Study on Hydrography and Small-Scale Process over Zhoushan Sea Area

WU He, DU Min, WANG Xiaoyong, and MENG Jie

National Ocean Technology Center, Tianjin 300112, P. R. China

Study on Hydrography and Small-Scale Process over Zhoushan Sea Area

WU He*, DU Min, WANG Xiaoyong, and MENG Jie

National Ocean Technology Center, Tianjin 300112, P. R. China

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2015

This paper mainly analyzes the tidal characteristics and small-scale mixing process near Zhoushan Islands. First, the spectral analysis and wavelet analysis are adopted for the measured tide level data and tidal current data from the Zhoushan sea area, which indicate that the main tidal cycle near Hulu Island and Taohua Island is semi-diurnal cycle, the diurnal cycle is subordinate. Both their intensities are changed periodically, meanwhile, the diurnal tide becomes stronger when semi-diurnal tide becomes weak. The intensity of baroclinic tidal current weakens at first and then strengthens from top to bottom. Then, in this paper, the Gregg-Henyey (G-H) parameterization method is adopted to calculate the turbulent kinetic energy dissipation rate based on the measured temperature and tidal current data. The results of which shown that the turbulent kinetic energy dissipation rate around Hulu Island is higher than that around Taohua Island. In most cases, the turbulent kinetic energy dissipation rate during spring tide is larger than that during the neap tide; the turbulent kinetic energy dissipation rate in the surface layer and the bottom layer are higher than that in the intermediate water; the changes of turbulent kinetic energy dissipation rate and tidal current are basically synchronous. The modeled turbulent kinetic energy dissipation rate gets smaller with the increase of the stratification, however, gets larger with the increase of shearing.

tidal characteristics; parameterization method; turbulent kinetic energy dissipation rate

1 Introduction

Tidology is a branch of marine science with long history. Initially, the study on tidal characteristics near seaport is mainly based on analysis of the measured tide data, such as Dong (2011) and Wu et al. (2008) respectively analyzed the tide characteristics of Dandong Port and Maoming Port. But the researches of large area by this way are realized through the limited data.

Because the modular technology is increasingly mature, more and more people start to use the three-dimensional numerical model of tidal current to analyze the characteristics of some sea areas, as well as how marine engineering construction (such as seaport construction, sea-closure engineering, etc.) effects on the surrounding tide current. Zhang et al. (2013) conducted a three-dimensional numerical simulation for the tidal characteristics and marine environment capacity of Liusha Bay based on POM model. Based on FVCOM model, Lin et al. (2013) and Jing et al. (2011) simulated the three-dimensional tidal current of the Quanzhou Bay and Xiamen Bay respectively. Based on Delft3D two-dimensional model, Peng et al. (2014) and Qi et al. (2014) studied respectively the cumulative response of the tidal influx in Shipu Port for thesea reclamation in Sanmen bay and the tidal current characteristics of Caofeidian marine engineering in different stages. In addition, based on MIKE21 commercial hydrodynamic model, Liu et al. (2013) studied the predictive analysis on impact of the construction of Zhangjiabu Port for nearby tidal current. Yang et al. (2012) studied the tidal dynamic characteristics of Yangshan Port and its response to engineering based on the measured data.

Relatively speaking, the study on small scale mixing process of tidal current is rarely. Seawater mixing may be an important process mechanism for controlling the physical properties of sea water, the flux distribution of nutritive salt and the concentration of particulate matters. In early 20th century, Taylor (1919) analyzed the tidal dissipation and turbulent kinetic energy dissipation rate of the Irish Sea. In recent years, many researchers have studied the tidal (or internal tidal) mixing in continent shelf seas. Mackinnon and Gregg (2002) analyzed the tidal mixing of the Late-Summer New England Shelf. Zhao et al. (2010) analyzed tide and mixing characteristics over continental slope at the mouth of Dalian Bay. But few people have performed related study in Zhoushan area, so, we are trying to do it in this paper.

2 Data Specification

All fourteen stations involved in this paper are locatedaround Zhoushan Islands, containing two temporary tide gauge stations, Hulu Island (30.0343?N, 30.0343?E) and Taohua Island (29.8397?N, 122.2967?E). The instrument of RBR XR-420-TG is adopted to measure the hourly tide level continuously in these two temporary tide gauge stations from 0:00 on August 5, 2013 to 23:00 on September 4, 2013. Fig.1 shows the geographical location of the two temporary tide level stations at the Changjiang estuary. Red star means the location of temporary tide level station in Taohua Island, yellow star means the location of temporary tide level station in Hulu Island.

Fig.1 Geographical locations of the two temporary tide level stations. Red star means Taohua Island and Yellow star means Hulu Island.

