Ichio Asanuma

Satellite ocean-color observations of the tropical Pacific Ocean

The Coastal Zone Color Scanner (CZCS) data set provided some insights into biological processes in the equatorial Pacific, but the sampling was too sparse to address questions on temporal and spatial variability. Since late 1996, the Ocean Color–Temperature Sensor (OCTS), the Polarization Detection Environmental Radiometer (POLDER), the Sea-viewing Wide Field-of-View Sensor (SeaWiFS), and the Moderate-Resolution Imaging Spectroradiometer (MODIS) have provided a nearly continuous record of biological processes in this region for the first time. This study summarizes the SeaWiFS observations of the tropical Pacific from September 1997 through March 2000, with particular emphasis on equatorial and mesoscale variability, the influence of biological processes on penetrating irradiance, and the performance of primary production algorithms in this region. Specific mesoscale phenomena described are the phytoplankton blooms along the west coast of Central America, in the vicinity of the Costa Rica dome, and south of the equator. The coastal Central American and Costa Rica dome blooms result from orographically steered coastal winds and Ekman divergence, respectively. An unusual bloom event occurred south of the equator and persisted for several months in 1999; specific mechanisms that would have sustained the bloom could not be identified. Also, the time-evolution of the equatorial bloom during the May–August 1998 transition from El Ni * no to La Ni * na is discussed. Again, no concise and broadly accepted explanation of the bloom’s genesis and migration has yet emerged. During this transition, the monthly mean diffuse attenuation coefficient decreased by a factor of 3 at some locations along the equator. This change in water transparency, coupled with large changes in mixed-layer depth, resulted in significant changes in surface layer heating rates that were substantiated with field observations. Finally, certain primary production algorithms designed to use remotely sensed chlorophyll-a concentrations are evaluated. None of the algorithms capture the observed variability in primary production, and all appear to underestimate the total primary production of the tropical Pacific. r 2002 Elsevier Science Ltd. All rights reserved.

Changes in longitudinal distribution of the partial pressure of CO2 (pCO2) in the central and western equatorial Pacific, west of 160°W

We describe spatial and temporal variations in the partial pressure of carbon dioxide (pCO2) in the central and western equatorial Pacific on the basis of measurements conducted for the periods between 1987 and 1994. Surface water pCO2 data indicate the significant differences in longitudinal distribution depending on the ocean conditions. We examine the relationship between the area showing higher surface pCO2 values and the El Nino/Southern Oscillation phenomenon by using the Southern Oscillation Index. Results indicate that the area showing higher surface pCO2 values correlates with the SOI, which suggests significant intra- and interannual fluctuations of CO2 outflux from the central and western equatorial Pacific.

Geographical distribution of new production in the western/central equatorial Pacific during El Niño and non-El Niño conditions

Rates of new production in the warm waters of the tropical Pacific and their seasonal to interannual variability are estimated by measurement of 15 N-nitrate uptake along the equator in the western and central (145°E-165°W) equatorial Pacific. Measurements were carried out during non-El Nino conditions and two examples of anomalous conditions: the moderate El Nino in 1994-1995 and the strong El Nino in 1997-1998. Variations in new production are explained in relation to changes in physical, chemical, and biological environments during the El Nino-Southern Oscillation cycle. During non-El Nino conditions in the western region the rates of integrated new production over the euphotic zone were low (0.15 mmol m -2 d -1 ) and likely limited by availability of nitrate. In the central region, new production was higher (1.37 mmol m -2 d -1 ) owing to upwelling of nitrate-rich subsurface waters. The east-west asymmetry of observed variables in the western and central equatorial Pacific is altered during El Nino conditions owing to the eastward expansion of the western Pacific Warm Pool. In the western region, during the moderate El Nino in 1994-1995, the integrated new production increased slightly compared to non-El Nino conditions, while during the strong El Nino in 1997-1998, it was higher by a factor of 10 than in non-El Nino conditions. In the central region during the 1994-1995 El Nino, the rates of new production markedly decreased compared to non-El Nino conditions. During the El Nino in 1997-1998, the integrated rates of new production were comparable to non-El Nino conditions, but the vertical distribution showed a downward displacement of maximum rates. The results of our study suggest that interannual variations in new production in the western and central equatorial Pacific correlate well with the change of the nitracline depth during the eastward expansion of the Warm Pool and depend strongly upon the severity of the El Nino event.

Distributions and variations in the partial pressure of CO2 in surface waters (pCO2w) of the central and western equatorial Pacific during the 1997/1998 El Niño event

Measurements of partial pressure of CO2 in surface waters (pCO2w) and overlying air (pCO2a) were made in the central and western equatorial Pacific from October 1997 to February 1998 within the period of the 1997/1998 El Nino, which was reported to be the strongest El Nino event on record. The distribution of the pCO2w showed a pattern driven by the eastward movement of western Pacific warm pool and thermodynamic effects (temperature and salinity), which was different from those of the moderate 1986/1987 El Nino and non-El Nino periods. Due to the eastward movement of the warm pool with sea surface temperature (SST) higher than 28.5 °C and sea surface salinity (SSS) lower than 34.5, the pCO2w between 180° and 163°W (347–364 μatm) was almost equal to that of the air (351 μatm). Between 143°E and 180°, the pCO2w tended to increase toward the west (387 μatm at 0°, 144°E in December 1997) along with the SST and SSS. West of 143°E in January 1998, a steep change in pCO2w ranging from 320 to 365 μatm occurred while retaining high SST (>28.5 °C) and SSS (>34.5). This was caused by the advection of surface water from the southern low latitudes that had been affected by biological activity (New Guinea Coastal Current). From December 1997 to January/February 1998, the SSS was usually higher than 34.5 west of 180°, which was significantly high compared to the western equatorial Pacific warm pool. This was probably due to the decrease of the net fresh water input for the western equatorial Pacific and/or the northward migration of surface water from the Southern Hemisphere. The CO2 outflux from the central and western equatorial Pacific (5.5°S–5.5°N, 139.5°E–159.5°W) was estimated to be 0.027 Pg-C/year in December 1997 and 0.038 Pg-C/year in January/February 1998. This presents a significant decrease from the CO2 outflux during the non-El Nino periods (0.34 Pg-C/year in January/February 1989, 0.11 Pg-C/year in September/November 1990) and a slight one from the moderate 1986/1987 El Nino period (0.055 Pg-C/year in January/February 1987). Following the El Nino–Southern Oscillation phenomena, CO2 outflux varied largely in the central equatorial Pacific and little in the western equatorial Pacific.

