Distribution features of the weather conditions in Nha Trang Bay (The South China Sea)

Mau Dinh Le1,2, Galina A. Vlasova3, Dung Thi Thuy Nguyen1, Hoan Sy Pham1, and Tuan Van Nguyen1

Received 2 December 2021; accepted 2 January 2022; published 19 May 2022.

Nha Trang Bay is a semi-envelop coastal basin. It is located on the west coast of the South China Sea as well as in the south of the central part of the Vietnamese coast. The climate of the bay belongs to the sub-equatorial zone or to the zone of equatorial-tropical monsoons. Determination of the meteorological characteristics of an ocean region, especially for nearshore bay waters, plays a very important role in the sustainable development of the social-economical system of Vietnam. This paper presents a spatial and temporal long-term analysis of several meteorological characteristics in Nha Trang Bay, such as wind, tropical cyclones, air temperature, air humidity, and rainfall. The meteorological data were collected from the Nha Trang meteorological station during 1977–2015 (wind data until 2020). The data for typhoons were extracted from the National Weather Service (USA) and the Japan Meteorological Agency during 1945–2015. Statistical methods for collecting and analyzing data to estimate the statistical parameters of meteorological characteristics were used in the study. A study of results shows that the meteorological characteristics in the Nha Trang Bay are a seasonal variation of the East Asian monsoon (northeastern-NE and southwestern-SW monsoons). The winds in Nha Trang Bay are the seasonal change, influenced by the NE monsoon from October to April and the SW monsoon from June to August. May and September are the transition periods. Nha Trang Bay has a temperate tropical climate because the temperature regime of the bay ranges from less than 25∘C in the winter and more than 28∘C in the summer. Extreme conditions, including typhoon activity and heavy rainfall (coinciding with high humidity), occurred in October, November, and December. KEYWORDS: Nha Trang Bay; the South China Sea; Meteorology; East Asian Monsoon.

Citation: Le, Mau Dinh, Galina A. Vlasova, Dung Thi Thuy Nguyen, Hoan Sy Pham, and Tuan Van Nguyen (2022), Distribution features of the weather conditions in Nha Trang Bay (The South China Sea), Russ. J.

Earth. Sci., 22, ES3001, doi:10.2205/2022ES000791.

Introduction

1Institute of Oceanography, Vietnam Academy of Science and Technology, Nha Trang, Vietnam

2Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Ha Noi, Vietnam

3V. I. Il’ichev Pacific Oceanological Institute, Far

Eastern Branch RAS, Vladivostok, Russia

Copyright 2022 by the Geophysical Center RAS. http://rjes.wdcb.ru/doi/2022ES000791-res.html

Nha Trang Bay is a semi-envelop coastal basin.

It is located on the west coast of the South China Sea as well as in the south of the central part of the Vietnamese coast (Figure 1). It ranges from 12∘11′ N to 12∘28′ N and 109∘08′ E to 109∘28′ E (from the Electronic information gate of Khanh Hoa Province, Vietnam). Furthermore, it is lo- cated opposite the sea and directly interacts with land-sea processes. The bay involves many islands

Figure 1. Location of Nha Trang Bay in the South China Sea and at the Vietnamese Coast.

and headlands. Fresh water discharges into the bay from the Cai River in the north and the Cua Be River in the south. The continental shelf is rela- tively narrow, with a depth contour of 50 m in the area of the bay mouth. The bottom sediment is fine sand. The total suspended sediment (TSS) concen- tration is low. The climate of the bay belongs to the sub-equatorial zone or to the zone of equatorial- tropical monsoons. A clearly pronounced seasonal variation of air temperature is observed here: win- ter is colder than summer, and the speed of win- ter wind is greater than summer. Three large- scale circulation atmospheric systems are observed in this zone: trade winds, equatorial troughs, and especially the monsoons. The first two are char- acterized by exceptional constancy and stability, and the monsoons are characterized by periodic- ity. In the conditions of the East Asian mon- soon, which prevails over the entire South China Sea and in particular over Nha Trang Bay, north- eastern winds prevail in winter and southwestern winds in summer. Therefore, the winter mon- soon is called the northeastern (NE), and the sum- mer monsoon, the southwestern (SW). Against the background of large-scale circulation systems, syn- optic and mesoscale systems are observed trop-

ical disturbances, which, under favorable condi- tions, develop into tropical storms. For exam- ple, in November 2017, the strongest typhoon ever recorded, Damrey, passed with catastrophic conse- quences.

