Michael Yu. Uleysky

Lagrangian study of temporal changes of a surface flow through the Kamchatka Strait

Using Lagrangian methods, we analyze a 20-year-long estimate of water flux through the Kamchatka Strait in the northern North Pacific based on AVISO velocity field. It sheds new light on the flux pattern and its variability on annual and monthly time scales. Strong seasonality in surface outflow through the strait could be explained by temporal changes in the wind stress over the northern and western Bering Sea slopes. Interannual changes in a surface outflow through the Kamchatka Strait correlate significantly with the Near Strait inflow and Bering Strait outflow. Enhanced westward surface flow of the Alaskan Stream across the 174°E section in the northern North Pacific is accompanied by an increased inflow into the Bering Sea through the Near Strait. In summer, the surface flow pattern in the Kamchatka Strait is determined by the passage of anticyclonic and cyclonic mesoscale eddies. The wind stress over the Bering basin in winter–spring is responsible for eddy generation in the region.

Specific Poincaré map for a randomly-perturbed nonlinear oscillator

The motion of a classical Hamiltonian oscillator, driven by a weak random force, is examined by means of a deterministic approach. We design the specific Poincarmap to find domains of finite-time stability in phase space. The trajectories belonging to these domains remain stable by Lyapunov criteria for the period of mapping T0 at least. We derive the lower border for the time of phase correlations between close trajectories. It is found that the lifetime of some stable domains significantly exceeds the correlation time of the external force. The randomly-driven Morse oscillator is used as an example.

Mesoscale circulation along the Sakhalin Island eastern coast

The seasonal and interannual variability of mesoscale circulation along the eastern coast of the Sakhalin Island in the Okhotsk Sea is investigated using the AVISO velocity field and oceanographic data for the period from 1993 to 2016. It is found that mesoscale cyclones with the horizontal dimension of about 100 km occur there predominantly during summer, whereas anticyclones occur predominantly during fall and winter. The cyclones are generated due to a coastal upwelling forced by northward winds and the positive wind stress curl along the Sakhalin coast. The anticyclones are formed due to an inflow of low-salinity Amur River waters from the Sakhalin Gulf intensified by southward winds and the negative wind stress curl in the cold season. The mesoscale cyclones support the high biological productivity at the eastern Sakhalin shelf in July– August.

Lagrangian study of transport of subarctic water across the Subpolar Front in the Japan Sea

The southward near-surface transport of transformed subarctic water across the Subpolar Front in the Japan Sea is simulated and analyzed based on altimeter data from January 1, 1993 to December 31, 2017. Computing Lagrangian indicators for a large number of synthetic particles, advected by the AVISO velocity field, we find preferred transport pathways across the Subpolar Front. The southward transport occurs mainly in the central part of the frontal zone due to suitable dispositions of mesoscale eddies promoting propagation of subarctic water to the south. It is documented with the help of Lagrangian origin and L-maps and verified by the tracks of available drifters. The transport of transformed subarctic water to the south is compared with the transport of transformed subtropical water to the north simulated by Prants et al. (Nonlinear Process Geophys 24(1):89–99, 2017c).

Mesoscale dynamics and walleye pollock catches in the Navarin Canyon area of the Bering Sea

Large canyons incise the shelf break of the eastern Bering Sea to be preferred sites of the cross-shelf exchange. The mesoscale eddy activity is particularly strong near the shelf-break canyons. To study the mesoscale dynamics in the Navarin Canyon area of the Bering Sea, the time series of velocities derived from AVISO satellite altimetry between 1993 and 2015, drifters, Argo buoys, and ship-borne data are analyzed. We demonstrate that the strength of anticyclonic eddies along the shelf edge in spring and summer is determined by the wind stress in March–April. The increased southward wind stress in the central Bering Sea forced a supply of low-temperature and low-salinity outer shelf water to the deep basin and formation of the anticyclonic mesoscale circulation seaward of the Navarin Canyon. Enhanced northwestward advection of the Bering Slope Current water leads to increase in an ice-free area in March and April and increased bottom-layer temperature at the outer shelf. The strong (weak) northwestward advection of the eastern Bering Sea waters, determined by eastern winds in spring, creates favorable (unfavorable) conditions for the pollock abundance in the western Navarin Canyon area in summer.

Lagrangian Tools to Study Transport and Mixing in the Ocean

The chapter introduces the procedure of simulation of large-scale transport and mixing in the real ocean which is used in the rest of the book. A large number of synthetic passive particles (tracers) are advected in satellite-derived or numerically generated velocity fields which are interpolated in space and time. Along with their trajectories, we compute a number of particle’s Lagrangian indicators which are trajectory’s functions containing information about the origin, history, and fate of water masses. The outputs are processed, analyzed, and represented as plots and Lagrangian maps in the geographical coordinates. In this chapter we introduce the important notion of hyperbolicity in the ocean and describe in detail our method for computing finite-time Lyapunov exponents based on a singular-value decomposition of the evolution matrix. The last section describes briefly the notion of Lagrangian coherent structures (LCS) and their role in organizing large-scale transport and mixing in the ocean.