Calibration and instrument status of ADEOS-II Global Imager

The Global Imager (GLI) on Advanced Earth Observing Satellite-II (ADEOS-II) launched on 14 December 2002 is an optical sensor to observe reflected solar radiation and infrared radiation. GLI has 36 channels from ultraviolet region (380nm) to thermal infrared (12micron). GLI data is used for understanding the global circulation of carbon, monitoring cloud, snow, ice, and sea surface temperature. NASDA carried out initial checkout to confirm GLI basic function until April 2003. Currently GLI calibration team that consists of sensor development division, ground system integration division, and science application group analyses calibration and validation to release L1 data at the end of this year. This report describes calibration and instrument status of GLI.

Microwave emission and reflection from the wind-roughened sea surface at 6.7 and 18.6 GHz

Microwave radiometric observations were made with specially designed microwave radiometers at 6.7 and 18.6 GHz, and the results were compared with those of other investigators, over the frequency range of 1-40 GHz. Dependences of sea surface emission and reflection on wind speed, frequency, incidence angle, and polarization type are discussed in detail, following discussions of the reflective processes of sky radiation and error estimation in the retrieval of mainlobe-averaged brightness temperature. The wind speed sensitivity of brightness temperature, emissivity, and reflectivity is formulated with respect to frequency and incidence angle in each polarization. The brightness temperature, emissivity and reflectivity at arbitrary wind speed are derived employing this formulation. Based on the results obtained it is suggested that the 10-19-GHz band may be optimal for satellite microwave radiometer observations of sea-surface wind. >

Satellite thermal observation of oil slicks on the Persian Gulf

Abstract A possibility of oil slicks detection is discussed for oil slicks spread in the vicinity of the Nowruz oil fields on the Persian Gulf since March 1983 to July 1983 with considering an apparent thermal inertia. The apparent thermal was computed from continuous observations of sea surface temperature and albedo by the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA-7 through day and night with 12 h interval. The apparent thermal inertia is defined as a function of a temperature difference between the daytime and the nighttime and an apparent albedo. Sea surface temperature used for computing the apparent thermal inertia was obtained through an atmospheric correction with an empirical equation which uses an energy difference between two thermal channels of the AVHRR. Although there was an ambiguity on a selection of same object on water body, the computed apparent thermal inertia showed the possibility of oil slicks detection from sea water.

GLI early calibration results for oceanographic applications

The Advanced Earth Observing Satellite-II (ADEOS-II) was launched on 14 December 2002, and its functions were checked until 2003 spring. The Global Imager (GLI) on board ADEOS-II has 36 channels (thirty 1-km resolution, six 250-m resolution) from ultraviolet to thermal infrared to facilitate understanding the global environmental changes in oceans, land and clouds with high accuracy. Ocean algorithms (e.g., ocean atmospheric correction and sea-surface temperature) need highly accurate sensor characterization coefficients because they retrieve sea-surface upward radiance precisely from the top of the atmosphere. The NASDA GLI calibration team includes members of sensor development, ground system integration, and science application groups. The team started investigating GLI characteristics and radio- and geo-correction processes in the initial verification period. In this paper, we will describe the initial results, radiometric accuracy, 12- or 48-detector dependency, scan-mirror surface, incident-angle dependency, and dynamic range related to oceanographic applications.

An Application of NOAA AVHRR for Oceanography in the East China Sea

Abstract Two sets of NOAA AVHRR imagery in spring and fall of 1980 are used to study hydrography of the sea southwest of Kuyshu, Japan. Comparison with hydrographic observations delineates two anticyclonic warm water eddies, which had diameters of 60 to 100 km determined from the satellite imagery. The high temperature and salinity of the eddies suggests that they originate from the Kuroshio. The bottom topography seems to influence strongly shape, spatial scales and locations of these eddies. The generation of the eddies may be caused by shear instabilities of the shore side edge of the Kuroshio. The eddies may supply the Kuroshio water to the Tsushima Current. This suggests that formation of the Tsushima Current southwest of Kyushu is of the episodic nature.

Chapter 4 Depth and Time Resolved Primary Productivity Model Examined for Optical Properties of Water

Abstract A depth and time resolved primary productivity model is proposed and validated with in situ and simulated in situ incubations using 13 C in the western Pacific Ocean. In this model, a vertical distribution of photosynthetically available radiation (PAR) is modeled based on the chlorophyll a concentration in the surface layer, where the assumption is that the surface chlorophyll a concentration determines the light field. A vertical distribution of chlorophyll a concentration is modeled with an empirical equation, in which a chlorophyll maximum is observed along the vertical distribution of PAR. A carbon fixation rate is modeled as a function of PAR and temperature, in contrast to previous studies, in which a temperature-dependent function is significantly improved. By finding an optimum vertical distribution of PAR for various combinations of chlorophyll a concentration and diffused attenuation coefficient, our depth and time resolved primary productivity model exhibited a good correlation with in situ and simulated in situ incubation.