Atmospheric processes (atmospheric circulation, trade winds, monsoons, typhoons, etc.) over the South China Sea, including meteorological param- eters (wind, air temperature, humidity, rainfall, etc.), form climatic conditions not only for all near- by countries but also affect the weather of the entire Asia-Pacific region. Tropical cyclones (typhoons) occupy a special place among atmospheric pro- cesses. The main destructive effect of typhoons oc- curs in the Southeast Asia region. However, a sig- nificant number of its are directed towards the Rus- sian Far East, with the outflow of a large amount of destructive energy [Vlasova et al., 2021].

Besides, specific meteorological conditions are formed in the coastal regions of the South China Sea (bays, fjords, river mouths, etc.), which affect on the weather of the territory.

The determination of the meteorological char- acteristics of nearshore ocean regions, especially for the Nha Trang Bay, plays a very important role in the sustainable development of the social-

Figure 2. Distribution of main marine meteorological stations of Vietnam.

economical infrastructure of the Vietnam govern- ment. In recent years, the economy in Nha Trang Bay has rapidly developed, especially in tourism, maritime, mariculture, shore protection, etc., and has become a significant social-economical place for Khanh Hoa Province in particular and Vietnam in general. In connection with the above, the study of the meteorological characteristics in Nha Trang Bay, the heart of the sea waters of South Vietnam, becomes obvious.

Phu Lien meteorological station in Hai Phong City has been the first meteorological station in Vietnam since 1902, and the Nha Trang meteo- rological station was established in 1906 by the French. However, the historic data of all meteoro- logical stations in the South of Vietnam (including Nha Trang station) was broken because of the war’s impact. So, the uninterrupted data collected from 1977 to the present. In addition, at present time, the number of main marine meteorological stations in Vietnam are 18. They are managed by the Viet-

nam Meteorological and Hydrological Administra- tion [Le et al., 2015]. These stations include (from north to south): Cua Ong ward, Hon Gai city, Co To Island, Bach Long Vy Island, Hon Dau Island, Sam Son city, Hon Ngu Island, Con Co Island, Da Nang city, Quy Nhon city, Nha Trang city, Phu Quy Island, Vung Tau city, Con Dao Island, Phu Quoc Island, Tho Chu Island, Southwest Cay Is- land, Spratly Island (Figure 2). At these stations, the meteorological parameters were observed and recorded, such as wind, air temperature, humidity, precipitation, visibility, fog, etc.

The first attempts to analyze meteorological char- acteristics in Vietnam were made in 1902 at Phu Lien station by the French [Youth Daily Newspa- per online in Vietnam, 2019]. The first weather station was opened in 1906, and the first analy- sis of meteorological parameters in Nha Trang Bay was done in 1906 by the French (Archived data in the Department of Meteorology and Hydrology for Southern Central Vietnam). In 2004, the Depart-

ment of Science and Technology, the Khanh Hoa Province published a monograph on climatic and hydrological features in the Khanh Hoa Province including Nha Trang Bay [Department of Science and Technology, Khanh Hoa Province, 2004]. But there are practically no papers in which the anal- ysis of meteorological parameters for a long-term period was carried out in Nha Trang Bay.

In recent years, the Institute of Oceanography, Vietnam was carried out different projects which were related to the marine meteorological condi- tions especially wind and typhoons, in the Nha Trang Bay.

The main projects were:

• the Vietnam–Russia collaboration project (2011–2012) on the study of marine meteo- rological, hydrological, and dynamical inves- tigations, Subproject No. 2, Project No. 19- Project 47;

• the Vietnam-Russia collaboration project (2019–2021) on the study of the structure and dynamics of the Vietnam waters (No. VAST19-020 and QTRU02.04/19-20);

• provincial project (2017–2019) on the study of the hydrometeorological features in Khanh Hoa Province waters;

• Vietnam National Project (2017–2022) on the study of some interaction processes between sea, atmosphere, and land, and environmen- tal variation corresponding to the context of global climate change within the frame- work of the IOC/WESTPAC program (No. DTDL.CN-28/17).

Research on these projects were published in the following papers [Le, 2005, 2008; Le and Pham, 2007; Le et al., 2010, 2020, 2021; Pham et al., 2015;

Vlasova et al., 2016, 2020].

