Co-Occurrence of Marine Extremes Induced by Tropical Storms and an Ocean Eddy in Summer 2016: Anomalous Hydrographic Conditions in the Pacific Shelf Waters off Southeast Hokkaido, Japan

Abstract

This study proposes an analysis methodology to address how very rare marine extremes can be understood using limited data. In summer 2016, extreme weather and marine events occurred simultaneously around the Pacific shelf off southeastern Hokkaido, Japan. Six successive tropical storms brought extreme precipitation and an anticyclonic mesoscale eddy of subtropical Kuroshio water closely approached the coast, locally causing marine heat waves. We examined how these compound extremes affected oceanographic conditions on the coastal shelf by analyzing data from ship surveys in October 2016 on the Pacific shelf and outputs from a realistic ocean model. Climatologically, warm, high-salinity (33.0–33.7) subtropical water from the Okhotsk Sea (modified Soya Warm Current water) is distributed near the sea surface on the Pacific shelf in October and transported by the along-shelf boundary current. In 2016, however, a vertically well-mixed low-salinity (<33.0) layer associated with the heavy rainfall was observed at 0–50 m depth on the shelf, salinity maxima (≥33.7) associated with Kuroshio water from the mesoscale eddy occurred at 50–150 m depth on the slope, and baroclinic jets formed along the salinity front near the shelfbreak. These observed salinity structures were reproduced by a 1/50° ocean model. Particle-tracking experiments revealed that the low-salinity water originated mainly off eastern Hokkaido, where heavy rainfall events occurred in August, and was modified by mixing with Soya Warm Current water before transport to the Pacific shelf.

Unprecedented Outbreak of Harmful Algae in Pacific Coastal Waters off Southeast Hokkaido, Japan, during Late Summer 2021 after Record-Breaking Marine Heatwaves

“…It is here emphasized that sea surface salinities simulated by the 1/50° ocean model had low-salinity bias of about 0.2 in autumn around the focal region (e.g., [25]), which was reconfirmed by comparison with in situ sea surface salinities along the A-line (e.g., [22]) that were measured on 4-10 October 2021 (not shown).…”

“…Meanwhile, subtropical, oligotrophic waters are transported from the subtropics to the study region, mainly through two pathways (Figure 1). Soya Warm Current water is supplied seasonally from the Sea of Okhotsk during summer-autumn by a downstream extension of the Soya Warm Current [23][24][25]. This water originates in the Kuroshio, which bifurcates south of Kyushu, intrudes into the Sea of Japan, flows northward as the Tsushima Warm Current, and reaches the Sea of Okhotsk.…”

“…Most of the configurations of the 1/50 • ocean model were identical to those used by Kuroda et al [25,33], except for the model domain, which was extended to include the whole of the Japanese main island (light blue square in Figure 2) [34]. As shown by Kuroda et al [25,33], the overall structures of oceanographic conditions-particularly temperature, salinity, and current velocity near the sea surface around Pacific coastal shelf waters off the south-eastern coast of Hokkaido-were reasonably reproduced.…”

“…Because the domain of the 1/50 • model was limited to the vicinity of Japan (Figure 2), daily mean reanalysis data from the 1/10 • FRA-ROMS were combined with daily mean outputs from the 1/50 • model to perform particle-tracking simulations over a wide area (i.e., green dashed box in Figure 2). For the particle-tracking simulations, we used the Larval TRANSport (LTRANS) model [45], which is configured for ROMS output, but the source codes were modified to suit our model's configuration and purpose [25,46,47].…”

Extensive Marine Heatwaves at the Sea Surface in the Northwestern Pacific Ocean in Summer 2021

“…A MHW known as "the blob," which occurred persistently in 2014-2016 in the northeastern Pacific, is especially famous (e.g., [16]). In contrast, a search for studies conducted in the northwestern Pacific, particularly east of Japan, with the keyword "MHW," yielded only three scientific studies [17][18][19]: one conducted in subtropical waters [17] and the others in subarctic waters [18,19]. Miyama et al [18] reported the occurrence of MHWs in summer during 2010-2016 near Hokkaido, northern Japan, which they attributed to warm mesoscale eddies that had become detached from the subtropical Kuroshio Extension and had been advected into the subarctic Oyashio region every summer of that period.…”