Chung-Ru Ho

Temperature and size variabilities of the Western Pacific Warm Pool

Variabilities in sea-surface temperature and size of the Western Pacific Warm Pool were tracked with 10 years of satellite multichannel sea-surface temperature observations from 1982 to 1991. The results show that both annual mean sea-surface temperature and the size of the warm pool increased from 1983 to 1987 and fluctuated after 1987. Possible causes of these variations include solar irradiance variabilities, EI Ni�o—Southern Oscillation events, volcanic activities, and global warming.

Satellite observation of upwelling along the western coast of the South China Sea

Abstract We observed the evolution of upwelling along the western coast of the South China Sea (SCS). The data we used are NOAA (National Oceanic and Atmospheric Administration) satellite AVHRR (Advanced Very High Resolution Radiometer) IR (infrared) images taken in 1996 and 1997 summer at the HRPT (High Resolution Picture Transmission) receiving station built on Tai-Ping Island, which is located in the central SCS. An upwelling intensity, defined by the total heat loss in the upwelling cold water region, is used to determine the relationship between coastal upwelling and the wind stress derived from the ERS-2 (European Remote Sensing Satellite) data. The results show that the upwelling intensity has a good linear relationship with the total alongshore wind stress while it has a low correlation with the cross-shore component of wind stress. These results imply that the alongshore wind stress is the main factor to pump the cold water up to the sea surface layer. Meanwhile, the satellite infrared images also indicate that the centroid of cold water moved southward from 15°N to 11°N during the observation period. The size of upwelling area changed as well, and finally evolved into a cold jet stretching offshore along 11°N–12°N in the mid-August 1997. Satellite infrared and altimeteric data show that the evolution of upwelling region is closely associated with the development of two anticyclonic circulations in the western SCS.

Statistical and dynamical analyses of generation mechanisms of solitary internal waves in the northern South China Sea

[1] This study analyzes the effects of thermocline shoaling on the ocean internal wave (IW) generation in the north South China Sea (NSCS). Seven years of satellite synthetic aperture radar (SAR) images from 1995 to 2001 are used for the statistical analysis of IW occurrence, and field measurements of sea surface wind, sea state, and vertical temperature profiles are used for analyzing IW generation and SAR imaging conditions. Latitudinal distribution of IW packets shows that 22% of IW packets distributed in the east of 118E obviously originate from the Luzon Strait, and 78% of IW packets west of 118E may propagate from the east or evolve into the solitons originating from the east boundary owing to the fission effect of shoaling thermocline. The yearly distribution of IW occurrence frequencies reveals an interannual variability, implying that there are long-term and large-scale processes modifying the SAR-observed IW occurrence. The monthly SAR-observed IWoccurrence frequencies show that the high frequencies are distributed from April to July and reach a peak in June with a maximum frequency of 20%. The low occurrence frequencies are distributed in winter from December to February of next year with a minimum frequency of 1.5% in February. This study proposes that the IW generation needs the necessary and sufficient conditions: initial disturbance formation and wave amplitude growth. Owing to the dissipation effect on the initial disturbance, only fully grown waves have a chance to radiate out of the source region. A physical model and PKdVequation are adopted for analyzing the sufficient conditions for solitary IW amplitude growth. The results indicate that the thermocline shoaling provides the forcing to soliton amplitude growth, so that the soliton amplitude growth ratio (SAGR) serves as a decisive factor for the IW occurrence frequency. Theoretical analysis predicts a linear relation between the two. Application of theoretical models to field measurements in the Luzon Strait gives a correlation coefficient as high as 0.845 with a confidence level of 99% for months from March to November. The linear regression gives a correlation coefficient (R 2 ) of 0.6519 and a SAGR threshold (minimum) value of

Seasonal variability of sea surface height in the South China Sea observed with TOPEX/Poseidon altimeter data

TOPEX/Poseidon altimeter measurements along 16 ground tracks in the South China Sea from November 1992 to October 1997 were used to observe the seasonal variability of sea surface height (SSH). In winter (November, December, January, and February) the SSH images are generally characterized by a sea surface tilting downward toward the east. High sea levels on the western side of the sea have a little monthly change, and a low (minus) sea level peak is centered in the northeastern deep basin. In spring (March and April), a single high sea level peak (HP) centered at 14°N, 114°E becomes a dominant feature. In summer (May, June, July, and August) the SSH images show a sea surface tilting downward toward the west. The HP moves from 14°N, 114°E in May to the northwest of Luzon Island in July. Low sea levels are centered in the offshore of the northern and southern Vietnamese coast. In fall (September and October) the appearance of an eastward low sea level jet in the western basin between 12° and 14°N constitutes a major feature of the SSH images. These results are comparable with previous results and drifter measurements.