The National Center for Hydro-Meteorological Forecasting (Vietnam) is forecasting the marine meteorological information for the different areas along the Vietnamese coast. However, the forecast- ing data from global, neighboring countries, and the Vietnam National Center for Hydro-Meteorolo- gical Forecasting are useful for the region as a whole and are not correct for the nearshore study basin of Nha Trang Bay. In this case, the grid resolution of forecasting data was coarse when compared to the nearshore basin area. Therefore, the data source

used for the meteorological study of the nearshore study basin is better collected from a local long- term observed data station.

The accumulated material by this time provided the basis for analyzing the spatial and tempo- ral variability of meteorological characteristics over a long-term period: for typhoons for the period 1945–2015 and for the main meteorological charac- teristics for the period 1977–2015 (for wind up to 2020) in the Nha Trang Bay.

An important aspect of the work is its novel ap- proach to a description of the features that give each ocean region its character. The two core prin- ciples are: the use of the most modern database for all maps of regional distributions of properties and a discussion of all observed features within a frame of reference developed for atmospheric dy- namics, rather than based on a simple geographical approach.

The purpose of our work is to study the long- term spatial and temporal variability of meteoro- logical characteristics in Nha Trang Bay for the pe- riod 1977–2020.

Data and Methods

Data Sources

We collected data on the average monthly air temperature, rainfall, and air humidity from the Nha Trang Meteorological Station during 1977– 2015 at six hourly intervals (0 h, 6 h, 12 h, and 18 h of Greenwich). We also collected wind data from 1977 to 2020 by the Department of Meteo- rology and Hydrology in Southern Central Viet- nam. Furthermore, we collected the data of ty- phoons that crossed the coastline from 12∘ to 13∘N, in the Nha Trang Bay for the period 1945–2015 from websites of the USA National Weather Service (https://sharaku.eorc.jaxa.jp/TYP DB/links1 e.ht ml) and of the Japanese Meteorological Agency (https://www.jma.go.jp/bosai/map.html#4/49.153

/132.979/&elem=root&typhoon=all&lang=en&con tents=typhoon). These websites provide access to a wealth of typhoon information, including charts on the track of the storm, plus a text, based table of tracking information. We considered the features, such as the velocity of forwarding motion and wind speed.

Table 1. The Beaufort Wind Scale

Table 2. Saffir-Simpson Hurricane Intensity Scale

Type Category Pressure (mb) Maximum sustained wind speed

Methods

We used in the study the statistical methods for collecting and analyzing data to estimate the statis- tical parameters of meteorological characteristics, such as wind, air temperature, rainfall, and hu- midity [WMO, 1966]. We used the classifications of the winds based on the Beaufort wind scale and of the typhoons based on the Saffir/Simpson ty- phoon scale for the correct estimation of the statis- tical parameters (Table 1, Table 2). We calculated the wind frequency based on the Beaufort scale and then drew the rose diagram.

We took the methods for analyzing the verti- cal structure of wind speed and monitoring ob- served wind characteristics from onshore stations to offshore from the “Shore Protection Manual”

[SPM, 1984] and the “Coastal Engineering Man- ual” [CEM, 2002]. These guides also provide a knowledge base on seasonal meteorological trends (hurricane season, winter storms) and long-term environmental trends (sea level rise, climate change).

Results and Discussion

Wind Regime

The wind in Nha Trang Bay is the seasonal change influenced under the NE monsoon (October– April), with the N, NE, and NW directions. And under the SW monsoon (June–August), with the SE direction of the influence of land-sea breeze, the wind direction is becoming east. May and Septem-

Figure 3. The wind rose diagram in Nha Trang Bay for the period 1977–2020 (%): a – the average long-term wind rose diagram in Nha Trang Bay; b – the average long-term wind rose diagram in January (NE monsoon) in the Nha Trang Bay; c – the average long-term wind rose diagram in July (SW monsoon) in Nha Trang Bay.

ber are the transition periods. The characteristics of the wind regime in the onshore region of Nha Trang Bay are for the period 1977–2020 (Figure 3, Table 3, Table 4, Table 5).

