Hu Dunxin

DYNAMIC AND THERMOHALINE PROPERTIES OF THE MINDANAO UNDERCURRENT I. DYNAMIC STRUCTURE

Hydrographic data from eleven 1986–1991 cruises at zonal sections near 8°N from the Philippine coast to 130°E were used to examine the dynamic structure of the western boundary currents there in the present study focusing on the Mindanao Undercurrent (MUC). The MUC with maximum velocity >10 cm/s was found to be a feature of both the individual cruises and multiyear mean velocity field as a countercurrent below the Mindanao Current (MC). Usually, the MUC occupies depths below 200 dbar and consists of more than one core. Its vertical and horizontal scales are 500–1000 m and 100–250 km, respectively, and vary greatly and irregularly. The spatial distribution of the MUC agreed with that of the westward deepening isopycnals in and below the thermocline. In the individual cruises, the volume transports of the MUC relative to 1500 dbar varied from 6.2 to 28.4×106 m3/s (average of 14.4 ×106 m3/s) while those relative to 3000 dbar varied from 15.4 to 43.9×106 m3/s (average of 25.4×106 m3/s). The multiyear mean MUC transport was 5.9×106 m3/s relative to 1500 dbar and 8.7×106 m3/s relative to 3000 dbar. The difference between the multiyear mean MUC transport and that of individual cruises resulted from the MUC spatial variation.

Interannual variability of the southern Yellow Sea Cold Water Mass

Temperature data collected in the sections of 34°N, 35°N and 36°N in August from 1975 through 2003 were analyzed using Empirical Orthogonal Function (EOF) to investigate interannual variability of the southern Yellow Sea Cold Water Mass (YSCWM). The first mode (EOF1) reveals variations of basin-wide thermocline depth, which is mainly caused by surface heating. The second mode (EOF2) presents fluctuations of vertical circulation, resulting mainly from interannual variability of cold front intensity. In addition, it is found that the upward extent of upwelling in the cold front is basically determined by wind stress curl and the zonal position of the warm water center in the southern Yellow Sea is correlated with spatial difference of net heat flux.

Upwelling and sedimentation dynamics: II. A simple model

It was found that the location and distributional pattern of certain sea bottom mud patches in the Yellow Sea and East China Sea are identical with the geographical location and flow pattern of the upwelling above. Following a general description by Hu (1984) of the role of upwelling in sedimentation, this study presents a two—dimensional numerical model to further examine the sedimentation dynamics in upwelling areas. The model shows that advection by ocean circulation, downward settling due to gravity and turbulent mixing are the main mechanisms controlling the movement of suspended matters, and that the upwelling on the shelf favors deposition near the bottom. It is concluded that on the shelf (1) wherever there is upwelling, mud sediment must exist; and (2) the closer to the center of upwelling, the greater the concentration of suspended matters in the water, the greater the depositing rate near the bottom and the finer the sediment on the sea bed.

Long-term variabilities of thermodynamic structure of the East China Sea Cold Eddy in summer

Based on more than 30 years observed sectional temperature data since the 1960s, and compared with multi-year wind and Changjiang (Yangtze) River discharge data, spatial-temporal variations of the East China Sea Cold Eddy (ECSCE) in summer was analyzed in relationship to ocean circulation and local atmospheric circulation. Empirical Orthogonal Function (EOF) and Singular Value Decomposition (SVD) analyses were applied to this study. The results show that; 1) The ECSCE in summer possesses significant interannual variabilities, which are directly associated with oceanic and atmospheric circulation anomaly. Main fluctuations demonstrate their falling in basically with El Niño events (interannual) and interdecadal variability. 2) The ECSCE in summer is closely related to the variation of the Yellow Sea Warm Current (YSWC) and the Changjiang River discharge. The stronger the YSWC, the more intensive the ECSCE with its center shifting westward, and vice versa. However, a negative correlation between the Changjiang River discharge and the ECSCE strength is shown. The ECSCE was strengthened after the abrupt global climate change affected by the interdecadal variation of the YSWC. 3) SVD analysis suggested a high correlation between the variation of the ECSCE in summer and the anomalous cyclonic atmospheric circulation over the ECS. Intensification of the cyclonic wind strengthens the ECSCE, and vice versa. 4) The cyclonic atmospheric circulation has dominant influence on the interannual variation of the ECSCE, and the influence of the ocean circulation takes the second in. The ECSCE was usually stronger in El Niño years affected by strong cyclonic circulation in the atmosphere. The variation in strength of the ECSCE resulted from the joint effect of both oceanic and atmospheric circulation.