Table 1 The specific location of tidal current observation stations

The remaining 12 stations are used for tidal current observation (their locations are shown in Table 1 and Fig.2), among them, A1-A5 are near Taohua Island, B1-B5 and C1-C2 are close to Hulu Island. The instruments of 400 kHz Nortek-AWAC are adopted in B1 station and B2 station for measurement, and the instruments of 400 kHz Nortek-AQP are adopted in other stations for measurement. Furthermore, the sea water temperature is also measured in A3, A4, B3, B4 and C1 stations. And the adopted measuring instruments are type RBR CTD. The observation period for tidal current and sea temperature in every station is the same, containing three time periods: neap tide period (from 10:00 on August 16, 2013 to 11:00 on August 17, 2013), moderate tide period (from 14:00 on August 19, 2013 to 15:00 on August 20, 2013), spring tide period (from 10:00 on August 23, 2013 to 11:00 on August 24, 2013). But the observation interval is different, 10 minute is needed for tidal current observation and 1 hour is needed for sea temperature observation. We observed from top to down vertically the tidal current and water temperature on surface layer, 0.2 H layer, 0.4 H layer, 0.6 H layer, 0.8 H layer and bottom layer (assuming H is the water depth).

Fig.2 Distribution of tidal current stations of measurement.

3 Data Processing and Analysis

In order to fully understand the tidal characteristics of this water, we processed firstly the tidal level data based on the following three methods.

Through the harmonic analysis of above data, which includes at least thirteen tidal constituents (containing MSf, Q1, O1, K1, P1, K2, N2, M2, S2, MK3, M4, MS4, M6), and indicates that the tide type values of Hulu Island and Taohua Island are 0.4615 and 0.4579 respectively, both them are semi-diurnal tide. The main tidal constituents are M2, S2, K1, O1 and the tidal amplitudes of which are 114.4968 cm, 44.43292 cm, 30.69351 cm and 22.14539 cm in Hulu Island and 114.3655 cm, 44.30544 cm, 30.70469 cm and 21.66886 cm in Taohua Island.

Fig.3 shows the results of spectrum analysis for hourly tide level data of Hulu Island and Taohua Island, both of which are within the 95% confidence bounds. The peak position can be more clearly seen when enlarging the red box, the tidal constituent of M2, S2, K1, O1 are still the main tidal constituent with dominant components in Hulu Island and Taohua Island.

Fig.4 shows the result of wavelet analysis on hourly tide level data, indicating that the main tidal cycle of the two Islands is semi-diurnal cycle, then is diurnal cycle. The intensities of both them are changed periodicallywith time period, about half a month. Diurnal tide becomes stronger when semi-diurnal tide becomes weak.

Fig.3 Analysis result of tidal level spectrum.

Fig.4 The wavelet analysis results of tidal level (dashed lines means semidiurnal tide M2 and diurnal tide K1).

Above is the overview of processing the hourly tide level data of two Islands. Hereinafter we divide the tidal current of each station into barotropic part and baroclinic part. The value of barotropic part is the average of six observed vertical tidal current values. The value of baroclinic part is the observed value subtracting the value of barotropic part.

Through the spectrum analysis of the u and v tidal constituents of barotropic tidal current, the result shows that the semi-diurnal period is dominant. Because the duration of the current data is only 25 h, no characteristics of diurnal period is detected from the tidal level data. In Fig.5, we take A1 during spring tide (from 10:00 on August 23, 2013 to 11:00 on August 24, 2013) as an example to show this result.

Fig.5 Result of spectrum analysis on barotropic tidal current of A1 during spring tide.

Through spectral analysis on each layer’s baroclinic tidal current (U and V tidal constituents) in station A1 during spring tide , the result indicates that the spectral peak is always at the semi-diurnal period and weaken at first and then strengthen from top to bottom (see Fig.6) .This result can not show clearly the peak of higher frequency, but can find a general phenomenon, namely the stronger spectral peaks are generally in surface layer and bottom layer and better that in the middle layer.

Fig.6 Spectral analysis of each layer’s baroclinic tidal current (U tidal constituent) in the station A1 during spring tide. The vertical line denotes M2 tidal constituent.

4 Turbulent Mixing

There are numerous turbulence parameterizations from some literature, and these formulas are partly empirical and partly based upon simple analytical models of the internal wave field. One of the most enduring of these is the eikonal model of Henyey et al. (hereinafter referred to as HWF). According HWF, the turbulent kinetic energy dissipation rate can be expressed as:

where m is the vertical wavenumber, E?(m)is the spectral energy density evaluated at some suitable high wavenumber (characterizing the energy of test waves), U(z)is the background velocity vector from larger-scale waves, and k is the wavenumber vector of the test wave. Neglecting the vertical component of background wave velocity, assuming the test wave energy is not correlated with background shear.

