The numerical simulation of seasonal variability of the upper circulation in the Okhotsk Sea

The general circulation of the Okhotsk Sea and its seasonal variability are studied using a three dimensional general circulation model. Model currents agree well with geostrophic currents derived from satellite data (AVISO) except the Kuril Basin and the southern part of the central Okhotsk Sea with dominant eddy activities. The volume transports of the second branch of the East Sakhalin Current, the West-Kamchatka Current, the Middle Current (north branch) and the inflow of the Kuril-Kamchatka Current through the Fourth Strait show same seasonal changes with a maximum in January and a minimum in summer. The West-Kamchatka Current is driven by the northward Sverdrup flow and the inflow through the Fourth Kuril Strait, feeding the Middle Current (north branch). The Middle Current (south branch) is driven by the Sverdrup flow and the inflow through the Kruzenshterna Strait. The inflow through the eastern Kuril straits and outflow through the western Kuril straits intensify the general cyclonic circulation of the Okhotsk Sea in winter as well as the wind and wind stress curl. A dramatic current reversal from a cyclonic to an anticyclonic circulation in the Shelihov Bay is initiated by the start of northeasterly monsoon wind in autumn. The heat and fresh water fluxes are dominant forces to drive the circulation of the northern shelf with the Northern Okhotsk Current and its countercurrent. Tidal forcing effects can be seen in significant reinforcements of cyclonic circulations in the Kuril Basin, Shelihov Bay and northern continental shelf with the Northern Okhotsk Current and its countercurrent

Simulation of Winter Deep Slope Convection in Peter the Great Bay (Japan Sea)

In wintertime, a high-density water forms on the shallow shelf in the vast Peter the Great Bay (Japan Sea). The steep continental slope with deep canyons and cold winters in the area provide suitable conditions for the implementation of deep slope convection—an important phenomenon in the formation of intermediate and bottom waters that occurs at a few locations in some semi-enclosed seas, including the Japan Sea. The descent of dense shelf water down the continental slope of Peter the Great Bay usually occurs to 1000–1200 m; however, in anomalously cold winters, it has been observed at greater than 2000 m depth supporting renewal and deep ventilation of intermediate and bottom waters in the Japan Sea. The deep slope convection is a rare episodic phenomenon with durations ranging from several hours to several days, that has never been simulated in Peter the Great Bay with a realistic numerical model of circulation. We apply the Regional Ocean Modeling System (ROMS) with a 600 m horizontal resolution to simulate the deep slope convection in the anomalously cold winter of 2001 when it has been observed in cruises. The results are compared with propagation of deep shelf water in the regular winter of 2010 when hydrological characteristics of this water were recorded by a profiler “Aqualog” installed at the shelf break. Using Lagrangian methods, we track and analyze the formation of dense shelf water, its advection to the slope edge in the bottom layer and descent down the slope. Special attention is payed to the role of coastal eddies arising due to a symmetric instability. These eddies promote the cross-shelf transport of the dense shelf water towards the continental slope edge. The simulation results are compared with rare observations of the deep slope convection in Peter the Great Bay

Physical and Biological Features of the Waters in the Outer Patagonian Shelf and the Malvinas Current

The aim of this study is to trace how the fine-thermohaline and kinematic structure, formed over a section along 45.8° S in the interaction zone of the outer Patagonian Shelf (PS) and Malvinas (Falkland) Current (MC) System waters, affect the spatial distribution of bio-optical characteristics, phyto/zooplankton, birds, and marine mammals. For the first time, simultaneous multidisciplinary observations at high spatial resolution (~2.5 km) were performed in this region during the cruise of the R/V “Akademic Mstislav Keldysh” in February 2022. A fine structure of alternating upwelling and downwelling zones over the PS and slope was identified, which resulted from the interaction between the MC inshore branch (MCi), bottom topography, and wind. This interaction significantly affects all the physical, and optical characteristics analyzed in the work, as well as the biota of the region. It was found that the euphotic zone is larger in the downwelling zones than in the upwelling zones, and all spatially local maxima of phytoplankton photosynthetic efficiency are observed in the zones between upwelling and downwelling. Phytoplankton along the section were represented by 43 species. A total of 30 zooplankton species/taxa were identified. Three species of marine mammals and 11 species of birds were recorded in the study site. Most of the phytoplankton species list were formed by dinoflagellates, and picoplankton Prasinoderma colonial quantitatively dominated everywhere. Two floristic and three assemblage groups were distinguished among the analyzed phytoplankton communities. High phytoplankton biodiversity was observed above the PS and low above the PS edge and in the MCi core. Copepods mostly dominated in zooplankton. Subantarctic species/taxa of zooplankton concentrated in the nearshore waters of the PS, while Antarctic species/taxa were most abundant in the zone between the MCi and the MC offshore branch (MCo). The relative abundance of birds in the PS was several times higher than in the MCo. The minimum abundance of birds was in the MCi in the zone of the strongest upwelling identified above the PS edge