Transport of Subtropical Waters in the Japan Sea

The Japan Sea is often called as the ocean in a miniature because it has many common features with the deep ocean. Transport of subtropical waters in the Japan Sea is simulated and analyzed in this chapter based on altimeter data for the period from 1993 to 2015. Computing Lagrangian indicators for synthetic tracers launched weekly for this period in the southern part of the Sea, we find preferred transport pathways across the Subpolar Front. This transport is shown to be meridionally inhomogeneous with “gates” and “barriers” whose locations are determined by the local advection velocity field. The gates “open” due to suitable dispositions of mesoscale frontal eddies facilitating propagation of subtropical waters to the north. It is documented for the western, central, and eastern gates with the help of different kinds of Lagrangian maps and validated by tracks of available drifters. The transport through the gates occurs by a portion-like manner, i.e., subtropical tracers pass the gates in specific places and during specific time intervals. “Forbidden” zones have been surprisingly found in the frontal area where the northward transport has not been observed during all the observation period. They exist due to long-term peculiarities of the advection velocity field there.

Oceans from the Space and Operational Oceanography

Satellites monitor now the global ocean in near real time and at high space resolution. The chapter starts with a brief survey of those aspects of satellite and operational oceanography which are extensively used in the book. The main attention is paid to satellite altimetry and its product, the near-surface velocity field provided by the Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) data distribution center. We also review briefly other satellite products and buoy programs.

Fukushima-Derived Cesium Isotopes in the North Western Pacific: Direct Observation and Altimetry-Based Simulation of Propagation

On March 11, 2011 tsunami caused a heavy damage to the Fukushima Nuclear Power Plant (FNPP). In this chapter we apply the Lagrangian approach to simulate propagation of Fukushima-derived radionuclides advected by altimetric velocity field in the North Western Pacific. The results of simulation are compared with in situ measurements of levels of134Cs and137Cs concentrations. In the first section we focus on simulation just after the accident and in situ measurements in research vessel cruises in June and July 2011 (Kaeriyama et al. Biogeosciences 10(2):4287–4295, 2013; Buesseler et al., Proc Natl Acad Sci 109(16):5984–5988, 2012). Backward-in-time Lagrangian techniques are used to find out the origin and pathways of water samples with measured high levels of radioactivity. The altimetry-based simulation provides the evidence of near-surface transport of radioactive contamination across the Kuroshio Extension jet just after the accident due to pinching off rings from the jet. In the second section we present the results of in situ measurements of134Cs and137Cs at different depths in a broad area in June and July 2012. It was found that 15 month after the incident concentrations of radiocesium in the Japan and Okhotsk seas were at background or slightly increased level, while they had increased values in the subarctic frontal area east off Japan. The highest concentrations have been found to exceed ten times the background levels before the accident. Maximal content of radiocesium was observed within subsurface and intermediate water layers inside the cores of mesoscale ACEs. Convergence and subduction of surface water inside eddies are main reasons of downward transport of radionuclides. Different Lagrangian diagnostics are used to reconstruct the pathways and origin of synthetic tracers imitating measured seawater samples collected in each of those eddies. The results of observations are consistent with the simulated results. It is shown that the tracers, simulating water samples with increased radioactivity measured in the cruise, really visited the areas with presumably high level of contamination. Fast water advection between ACEs and convergence of surface water inside eddies make them responsible for spreading, accumulation, and downward transport of cesium rich water to the intermediate depth in the frontal zone.

Lagrangian Fronts and Coherent Structures Favorable for Fishery and Foraging Strategy of Top Marine Predators

Lagrangian fronts (LFs) in the ocean are defined as boundaries between surface waters with strongly different values of Lagrangian indicators. They can be accurately detected in a given velocity field by computing different Lagrangian indicators for synthetic tracers. We report here our results on connection of the LFs with fishing grounds and catch locations. Imposing on the altimetry-based Lagrangian maps available catch data on Pacific saury and neon flying squid in the region of the North Western Pacific with one of the richest fisheries in the world, it is shown that the LFs could serve as a new indicator for potential fishing grounds. It is shown statistically that the catch locations are not randomly distributed over the region but concentrated mainly along the strong LFs where productive cold waters of the Oyashio Current, warmer waters of the southern branch of the Soya Current, and waters of warm-core Kuroshio eddies converge. Electronic tagging and tracking of marine animals and seabirds provides a new source of information on their foraging behavior and its relationship with the marine environment. It is discussed how some top marine predators as great frigates, elephant seals, and Mediterranean fin whales could be able to track somehow mesoscale and submesoscale Lagrangian coherent structures (LCSs) and use them in the foraging strategy and to feed on. Possible biophysical reasons for aggregation of food near strong LFs and LCSs are discussed.