Impact of climate change on occurrence and characteristics of coastal upwelling in Listvennichny Bay (Southern Baikal) from 1941 to 2023

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1. Introduction

Upwelling is the organized upward movement of waters toward the surface of a water body. It is one of the mechanisms of vertical water exchange that affects the distribution of physical and chemical parameters of water (temperature, salinity, density, chemical composition, pollutants etc.) as well as biological processes (for example, the life cycles of bacteria, phyto- and zooplankton). It is known that upwelling can develop in the coastal and pelagic zones. Coastal upwelling in large lakes is quite well studied (e.g., Boyarinov and Petrov, 1991; Bell and Eadie, 1983; Corman et al., 2010; Plattner et al., 2006).

The coastal upwelling in Baikal was first described by G. Yu. Vereshchagin about a hundred years ago (1927). He measured water temperature and dissolved oxygen concentrations in the coastal surface waters, which corresponded to the values observed in the pelagic zones at 50-200 m depth. Later, upwellings were identified in the coastal zones of Northern Baikal using NOAA/AVHRR satellite imagery (Semovski et al., 2001). Katz and co-authors (2011) suggested that variability of currents is one of the factors influencing water dynamics in the lake, especially the development of the coastal upwelling. The first quantitative assessments (water temperature decrease, duration and depth of upwelling development, vertical water velocities) of the coastal upwelling in the Southern Baikal have been obtained relatively recently (Shimaraev et al., 2012).

Some biological studies have reported the effect of upwelling on plankton distribution in Lake Baikal. For example, M.M. Kozhov (1962) observed upwelling in the Maloye More Strait and its adjacent areas of the lake after a strong NW storm on September 4-9, 1951. The water surface temperature dropped from 12-13°C to 7-7.5°C in the Maloye More Strait and to 8-9°C near the eastern shore of Olkhon Island. After this upwelling, plankton “was very sparse and rather equally distributed in the upper 100-meter layer”. In August 1963 (Kozhov et al., 1970), a strong NW wind was over the Southern Baikal near the Bolshiye Koty settlement, in result the water surface temperature decreased to 5-6°C. The zooplankton biomass decreased to 1.2 g/m² compared to the average annual value of 40-50 g/m². Further, E.L. Afanasyeva (1977) also showed that vertical water movements in the upwelling zone could transport nauplii of the copepod Epishura baikalensis from deep water layers to the surface. Based on phytoplankton measurements and satellite SeaWiFS observations in 2001-2003, B. Heim et al. (2005) concluded that reductions in concentration of chlorophyll a along the eastern shore of the Northern Baikal were associated with upwelling events. Complex studies at the testing site near Cape Elokhin (the western shore of the Northern Baikal) in August 1988 showed that the concentration of chlorophyll a could increase after upwelling relaxation (Verbolov et al., 1992).

In the past several decades, Baikal, as well as the entire Northern Hemisphere, has been experiencing changes in the thermal and ice regimes. (Livingstone, 1999; Magnuson et al., 2000; Shimaraev et al., 2002; Todd and Mackay, 2003; Kouraev et al., 2007; Hampton et al., 2008; Shimaraev et al., 2018; Sharma et al., 2021). The under-ice period has been shortened by almost three weeks (Livingstone, 1999; Magnuson et al., 2000; Shimaraev et al., 2002). This resulted in earlier dates of summer stratification and gradual increase of water surface temperature. The transition to winter stratification was shifted correspondingly to later dates (Aslamov et al., 2024). Recent studies of water temperature in individual lake basins (Shimaraev et al., 2009) and water column heat content in the Southern Baikal (Troitskaya et al., 2022) indicate the transformation of the temperature field and redistribution of heat content under climate change conditions. Consequently, this should be reflected in the intensity of vertical heat and water exchange processes in Baikal, which are of particular importance for the littoral, which is the habitat of the largest number of hydrobionts.

The aim of this work was to quantify the characteristics of the coastal upwelling in Listvennichny Bay, and their correlation with the wind activity and climate change for the last 80 years.