Dynamic and thermohaline properties of the Mindanao Undercurrent, part II: Thermohaline structure

Hydrographic data from eleven 1986–1991 cruises at zonal sections near 8°N from the Philippine coast to 130°E were used to examine thermohaline structures and water mass properties of the western boundary currents there, especially those of the Mindanao Undercurrent (MUC). The finding that the MUC consisted of two water masses with salinity of 34.6 at 26.9 σt and 34.52 at 27.2 σt which were remnants of the lower part of the Southern Pacific Subtropical Water (SPSW) and of the Antarctic Intermediate Water (AAIW) of South Pacific origin, respectively, showed that the MUC was not a local transient but originated elsewhere. As the MUC flowed from 7.5°N to 8°N, part of it carrying the SPSW turns anticyclonically and eastward. The Northern Pacific Intermediate Water (NPIW) often joins the MUC, which suggests that the NPIW carried by the MC partly returns northward as a result of the shear between the MC and the MUC or other processes. The shear instability provides the energy for the irregular fluctuation of the MUC.

Surface circulation patterns observed by drifters in the Yellow Sea in summer of 2001, 2002 and 2003

In summer of 2001, 2002 and 2003, ten, six and seventeen satellite-tracked surface drifters with drogues centered at 15 and 4 m were deployed, respectively, in the southern Yellow Sea (YS). 23 drifters of them transmitted useful data of at least 30 days. The wind-driven component of the drift was removed from the original drift velocity of drifters. The wind data used are from NCEP (National Center for Environmental Prediction), USA.Trajectories and drift velocities of the 23 drifters depicted the upper circulation structure in the southern YS. There exists an anti-cyclonic eddy with a mean speed and radius of 0.063 m/s and 50km in the central southern YS, whose center lingered within 35.3–36.0°N/123.5–124.0°E. Showed by 6 drifters, a basin-scale elliptic cyclonic gyre with a mean speed of 0.114 m/s, long and short radius of 250 and 200 km surrounds the anti-cyclonic eddy. In the southwestern part of the southern YS has obvious frontal eddy activities within about 100 km with a mean speed about 0.076 m/s. All the drifters passing Korean coast were staggering for more than 10 days west of a protruding cape of central Korea. A small-scale cyclonic eddy centered at around 120.5°E/35.1°N with a mean speed of 0.048 m/s was observed in western part of the southern YS.

Upwelling and sedimentation dynamics III: Coincidence of upwelling areas with mud patches in north hemisphere shelf seas

The determinant role of upwelling in fine sediment patches is examined from the viewpoints of physical process and biological process respectively. It is pointed out that physical and biological processes are usually coexistent and interact with each other during the sedimentation of suspended matters. This study used available figures showing the circulation pattern and surface sediment distribution in the whole China Seas, the Gulf of Maine, the Irish Sea and the North Sea, and additional data to verify that wherever upwelling exists on the continental shelf, mud must occur; and that wherever downwelling occurs, coarse sediment substitutes for mud.

Outflow of suspended materials from the Changjiang River mouth

Typical wind fields in winter and summer were applied in a three-dimensional model for simulating the wind-driven current fields in the study region 27.0 °N to 34.5 °N, 120.0 °E to 126.0 °E. Data from three cruises in 1993 and 1994 were analyzed to obtain the horizontal and vertical structures of total suspended matter (TSM) there. Based on this study, a numerical method was derived to show that it is in winter, not summer, that the suspended matter (SM) is carried out from the mouth of the Changjiang River to the deep ocean, and yield some other important conclusions.

Heat center of the western Pacific warm pool

A heat center (HC) of the western Pacific warm pool (WPWP) is defined, its variability is examined, and a possible mechanism is discussed. Analysis and calculation of a temperature dataset from 1945–2006 show that the mean position of the HC during this period was near 0.4°S/169.0°E, at 38.0 m depth. From a time series of the HC, remarkable seasonal variability was found, mainly in the meridional and vertical directions. Interannual variabilities were dominant in the zonal and vertical directions. In addition, semiannual variation in the HC depth was discovered. The longitude of the HC varies with ENSO events, and its latitude is weakly related to ENSO on time scales shorter than a decade. The variation of the HC longitude leads the Niño-3 index by about 3–4 months, and its depth lags the index for approximately 3 months. It is concluded that the HC depth results from a combination of its longitudinal and latitudinal variations. Low-pass-filtered time series reveal that the HC has moved eastward since the mid 1980s.

Seasonality and causes of the Yellow Sea Warm Current

To study the seasonality and causes of the Yellow Sea Warm Current (YSWC) in detail, rotated empirical orthogonal function (REOF) and extended associate pattern analysis are adopted with daily sea surface salinity (SSS), sea surface temperature (SST) and sea surface height (SSH) datasets covering 1126 days from American Navy Experimental Real-Time East Asian Seas Ocean Nowcast System in the present paper. Results show that in the Yellow and East China Seas, the YSWC is a mean barotropic flow as compensation of winter-monsoon-driven surface currents, which has been directly observed. When East Asia winter monsoon weakens, so do the meridional pressure gradient of the surface seawater and the YSWC, while the transversal pressure gradient changes rather slowly that results in the YSWC left turning. In addition, there is southward mean flow compensation of summer-monsoon-driven surface currents, which actually was also directly observed.