Futoki Sakaida

Sea surface cooling caused by typhoons in the Tohoku area in August 1989

A remarkable sea surface cooling event was observed in the north western Pacific off Japan coast, which is known as the Tohoku Area, by using well-calibrated satellite sea surface temperature images, when typhoons T8914 and T8915 passed over the Tohoku Area in August 1989. The sea surface cooling maximum reached up to 9°C, which is much larger than that reported in the previous studies. The corresponding hydrographic data show that the sea surface cooling state is closely related with the ocean structure in the Tohoku Area; the larger sea surface cooling occurs in the Oyashio area. The numerical experiments taking account of the ocean structure show that the surface cooling maximum is 3–4 times larger in case of the Oyashio profile than that of the Kuroshio profile and clarify the mechanism of the sea surface cooling.

Estimation of sea surface temperatures around Japan using the Advanced Very High Resolution Radiometer (AVHRR)/NOAA-11

The accuracy of sea surface temperatures (SSTs) derived from the Advanced Very High Resolution Radiometer (AVHRR)/NOAA-11 is examined by comparison with sea-truth SSTs obtained from ocean data buoys durings November 1988 through December 1989. We made a 122 point data set of buoy SSTs from the oceans around Japan and the corresponding brightness temperatures of channels 4 and 5 during cloud free periods. The satellite temperatures are corrected for atmospheric effects using the NOAA Multi-Channel SST (MCSST) and Cross Product SST (CPSST) algorithms. The two algorithms give similar results for our data set and result in biases of about −0.1°C with rms errors of about 0.6°C relative to buoy SSTs. It is found that MCSSTs and CPSSTs tend to be higher than SSTs from the buoy in the Japan Sea in summer. New coefficients for the MCSST equations suitable for our data set are determined and the resultant rms error is 0.49°C. If we eliminate the cluster of anomalous summer data in the Japan Sea, the rms error becomes 0.43°C.

Seasonal phytoplankton blooms associated with monsoonal influences and coastal environments in the sea areas either side of the Indochina Peninsula

The Gulf of Thailand (GoT) is a semienclosed sea on the west and southwest side of the Indochina Peninsula and connects with the near-coastal waters of the South China Sea (SCS) on the east and northeast side of the Malay Peninsula. The objective of the present study is to understand dynamic features of the phytoplankton biology in the GoT and the nearby SCS, on both sides of the Indochina Peninsula, using remote-sensing measurements of chlorophyll-a (Ch1 a), sea surface temperature (SST), and surface vector winds obtained during the period from September 1997 to March 2003. Results show that seasonal variations of the phytoplankton blooms are primarily controlled by the monsoonal winds and related coastal environments. The GoT and the near-coastal SCS have a peak in the averaged monthly Ch1 a in December and January, which is associated with the winter northeaster monsoon. The near-coastal SCS have another big peak in the averaged monthly Ch1 a in summer (July to September), which is associated with the summer southwest monsoon. The offshore bloom in the GoT occurs in its southern part and enhances the December-January peak of averaged monthly Ch1 a. By contrast, the offshore bloom in the nearby SCS is observed northeast of the Peninsula, and represents the primary source of the July-September peak Ch1 a. Here the coastal upwelling associated with the offshore Ekman transport caused by the coastal surface winds parallel to the Vietnam east coast gives physical conditions favorable to the development of offshore phytoplankton blooms. The Mekong River discharge waters flow in different directions, depending on the monsoon winds, and contributes to seasonal blooms on both sides of the Peninsula.

Wintertime sea surface temperature fronts in the Taiwan Strait

[1] We present wintertime variations and distributions of sea surface temperature (SST) fronts in the Taiwan Strait by applying an entropy-based edge detection method to 10-year (1996-2005) satellite SST images with grid size of 0.01°. From climatological monthly mean maps of SST gradient magnitude in winter, we identify four significant SST fronts in the Taiwan Strait. The Mainland China Coastal Front is a long frontal band along the 50-m isobath near the Chinese coast. The sharp Peng-Chang Front appears along the Peng-Hu Channel and extends northward around the Chang-Yuen Ridge. The Taiwan Bank Front evolves in early winter. As the winter progresses, the front becomes broad and moves toward the Chinese coast, connecting to the Mainland China Coastal Front. The Kuroshio Front extends northeastward from the northeastern tip of Taiwan with a semicircle-shape curving along the 100-m isobath.

Accuracies of NOAA/NESDIS Sea Surface Temperature Estimation Technique in the Oceans around Japan

We have examined accuracies of nine nighttime National Oceanic and Atmospheric Administration/National Environmental Satellite, Data and Information Service (NOAA/NESDIS) equations for SST estimation using the Advanced Very High Resolution Radiometer (AVHRR)/NOAA-11 dataset produced by Sakaida and Kawamura (1992). Among the nine equations, the revised triple-window CPSST algorithm gives the smallest rms error, which is 0.38°C. The dual-window MCSST algorithm gives the largest rms error 0.56°C. Rms errors of the other algorithms are smaller than 0.5°C.