2. Materials and methods

Water temperature data obtained in the period of 1941-2023 at the pier of LIN SB RAS in the settlement of Listvyanka, located on the shore of Listvennichny Bay in the Southern Baikal (Fig. 1), were used to identify upwelling events and assess their characteristics. From 1941 to 2005, temperature was measured with a mercury thermometer (accuracy ±0.02°C) at 8, 14, and 20 h, from 2006 to the present time with electronic temperature sensors (accuracy ±0.002°C, measurement discreteness from 1 s to 2 min). The study analyzes only the period of summer stratification, when the surface water is warmer than the deep layers and upwellings cause a sharp temperature drop, which makes them easier to identify. Daily average water temperatures were used to analyze the development of the coastal upwelling. The start and end dates of upwelling, its duration, and the value of the temperature decrease were determined.

 

Fig.1. Map of Listvennichny Bay and measurement locations (https://earth.google.com/web – accessed 29.11.2023). The inset shows the location of Listvennichny Bay in the Southern Baikal.

 

Upwelling was considered in those cases when temperature dropped sharply by one or more degrees and it persisted for more than 3 days. The upwelling start date was taken as the day when the water temperature dropped. The date of its end was considered to be the day when the water surface temperature became close to that before the upwelling. The difference between these dates determines the duration of upwelling. The value of water temperature decrease was calculated as the difference between the water temperature before upwelling and at the moment when it reached the minimum value during upwelling.

Depending on the depth from which the water rises to the surface of a water body, full and partial upwelling events are distinguished. Full upwelling is formed by subthermocline waters rising towards the surface, i.e. hypolimnion waters. The subthermocline waters does not reach the surface at partial upwelling. In this regard, we identified full upwellings, determined their characteristics and analyzed the conditions of their development.

To analyze wind conditions, we used daily data on wind speed and direction from 1954 to 2010 from the Angara River Head meteorological station and from 2011 to the present time from an automatic weather station installed on the pier of LIN SB RAS (Listvyanka settlement). As the distance between them is about 4 km and the morphometric conditions are similar, the wind data series are likely uniform. We determined the directionally stable wind with a speed of at least 3 m/s and a duration of at least 6 h observed during the day before the development of upwelling and on the day of its beginning. The average wind direction and speed at which the upwelling started to develop were then calculated. In 2011-2023, modern data enabled us to determine the maximum values of wind speed at wind gusts.

3. Results

Taking into account the influence of climate on Baikal’s ice-thermal regime (Livingstone, 1999; Magnuson et al., 2000; Shimaraev et al., 2002; Todd, Mackay, 2003; Kouraev et al., 2007; Hampton et al., 2008; Shimaraev et al., 2018; Sharma et al., 2021), we divided the available water temperature over the observation period into two intervals: 1941-1969 and 1970-2023. The period of 1941-1969 is characterized by a decrease in water surface temperature, while the period from 1970 to the present is characterized by its increase (Fig. 2). The value of the trends in 1941-1969 was –0.39°C/10 years (r = 0.38, p = 0.04), and it was +0.26°C/10 years (r = 0.38, p = 0.009) in 1970-2023.

 

Fig.2. The average values of water temperature for the period of summer stratification at the pier of LIN SB RAS in the Listvyanka settlement for 1941-2023 (solid blue line) and linear trends for 1941-1969 and 1970-2023 (dotted green lines).

 

A total of 285 upwelling events were identified and treated between May and October 1941-2023. Up to four upwelling events were recorded in each individual month. During the year, 1-3 upwelling events were most often recorded (Fig. 3). The maximum number of upwelling events was 13 within one year (in 1943).

 

Fig.3. The number of identified upwellings in individual years for 1941-2023.

 

A total of 129 upwelling events (an average of 4.5 events per year) were recorded for the period of 1941 to 1969, and 157 upwelling events (an average of 2.9 events per year) were detected from 1970 to 2023. Figure 4 shows the frequencies of upwelling events for selected months in two periods. Comparison of the two periods revealed a change in the distribution of upwellings by month: compared to the 1941-1969, 1970-2023 shows a greater concentration of upwelling in August (from 38.0% to 46.5%), neighboring July and September showed minor changes in the number of upwellings, and in all other months the number of upwelling decreased significantly (from 0.8% to 0% in May, from 7.0% to 0.6% in June, and from 7% to 3% in October).

 

Fig.4. The frequencies of upwelling events for selected months in 1941-1969 and 1970-2023.