The three factors on the rhs are related tothe energy density of the test waves (local derivative of spectral density),the rms background shear, and kHthe horizontal wavenumber of the test waves, respectively.

According to the relationship between GM spectrum and the dispersion, the dissipation rate at a particular latitude scales can be expressed as:

To compare this scaling with oceanic measurements at a variety of mid-latitude locations, G89 uses the ratio of measured shear at a fixed wavenumber (10 m) to the modeled Garrett-Munk shear at that scale as a proxy or the spectral energy level to get what we will subsequently refer to as the Gregg-Henyey (G-H) scaling.

where N0= 5.23×10-3s-1is a reference buoyancy frequency, f = 2Ωsin(latitude) is the Coriolis frequency, when calculating the stratification, the given salinity is 25.5 PSU. Because the data of hydrology investigation conducted around Chengzigang Port is quite near that of Taohua Island and the salinity in this area is with little change, the average of which is adopted in this paper for calculating water stratification.

Through comparison it is found that the turbulent dissipation rate around Hulu Island (B3, B4, C1) is higher than that around Taohua Island (A3, A4), in most cases, the turbulent dissipation rate during spring tide is larger than that during neap tide (as Fig.7). After calculation, the proportion of turbulent dissipation rate during spring tide and that during neap tide is 11 times in station A3, 3 times in station A4, 17 times in station B3 and 6 times in station B4. For the station C1, the large turbulent dissipation rate during neap tide may be related to the local wind.

Fig.7 The averaged turbulent dissipation rate vertically and spatially.

In most cases, the turbulent dissipation rate near the surface layer and the bottom layer is larger than that near the intermediate water layer. Turbulent dissipation rate in the bottom layer near station C1 is larger than that of other stations, especially during moderate tide and neap tide.

In Fig.9, the turbulent dissipation rate and tidal velocity during spring tide, moderate tide and neap tide are drawn as the data of A3, their changes are basically synchronous. The correlation coefficients between them during three periods are 0.8098, 0.5443 and 0.4643.

Fig.8 The turbulent dissipation rate of each layer.

Fig.9 Comparison chart of the turbulent dissipation rate and the tidal current of station A3 during spring to neap tide period.

The changes of turbulent dissipation rate with shear and stratification are described in Fig.10, and it is found that the turbulent dissipation rate can increase with the increase of shear and decrease with the increase of stratification. This result is consistent with the findings of MacKinnon and Gregg (2002), but a difference existed in their simulated and observed dissipation rate, namely the observed turbulent dissipation rate gets larger with the increase of the stratification and shear.

Fig.10 The relation between turbulent dissipation rate and shear with stratification.

5 Conclusion

The spectral analysis and wavelet analysis are firstly adopted for the measured tide level data and tidal current data of the Zhoushan sea area, which indicate that the main tidal cycle near two Islands of this sea area is semi-diurnal, the diurnal cycle is subordinate. Both their intensities are changed periodically, and meanwhile, the diurnal tide becomes stronger when semi-diurnal tide becomes weak. The intensity of baroclinic tidal current weakens at first and then strengthens from top to bottom.

In this paper, then, the Gregg-Henyey (G-H) parameterization method is adopted to calculate the turbulent kinetic energy dissipation rate based on the measured temperature and tidal current data, it is found that the turbulent kinetic energy dissipation rate around Hulu Island is higher than that around Taohua Island. In most cases, the turbulent kinetic energy dissipation rate during spring tide is larger than that during the neap tide; the turbulent kinetic energy dissipation rate in the surface layer and the bottom layer are higher than that in the intermediate water; the changes of turbulent kinetic energy dissipation rate and tidal current are basically synchronous. The modeled turbulent kinetic energy dissipation rate gets smaller with the increase of the stratification, however, gets larger with the increase of shearing.

Acknowledgements

This study is supported by the foundation items: The Chinese Marine Renewable Energy Special Fund (GHME 2012ZC05, GHME2013GC03, GHME2013ZC01, GHME 2014ZC01).

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(Edited by Ji Dechun)

(Received October 11, 2014; revised April 14, 2015; accepted May 2, 2015)

J. Ocean Univ. China (Oceanic and Coastal Sea Research)

DOI 10.1007/s11802-015-2779-6

ISSN 1672-5182, 2015 14 (5): 829-834

http://www.ouc.edu.cn/xbywb/

E-mail:xbywb@ouc.edu.cn

* Corresponding author. E-mail: wh_crane@163.com