Temporal Scale of Sea Surface Temperature Fronts Revealed by Microwave Observations

Sea surface temperature (SST) data for three years from the Advanced Microwave Scanning Radiometer for the Earth Observing System are used to statistically evaluate the temporal scales of the global SST fronts (SSTFs). Using the entropy-based edge detection method which is very resistant to impulsive noises, temporal autocorrelation of the dissimilarities of two SST groups across the SSTF is calculated in 10-km-gridded map. In general, the derived temporal scales, defined as e-folding scales in this study, range from 10 to 40 days. Long temporal scales of up to 100 days are found in areas where the stationary ocean currents maintain the frontal structures.

Wintertime high-resolution features of sea surface temperature and chlorophyll-a fields associated with oceanic fronts in the southern East China Sea

The southern East China Sea (ECS) and the sea north of Taiwan comprise an important transit area between the coastal waters of mainland China and the open water of the western North Pacific. Their wintertime dynamical features were examined using long‐term high‐resolution satellite‐derived sea surface temperature (SST) and chlorophyll‐a (Chl‐a) images. Along the Chinese coast, a cold‐water tongue with an SST of less than 15°C extended southwestwards from the ECS in December, intruded into the Taiwan Strait (TS) in January, and extended further southwestwards in February. Conforming with the cold SST development, the Chl‐a concentration starts to increase along the 50‐m isobath. However, in the sea north of Taiwan, the SST and the Chl‐a front between the shelf region and the eastern open ocean was semicircular in shape, left the shelf break and extended northeastwards. It was found that, along the semicircular Kuroshio front in the sea north of Taiwan, alternative cold and warm fronts appeared in the geographically fixed area in January and February. Based on the investigation of the snapshot SST images, their systematic appearance is attributed to an SST pattern similar to frontal eddies at the Kuroshio northern front. High concentrations of Chl‐a (>2.0 mg/m3) were found at the warm front and the offshore cold front patch.

The production of the new generation sea surface temperature (NGSST-O ver.1.0) in Tohoku University

Satellite sea surface temperature (SST) observations from infrared radiometers (AVHRR, MODIS) and a microwave radiometer (AMSR-E) are objectively merged to generate the new generation SST product (NGSST-O Ver.1.0). The product is a quality-controlled, cloud-free, high-spatial resolution (0.05 degree-grided), wide-covering (13-63N, 116-166E), and daily SST digital map. The real-time production and distribution of the NGSST-O has started from September 2003 as a part of the GODAE High Resolution Sea Surface Temperature Pilot Project. To examine the accuracy of the NGSST-O product, the NGSST-O SST is compared with the in-situ SST that is obtained by the buoys drifting in the analyzed area for July 2002 to October 2004. The match-ups of 396,254 points show that the bias of the NGSST-O is -0.15 K against the buoy observations and the rms error, 0.85 K.

Improvement of New Generation Sea Surface Temperature for Open ocean (NGSST-O) : a new sub-sampling method of blending microwave observations

An improved version (version 1.6) of the blended optimum interpolation sea surface temperature (SST), obtained by use of infrared (IR) and microwave (MW) data of the New Generation SST for Open ocean (NGSST-O) product, has been developed. A major improvement is the introduction of a sub-sampling scheme for MW-SST with a finer grid; this has resulted in reduction of the blocky patterning occasionally found in blended SST products with finer grids. Spectral comparison of along-track sea surface height and NGSST-O suggests that mesoscale turbulence was reproduced in the updated NGSST-O in a wide wavelength range.

Kuroshio-induced cold eddy streets in the lee of isolated islands

Cold eddy streets formed in the lee of isolated islands were investigated in relation to the Kuroshio current. Multi- temporal SAR images acquired over a one-month period revealed island wake patterns, such as meandering disturbances and eddy streets. These patterns corresponded to the Kuroshio path, indicating a Kuroshio-island interaction. We constructed high-spatial-resolution sea-surface temperature (SST) images for the time when the Kuroshio impinged on the islands by regressing the LANDSAT infrared images on the AVHRR-derived SST. The images revealed low-SST island wakes, some of which included cold eddy streets. A numerical simulation was performed to investigate their formation mechanism. The simulation qualitatively reproduced the cold eddy pattern, with eddy-driven mixing developing a mixed layer down to 100 m, causing the low- SST island wakes. The shedding frequency and distance of the model-produced eddies were close to those of the Karman vortex theory, suggesting that Karman-like cold eddy streets developed behind the islands when the Kuroshio passed.