 

Figure 5 shows the frequency of upwelling events with different durations for the two analyzed periods. With the same average duration of upwellings (7 days), a shift in duration to a greater extent since 1970 was revealed (Fig. 5). In the period of 1941-1969, upwellings lasting 4-6 days were more frequent (57.7%), whereas in the period of 1970-2023, the duration of upwellings increased to 5-8 days (56.1%). The number of upwellings with durations of 10 days or more has increased one and a half times in recent decades (from 11.5% to 17.8%).

 

Fig.5. The frequency of upwelling events with different durations for 1941-1969 and 1970-2023.

 

The magnitude of water temperature drop during upwelling has also changed. Thus, in 1941-1969 and 1970-2023, the mean values were 4.2 and 5.3°C, and the maximum was 12.0 and 13.5°C, respectively. The pattern of the frequency distribution of upwellings by temperature drops has also undergone a transformation (Fig. 6). While, in 1941-1969, 60% of upwellings were characterized by a 1-4°C temperature drop, in 1970-2023, 61% of upwellings showed a temperature drop of 3-7°C.

 

Fig.6. The frequency of upwelling events with different temperature decreases in 1941-1969 and 1970-2023.

 

Analyses of available wind data (since 1954) also enabled us to assess changes in wind characteristics. In 1954-1969 (Fig. 7), upwelling most often developed under N and SW winds (64% of events), and from 1970 under W and SW winds (55% of events). The mean and maximum wind speeds for the considered periods differ a little and are 6 (12) m/s during 1954-1969 and 5 (14) m/s during 1970-2023.

 

Fig.7. The frequency of upwelling events (%) that developed under winds of a certain direction during two periods.

 

There were 42 full upwelling events identified from 1941 to 2023; of these, 29 occurred from 1941 to 1969 and 13 occurred from 1970 to 2023. The maximum number of full upwelling events 18 and 13 occurred in August and September, respectively, while 7 events were detected in October. Two full upwellings each were recorded in June and July, and one in May. The distribution of full upwelling by decades is shown in Fig. 8.

 

Fig.8. The number of full upwellings by decades.

 

Analysis of available wind data for 24 total upwelling events showed that full upwellings occurred most often (10 cases) with SW winds blowing at 4-7 m/s. Four events of full upwellings were connected with W winds at wind speeds of 5-9 m/s.

4. Discussion

Thus, we identified summer upwellings in Listvennichny Bay, ranked them by duration, water temperature drop and direction and speed of accompanying winds. Cases of full upwellings were processed individually. The resulting data were divided into two time periods corresponding to observed climatic trends.

Analyzing the morphometry of Listvennichny Bay, it can be concluded that nearshore upwellings can develop under N-NW winds causing the runoff, or due to the Ekman transfer of surface water under W-SW winds and formation of anticyclonic eddy in the bay. At the development of the anticyclonic vortex, there is a sinking of water in its center and a compensating rise of water at the periphery near the bay shores. The analyzed data of winds accompanying upwelling confirm the above assumptions (Fig. 7).

Analysis of upwelling distribution by years showed that until 1960 there was a gradual decrease in the number of upwellings per year. Then, until 2016 their average number remained constant at the rate of about 2.5 upwellings per year, and in the last 8 years there was a certain increase (Fig. 3). Of particular interest was the narrowing of the frequency distribution of upwelling events by month and the concentration of upwellings in August in 1970-2023 (Fig. 4). A redistribution of upwellings toward longer duration and a larger temperature drop after 1970 compared to the previous period was also noted (Fig. 5, 6).

The number of full upwellings has decreased with each decade, from 12 in the 40s to only one in the 2010s. After 2020, no full upwellings have been observed yet.

To understand the possible reasons for these tendencies, it is necessary to analyze how the winds changed during these periods. It should be noted that wind speeds in recent decades have become noticeably lower than in the mid-40s-50s of the last century (Atlas ..., 1977). Comparison of average wind speeds for individual months in 1959-1968 and in 2000-2022 revealed their decrease in June-August by 0.8-1.1 m/s, and by 1.1-2.0 m/s in September-November. Such changes are observed over most territory of Russia (Bulygina et al., 2013) and are probably caused by the rapid warming in the Arctic and a decrease in the poleward temperature gradient which could influence mid-latitude atmospheric circulation and intensity of winds (Coumou et al., 2015).