The three wind rose diagrams (Figure 3) illus- trate the direction and wind speed changes in Nha Trang Bay from 1977 to 2020. There are three main features during this period. Figure 3a (Table 3) shows the average long-term data affected by the two monsoons (NE and SW). Figure 3b (Table 4) shows the average long-term data in January, the typical month for the NE monsoon. And Figure 3c (Table 5) shows the average long-term data in July, the typical month for the SW monsoon. Let’s take a closer look at this.

The average long-term wind rose diagram and average long-term wind frequency in the Nha Trang Bay during the period show the influence of the NE

and SW monsoons (Figure 3a, Table 3). The per- centages of the NW, N, and NE directions show the impact of the NE monsoon, and the figure of SE shows the impact of SW. At the top of the di- rections was the NW direction, with just nearly 10%. The second most popular was the SE di- rection, at 9%, followed by the NE and N di- rection, both at just near 8%. Additionally, the wind calm had a frequency of 30.7% (100%–69.3%). At the same time, the wind speed changed from

1.5 m/s to 8 m/s with a dominant frequency of 47% (22.7% + 14.4% + 9.8%), and the wind speed changed from 0 to 1.5 m/s and over 8.0 m/s with a negligible frequency of 22% (17.7% + 4.2% + 0.5%). Figure 3b (Table 4) shows the wind rose diagram

(wind frequency) of January – the typical month in the NE monsoon. The wind from the N di- rection had the largest frequency at 19.7%.

Table 3. Average Long-Term Wind Frequency (%) at the Nha Trang Station for the Entire Period 1977–2020

𝑉 – wind speed ∑︀ = 100

The next tops of the frequencies were NE and NW di- rections have the frequencies respectively 13.5 and 10.0%. In addition, ENE and W directions was at just under 5%. The wind speed from 0.3 m/s to 8 m/s changed with the dominat frequency of 70.7% (12.6% + 24.2% + 17.0% + 16.9%). And the wind speed over 8.0 m/s changed with the negligible frequency of 9.6% (8.9% + 0.7%). The wind calm had a frequency of 19.7% (100% 80.3%).

Figure 3c (Table 5) shows the wind rose diagram (wind frequency) of July – the typical month of the SW monsoon. In this connection, the wind direc- tion becomes east. There were two main directions (SE and NW), which had frequencies of 17.9% and 8.4%, respectively. The total of E, S, and W direc- tions was just around 7.7%. Additionally, the wind speed changed from 0.3 to 5.5 m/s with a frequency

of 54.5% (20.1% + 21.5% + 12.9%). The wind calm – was 41.1% (100% 58.9%).

As can be seen from Table 6, the wind reaches

its maximum values in winter. The highest wind speed was observed in November at 30 m/s.

At the Nha Trang station, during the period from 1977 to 2020, the average long-term wind speed was

3.4 m/s, with monthly fluctuations of 2.6–4.7 m/s (Table 7).

The month with the highest average wind speed is usually in the period of the strong northeast monsoon (from November to February next year). The strongest wind speeds of the day can reach lev- els of 7–8 on the Beaufort Scales, which can knock down trees, houses, etc., especially under the in- fluence of the strong northeast monsoon and ty- phoons.

Table 4. Average Long-Term Wind Frequency (%) at the Nha Trang Station in January (1977–2020)

𝑉 – wind speed ∑︀ = 100

Typhoon Regime

The 58 tropical cyclones that reached Nha Trang Bay for the period 1945–2015 (over a 71-year), had both directional and non-directional impacts on the bay (Figure 4a). Among them, 12 typical direc- tional typhoons (Figure 4b) from tropical depres- sions to typhoons of Category 5 (hurricane) were observed in the Nha Trang Bay. In general, during this period, there were 27 typhoons with a maxi-

mum wind speed (𝑉max) of more than 33 m/s, 16 tropical storms with 17 m/s < 𝑉max < 33 m/s and 15 subtropical storms with a 𝑉max < 17 m/s (Ta- ble 8).

From Figure 5, the average long-term distribu- tions of tropical cyclones at Nha Trang Bay were

not regular. There was a long period when the av- erage long-term number of cyclones was low just around 0.8 cyclones/year. From 1945 to 1952, the number was lower, then the number from 1963– 1999 was a little higher. Finally, between 2005 and 2012, the number was approximately the average value. The number of tropical cyclones reached a peak (3 tropical cyclones) in 1959. However, there were lots of years that did not appear storms, es- pecially those allocated to two periods: 1953–1958 and 2000–2004. In the rest of the years, there were two cyclones per year.

