Kunio Kutsuwada

Japanese Ocean Flux Data Sets with Use of Remote Sensing Observations (J-OFURO)

We have constructed ocean surface data sets using mainly satellite data and called them Japanese Ocean Flux data sets with Use of Remote sensing Observations (J-OFURO). The data sets include shortwave radiation, longwave radiation, latent heat flux, sensible heat flux, and momentum flux etc. This article introduces J-OFURO and compares it with other global flux data sets such as European Centre for Medium Range Weather Forecasting (ECMWF) and National Center for Environmental Prediction (NCEP) reanalysis data and da Silva et al. (1994). The usual ECMWF data are used for comparison of zonal wind. The comparison is carried out for a meridional profile along the dateline for January and July 1993. Although the overall spatial variation is common for all the products, there is a large difference between them in places. J-OFURO shortwave radiation in July shows larger meridional contrast than other data sets. On the other hand, J-OFURO underestimates longwave radiation flux at low- and mid-latitudes in the Southern Hemisphere. J-OFURO latent heat flux in January overestimates at 10°N–20°N and underestimates at 25°N–40°N. Finally, J-OFURO shows a larger oceanic net heat loss at 10°N–20°N and a smaller loss north of 20°N in January. The data of da Silva et al. in July show small net heat loss around 20°S and large gain around 20°N, while the NCEP reanalysis (NRA) data show the opposite. The da Silva et al. zonal wind speed overestimates at low-latitudes in January, while ECMWF wind data seem to underestimate the easterlies.

Relationships between Antarctic krill (Euphausia superba) variability and westerly fluctuations and ozone depletion in the Antarctic Peninsula area

An assessment of the environmental processes influencing variability in the recruitment and density of Antarctic krill (Euphausia superba) is important, as variability in krill stocks affects the Antarctic marine ecosystem as a whole. We have assessed variability in krill recruitment and density with hypothesized environmental factors, including strength of westerly winds (westerlies) determined from sea level pressure differences across the Drake Passage, sea ice cover, and ozone depletion. We found a significant positive correlation between krill recruitment in the Antarctic Peninsula area and the strength of westerlies during 1982–1998. Years with strong westerlies during the austral summer season resulted in high krill recruitment in 1987/1988, 1990/1991, and 1994/1995, while the years of weak westerlies resulted in low krill recruitment in 1982/1983, 1988/1989, 1992/1993, and 1996/1997. The strength of westerlies was significantly related to recruitment of 1-year-old krill (r = 0.57) and 2-year-old krill (r = 0.69) with a level of significance of 5%. In addition, the strength of westerlies also had a strong correlation with chlorophyll a (r = 0.63) and sea ice cover with a 1-year time lag (r = 0.67). The strength of westerlies is considered to be a key environmental factor. We also found significant correlations between krill density in the Antarctic Peninsula area and the Antarctic ozone depletion parameters during 1977–1997 (e.g., total ozone in October at Faraday/Vernadsky Station of r = 0.76 with a level of significance of 1%). We suspect that ozone depletion impacts directly and/or indirectly on the variability in krill density.

Impact of wind/wind-stress field in the North Pacific constructed by ADEOS/NSCAT data

Data sets of surface wind and wind-stress fields in the North Pacific from September 1996 to June 1997 have been constructed using NASA Scatterometer (NSCAT) data on-board ADEOS to investigate their variability and implications for the wind-driven oceanic circulation. Using a weighting function decreasing with the distance between each grid and data points, and of Gaussian type for time, daily, 10-day and monthly averages are calculated for each 1°×1° grid. Products are validated by comparison with those calculated from in-situ measurement data at oceanic buoys around Japan (JMA) and in the equatorial area (TAO). The RMS differences for wind direction and speed never exceed 20° and 2 ms−1, respectively, for the TAO buyos. This does not hold for data taken by JMA buoys, suggesting that the reliability in the mid-latitudes is not good for time averages shorter than several days. Zonal integration of the Sverdrup transport in a zone of 28°–30°N calculated from the monthly-mean products ranges between 25 and 60×106 m3s−1 (Sv) around its mean of 38 Sv. These are not so different from the Kuroshio transport values calculated from oceanic measurements.

