Yu-Hsin Cheng

Modulation of Kuroshio transport by mesoscale eddies at the Luzon Strait entrance

Measurements of Kuroshio Current velocity at the entrance to Luzon Strait along 18.75°N were made with an array of six moorings during June 2012 to June 2013. Strong positive relative vorticity of the order of the planetary vorticity f was observed on the western flank of the Kuroshio in the upper 150 m. On the eastern flank, the negative vorticity observed was about an order of magnitude smaller than f. Kuroshio transport near its origin is computed from direct measurements for the first time. Kuroshio transport has an annual mean of 15 Sv with a standard deviation of 3 Sv. It is modulated strongly by impinging westward propagating eddies, which are identified by an improved eddy detection method and tracked back to the interior ocean. Eight Kuroshio transport anomalies >5 Sv are identified; seven are explained by the westward propagating eddies. Cyclonic (anticyclonic) eddies decrease (increase) the zonal sea level anomaly (SLA) slope and reduce (enhance) Kuroshio transport. Large transport anomalies of >10 Sv within O(10 days) are associated with the pairs of cyclonic and anticyclonic eddies. The observed Kuroshio transport was strongly correlated with the SLA slope (correlationu2009=u20090.9). Analysis of SLA slope data at the entrance to Luzon Strait over the period 1992–2013 reveals a seasonal cycle with a positive anomaly (i.e., an enhanced Kuroshio transport) in winter and spring and a negative anomaly in summer and fall. Eddy induced vorticity near the Kuroshio has a similar seasonal cycle, suggesting that seasonal variation of the Kuroshio transport near its origin is modulated by the seasonal variation of the impinging mesoscale eddies.

Statistical Characteristics of Mesoscale Eddies in the North Pacific Derived from Satellite Altimetry

The sea level anomaly data derived from satellite altimetry are analyzed to investigate statistical characteristics of mesoscale eddies in the North Pacific. Eddies are detected by a free-threshold eddy identification algorithm. The results show that the distributions of size, amplitude, propagation speed, and eddy kinetic energy of eddy follow the Rayleigh distribution. The most active regions of eddies are the Kuroshio Extension region, the Subtropical Counter Current zone, and the Northeastern Tropical Pacific region. By contrast, eddies are seldom observed around the center of the eastern part of the North Pacific Subarctic Gyre. The propagation speed and kinetic energy of cyclonic and anticyclonic eddies are almost the same, but anticyclonic eddies possess greater lifespans, sizes, and amplitudes than those of cyclonic eddies. Most eddies in the North Pacific propagate westward except in the Oyashio region. Around the northeastern tropical Pacific and the California currents, cyclonic and anticyclonic eddies propagate westward with slightly equatorward (197° average azimuth relative to east) and poleward (165 °) deflection, respectively. This implies that the background current may play an important role in formation of the eddy pathway patterns.

The Use of Neural Networks in Identifying Error Sources in Satellite-Derived Tropical SST Estimates

An neural network model of data mining is used to identify error sources in satellite-derived tropical sea surface temperature (SST) estimates from thermal infrared sensors onboard the Geostationary Operational Environmental Satellite (GOES). By using the Back Propagation Network (BPN) algorithm, it is found that air temperature, relative humidity, and wind speed variation are the major factors causing the errors of GOES SST products in the tropical Pacific. The accuracy of SST estimates is also improved by the model. The root mean square error (RMSE) for the daily SST estimate is reduced from 0.58 K to 0.38 K and mean absolute percentage error (MAPE) is 1.03%. For the hourly mean SST estimate, its RMSE is also reduced from 0.66 K to 0.44 K and the MAPE is 1.3%.

An Algorithm for Cold Patch Detection in the Sea off Northeast Taiwan Using Multi-Sensor Data

Multi-sensor data from different satellites are used to identify an upwelling area in the sea off northeast Taiwan. Sea surface temperature (SST) data derived from infrared and microwave, as well as sea surface height anomaly (SSHA) data derived from satellite altimeters are used for this study. An integration filtering algorithm based on SST data is developed for detecting the cold patch induced by the upwelling. The center of the cold patch is identified by the maximum negative deviation relative to the spatial mean of a SST image within the study area and its climatological mean of each pixel. The boundary of the cold patch is found by the largest SST gradient. The along track SSHA data derived from satellite altimeters are then used to verify the detected cold patch. Applying the detecting algorithm, spatial and temporal characteristics and variations of the cold patch are revealed. The cold patch has an average area of 1.92 × 104 km2. Its occurrence frequencies are high from June to October and reach a peak in July. The mean SST of the cold patch is 23.8 °C. In addition to the annual and the intraseasonal fluctuation with main peak centered at 60 days, the cold patch also has a variation period of about 4.7 years in the interannual timescale. This implies that the Kuroshio variations and long-term and large scale processes playing roles in modifying the cold patch occurrence frequency.