As for the average long-term monthly distribu- tion of tropical cyclones, then occurrences of tropi- cal cyclones were mostly in September (10.34%), October (27.59%), November (41.38%), and De-

cember (12.07%) (Table 9, Figure 6).

Table 5. Average Long-Term Wind Frequency (%) at the Nha Trang Station in July (1977–2020)

Table 6. Maximum Long-Term Wind Speed at the Nha Trang Station (1977–2020)

According to Le [1999], the typhoon season in the South China Sea is from June to December. And in October, the typhoon’s track is usually to the

west and direct towards the Central Coast. Also, in November and December, the average trajectory of the typhoon completely moved to the south (12– 13∘N) and slightly to the southwest, heading to the coast of South-Central and Southern Vietnam due to the influence of the continental high-pressure tongue.

Air Temperature in Nha Trang Bay

Table 10 shows the average long-term monthly air temperature in Nha Trang for the period 1977– 2015, from which it can be seen that the minimum values (less than 25∘C) are in the winter period, and the highest (more than 28∘C) – in the sum- mer. The hottest month is June, the coldest one is January. From Table 10, we also see that the av- erage long-term range of daily temperature change in Nha Trang Bay was just over 6∘C. The average long-term range of daily temperature change was

Table 7. The Average Long-Term Monthly Wind Speed at the Nha Trang Station (1977–2020)

Figure 4. The trajectories of tropical cyclones that affected Nha Trang Bay for the period 1945–2015: a – the trajectories of 58 of tropical cyclones; b – the trajectories of 12 typical tropical cyclones.

Table 8. Statistics of Tropical Cyclones Based on Saffir-Simpson Scale

the lowest in November and December, at 4.9∘C. August was the highest, with 6.9∘C. The maxi- mum long-term range of daily temperature change in Nha Trang Bay happened in May and June, at 12.0∘C.

Compared with all of Vietnam, the weather in Nha Trang is pleasant. It is not too hot in the sum- mer, and the short winter is not very cold. The maximum temperature in the Nha Trang station between 1977 and 2015 reached a peak of 37.9∘C on August 7th, 1976. Additionally, the minimum temperature in the Nha Trang station was the low-

est at 15.8∘C on January 10th, 1984 and December 25th, 1999 (Table 10).

Table 11 shows the average long-term annual and monthly rainfall, as well as rainfall in dry and rainy seasons for the period 1977–2015. The rainy period was observed in the fall-early winter (more than 350 mm), the dry period – the rest of the time (less than 60 mm), except for the month of May (more than 100 mm). There are many reasons why it rains in Nha Trang Bay, such as storms, mon- soon, and the intertropical convergence zone. From January to April, a gradually weakening northeast

Figure 5. The average long-term distribution of tropical cyclones affected on Nha Trang Bay for the period 1945–2015.‌

Table 9. The Average Long-Term Monthly Distribution of Tropical Cyclones in Nha Trang Bay for the Period 1945–2015

monsoon dominates Nha Trang, so the values of average monthly rainfall are often lower than 60 mm. May (after May 21st) and June usually have rain and small floods in a short period because of the tropical convergence zone in early summer. The SW monsoon weakens slightly in August and September. For several reasons, from September to December, was the wet season in Nha Trang Bay. The southwest monsoon is gradually weaken- ing in these months, and the northeast monsoon, on the contrary, has become is active strongly. The cold air masses are strengthening continuously to the south, combining the activities of tropical con- vergence zone, tropical cyclones, and high-altitude east wind disturbances.

Figure 6. The average long-term monthly distri- bution of tropical cyclones affected on Nha Trang Bay during 1945–2015.

Air Humidity

The average long-term monthly air humidity for the period 1977–2015 is illustrated in Table 12. The

Table 10. The Average Long-Term Monthly Air Temperature and the Range of Average (Maximum) Long-Term Daily Air Temperature Change in Nha Trang Bay (1977–2015)

Monthly Daily range

Table 11. The Average Long-Term Monthly Rainfall in Nha Trang (1977–2015)

Table 12. The Average Long-Term Monthly air humidity (%) in Nha Trang Bay (1977–2015)

∑︀ 

year 1448.4 year 79 39

Rainy season rainfall 1095.3 (September–December) Dry season rainfall 353.1 (January–August) long-term annual variation of relative humidity is both influenced by the temperature regime and also by the rain regime. The period with the lowest hu- midity is usually in the months of June, July, and August at around 77%. While the period of highest the humidity is from September to December, reaching 83% (coinciding with the rainy season). The average humidity varies from month to month, by only 1–2%. At the end of the dry season and the beginning of the rainy season, the difference in air humidity is 3–6%. The long-term annual range of relative humidity averages 6–7%.