Since the initial wind data were available only for the upwelling dates, we utilized the well-known reanalysis of the European Centre for Medium-Range Weather Forecasts, in its latest detailed release ERA5-Land (Muñoz-Sabater et al., 2021), which aims to summarize the global meteorological monitoring network based on a four-dimensional variational assimilation system of retrospective data collected in the most complete database (with a grid spacing of 0.1 degree and a temporal resolution of 1 h).

A homogeneous hourly series of winds in Listvyanka from 1950 to 2022 was sampled and obtained. Then a daily averaging was carried out, and the temporal variability of wind frequencies during summer months was plotted for 8 main directions with a step of 45 degrees, as well as the average vector and scalar velocities and wind stability were calculated. Since in recent years there has been an increase in the frequency of upwellings, it was decided to analyze this period separately.

The study of winds for July-September (when most of the upwellings occurs) showed that the average monthly wind speed gradually increases from July to September (2.2, 2.5 and 3 m/s, respectively), wind stability also rises (0.27, 0.33 and 0.52, respectively). At the same time, there is a reorganization of the main wind directions: in July, westerly winds prevail, in August the share of north-westerly winds increases significantly, and by September they become dominant. At the same time, a gradual transformation of the wind rose for August was noted. If in the 50-60s of the last century, westerly winds accounted for 33.5% and north-westerly winds for 20.1% of the total number of winds, then in the last decade, due to climatic changes, the timing of wind reorganization was shifted, and, accordingly, the frequency of occurrence of these winds in August has changed: 28.6% and 29.4%, respectively.

An examination of the proportion of the two main wind directions in August causing upwelling in Listvennichny Bay (Fig. 7) revealed a slight gradual increase in the proportion of northerly winds causing runoff (from 1% in the 1950s, 2% in the 1960s-2010s and 3% in the last decade). The frequency of southwesterly winds also slightly increased (from 2% in the 1950s to 3.1% in the last decade), but dropped to 1.7% during the 1960-2010s, which is likely one of the reasons for the reduced amount of upwellings during this period (Fig. 3).

If one calculates the total frequency of N and SW winds in August for two periods: before 1969 and since 1970, one can note that it increased from 3.6% to 5%, respectively. The observed rise in the proportion of winds causing upwellings coincides with the growth in the number of recorded upwelling events in August 1970-2023 compared to the previous period (Fig. 4). The smaller number of upwellings in July are explained by the fact that this month is characterized by minimum wind speeds for the whole summer-autumn period and minimum wind stability. The share of upwellings in September is also less in spite of the general wind intensification. The reason for this is a significant increase in the stability of north-westerly winds (>0.5), and a reduced share of SW winds to 2% (compared to 4-5% in August).

The observed redistribution of upwellings towards longer duration and with a larger temperature drop in 1970-2023 compared to the previous period (Figs. 5, 6) has a common reason. This is directly related to the increasing share of upwellings around August. August is characterized by the warmest water in the littoral, and, accordingly, the upwelling that occurs, with the same temperature of rising waters, will cause a greater temperature drop in August than in other months. And, accordingly, the incoming cold water, due to its high heat capacity, will need much more time to warm up to its initially high temperature values.

The influence of wind parameters on the development of full upwellings can be judged by analyzing the meteorological data during the full upwelling on September 11-21, 2011, when the wind parameters were measured with high discreteness by an automatic weather station. Sustained winds of W-NW direction began in the evening of September 11 and lasted for three days. Wind speeds reached 17 m/s and averaged 8.7 m/s. Water temperature at the beginning of the upwelling was 10.6°C, dropped to 4.0°C by September 15 and after relaxation of the upwelling was 6.8°C on September 21. It should be noted that not only wind strength and duration, but also its stability is necessary for the full upwelling development. For example, on August 16-27, 2023, at the maximum observed western wind speeds (average speed of 14 m/s, gusts up to 35 m/s), the water temperature dropped from 19.4°C to 4°C. However, since the wind decreased and increased again many times during 10 days, water temperature fluctuations from 4 to 10-15°C with a period of 12 to 24 hours were registered, forming a so-called intermittent upwelling. Despite the fact that water from hypolimnion was coming to the surface, the unstable winds did not allow large water masses to be involved in this movement, and the water was immediately replaced by warm surface water at the slightest wind attenuation. Therefore, we did not refer this case to a full upwelling.

