Motoyo Itoh

Pacific Ocean inflow: Influence on catastrophic reduction of sea ice cover in the Arctic Ocean

Received 27 December 2005; revised 7 March 2006; accepted 13 March 2006; published 21 April 2006. [1] The spatial pattern of recent ice reduction in the Arctic Ocean is similar to the distribution of warm Pacific Summer Water (PSW) that interflows the upper portion of halocline in the southern Canada Basin. Increases in PSW temperature in the basin are also well-correlated with the onset of sea-ice reduction that began in the late 1990s. However, increases in PSW temperature in the basin do not correlate with the temperature of upstream source water in the northeastern Bering Sea, suggesting that there is another mechanism which controls these concurrent changes in ice cover and upper ocean temperature. We propose a feedback mechanism whereby the delayed sea-ice formation in early winter, which began in 1997/1998, reduced internal ice stresses and thus allowed a more efficient coupling of anticyclonic wind forcing to the upper ocean. This, in turn, increased the flux of warm PSW into the basin and caused the catastrophic changes. Citation: Shimada, K., T. Kamoshida, M. Itoh, S. Nishino, E. Carmack, F. A. McLaughlin, S. Zimmermann, and A. Proshutinsky (2006), Pacific Ocean inflow: Influence on catastrophic reduction of sea ice cover in the Arctic Ocean, Geophys. Res. Lett., 33, L08605,

Beaufort Gyre freshwater reservoir: State and variability from observations

[1] We investigate basin-scale mechanisms regulating anomalies in freshwater content (FWC) in the Beaufort Gyre (BG) of the Arctic Ocean using historical observations and data collected in 2003–2007. Specifically, the mean annual cycle and interannual and decadal FWC variability are explored. The major cause of the large FWC in the BG is the process of Ekman pumping (EP) due to the Arctic High anticyclonic circulation centered in the BG. The mean seasonal cycle of liquid FWC is a result of interplay between the mechanical (EP) and thermal (ice transformations) factors and has two peaks. One peak occurs around June–July when the sea ice thickness reaches its minimum (maximum ice melt). The second maximum is observed in November–January when wind curl is strongest (maximum EP) and the salt input from the growing ice has not yet reached its maximum. Interannual changes in FWC during 2003–2007 are characterized by a strong positive trend in the region varying by location with a maximum of approximately 170 cm a � 1 in the center of EP influenced region. Decadal FWC variability in the period 1950–2000 is dominated by a significant change in the 1990s forced by an atmospheric circulation regime change. The center of maximum FWC shifted to the southeast and appeared to contract in area relative to the pre-1990s climatology. In spite of the areal reduction, the spatially integrated FWC increased by over 1000 km 3 relative to climatology.

Penetration of the 1990s warm temperature anomaly of Atlantic Water in the Canada Basin

[1] Penetration of the 1990s warm temperature anomaly (WTA) of the Fram Strait branch of Atlantic Water (FSBW) in the Canada Basin is described using available temperature, salinity, and velocity data. The core temperatures of FSBW show distinct pathways. Over the Chukchi Borderland advective velocities of the FSBW are well-correlated with bottom topography. The resulting multifarious pathways over the Chukchi Borderland act to modulate and substantially increase the time scale of WTA spreading and advancement. Further downstream two WTA tongues are observed. One tongue followed the Beaufort Slope and, along this pathway, the core temperatures of FSBW decreased rapidly. The depth integrated value of heat content remained near constant however, suggesting enhanced vertical mixing. The second tongue debouched from the northern tip of the Northwind Ridge and spread eastward into the deep Canada Basin, suggesting a complex recirculation structure within the Beaufort Gyre. INDEX TERMS: 4532 Oceanography: Physical: General circulation; 4536 Oceanography: Physical: Hydrography; 4512 Oceanography: Physical: Currents. Citation: Shimada, K., F. McLaughlin, E. Carmack, A. Proshutinsky, S. Nishino, and M. Itoh (2004), Penetration of the 1990s warm temperature anomaly of Atlantic Water in the Canada Basin, Geophys. Res. Lett., 31, L20301,

Seasonal Variations of Water Masses and Sea Level in the Southwestern Part of the Okhotsk Sea

A new grid data set for the southwestern part of the Okhotsk Sea was compiled by using all the available hydrographic data from the Japan Oceanographic Data Center, World Ocean Atlas 1994 and the other additional data sources with the resolution of about 10 km. We examine the seasonal variations of areas and volumes of Soya Warm Current Water (SWCW) and East Sakhalin Current Water (ESCW) and show that the exchanges of these water masses drastically occur in April and November. The peculiar variation of sea level in this region is also related with the water mass exchange. Sea level at the Hokkaido coast of the Okhotsk Sea reaches its minimum in April about two months later than in the case of ordinary mid-latitude ocean, and its maximum in December besides the summer peak. The winter peak of sea level in December is caused by the advent of fresh and cold ESCW which is accumulated at the subsurface layers (20–150 m) through the Ekman convergence by the prevailing northerly wind. Sea level minimum in April is caused by the release of the convergence and the recovery of dense SWCW that is saline and much colder than that in summer.