Conclusions

All investigations of meteorological parameters (wind, typhoons, air temperature, air humidity, and rainfall) in Nha Trang for the period 1977–2015 confirmed the influence of the East Asian monsoon (NE and SW monsoons).

• Winds in Nha Trang Bay have the seasonal variability. In the NE monsoon (October– April) with the main N, NE, and NW di- rections, in the SW monsoon (June–August) with the main SE direction. May and Septem- ber are the transition periods. A remark- able phenomenon is that during SW monsoon (summer) Nha Trang Bay is influenced by the SE wind direction (it is blowing from sea to land). In addition to this, in summer, all ar- eas along the coast of Central Vietnam are strongly influenced by hot and dry southwest- ern winds. Therefore, the weather in Nha Trang is windy;

   

• The long-term annual distributions of tropical cyclones in Nha Trang Bay were not regular. In the period the average occurrence of cy- clones was low, just around 0.8 cyclones/year. Occurrences of tropical cyclones were mostly in September (10.34%), October (27.59%), November (41.38%), and December (12.07%). The extreme conditions, including typhoon activity, and heavy rainfall, occurred in Oc- tober, November, and December;

   

• The air temperature regime in Nha Trang Bay shows that the minimum values are less than 25∘C in winter, and the highest, more than 28∘C in summer. The hottest month is June, the coldest is January. That means it shows that Nha Trang Bay has a temperate climate;

   

• The rainy period was observed in the fall-early winter (more than 350 mm). The dry period is the rest of the time (less than 60 mm), except for the month of May (more than 100 mm). The long-term average yearly rain- fall was 1449.4 mm, the average long-term rainy season rainfall (from September to De- cember) was 1095.3 mm, and the average long-term dry season rainfall (from January to August) was 354.1 mm;

• The period with the lowest humidity is usu- ally in the months of June, July, and August at around 77%, while the period of the high- est humidity is from September to December, reaching 83% (coinciding with the rainy sea- son).

In general, Nha Trang Bay is influenced by the tropical climate. The typical weather characteris- tics are relatively harmonious, especially during the windy weather (summer). The weather is typically warm and mild nearly all year round. As such, it has long been a popular destination for sunbathing, swimming, and enjoying beach activities for both domestic and foreign tourists.

The long-term meteorological characteristics in Nha Trang Bay are important primarily for the economy of Vietnam, but also for the climatic con- ditions of the neighboring region, as well as for the navigation of ships, both domestic and inter- national.

Acknowledgments.The authors gratefully acknowledge the support of the joint project be- tween the Vietnam Academy of Science and Tech- nology (VAST) and Il’ichev Pacific Oceanological Institute (POI) FEB RAS “Structure and dynam- ics of the Vietnam waters and their variability due to modern climatic tendencies” (Project No: VAST19-020 and QTRU02.04/19-20). And Viet-

nam National project on “Study some interaction processes between Sea-Atmosphere-Land and en- vironmental variation corresponding to the con- text of global climate change within the frame- work of the IOC/WESTPAC program” (Project No: DTDL.CN-28/17). This work also was car- ried out with the financial support of the state budgetary theme “Investigation of the main pro- cesses that determine the state and variability of the oceanological characteristics of the marginal seas of Asia and the adjacent regions of the Pa- cific and Indian Oceans” of POI FEB RAS (Project No:121021700341-2). The main author, the Se- nior Researcher Dr. Mau Le Dinh, is also thank- ful to the Vietnam Academy of Science and Tech- nology (VAST) for supporting his scientific activi- ties for the year 2021 (Code: NCVCC17.04/21-21). The authors are also thankful to all colleagues for their kind help and encouragement throughout the preparation of this paper.

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Corresponding author:

Galina A. Vlasova, Il’ichev Pacific Oceanological In- stitute, Far Eastern Branch, Russian Academy of Sci- ences, Vladivostok, Russia. (gavlasova@mail.ru)

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