Jiayi Pan

River Influences on Shelf Ecosystems: Introduction and synthesis

[1] River Influences on Shelf Ecosystems (RISE) is the first comprehensive interdisciplinary study of the rates and dynamics governing the mixing of river and coastal waters in an eastern boundary current system, as well as the effects of the resultant plume on phytoplankton standing stocks, growth and grazing rates, and community structure. The RISE Special Volume presents results deduced from four field studies and two different numerical model applications, including an ecosystem model, on the buoyant plume originating from the Columbia River. This introductory paper provides background information on variability during RISE field efforts as well as a synthesis of results, with particular attention to the questions and hypotheses that motivated this research. RISE studies have shown that the maximum mixing of Columbia River and ocean water occurs primarily near plume liftoff inside the estuary and in the near field of the plume. Most plume nitrate originates from upwelled shelf water, and plume phytoplankton species are typically the same as those found in the adjacent coastal ocean. River-supplied nitrate can help maintain the ecosystem during periods of delayed upwelling. The plume inhibits iron limitation, but nitrate limitation is observed in aging plumes. The plume also has significant effects on rates of primary productivity and growth (higher in new plume water) and microzooplankton grazing (lower in the plume near field and north of the river mouth); macrozooplankton concentration (enhanced at plume fronts); offshelf chlorophyll export; as well as the development of a chlorophyll ‘‘shadow zone’’ off northern Oregon.

Interpretation of scatterometer ocean surface wind vector EOFs over the Northwestern Pacific

Abstract Satellite scatterometer winds over the northwestern Pacific were analyzed with the vector empirical orthogonal function (VEOF) method. The Hilbert–Huang transform (HHT), a newly developed non-linear and non-stationary time series data processing method, was also employed in the analysis. A combination of European Remote Sensing Satellite (ERS) −1/2 scatterometer, NASA Scatterometer (NSCAT) and NASAs Quick Scatterometer (QuikSCAT) winds covering the period from January 1992 to April 2000 and the area of 0–50°N, 100–148°E constitutes the baseline for this study. The results indicate that annual cycles dominate the two leading VEOF modes. The first VEOF shows the East Asian monsoon features and the second represents a spring–autumn oscillation. We removed the annual signal from the data set and calculated the interannual VEOFs. The first interannual VEOF represents the interannual variability existing in the spring–autumn oscillation. The temporal mode is correlated with the Southern Oscillation Index (SOI), but has a half-year lag with respect to the SOI. The spatial mode of the first interannual VEOF reflects the response of the tropical and extratropical winds to ENSO events. The second interannual VEOF is another ENSO related mode, and the temporal VEOF mode is correlated with the SOI with a correlation coefficient of 0.78, revealing the wind variability over mid-latitudes, which is associated with ENSO events. Further analysis indicated that the wind variability over the coast of East Asia represents anomalies of a Hadley cell. The quasi-biennial oscillation (QBO) was found in the temporal mode, indicating and verifying that the QBO in the wind fields is related to ENSO events. The third VEOF shows the interannaul variability in the winter–summer mode and displays the interannual variability of the East Asian monsoon. The three leading interannual VEOFs are statistically meaningful as confirmed by a significance test.

Nonlinear evolution of ocean internal solitons propagating along an inhomogeneous thermocline

Measurements of the upper ocean thermal structure show that on the continental shelves the thermocline depth may shoal or deepen generally depending on the bottom topography. Thermocline shoaling and deepening cause changes in the phase speeds of internal waves as described by linear wave theories. On the other hand, the ocean area where internal waves have variable phase speeds may be treated as a dynamically inhomogeneous medium. In this case, theories of nonlinear dispersive wave propagation in inhomogeneous media developed by Tappert and Zabusky [1971] may stand. We used these theories to analyze the evolution of ocean internal solitary waves passing over a seamount in the Gulf of Aden. The results indicate that a surprisingly sharp recess of an internal solitary wave packet, imaged by the space shuttle Discovery, is a signature of spatial phase delay caused by thermocline shoaling over the seamount. Soliton fission due to thermocline shoaling was also observed in the imagery. The observed number of transmitted solitons over the seamount agrees with theoretical predictions. Relative soliton amplitudes measured from the imagery also agree qualitatively with predictions.

