Hirotaka Otobe

Subsurface ammonium maximum in the northern North Pacific and the bering sea in summer

Abstract Ammonium data collected on three summer cruises of the R.V. Hakuho Maru , University of Tokyo, in offshore shallow waters (200 m or less) of the northern North Pacific and the Bering Sea are summarized. Three types can be distinguished with respect to vertical distribution of ammonium: (1) the maximum concentration of ammonium appears in subsurface layer of 30 to 50 m, (2) the maximum appears near the sea floor, and (3) ammonium concentration is low throughout the water column and shows no vertical trend. The subsurface ammonium maximum is commonly observed in the areas where the depth is > 100 m. It is associated with a sharp pycnocline near the bottom of the euphotic zone. The regeneration of ammonium by zooplankaton and/or bacteria together with the stratification of the water column in summer are responsible for the formation of the subsurface ammonium maximum. Even with the utilization of this ammonium by phytoplankaton, the supply is sufficient for the maximum to persist. The second type of ammonium distribution is found in the midshelf region of the eastern Bering Sea. Different processes are responsible for the formation of the subsurface and sea-floor ammonium maxima. The ammonium near the sea floor is probably produced by benthic organisms. Implications of the subsurface ammonium ammonium to the fertility of the subarctic Pacific and the Bering Sea are discussed.

OCTOPUS, an octo parameter underwater sensor, for use in biological oceanography studies

A realtime monitoring system for observing physical, biological, and optical oceanographic parameters in shallow waters was developed. It consists of a CTD with oxygen sensor, anin situ fluorometer, anin situ transmissometer, and anin situ quanta meter together with an on-deck quanta meter. The system is also equipped with a water sampling device. Water sampling can be done during the upward cast on the basis of depth profiles of temperature, salinity, oxygen,in situ fluorescence, transmittance, and underwater irradiance taken during the downward cast.

Recent Occurrences of Dinophysis fortii(Dinophyceae) in the Okkirai Bay, Sanriku, Northern Japan, and Related Environmental Factors

Occurrence of Dinophysis fortii, a causative organism of diarrhetic shellfish poisoning, in the Okkirai Bay, Sanriku was surveyed in 1995, 1996, 1998 and 1999. In each year, its major occurrence was detected from the late May or early June and continued until the late June or early July. Seawater temperature, salinity and nutrients measurements suggested that inflows of offshore water into the bay played key role on the first major occurrence of D. fortii. With an analysis of continuous temperature data in the Otsuchi Bay which locates north of the Okkirai Bay, this influent was considered to be intermittent inflow of the offshore water by internal tidal waves which propagated from north to south. First occurrence peak of D. fortii was synchronous with phycobilin containing microalgae, synechococcoid cyanobacteria and cryptomonad, in all years. In vivo fluorescence measurement of D. fortii cells in 1995 and 96 showed that the cells in these microalgal-rich water contained more phycobilin pigment than those in the microalgal-poor water. The result may support a hypothesis that D. fortii acquires phycobilin by an uptake of these microalgae. After the first major occurrence in the bay, D. fortii sometimes occurred in rather inshore waters where showed elevated ammonium level possibly due to increased heterotrophic activity. Together with another finding that D. fortii is mixotrophic, it could be assumed that the environment being suited to heterotrophic nutrition also stimulates D. fortii growth in the bay.

Observation of temperature and velocity from a surface buoy moored in the Shikoku Basin (OMLET-88) —An oceanic response to a Typhoon

A surface buoy was moored from 20 April to 2 November 1988 at 28°48′ N and 135°01′ E where the water depth was 4900 m to measure temperature and velocity in the upper 150 m. The Typhoon 8824 passed at 0300 (JST) on 8 October about 50 km north to the mooring station with a maximum wind speed of 43.5 m s−1. The buoy was shifted about 30 km to southwest, and the instruments were damaged. The records of temperature at 0.5 m and velocity at 50 m were obtained. The inertial oscillation caused by the typhoon is described using the current record. The oscillation endured for about 20 days. Deep mixing and vertical, heart transport by the typhoon are discussed based on the data from the Ocean Data Buoy of the Japan Meteorological Agency moored at 29°N and 135°E.

