstract

Large-ensemble climate experiments were performed to simulate future climates for a + 1.5 K rise in the global mean surface air temperatures relative to preindustrial levels as a subset of the database for Policy Decision making for future climate change (d4PDF), using the Non-Hydrostatic Regional Climate Model (NHRCM) with 20 km grid spacing. Along with present climate, + 2 K and + 4 K experimental outputs from the d4PDF already available, we investigated responses of surface air temperature (SAT) and precipitation on regional scales over Japan to global warming. The reproducibility of the present climate experiment is satisfactory to investigate future changes in the Japanese climate, and dynamical downscaling from the global to the regional climate states improves the frequency of heavy daily precipitation. In the future, SAT over Japan rises linearly with and faster than the global mean SAT. The meridional contrast of SAT rises becomes more pronounced as global warming progresses. Winter precipitation decreases/increases linearly in the western/eastern Japan, reflected by weakening of future winter monsoons. Annual maximum daily precipitation (R1d) shows a closely linear increase with the global SAT rise, but annual precipitation is mostly unchanged. The global mean SAT change from + 1.5 to + 2 K enhances R1d by 2.7% over the Japanese Islands, although the increase of R1d varies by regions. The increase in R1d is 5% in northern Japan, where the SAT increases are greater than those in other regions.

d4PDF: large-ensemble and high-resolution climate simulations for global warming risk assessment

“…Higher-resolution regional models perform better in representing heavy precipitations, resolving complex topography, coastal lines, and dynamical process on mesoscales (Giorgi 2019). The model of the 20 km spatial resolution used in d4PDF satisfactorily simulated temperature and precipitation variations on the daily time scale (Nosaka et al 2020;Takayabu and Hibino 2016). In addition, the global model provided NHRCM with good-quality lateral boundary conditions for dynamical downscaling.…”

“…2 and Table 2. The individual SST trends were rescaled by multiplying by a factor (Fujita et al 2019;Mizuta et al 2017;Nosaka et al 2020;Shiogama et al 2010) that forces MRI-AGCM3.2 to simulate the global mean SATs equivalent to each warming level. Future SIC varies consistently with SSTs at geographical locations, which is determined by using quadratic relationships between SST and SIC as used in Hirahara et al (2014).…”

“…The oceanic boundary conditions are the same as in the global model. An error in SIC used by NHRCM was reported by Nosaka et al (2020), that is, no sea ice in the Okhotsk Sea given for all NHRCM simulations. The error Table 2 List of CMIP5 models whose future climate projections are used in d4PDF.…”

“…The model having detailed topography shows SAT and precipitation climatologies (Nosaka et al 2020), snow coverage (Kawase et al 2016), and extreme precipitation (Hatsuzuka and Sato 2019; Osakada and Nakakita 2018b) more realistically than the global model. Meanwhile, seasonal biases unavoidably exist in the NHRCM simulations.…”

“…In addition, scalable relationships help us determine future changes at any warming levels without additional model simulations. Fujita et al (2019) compared future changes in precipitation indices between +2 K and +4 K simulations separately for wet and dry seasons across the globe using d4PDF-G. Nosaka et al (2020) investigated the scalability of various aspects in future regional climate changes using simulation results at four warming levels of d4PDF-R. In many cases, future changes of climatological states and extremes have scalability.…”