Pacific warm pool excitation, earth rotation and El Niño southern oscillations

The interannual changes in the Earths rotation rate, and hence in the length of day (LOD), are thought to be caused by the variation of the atmospheric angular momentum (AAM). However, there is still a considerable portion of the LOD variations that remain unexplained. Through analyzing the non-atmospheric LOD excitation contributed by the Western Pacific Warm Pool (WPWP) during the period of 1970-2000, the positive effects of the WPWP on the interannual LOD variation are found, although the scale of the warm pool is much smaller than that of the solid Earth. These effects are specifically intensified by the El Nino events, since more components of the LOD-AAM were accounted for by the warm pool excitation in the strong El Nino years. Changes in the Earths rotation rate has attracted significant attention, not only because it is an important geodetic issue but also because it has significant value as a global measure of variations within the hydrosphere, atmosphere, cryosphere and solid Earth, and hence the global changes.

Vector empirical orthogonal function modes of the ocean surface wind variability derived from satellite scatterometer data

Ocean surface winds derived from NSCAT, QuikSCAT and ERS-1/2 scatterometer observations during a period from January 1992 to April 2000 were analyzed using the vector empirical orthogonal function (VEOF) method. With the boreal winter and summer oscillation, the first VEOF is dominated by the Indian and East Asian monsoons and also shows an annual cycle of the trade winds. The second VEOF represents the boreal autumn and spring oscillation, and reveals a transition state between winter and summer. The third VEOF indicates the wind variability associated with El Nino Southern Oscillation (ENSO) events, because the temporal mode has a high correlation coefficient of 0.8 with the Southern Oscillation Index (SOI). Furthermore, the third mode reveals the teleconnection of the Indian monsoon and wind variability over high latitude oceans, such as the Aleutian Low system, with ENSO events.

Temporal and spatial variability of the surface temperature anomaly in the Levantine Basin of the Eastern Mediterranean

In this study we use the monthly mean sea surface temperature (SST) fields (1985-1997), calculated by the National Oceanic and Atmospheric Administration (NOAA) Pathfinder project analysis of advanced very high resolution radiometer (AVHRR) data, to investigate the temporal and spatial variability of warm/cool anomalies in the Levantine Basin (LB). Wind data from European Remote Sensing Satellite-1/2 (ERS-1/2) over the LB are analysed and surface wind stress and wind stress curls are calculated. First, we employed an optimal interpolation scheme to build up a dataset for 1985-1997 on an 18 km 2 18 km grid. An ensemble mean SST field ( T ) has been established with a rather weak horizontal gradient (19.4° C south-east of Rhodes to 22.4° C southeast of the LB). Second, we performed a composite analysis to obtain the monthly averaged SST anomaly field ( T ) deviation from the ensemble mean. Three areas of evident SST anomalies were found: south of Rhodes, south-east of Crete and north-east of Cyprus. During the spring-to-summer transition (March to May), a small low anomaly is formed south-east of Crete ( T <-1.4° C in May), and a small high anomaly is formed south of Rhodes ( T >-0.4° C in May). During the autumn-to-winter transition (October to November), a small high anomaly forms south-east of Crete ( T >1.2° C in November), and a small low anomaly forms between Rhodes and Antalia Bay ( T <-0.2° C in November). Third, we performed an empirical orthogonal function (EOF) analysis on the residual T relative to T + T in order to obtain transient and interannual variations of the SST fields. EOF1 explains 73% of the total variance and represents the warm/cool anomaly pattern, with a maximum value in the region between the south of Rhodes and Cyprus and a minimum value off the western part of the Egyptian coast. EOF2 explains 6% of the total variance. A strong LB warm anomaly (1° C warmer) appears during August 1988, May 1994 and September-October-November 1994. A strong cool anomaly (1° C cooler) occurs during May 1987, November 1988, August 1989 and May 1992. The cross-correlation coefficient between wind stress curl from ERS-1/2 over the LB in the period from January 1992 to December 1997 and the SST anomaly were calculated, and it reveals the air-sea interaction in the LB.

