Gregg A. Jacobs

A statistical comparison of wind speed, wave height, and wave period derived from satellite altimeters and ocean buoys in the Gulf of Mexico region

The capability of spaceborne altimeters to provide precise measurement of significant wave height and wind speed has been demonstrated repeatedly. It is shown in this paper that in addition to the significant wave height and wind speed, the wave period can be calculated from the semiempirical functions established from earlier wave research. The calculated characteristic wave period using the altimeter-derived wind speed and wave height are found to be in excellent agreement with the peak wave period and average wave period from the ocean buoy measurements in the Gulf of Mexico. Also, with the long time series of collocated data set, it is possible to compare altimeter output of wind and wave parameters with ocean buoy measurements taking into consideration the spatial lags between the buoy locations and the altimeter footprints, and the temporal lags between the two sensor systems. It is found that when the spatial lags are less than 10 km, the RMS difference of the significant wave height is approximately 0.1 m, which is the digitization resolution of the output from both altimeter and ocean buoy. For the wind speed, the RMS difference approaches 1.2 m/s in the Gulf of Mexico using the empirical algorithms. The wind speed agreement is significantly improved to 0.8 m/s when the tilting effect on the altimeter cross section is accounted for. In contrast to the spatial lags, temporal lags of up to 1 hour do not appear to produce significant difference in the statistics of comparison based on this study.

Ocean circulation variations associated with the Antarctic Circumpolar Wave

Altimeter data analysis indicates persistent sea surface height (SSH) anomalies propagating eastward about the Antarctic continent. The spatial and temporal characteristics match variations in the atmosphere and sea surface temperature (SST) that are associated with the Antarctic Circumpolar Wave (ACW). During the observation time period, the SSH appears quasiperiodic with a dominant 4 year period and 180° longitude wavelength. Thus, the SSH signature of the ACW appears as two anomalies on opposite sides of the Antarctic continent propagating eastward at 10 cm/s. The SSH response to observed wind forcing agrees in terms of amplitude and phase with simple quasigeostrophic dynamics.

Low-Frequency Current Observations in the Korea/Tsushima Strait*

Abstract High resolution, continuous current measurements made in the Korea/Tsushima Strait between May 1999 and March 2000 are used to examine current variations having time periods longer than 2 days. Twelve bottom-mounted acoustic Doppler current profilers provide velocity profiles along two sections: one section at the strait entrance southwest of Tsushima Island and the second section at the strait exit northeast of Tsushima Island. Additional measurements are provided by single moorings located between Korea and Tsushima Island and just north of Cheju Island in Cheju Strait. The two sections contain markedly different mean flow regimes. A high velocity current core exists at the southwestern section along the western slope of the strait for the entire recording period. The flow directly downstream of Tsushima Island contains large variability, and the flow is disrupted to such an extent by the island that a countercurrent commonly exists in the lee of the island. The northeastern section is marked b...

Connectivity of the Taiwan, Cheju and Korea Straits

Insight into the circulation of the East China Sea and origin of the Tsushima Current are investigated through direct, concurrent measurements of velocities through the Taiwan, Cheju, and Korea Straits. Current data are obtained from six bottom-mounted acoustic Doppler current profilers (ADCPs) arrayed along a section spanning the Korea Strait, a single bottom-moored ADCP in the Cheju Strait, and four bottom-moored ADCPs along a section spanning the Taiwan Strait. Mass transports are computed for the October–December, 1999 time period. In addition, temperature and salt transports are examined in conjunction with climatological values of temperature and salinity. Average volume transport is 0.14 Sverdrups (Sv) through the Taiwan Strait, 0:59 Sv for the Cheju Strait, and 3:17 Sv for the Korea Strait. Salt and temperature transport through the Korea Strait and into the Japan/East Sea are 110:48 � 10 6 kg=s and 0:24 � 10 15 watts (W), respectively. Heat loss in the East China Sea is approximately 200 W=m 2 : Winds affect the transports in each of the straits. Most noticeable wind effects are observed in the Taiwan Strait where strong north wind events force flow into the South China Sea. The main source for the Tsushima Current and its flow into the Japan/East Sea is clearly the Kuroshio for fall, 1999. Published by Elsevier Science Ltd.

Monthly Variations of Water Masses in the Yellow and East China Seas, November 6, 1998

The monthly water mass variations in the Yellow Sea and the East China Sea are investigated using over 40 years of historical temperature and salinity observations via a cluster analysis that incorporates geographical distance and depth separation in addition to the temperature and salinity. Results delineate monthly variations in the major water masses and provide some insight into formation mechanisms and intermixing. The major water masses include the Kuroshio-East China Sea water (KE), the Yellow Sea surface water (YSS) and bottom cold water (YSB), mixed water (MW), and coastal water (CW). The distribution of the KE water mass reveals the intrusion pattern into the area west of Cheju. A separate mixed water type appears between the KE water mass and the Yellow Sea water masses during winter. The formation mechanism of the YSB appears to be the surface cooling and active mixing in winter. In the East China Sea, during summer, surface water is differentiated from the subsurface water while there is no differentiation during winter. In the Yellow Sea, a three layer system exists in the summer and fall (May–November) while a two layer system exists during the rest of the year. A fresh water mass generated by Yangtze River discharge (YD) is present over the northern East China Sea and the southern Yellow Sea during summer.