The revealed regularities of changes in conditions of the formation and existence of upwellings in Listvennichny Bay allow us to state the following suggestions. On the one hand, reduction in the number of full upwellings should influence the amount of biogens transport from the hypolimnion to the littoral. On the other hand, a decrease in the frequency of upwellings should accompanied by an increase in the average surface temperature in the coastal zone during the summer season, which increases vertical density gradients in the upper layers and may cause eutrophication of littoral. As a consequence, the sharp growth spike of Spirogira algae in Listvennichny Bay observed in recent years (Kravtsova et al., 2012; Timoshkin et al., 2014; Timoshkin et al., 2018) may be related to the complex influence of increasing anthropogenic pressure (and rich input of biogens with wastewaters) and climatic changes (Shimaraev and Troitskaya, 2018). The latter, in turn, have a twofold effect on the increase of littoral temperature, both through greater warming due to higher air temperatures and less frequent water exchange (and consequent cooling) with the deep waters of Lake Baikal. Thus, more favorable physical and trophic conditions may be formed, for the development of algae not typical for the littoral of Baikal.

5. Conclusions

As a result of the presented work, we analyzed data on coastal water temperature and winds in Listvennichny Bay. We identified summer coastal upwellings, ranked them by duration, water temperature drops and direction and strength of accompanying winds. The obtained data were divided into two periods corresponding to the observed climatic trends.

It was found, that the main winds causing upwelling are from north and southwest directions, which is confirmed by the morphometry of the bay. Analysis of upwellings distribution by years showed that until 1960 there was a gradual decrease in the number of upwellings per year. From 1960 until 2016 the average number of events roughly constant at about 2.5 upwellings per year, with a slight increase in the last 8 years. The maximum number of upwellings (13) was recorded in 1943. The duration of upwellings averages 7 days, with a maximum of 21 days in 1979. Water temperature usually drops by about 5°C, with a maximum of 13.5°C recorded in 2016.

An increase in the proportion of upwellings in August compared to other months during 1970-2023 was found. A redistribution of upwelling events toward their longer duration and greater temperature drops after 1970 compared to the previous period was also revealed. The cause of these changes was the global course of decreasing wind activity associated with observed climatic changes, and a particular redistribution of the proportion of N and SW winds in the bay during the study period.

The number of full upwellings has decreased with each decade, from 12 in the 40s to just one in the 2010s. After 2020, no full upwellings have been observed yet. Two cases of full and intermittent upwellings were described and compared. Possible ecological impacts on the Listvennichny Bay due to the combined effects of increasing anthropogenic pressure and less frequent upwellings have been hypothesized.

6. Acknowledgments

The authors thank all the staff members of the Laboratory of Hydrology and Hydrophysics of LIN SB RAS for their participation in the expeditionary works and data collection as well as for productive discussions of the study results. Special thanks to Ruslan Gnatovsky for preparation of ERA5-Land data for analysis.

The study was carried out within the State Assignment of LIN SB RAS (0279-2021-0004).

Conflict of interests

The authors declare no conflicts of interest.

About the authors

E. S. Troitskaya

Limnological Institute of the Siberian Branch of the Russian Academy of Sciences; Irkutsk State University

Author for correspondence.
Email: elena.troitskaya@lin.irk.ru
Russian Federation, Ulan-Batorskaya Str., 3, Irkutsk, 664033; Karl Marx Str., 1, Irkutsk, 664003

M. N. Shimaraev

Limnological Institute of the Siberian Branch of the Russian Academy of Sciences

Email: elena.troitskaya@lin.irk.ru
Russian Federation, Ulan-Batorskaya Str., 3, Irkutsk, 664033

I. A. Aslamov

Limnological Institute of the Siberian Branch of the Russian Academy of Sciences

Email: elena.troitskaya@lin.irk.ru
Russian Federation, Ulan-Batorskaya Str., 3, Irkutsk, 664033

References

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