Abstract:

Based on daily minimum temperature of 66 meteorological stations in Zhejiang province from 1971 to 2022 and projected results stem the BCC−CSM2−MR model, a series of cold events for Zhejiang was constructed using the absolute threshold values and standard deviations. Furthermore, spatiotemporal variability of cold events was investigated by diagnostic and statistical methods, and agricultural freezing risk under the SSP245 and SSP585 scenarios during 2023−2100 was quantitatively assessed in order to effectively support regional climate adaptation and disaster risk management. It was found that during 1971−2022, the annual mean number of cold days, frequency of cold events, and frequency of extreme cold events in Zhejiang all exhibited significant decreasing trends (P<0.01), with decreasing rates of 6.1d·10y⁻¹, 0.6times·10y⁻¹ and 0.2times·10y⁻¹, respectively. There displayed distinct spatial differences for cold days, cold events, and extreme cold events, with showing patterns of "more in the north and less in the south, more in the west and less in the east" "more in the northeast and less in the southwest" and "more in the central−north and less in the southwest", respectively. As for the long-term trends, climatic tendency rates of cold events' frequency in spring and autumn were −0.4times·10y⁻¹ and −0.3times·10y⁻¹ (P<0.01) respectively, while no significant trend was observed in winter. Empirical orthogonal function (EOF) analysis of extreme cold events indicated that the first mode was a province-wide consistent pattern with a variance contribution of 79.7%, and the second, third and fourth modes exhibited the northwest−southeast dipole patterns, near−tripl and north−south dipole, respectively. In the projection results of the BCC−CSM2−MR model, under the SSP245 and SSP585 scenario, the annual average frequency of extreme cold events in Zhejiang during 2023−2100 would be 0.9times and 1.0times with climatic tendency rates of −0.1times·10y⁻¹ and −0.2times·10y⁻¹ (P<0.01), respectively. During the same period, spatial distribution of the cumulative frequency of extreme cold events under SSP245 and SSP585 scenario indicated that a south-north gradient (higher in the south and lower in the north) and an inland−coastal gradient (higher inland and lower along the coast), and the high−values would be located in southwestern Zhejiang. Under the SSP245 scenario, cold events could result in an agricultural freezing−affected rate ≥5% in 19y and a freezing-suffering rate >5% in 2y over Zhejiang in the future 78 years. Especially, the agricultural freezing−affected rate would reach 18.1% and 25.9%, and the freezing−suffering rate would be 6.7% and 11.0% in 2030 and 2066, respectively. Under the scenario of SSP585, there might be 17y with an agricultural freezing−affected >5% and 3y with a freezing−suffering rate >5%. In particular, the agricultural freezing−affected rate would be 24.9% and the freezing−suffering rate would be 11.9% in 2079.

Key words: Cold day, Cold event, Climate Change, Agriculture, CMIP6