Intraseasonal Variations in the Upper Equatorial Pacific Ocean prior to and during the 1997-98 El Nino

Abstract Time series data from the Tropical Atmosphere–Ocean (TAO) array of moored buoys and data from other sources are used to document the oceanic dynamical response to intraseasonal (periods centered near 60 days) wind stress forcing in the equatorial Pacific. We focus on the period from October 1996 to December 1997, encompassing the time just prior to and during the onset of the 1997–98 El Nino, when both atmospheric forcing and ocean response were exceptionally strong. Analysis reveals that the oceanic response to intraseasonal wind forcing is characterized by two district regimes. West of the international date line, the response is dominated by reversing zonal jets in both time and depth in the upper 200 m. East of the date line, variability is controlled primarily by first baroclinic mode equatorial Kelvin waves propagating eastward at phase speeds of 2.4–2.7 m s−1. However, amplitude structures in the eastern Pacific deviate from those expected for first baroclinic Kelvin waves in a resting oce...

Seasonal Variation in Volume Transport of the Kuroshio South of Japan

Abstract A two-layer numerical model driven by the observed wind stress compiled by K. Kutsuwada and T. Teramoto is used to study the seasonal variation in volume transport of the Kuroshio south of Japan. The authors focus on the observational evidence that the geostrophic volume transport of the Kuroshio is maximum in summer and minimum in winter; however, Sverdrup transport estimated by the observed wind stress indicates a maximum in winter and a minimum in summer. It is shown from a flat-bottom model that the western boundary current has a maximum transport of 90 Sv (Sv ≡ 106 m3 s−1) in late winter and a minimum transport of 40 Sv from summer to early autumn. The time lag of the calculated transport from Sverdrup transport is about one month, which shows that the barotropic response is dominant for the seasonal change in wind stress. However, the vertical velocity difference at the western boundary region reaches a maximum in summer and a minimum in autumn, of which the summer maximum coincides with th...

Observation of mixed Rossby-gravity waves in the Western equatorial Pacific

During the IOP (Intensive Observation Period) of TOGA/COARE (Tropical Ocean and Global Atmosphere/Coupled Ocean Atmosphere Response Experiment) from December 1992 to February 1993, four Japanese moored ADCPs (Acoustic Doppler Current Profilers) measured vertical profiles of three-component velocities at the stations 2S (2°S, 156°E), 2N (2°N, 156°E), 154E (0°N, 154°E) and 147E (0°N, 147°E). Power spectra of the surface current showed a pronounced peak having a period of around 14 days for both the zonal and meridional velocities at the stations 2S and 2N near the equator, and for only the meridional velocity at the equator. This 14-day phenomenon is considered to be a kind of equatorial wave of the first baroclinic mode, from a comparison of the result of the vertical mode analysis and the vertical distribution of the standard deviation of band-pass filtered velocity fluctuations. A dispersion relationship obtained from the horizontal mode analysis of this wave confirmed that the 14-day phenomenon is a mixed Rossby-gravity wave with the westward propagating phase speed and eastward propagating group velocity. From the cross-spectral analysis of velocity data, the average phase speed and wavelength of the wave were estimated as 3.64 m s−1 and 3939 km, respectively, for station pair 2S∼147E. These values were in good agreement with the average phase speed and wavelength of 3.58 m s−1 and 3836 km estimated from the dispersion curve and the observed period. A northerly wind burst blew over all the mooring sites during the middle of the observation period. The mixed Rossby-gravity wave, which is anti-symmetric for the zonal velocity about the equator, is likely to be forced by this northerly wind burst crossing the equator. Generation of the oceanic mixed Rossby-gravity wave of the first baroclinic mode is discussed in association with the atmospheric Rossby wave having the same period.