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...

Numerical study of winter water formation in the Chukchi Sea: Roles and impacts of coastal polynyas

Winter water formation is examined in the Chukchi Sea for the winters of 1992–2006 using a primitive equation ocean model forced by NCEP wind and surface salinity flux derived from SSM/I thin ice thickness estimates. The model is also forced by an external inflow of 0.8 Sv through the Bering Strait. The model successfully reproduces the oceanic circulation on the Chukchi shelf, thus providing numerous insights into behaviors of salt‐enriched water produced on the shelf. The experiments show that under northeasterly winds, northward throughflow across Barrow Canyon is reduced. This results in salinity buildup under freezing conditions and ultimately in greater enhancement of salinity of the waters carried into the Arctic Basin. The flow and salinity enhancement of the flow through Herald Canyon is less extreme but more steady than through Barrow Canyon. Together with moored salinity in the Bering Strait, the model results estimate the actual salinity to be 32.9 ± 0.8 psu and 32.7 ± 0.3 psu, respectively, for waters moving through the Barrow and Herald Canyons. Both estimates are less than 33.1 psu that is typically observed for the cold halostad layer in the Canada Basin, suggesting the importance of diapycnal mixing with saltier Atlantic origin water.

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.

Formation and spreading of Eurasian source oxygen‐rich halocline water into the Canadian Basin in the Arctic Ocean

[1] We identify the source region and spreading pattern of cold, oxygen-rich water observed in the halocline of the northern Canada Basin using both Joint Western Arctic Climate Studies 2002-2005 and other data. This water originates in the winter mixed-layer in the Nansen Basin and, because of its convective origin, can be traced by its cold, oxygen-rich properties together with a signature of low potential vorticity. This water, a component of the cold halocline complex, spreads into the Makarov Basin across the Lomonosov Ridge between 82°N and 86°N, enters the Canada Basin between the Alpha and Mendeleyev ridges, and continues eastward into the Beaufort Gyre north of Chukchi Plateau.

Variability in dissolved organic matter optical properties in surface waters in the Amerasian Basin

Surface absorption and fluorescence measurements of Dissolved Organic Matter (DOM) were conducted along with hydrographic parameters in the Canada and Makarov Basins. Parallel factor analysis of DOM fluorescence identified four humic-like and one protein-like component in all 107 surface samples. Based on strong negative trends observed between the spectral slope in the 275-295 nm range and absorption at 370 nm, and four humic-like components C1-C4, the DOM character was found to be basin-dependent. The Makarov basin surface DOM was largely dominated by high molecular weight and humic-rich material whereas the Canada Basin surface DOM was more heterogeneous with a marked influence of in situ production. This study highlights that absorbing and fluorescing measurements can be used successfully to trace and differentiate DOM from diverse sources and across frontal zones, and as such can be convenient and complementary tools for the better understanding of marine biogeochemical cycling of carbon.

Shelf-Break Exchange in the Bering, Chukchi and Beaufort Seas

The Bering and Chukchi/Beaufort shelf-breaks form the beginning and end of the dramatic sea-level and wind-forced flow of Pacific Ocean water across the Bering and Chukchi continental shelves between the Pacific and Arctic Oceans. Recent model results suggest that the on-shelf flow in the Bering is distributed along the shelf-break, wind-dependant, focused by Zhemchug and Bering canyons and modified by shelf-break eddies. Similarly, the off-shelf flow in the Chukchi/Beaufort is mediated by canyons, shelf-break jets, eddies, and wind forcing. In the Chukchi, flow is channeled through Barrow and Herald canyons to the shelf-break, where across-slope flow in Ekman boundary layers and instabilities result in the exchange of water and properties across the slope. In addition, dense shelf-water, created from brine rejection during ice formation in coastal polynyas, has been observed to flow downslope through Barrow Canyon. Along the Beaufort shelf, the shelf-break current is unstable, shedding eddies that populate the deep Beaufort basin. Upwelling favorable winds in summer have been observed to modify the structure of the shelf-break current and drive exchange across the Chukchi and Beaufort slopes. Most of our understanding of shelf-break flow in Bering and Chukchi Seas is based on numerical model results and broad-scale observations. There is thus a need for more detailed shelf-break observational programs.