Statistical features of eddies approaching the Kuroshio east of Taiwan Island and Luzon Island

This study examined the statistical features of eddies approaching the Kuroshio east of Taiwan Island and Luzon Island. In total, 315 eddies (138 anticyclonic and 177 cyclonic eddies) were detected from 19.5xa0years of satellite altimeter sea-level data, with more than 95% of these eddies being generated in the ocean west of the Mariana Islands. Eddy trajectory statistics indicated that eddies frequently intrude into the Kuroshio regime at two latitude bands, namely 18°N–19°N and 22°N–23°N, with periods of 146xa0±xa062 and 165xa0±xa046xa0days, respectively. The interaction time is longer within the two active bands (33xa0±xa010xa0days at 18°N–19°N and 45xa0±xa017xa0days at 22°N–23°N) than at other latitudes. These two eddy-intrusion bands are associated with the northern and southern Subtropical Countercurrents (STCCs). These STCCs have a vertically reversed sign of the meridional potential vorticity gradient, thus providing a key energy source for eddy generation. In addition, when westward-propagating eddies approach the Ryukyu Islands, the southwestward recirculation flow east of the island chain as well as topographic effects cause some eddies to head southwestward to the east of Taiwan and intrude into the Kuroshio at 22°N–23°N, rather than to dissipate directly. Therefore, we suggest that the STCCs play a key role in inducing the eddies to frequently intrude into the Kuroshio at 18°N–19°N and 22°N–23°N. In addition, the Ryukyu Islands are responsible for concentrating the eddies within 22°N–23°N.

Interannual variability of cold domes northeast of Taiwan

ABSTRACT Multi-sensor data from different satellites were analysed to study the interannual variability of the cold domes northeast of Taiwan. From the empirical orthogonal function analysis of along-track satellite altimeter data and sea surface temperature, both revealed a cold dome phenomenon located at approximately 122°36´E and 25°24´N, and their principal components demonstrated a variation of 4–6 years. The cold dome variation correlates best with the Oceanic Niño index and poorly with Pacific Decadal Oscillation index. The forcing could be wind stress curl and Kuroshio volume transport, which produce an upwelling around the Men-Hua Canyon. Positive wind stress curl and the on-shelf water transport of the Kuroshio Current through the North Men-Hua Canyon increase the chance of the cold dome formation which may be responsible for the interannual variability of cold domes.

Observations of Typhoon Eye on Ocean Surface Using SAR and Other Satellite Sensors

In this study, typhoon eyes have been delineated using wavelet analysis from the synthetic aperture radar (SAR) images of ocean surface roughness and from the warmer area at the cloud top in the infrared (IR) images, respectively. RADARSAT and ENVISAT SAR imagery, and multi-functional transport satellite (MTSAT) and Feng Yun (FY)-2 Chinese meteorological satellite IR imagery were used to examine the typhoons in the western North Pacific from 2005 to 2011. Nine cases of various typhoons in different years, locations, and conditions have been used to compare the typhoon eyes by SAR (on the ocean surface) with IR (at the cloud-top level) images. Furthermore, the best track data getting from the Joint Typhoon Warning Center (JTWC), Chinese Meteorological Administration (CMA), and the Japan Meteorological Agency (JMA) are checked for the calibration and validation along with the Moderate Resolution Imaging Spectroradiometer (MODIS) image. Because of the vertical wind shear, which acts as an upright tilt, the location of the typhoon eye on the ocean surface differs from that at the top of the clouds. Consequently, the large horizontal distance between typhoon eyes on the ocean surface and on the cloud top implies that the associated vertical wind shear profile is considerably more complex than generally expected. The upright tilt structure may be caused by the ocean’s feedback or the effect of island obstruction. This result demonstrates that SAR can be a useful tool for typhoon monitoring study over the ocean surface.

Typhoon Eye Observations Using SAR and MTSAT

Typhoon eyes have been delineated from the smoother area in the Radarsat Synthetic Aperture Radar (SAR) images of ocean surface roughness and from the warmer area in the Multi-functional Transport Satellite (MTSAT) infrared images by using wavelet analysis. Case studies for different typhoons and environment have been investigated to demonstrate that SAR can be a powerful tool to help in typhoon tracking and prediction, especially at the ocean surface. It is found that the distance between the center locations of these typhoon’s eyes, as determined by SAR and MTSAT, respectively, is quite significant (14–26 km) for all five cases. The result of large center distance between typhoon eyes at the cloud level from MTSAT data and on the ocean surface from SAR data implies that the eyewall shaft may be highly tilted and the vertical wind shear profile is more complex than generally expected. Some of the issues concerning the definition of typhoon eye and typhoon tracking/prediction have been identified and compared with other data sets. Also, the tilted structure and associated vertical wind shear, especially during typhoon turning and staggering, may be caused by the ocean feedback or island blocking effects.