Evaluation of Vector Winds Observed by NSCAT in the Seas around Japan

In order to validate wind vectors derived from the NASA Scatterometer (NSCAT), two NSCAT wind products of different spatial resolutions are compared with observations by buoys and research vessels in the seas around Japan. In general, the NSCAT winds agree well with the wind data from the buoys and vessels. It is shown that the root-mean-square (rms) difference between NSCAT-derived wind speeds and the buoy observations is 1.7 ms−1, which satisfies the mission requirement of accuracy, 2 ms−1. However, the rms difference of wind directions is slightly larger than the mission requirement, 20°. This result does not agree with those of previous studies on validation of the NSCAT-derived wind vectors using buoy observations, and is considered to be due to differences in the buoy observation systems. It is also shown that there are no significant systematic trends of the NSCAT wind speed and direction depending on the wind speed and incidence angle. Comparison with ship winds shows that the NSCAT wind speeds are lower than those observed by the research vessels by about 0.7 ms−1 and this bias is twice as large for data observed by moving ships than by stationary ships. This result suggests that the ship winds may be influenced by errors caused by ships motion, such as pitching and rolling.

GRIDDED SURFACE WIND-STRESS PRODUCT OVER THE WORLD OCEAN CONSTRUCTED BY SATELLITE SCATTEROMETER DATA AND ITS COMPARISON WITH NWP PRODUCTS

Products of gridded surface wind and wind-stress vectors over the world ocean are constructed by satellite scatterometer (ERS-1/2 and Qscat/SeaWinds) data with highly temporal and spatial resolutions. Data of the ERS-1/2 and the Qscat/SeaWinds cover periods of 1992-2000 and since August 1999, respectively, and permit us to establish a long-term time series. We make validation for our products by inter-comparison with in-situ data (TAO and NDBC buoys), and find that our Qscat product has high reliability in the almost whole area around buoy locations. For areas where there are no in-situ data such as the high-latitude southern ocean, we also make inter-comparison of our products with numerical weather prediction(NWP) ones (NCEP and ECMWF). Results reveal that there are significant differences in the westerly region of 40°- 60°S, suggesting that the wind-stress magnitudes calculated from the NCEP reanalysis 6-hourly product are overestimated in the high latitudes

Long-term variation in the North Pacific using satellite-derived wind data set/J-OFURO over the last decade and other data sets over a longer record

Time series of gridded data sets of surface winds (from Qscat (J-OFURO)) constructed by satellite microwave sensors covering almost a decade (1999–2009) are used to examine long-term change in surface wind fields over the world’s oceans. Evidence has been provided by most previous studies that wind speeds have a tendency to increase over time in many area, and we verify whether or not this tendency persists. Results reveal that zonal winds tended to be weaker over the study period in the region of the North Pacific where westerly winds prevail. Time series of different types of data sets based on numerical model products and voluntary ship measurements present similar features of weakening westerly winds, even allowing for discrepancies among the data sets. These time series also exhibit a tendency of enhanced westerly winds in periods prior to the start of the twenty-first century, which means that the long-term trend in wind speed has changed from positive in the 1980s/1990s to negative in the 2000s. Examinations of time series for each season reveal that the above feature is found in winter, suggesting that it is related to the strength of the Aleutian Low.

Construction of long-term data set of sea surface wind speed/stress vectors by continuous satellite observations

Using scatterometer data from three sources (QSCAT/SeaWinds, MetOp-A/ASCAT, and ERS-1,2), we construct a gridded data set of wind/wind-stress vectors with continuous time series for investigation of long-term variation in ocean surface wind. The present study investigates differences in wind fields between two products in the overlapping periods 2008 (for QSCAT and ASCAT products) and 2000 (for QSCAT and ERS products). Systematic differences are found in the meridional components, showing that anomalies in the ASCAT and ERS products from that in QSCAT’s are positive (northward) in the northern hemisphere and negative (southward) in the southern hemisphere, corresponding to poleward anomalies. These poleward anomalies are also found in the monthly mean fields, and less clearly in the daily mean ones, in which there are meridionally striped patterns in the mid-latitude areas, suggesting that they are due to differences in satellite orbits and measurement timings. Similar comparisons are also made in the wind-stress curl field, which is a driving force of oceanic currents. The poleward anomalies are not so marked in the annual mean field of the wind-stress curl field, but there are meridionally striped features in the mid-latitude areas. Time series of the wind-stress curl are made in the subtropical and subarctic circulation regions of the North Pacific for investigation of long-term variation over several years using corrected times series. The results reveals that the magnitude of wind-stress curl in both regions has been weakening since the beginning of this century, indicating that they would be a result of weakening of the westerlies.