Outbreak of warm water from the Kuroshio South of Japan: a combined analysis of satellite and OMLET oceanographic data

During the concentrated observation (April–May 1988) conducted as a part of the Ocean Mixed Layer Experiment (OMLET) in the sea area south of Japan, a conspicuous outbreak of warm water occurred from the large-meander region of the Kuroshio toward the southwest in the direction of the former Ocean Weather Station “T”. A series of NOAA-AVHRR infrared images clearly showed the process of this event. A surface buoy-mooring system deployed in this experiment recorded the arrival of this outbreak of water, in terms of the rise of sea-surface temperature (SST) of 1.5°C and the flow of warm water of 1.5kt toward the northwest at “T”. We studied this phenomenon by combining time series of infrared SST images with the oceanographic data obtained by two research vessels. The warm water was about 100 m deep in the section at 137°E along the edge of the Off-Shikoku Warm Water. It was estimated that about twenty outbreaks of this kind in a year can compensate a large heat loss to the atmosphere above this ocean region.

In situ Measurement of Incoming Solar Radiation by Voluntary Ships in the Western Pacific

We have been performingin situ measurement of downward short wave radiation (solar radiation) in the western Pacific Ocean in cooperation with voluntary ships since autumn 1990 in order to obtain much more precise knowledge of downward short wave radiation at the seas surface than before. Preliminary result of the observation from autumn 1990 through spring 1992 is shown in this paper. The comparison of observed daily mean downward short wave radiation with that estimated from observed cloudiness by using Reed (1977) formula is also presented to show the necessity ofin situ measurement in the study of the downward short wave radiation at the seas surface.

Variability of Upper Ocean Heat Balance in the Shikoku Basin during the Ocean Mixed Layer Experiment (OMLET)

The heat balance of the surface layer in the vicinity of the former Ocean Weather Station “Tango” (OWS-T; 29°N, 135°E), where a large amount of heat is transported by the Kuroshio and transferred to the atmosphere, was studied by during Ocean Mixed Layer Experiment (OMLET) as an oceanographic component of the Japanese World Climate Research Program (1987–1991). Temperature and velocity in the upper ocean measured using a surface moored buoy system deployed by the Ocean Research Institute, the University of Tokyo, in total 668 days of four time series namely the periods of April 1988–November 1988 (OMELET-88), August 1989–February 1990 (OMLET-89), April 1990–September 1990 (OMLET-901) and September 1990–January 1991 (OMLET-902). We have analyzed the moored buoy data of the upper 100 m for the latter three time series (OMLET-89, -901 and -902) and here we discuss the heat balance of the upper 100 m, in combination with surface heat flux and oceanographic data provided by the Japan Meteorological Agency. A large fluctuation of oceanic heat convergence/divergence of 200–300 W/m2 in amplitude with predominant period of 20–30 days occurred in the first half of OMLET-89 period, which was just the early stage in the formation process of a large meander path of the Kuroshio. A large amount of heat convergence of 71 and 79 W/m2 on average was detected in observation period of OMLET-89 and -901, respectively. During OMLET-902, relatively small heat convergence of 13 W/m2 was obtained. It is suggested that these variations of oceanic heat convergence in this region were closely related to the fluctuation of the Kuroshio axis to the south of Japan.

