Characteristics of Explosive Cyclones over the Northern Pacific

Abstract

Explosive cyclones (ECs) over the northern Pacific Ocean during the cold season (October–April) over a 15-yr (2000–15) period are analyzed by using the Final (FNL) Analysis data provided by the National Centers for Environmental Prediction. These ECs are stratified into four categories according to their intensity: weak, moderate, strong, and super ECs. In addition, according to the spatial distribution of their maximum-deepening-rate positions, ECs are further classified into five regions: the Japan–Okhotsk Sea (JOS), the northwestern Pacific (NWP), the west-central Pacific (WCP), the east-central Pacific (ECP), and the northeastern Pacific (NEP). The occurrence frequency of ECs shows evident seasonal variations for the various regions over the northern Pacific. NWP ECs frequently occur in winter and early spring, WCP and ECP ECs frequently occur in winter, and JOS and NEP ECs mainly occur in autumn and early spring. The occurrence frequency, averaged maximum deepening rate, and developing and explosive-developing lifetimes of ECs decrease eastward over the northern Pacific, excluding JOS ECs, consistent with the climatological intensity distributions of the upper-level jet stream, midlevel positive vorticity, and low-level baroclinicity. On the seasonal scale, the occurrence frequency and spatial distribution of ECs are highly correlated with the intensity and position of the upper-level jet stream, respectively, and also with those of midlevel positive vorticity and low-level baroclinicity. Over the northwestern Pacific, the warm ocean surface also contributes to the rapid development of ECs. The composite analysis indicates that the large-scale atmospheric environment for NWP and NEP ECs shows significant differences from that for the 15-yr cold-season average. The southwesterly anomalies of the upper-level jet stream and positive anomalies of midlevel vorticity favor the prevalence of NWP and NEP ECs.

Physical Process Contributions to the Development of a Super Explosive Cyclone Over the Gulf Stream

“…The EC definition modified by Zhang et al (2017) is used in this study and the deepening rate (DR in Bergeron) of a cyclone SLP can be calculated using the following formula: DR P t−6 − P t+6 12 × sin 45 °sin…”

“…Sanders and Gyakum (1980) defined the explosive cyclone (EC) as the one with the central SLP drops more than 24 hPa within 24 h when adjusted geostrophically to 60 °N. The latitude adjustment is performed to account for the variation of the geostrophic wind with latitude for a given pressure gradient. Given the facts that ECs usually occur in the mid-latitude and that the high-resolution reanalysis data are widely used, Zhang et al (2017) adjusted the latitude to 45 °N and set the pressure fall to 12 hPa/12 h. The one "Bergeron" of a 24 hPa/24 h at 60 °N in Sanders and Gyakum (1980) was modified into a 12 hPa/12 h at 45 °N. In addition, these ECs were classified into four categories using K-means clustering algorithm based on the maximum deepening rate: Super (≥2.30 Bergeron), strong (1.70-2.29 Bergeron), moderate (1.30-1.69 Bergeron) and weak (1.00-1.29 Bergeron).…”

“…In the Northern Hemisphere, ECs occurred frequently over the Northwestern Pacific and the Northwestern Atlantic (Rogers and Bosart, 1986;Lim and Simmonds, 2002;Iwao et al, 2012;Zhang et al, 2017;Fu et al, 2020), especially over the Gulf Stream and Kuroshio/Kuroshio Extension regions (Roebber, 1984;Gyakum et al, 1989;Wang and Rogers, 2001;Allen et al, 2010;Zwiers, 2016a, 2016b). The Gulf Stream and Kuroshio/ Kuroshio Extension supply abundant moisture and turbulent (sensible and latent) heat fluxes, which favor the rapid development of ECs through increasing the latent heat release and decreasing atmospheric stability (Nuss and Kamikawa 1990;Kuo et al, 1991b;Reed et al, 1993b;Takayabu et al, 1996;Booth et al, 2012).…”

Surface wind and vertical extent features of the explosive cyclones in the Northern Hemisphere based on the ERA‐I reanalysis data

“…The Sea level pressure (SLP) and geopotential height at the pressure levels from 1,000 to 200 hPa are used to identify ECs. Similar to the region used in Ding et al (1998), Allen et al (2010), and Zhang et al (2017), we focus only on the ECs that form in the Northern Hemisphere within the range of 25 -80 N, 0 -360 . In order to exclude tropical cyclones, all ECs identified in this study were confirmed to form north of 25 N (i.e., they are not generated in tropical regions) and have intense lower-level temperature gradients (i.e., their baroclinity was strong).…”

“…Iwao et al (2012) showed that there was an increasing trend in the numbers of ECs over the northwest Pacific from 1979 to 2010 based on a manually verified objective tracking algorithm using the 6-hourly and 1.25 × 1.25 JRA-25 reanalysis data. Zhang et al (2017) used NCEP-FNL data to analyse ECs over the North Atlantic during a 15-coldseason period, and showed that ECs' occurrence frequency was highly correlated with the intensity of lower-level baroclinity. On the basis of a 34-extended-winter statistical analysis (from 1979 to 2012) using ERA-I reanalysis data, Hart et al (2017) found that ECs were a main cause of cyclone-related strong wind events over the British Isles.…”