Yasushi Fukamachi

A numerical investigation of jets and eddies near an eastern ocean boundary

The dynamics of the circulation near an eastern-ocean boundary are investigated using a 2½-layer numerical model that includes entrainment of cool water into the upper layer. Solutions are found in a regional ocean basin and are forced by an upwelling-favorable, alongshore wind field without curl. Dynamically simpler versions of the model are also utilized in order to isolate the effects of various physical processes; in particular, a linearized instability model is used to study the unstable waves associated with various background coastal circulations. The model is quite successful in simulating many features of the observed flow and SST fields. Initially, the “main run” solution spins up as in a linear model, generating a surface jet, an undercurrent, and an upwelling front. Subsequently, small-scale, “fingerlike” disturbances appear along the coast and grow rapidly in amplitude and scale. The growth in scale has two causes: the slower development of larger-scale disturbances, and eddy coalescence via nonlinear interactions. Eventually, the solution adjusts to a realistic equilibrium state that contains upwelling filaments, squirts, eddies, dipole eddy pairs, and a realistic mean circulation. Entrainment is a crucial process in the dynamics of the mean flow. In particular, entrainment damps the westward propagation of Rossby waves, thereby ensuring that the currents remain coastally trapped. (In this aspect entrainment plays a dynamical role similar to vertical mixing in continuously stratified models.) Eddies influence the mean flow primarily by providing a strong heat source near the coast, but they have very little effect on the mean momentum field. The small-scale, fingerlike disturbances are caused by a frontal instability that requires the existence of an upper-layer temperature gradient. Some dynamically simpler solutions indicate that the larger-scale disturbances are caused by baroclinic instability. On the other hand, unstable-wave solutions to the linearized instability model suggest that their cause is frontal instability, becoming pure baroclinic instability only when there is no temperature gradient.

Structure and Seasonal Variability of the East Sakhalin Current

Abstract In order to clarify the structure and seasonal variability of the flow field near the western boundary of the Sea of Okhotsk, long-term mooring measurements were carried out from 1998 to 2000 in this region. In most of the mooring period a persistent southward flow (the East Sakhalin Current) was observed, which extends from the surface to a depth around 1000 m. The speed of this southward flow clearly changed seasonally. The peak monthly mean speed along 53°N at a depth of 200 m attained a maximum of 37 ± 9 cm s−1 in January and a minimum of 10 ± 8 cm s−1 in July. Three different cores of intense flow were identified in the southward flow. The first core was centered over the continental slope and had rather large vertical extent, reaching the bottom on the slope. The second core was trapped over the shelf near the surface and was observed from October to November. This core was associated with less saline surface water affected by the Amur River discharge. The third core was intensified toward ...

Sverdrup Balance and the Cyclonic Gyre in the Sea of Okhotsk

Abstract It is proposed that the cyclonic gyre over the northern half-basin of the Okhotsk Sea is driven by the wind stress curl and that a major part of the East Sakhalin Current (ESC) can be regarded as its western boundary current. Both from the high-resolution ECMWF and Comprehensive Ocean–Atmosphere Dataset (COADS) data, the annual mean wind stress curl is positive over the sea. When the Sverdrup streamfunction is calculated by excluding the shallow shelves, the streamfunction shows a cyclonic pattern over the central basin, which is roughly consistent with the geopotential anomaly distribution from all the available hydrographic data. Profiling floats suggest that the cyclonic gyre extends to at least a depth of 500 m: a relatively intense southward flow (ESC) with an average speed of approximately 10 cm s−1 near the western boundary and slow northward flow with an average speed of approximately 2 cm s−1 in the east. Climatological data show that along zonal sections at 50°–53°N isopycnal surfaces g...

Instability of Density Fronts in Layer and Continuously Stratified Models

The instability of density fronts is investigated as a possible generation mechanism for the small-scale, wavelike patterns that are commonly observed along up welling fronts and filaments. Unstable-wave solutions are obtained in two linearized models: a 1½-layer model, and a continuously stratified model confined to the surface region of the ocean. In both systems the thickness of the upper region is held constant for the background state, the front being specified by allowing the temperature field T within the region to vary zonally. The background state in the layer model consists of vertically oriented isotherms associated with a depth-independent current, whereas in the continuously stratified model it consists of steeply tilted isotherms and a vertically sheared current. Solutions are found both when the background velocity field V is zonally uniform and when it is zonally sheared. When V is weak and zonally uniform, approximate solutions are derived analytically for both models that are valid for low-frequency, low-wavenumber waves. These solutions demonstrate that the unstable waves in the two systems are dynamically related, both being representations of ageostrophic baroclinic instability. Numerical solutions corroborate the analytic results and extend their range of validity. Energetics analyses confirm that the energy source for the waves is the background potential energy associated with the zonally varying T field. When V is a zonally sheared jet, the models still exhibit a band of instability, which is identifiable with ageostrophic baroclinic instability. The most unstable wave in this band has a short wavelength, a frequency near ƒ/2, and a rapid growth rate consistent with observed features. The layer model also has a band of larger-scale waves that is a mixed, baroclinic-barotropic instability; however, for a typical frontal structure this band is weaker than the baroclinic band.