Heat balance of the upper ocean under a land and sea breeze in Sagami Bay in summer

The heat balance of the upper ocean under a land and sea breeze was investigated based on observations of sea water temperature in the upper 300 m layer and heat flux across the sea surface at a fixed station in Sagami Bay (35‡10′N, 139‡25′E) during two periods of two days in August 1980 and three days in August 1981. During both periods, a typical land and sea breeze of 4–6 m sec−1 at maximum prevailed in the observation area. Large diurnal variation of sea surface temperature with a maximum peak around noon LST was observed during both periods (the daily value of the range was 0.9‡C and 2.5‡C in 1980, and 1.2‡C, 1.5‡C and 1.7‡C in 1981). It was found that these large temperature variations were caused by diurnal variation of the wind speed which dropped to 0–3 m sec−1 at noon when the strongest insolation (−270 Wm−2) penetrated the sea and at midnight in association with alternations of the land breeze and the sea breeze. On the other hand, vertical mixing of the sea water caused by the wind stress and/or convection due to cooling at night extended down only to the surface 10 m layer. Horizontal heat advection was negligibly small. Therefore the local time change of the heat content in the upper 10 m water column was affected mainly by the heat flux across the air-sea interface which was estimated from data on radiation fluxes measured directly on board and latent and sensible heat fluxes calculated by the aerodynamic bulk method. The water temperature below the 10 m layer also varied with time and the temperature variation in the thermocline (20–50 m depth) was frequently larger than that of the sea surface temperature. However, the variation in the upper 10 m layer was little influenced by that below the layer.

Radiation balance and heat budget at the ocean/atmosphere interface in the western North Pacific

The downward short- and long-wave radiation fluxes at the sea surface (S↓, εL↓) were measured aboard the R/VHakuho Maru, University of Tokyo, for the period of 117 days on six cruises from 1981 to 1985 in the western North Pacific near Japan. The upward fluxes of short- and long-wave radiation (S↓, εL↑) were calculated by Paynes (1972) table and the Stefan-Boltzmanns law, respectively. The sensible and laten heat fluxes (Qh,Qe) were also estimated from an aerodynamic bulk method.From April to August, the daily mean value ofS↓ varied with the amplitude of 100∼200 Wm−2. The value ofS↑ was estimated approximately 6% ofS↓ in all seasons. The difference betweenεL↓ andεL↑ was so small that the net radiation flux (Qn) was dominated byS↓. In addition, the net heat flux at the sea surface was also dominated byS↓ due to small values ofQh andQe, and then the ocean was warmed at the rate of 111 Wm−2 in April and 63 Wm−2 in August in the Oyashio Area, and 132 Wm−2 in May and 164 Wm−2 in June in the Kuroshio Area, respectively.From September to March, a remarkable negative correlation between the day to day variation ofS↓ and that ofεL↓ was observed except when an intense cold air outbreak occurred. It was found that the correlation was caused by the cloud climatological feature of the western North Pacific in this period.S↓ was not a dominant factor in the net heat flux. The value ofQh+Qe in the Kuroshio Area ranged from 260 Wm−2 to 630 Wm−2, much larger thanQn which ranged from −8 Wm−2 to 92 Wm−2 in the leg mean values (each leg period was about 10 days). Then the ocean was cooled at the rate of −160∼−620 Wm−2 during this period. The net heat flux in the Kuroshio Area averaged over five legs from late November to February was −473 Wm−2. This value is 50∼100% larger than the climatological values reported so far.The temporal and spatial variability of radiation fluxes and heat fluxes during each leg was also discussed.

Heat energy exchange across the sea surface of the Bering Sea and the northern North Pacific in summer: estimates from direct measurements of radiation fluxes

Net heat flux across the sea surface in the Bering Sea and the northern North Pacific was estimated from observations during 21 days in the Bering Sea and 11 days in northern North Pacific during two summer cruises of the R.V.Hakuho Maru. Radiation fluxes were measured directly, and latent heat and sensible heat fluxes were calculated by an aerodynamic bulk method. There was little difference in the values of net flux between the two areas. The mean net radiation flux (0.22 ly min−) over the 32 days was in good agreement with other estimates in a region of the Bering Sea and at Station PAPA in the northern North Pacific. The mean values for latent heat flux (0.02 ly min−1) and sensible heat flux (0.00 ly min−1) were also identical with the estimates by Reed and in Budykos Climatic Atlas. On the basis of these estimates and other data, the vertical thermal eddy diffusion coefficient at the top of the seasonal thermocline of the eastern Bering Sea was estimated to be between 0.12 and 3.1 cm2s−1.