Dynamically Active Areas in the South China Sea Detected from TOPEX/POSEIDON Satellite Altimeter Data

Abstract TOPEX/POSEIDON (T/P) altimeter measurements from December 1992 to April 1998 were processed to study the sea surface height (SSH) variation in the South China Sea (SCS). Patterns of variance of the SSH deviations from the four-year (1993–1996) mean show that there are three high-variation areas in the deep basin. The first is at the northern basin west of Luzon Island (16°–18° N, 117°–119° E), the second is at the center of the SCS (14°–16° N, 113.5°–114.5° E), and the third is off the central Vietnam coast (12°–13° N, 110°–112° E). From empirical orthogonal function (EOF) analysis, we found that the variations of these three active areas are dominated by annual cycles, but out of phase. The first two areas have the highest SSH in June and the lowest SSH in December. The third one off the Vietnam coast has the highest SSH in February and the lowest SSH in August. The EOF analysis also reveals that the variation of SSH in the SCS has a response to El Nino events.

Coastal lee waves on ERS-1 SAR images

Alternative dark-bright patterns on ERS-1 synthetic aperture radar (SAR) images of the west side of the Taiwan Strait taken on December 8, 1994, were recognized to be the sea surface signature of a coastal lee wave. Such waves are called coastal lee waves because they occur along the lee side of the coast. The coastal lee waves appeared in the form of a wave packet distributed within an offshore band 20–40 km wide. The first packet, which occurred in the northern portion of the observed area, contained six waves with variable wavelengths (defined as the spatial separation between two waves) from 1.7 to 2.7 km. The second packet, in the middle, contained 10 waves with a relatively uniform wavelength of 4.2 km. The third packet, in the southern portion, contained 17 waves with an average wavelength of 2.0 km. The crest lengths were from 20 to 80 km. Local meteorologic parameters observed simultaneously at Fuzhou, China, close to the imaged area, showed an offshore wind of 1.5–3.5 m/s and a land surface air temperature of 19°C, which was 4°C lower than the sea surface temperature (SST). Thus the lower atmospheric boundary conditions at imaging time were very favorable both for generating the land breeze circulation and small wind waves on the sea surface, which are in the Bragg-scattering wavelength band of the C band ERS-1 SAR. A physical model of a three-layer atmosphere was developed in order to explain how the land breeze circulation can generate the coastal lee waves. The results showed that the vertical velocity disturbance caused by the wind convergence at the land breeze frontal zone is of vital importance for the generation of coastal lee waves, and the model gave very good estimates of the processes observed. The SAR imaging mechanisms of the waves were analyzed in detail. The differences between coastal lee waves and ocean internal waves, which appear as similar alternative dark-bright patterns on SAR images, were also discussed.

Observation of the Kuroshio intrusion region in the South China Sea from AVHRR data

The variability of sea surface temperature in the region of the Kuroshio intrusion into the South China Sea (SCS) through the Luzon Strait was studied using sea surface temperature (SST) derived from Advanced Very High Resolution Radiometer (AVHRR) from 1985 to 2002. The covariance empirical orthogonal function (CEOF) method was applied for analysing the temporal and spatial variability in the study area. The results show that the Kuroshio intrusion during El Niño periods is weaker than that in La Niña periods. The calculation of surface layer heat of the Kuroshio intrusion region also shows response to the El Niño-La Niña events. The variation is attributed to the changes in wind fields during those events.

Centroid motion of the Western Pacific warm pool during three recent El Nino-Southern Oscillation events

Abstract The centroid motion of the Western Pacific warm pool for 1982–83, 1986–87 and 1991–93 El Nino–Southern Oscillation events was investigated by analyzing satellite multichannel sea surface temperature and satellite pathfinder sea surface temperature data in conjunction with wind data from The Florida State University. The authors found that the direction of motion of the warm pool’s center changes shortly before the onset of a new El Nino event and that this change was quite different during the 1991–93 El Nino than during the 1982–83 and 1986–87 El Nino events. The possible cause of these changes and differences may be the El Nino–related wind pattern shifts in the western equatorial Pacific Ocean before and during the events, and the seasonal phase-lock of the El Nino events.

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 Effects of Shear Flow on Propagation of Rossby Waves in the Equatorial Oceans

Abstract The effects of strong meridional shear of mean flow on propagation of long waves having a wavelength of about 1000 km and period of 25 days observed in the equatorial Pacific and Atlantic Oceans are analyzed in this paper. Information from the weekly multichannel sea surface temperature images derived from the Advanced Very High Resolution Radiometer on board the NOAA series satellites from January 1986 to March 1992 is used for the statistics of the visual parameters of the waves. The characteristic scales of the waves are estimated based on the results by previous investigators and the statistical results of the present study. Solving a potential vorticity equation, the authors obtain a dispersion relation with an additional term depending on the flow shear strength compared with that of the free Rossby wave. The theoretical model provides a good explanation for the variable behavior of long-wave propagation.