Winter oceanographic conditions in the southwestern part of the Okhotsk Sea and their relation to sea ice

In the southwestern part of the Okhotsk Sea, oceanographic and sea-ice observations on board the icebreaker Soya were carried out in February 1997. A mixed layer of uniform temperature nearly at the freezing point extending down to a depth of about 300 m was observed. This is much deeper than has previously been reported. It is suggested that this deep mixed layer originated from the north (off East Sakhalin), being advected along the shelf slope via the East Sakhalin Current, accompanied with the thick first-year ice (average thickness 0.6 m). This vertically uniform winter water, through mixing with the surrounding water, makes the surface water more saline (losing a characteristic of East Sakhalin Current Water) and the water in the 100–300 m depth zone less saline, colder, and richer in oxygen (a characteristic of the intermediate Okhotsk Sea water). The oceanographic structure and a heat budget analysis suggest that new ice zone, which often appears at ice edges, can be formed through preconditioning of thick ice advection and subsequent cooling by the latent heat release due to its melting.

Seasonal variability of bottom water properties off Adélie Land, Antarctica

The region off Adelie Land is considered as one of the sources of Antarctic Bottom Water. Hydrographic observations were carried out during two cruises in December 1994 and January 1995 and January and February 1996 in this region. Vertical sections along 140°E show that bottom water is colder and fresher than the water above. This bottom water also has higher dissolved oxygen and lower silicate concentrations. The saline bottom water that originated from Ross Sea is not found in these hydrographic data obtained west of 142°E. Current meter moorings were also carried out at three locations on the continental slope in this region. At one of these moorings (139°59′E, 65°10′S, 2665 m deep), data were successfully obtained from January 1995 to March 1996. Three current meters were deployed at 1075, 1778, and 2632 m deep in this mooring. The data show that the average current speed at the lower current meter is 16.2 cm s−1, and it is about 3 times larger than those at the upper two current meters. Also, variability of speed and temperature is largest at the lower current meter. In addition, seasonal variability of speed and temperature is evident only at this current meter. From August to December, speed is larger by 5.7 cm s−1 and temperature is lower by 0.27°C. Also, their variability is larger during the same period. This seasonal variability observed near the bottom suggests seasonal variability of bottom water formation in this region.

The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay

A fourth production region for the globally important Antarctic bottom water has been attributed to dense shelf water formation in the Cape Darnley Polynya, adjoining Prydz Bay in East Antarctica. Here we show new observations from CTD-instrumented elephant seals in 2011–2013 that provide the first complete assessment of dense shelf water formation in Prydz Bay. After a complex evolution involving opposing contributions from three polynyas (positive) and two ice shelves (negative), dense shelf water (salinity 34.65–34.7) is exported through Prydz Channel. This provides a distinct, relatively fresh contribution to Cape Darnley bottom water. Elsewhere, dense water formation is hindered by the freshwater input from the Amery and West Ice Shelves into the Prydz Bay Gyre. This study highlights the susceptibility of Antarctic bottom water to increased freshwater input from the enhanced melting of ice shelves, and ultimately the potential collapse of Antarctic bottom water formation in a warming climate.

Taking a look at both sides of the ice: comparison of ice thickness and drift speed as observed from moored, airborne and shore-based instruments near Barrow, Alaska

Abstract Data from the Seasonal Ice Zone Observing Network (SIZONet) acquired near Barrow, Alaska, during the 2009/10 ice season allow novel comparisons between measurements of ice thickness and velocity. An airborne electromagnetic survey that passed over a moored Ice Profiling Sonar (IPS) provided coincident independent measurements of total ice and snow thickness and ice draft at a scale of 10 km. Once differences in sampling footprint size are accounted for, we reconcile the respective probability distributions and estimate the thickness of level sea ice at 1.48 ± 0.1 m, with a snow depth of 0.12 ± 0.07 m. We also complete what we believe is the first independent validation of radar-derived ice velocities by comparing measurements from a coastal radar with those from an under-ice acoustic Doppler current profiler (ADCP). After applying a median filter to reduce high-frequency scatter in the radar-derived data, we find good agreement with the ADCP bottom-tracked ice velocities. With increasing regulatory and operational needs for sea-ice data, including the number and thickness of pressure ridges, coordinated observing networks such as SIZONet can provide the means of reducing uncertainties inherent in individual datasets.

A Numerical Investigation of Formation and Variability of Antarctic Bottom Water off Cape Darnley, East Antarctica

AbstractAt several locations around Antarctica, dense water is formed as a result of intense sea ice formation. When this dense water becomes sufficiently denser than the surrounding water, it descends the continental slope and forms Antarctic Bottom Water (AABW). This study presents the AABW formation off the coast of Cape Darnley [Cape Darnley Bottom Water (CDBW)] in East Antarctica, using a nonhydrostatic model. The model is forced for 8 months by a temporally uniform surface salt flux (because of sea ice formation) estimated from Advanced Microwave Scanning Radiometer for Earth Observing System (EOS; AMSR-E) data and a heat budget calculation. The authors reproduce AABW formation and associated periodic downslope flows of dense water. Descending pathways of dense water are largely determined by the topography; most dense water flows into depressions on the continental shelf, advects onto the continental slope, and is steered downslope to greater depths by the canyons. Intense sea ice formation is the ...

Antarctic Bottom Water production from the Vincennes Bay Polynya, East Antarctica

One year moorings at depths greater than 3000m on the continental slope off Vincennes Bay, East Antarctica, reveal the cold (<-0.5 degrees C) and fresh (<34.64) signals of newly formed Antarc ...