A New Method for Estimation of the Sensible Heat Flux under Unstable Conditions Using Satellite Vector Winds

It has been difficult to estimate the sensible heat flux at the air‐sea interface using satellite data because of the difficulty in remotely observing the sea level air temperature. In this study, a new method is developed for estimating the sensible heat flux using satellite observations under unstable conditions. The basic idea of the method is that the air‐sea temperature difference is related to the atmospheric convergence. Employed data include the wind convergence, sea level humidity, and sea surface temperature. These parameters can be derived from the satellite wind vectors, Special Sensor Microwave Imager (SSM/I) precipitable water, and Advanced Very High Resolution Radiometer (AVHRR) observations, respectively. The authors selected a region east of Japan as the test area where the atmospheric convergence appears all year. Comparison between the heat fluxes derived from the satellite data and from the National Centers for Environmental Prediction (NCEP) data suggests that the rms difference between the two kinds of sensible heat fluxes has low values in the sea area east of Japan with a minimum of 10.0 W m22. The time series of the two kinds of sensible heat fluxes at 10 locations in the area are in agreement, with rms difference ranging between 10.0 and 14.1 W m22 and correlation coefficient being higher than 0.7. In addition, the National Aeronautics and Space Administration (NASA) Goddard SatelliteBased Surface Turbulent Flux (GSSTF) was used for a further comparison. The low-rms region with high correlation coefficient (.0.7) was also found in the region east of Japan with a minimum of 12.2 W m22. Considering the nonlinearity in calculation of the sensible monthly means, the authors believe that the comparison with GSSTF is consistent with that with NCEP data.

Tidal Variability Related to Sea Level Variability in the Pacific Ocean

Ocean tides are changing worldwide for reasons unrelated to astronomical forcing. Changes in tidal properties coupled with altered mean sea level (MSL) may yield higher peak water levels and increased occurrence of short-term exceedance events such as storm surge and nuisance flooding. Here we investigate the hypothesis that changes in relative sea-level are correlated with alterations in tidal amplitudes. Our approach focuses on the correlation between short-term (monthly to interannual) fluctuations in sea-level with changes in tidal properties of major ocean tides (M2, and K1; S2 and O1) at 152 gauges. Results suggest that sea-level variability is correlated to inter-annual tidal variability at most (92%) of tide gauges in the Pacific, with statistically significant rates between ±10 and ±500 mm per meter sea-level rise observed. These tidal anomalies, while influenced by basin-scale climate processes and sea-level changes, appear to be locally forced (in part) and not coherent over amphidromic or basin-wide scales. Overall, the Western Pacific shows a greater concentration of tide/sea level correlations at interannual time scales than the Eastern Pacific; 44% and 46% of gauges are significant in K1 and O1 in the west compared to 29% and 30% in the east, and 63% and 53% of gauges in the west are significant in M2 and S2 versus 47% and 32% in the east. Seasonal variation in tidal properties is less apparent in the empirical record, with statistically significant seasonal variations observed at only 35% of all gauges, with the largest concentrations in Southeast Asia.

Scatterometers observe the variation of the Walker circulation

The variation of the Walker circulation is closely related to the onset of El Nino-Southern Oscillation (ENSO) events [Kousky, 1984], so observing the Walker circulation can lead to better understanding of ENSO events. Wind data from the NASA Scatterometer (NSCAT), Quick Scatterometer (QuikSCAT), and European Remote Sensing Satellite-1/2 (ERS-1/2) were merged into one data set in a 1°×1° grid covering a period from January 1992 to December 2000. The study region was confined to the tropical oceans between 20°S and 20°N. The vector empirical orthogonal function (VEOF) method [Hardy and Walton, 1978] was used to process the wind data set and extract principal modes of the wind fields.

A Method for Processing Acoustic Doppler Current Profiler Velocity Data from Towed, Undulating Vehicles

The utility of the acoustic Doppler current profiler (ADCP) for sampling small time and space scales of coastal environments can be enhanced by mounting a high-frequency (1200 kHz) ADCP on an oscillating towed body. This approach requires both an external reference to convert the measured shears to velocities in the earth coordinates and a method to determine the towed body velocities. During the River Influence on the Shelf Ecosystems (RISE) project cruise, a high-frequency (1200 kHz) and narrowbeam ADCP with mode 12 sampling was mounted on a TRIAXUS oscillating towfish, which steers a 3D path behind the ship. This deployment approach extended the vertical range of the ADCP and allowed it to sample near-surface waters outside the ship’s wake. The measurements from a ship-mounted 1200-kHz narrowbeam ADCP are used as references for TRIAXUS ADCP data, and a method of overlapping bins is employed to recover the entire vertical range of the TRIAXUS ADCP. The TRIAXUS vehicle horizontal velocities are obtained by removing the derived ocean current velocity from the TRIAXUS ADCP measurements. The results show that the method is practical.