The global structure of the annual and semiannual sea surface height variability from Geosat altimeter data

The global structure of the annual and semiannual sea surface height variability is constructed from the first 2 years of the Geosat Exact Repeat Mission (ERM) altimeter data. The GEM-T2 orbits available for the first 2 years of the ERM have an accuracy of 40 cm RMS. Residual orbit error is modeled as one cycle per orbital revolution and is removed from collinear differences of arcs made of data from one full orbital revolution of the satellite. The error in the Schwiderski M2 tidal model must be estimated due to the fact that the tidal variability aliases to 1.15 cycles per year (cpy), and this frequency is not separable from 1 cpy with the 2 years of data. An estimate of the M2 tidal error is made based on the particular temporal and spatial aliasing of the tide. With this error removed, a least squares fit of sine and cosine waves with annual and semiannual frequencies is made to the time series at every point along the ground track of the satellite. This produces sine and cosine coefficients at the ground track points which are interpolated to a regularly spaced ½° grid over the globe. From the sine and cosine maps, amplitude and phase may be obtained. Interpolation of the sine and cosine coefficients using different spatial scales is done to better observe the large-scale phase changes and to remove small-scale noise. Results show the phase relationships between major current systems, large-scale variations near the equator in the Intertropical Convergence Zone (ITCZ), a 180° phase difference between the northern and southern hemispheres for the annual variability, large-scale westward propagating waves, and other large-scale gyre features.

Mesoscale variability in the boundary currents of the Alaska Gyre

Abstract Measurements of sea-surface height anomalies acquired during the GEOSAT, ERS-1, and TOPEX altimeter missions show that the boundary currents of the Alaska gyre exhibit interannual variability with respect to the occurrence, size, and propagation of mesoscale, eddy-like features. Observations and model results suggest that eddies are generated in the Alaska Current during years in which the wind forcing in the eastern Gulf of Alaska promotes strong downwelling along the British Columbia–Alaska coast. Wind forcing conditions that support eddy formation and intensification often occur in years that coincide with El Nino–Southern Oscillation events. Eddy variability is significantly more deterministic in the Alaska Current than in the Alaskan Stream.

Yellow and East China Seas response to winds and currents

The influences of the Kuroshio Current (KUC), Taiwan Warm Current (TWC), and surface wind stress on the Yellow and East China Seas (YES) are examined using tracers in a Princeton Ocean Model. Two experiments are performed: one with wind stress and one without. Seasonal variations in the inflow and outflow of the TWC and Tsushima Current are specified to examine the effects of the current transports. Two separate tracers are inserted into each model experiment to track the pattern of KUC and TWC waters into the YES. The two main areas of KUC and TWC water movement into the Yellow Sea are the southern entrance to the Yellow Sea trough and the Yangtze Relict River valley. Results indicate that KUC and TWC waters advect into the Yellow Sea regardless of wind stress. However, in the Yellow Sea during winter the wind stress increases KUC and TWC concentration at 20 m. The wind stress also produces short time period events that horizontally advect water masses and spatially smooth seasonally averaged water mass concentrations. The bottom Ekman layer appears to be one of the mechanisms driving the northward bottom flow across the East China Sea shelf. The bottom friction layer is stronger in summer when the TWC velocity is high. The bottom friction layer draws KUC water across the bottom of the continental shelf into the Yangtze Relict River valley and generates upwelling along the Chinese coast.

Transport reversals at Taiwan Strait during October and November 1999

[1] The observed transport reversals at Taiwan Strait during October and November 1999 are examined by analytic solutions, a numerical ocean model, and the prediction from a real-time, North Pacific Ocean, data-assimilating model. Wind stress explains a majority of the transport reversals. The reversals are forced by a combination of the local wind and the remote wind in the Yellow and East China Seas. The connection between the Yellow and East China Seas wind stress and transport reversals at Taiwan Strait is provided by coastally trapped waves. The waves are generated by the northerly winter wind bursts in the Yellow Sea and are enhanced in the East China Sea by alongshore northerly wind.

Monthly Variations of Water Masses in the Yellow and East China Seas

The monthly water mass variations in the Yellow Sea and the East China Sea are investigated using over 40 years of historical temperature and salinity observations via a cluster analysis that incorporates geographical distance and depth separation in addition to the temperature and salinity. Results delineate monthly variations in the major water masses and provide some insight into formation mechanisms and intermixing. The major water masses include the Kuroshio-East China Sea water (KE), the Yellow Sea surface water (YSS) and bottom cold water (YSB), mixed water (MW), and coastal water (CW). The distribution of the KE water mass reveals the intrusion pattern into the area west of Cheju. A separate mixed water type appears between the KE water mass and the Yellow Sea water masses during winter. The formation mechanism of the YSB appears to be the surface cooling and active mixing in winter. In the East China Sea, during summer, surface water is differentiated from the subsurface water while there is no differentiation during winter. In the Yellow Sea, a three layer system exists in the summer and fall (May– November) while a two layer system exists during the rest of the year. A fresh water mass generated by Yangtze River discharge (YD) is present over the northern East China Sea and the southern Yellow Sea during summer.