The frequency of severe droughts in Crimea and their relationship with El Niño

Degterev Andrey Kharitonovich Professor, Department of Radioecology and Environmental Compliance, Sevastopol State University 299033, Russia, Sevastopol', g. Sevastopol', ul. Kurchatova, 7 degseb@yandex.ru Other publications by this author

Introduction

One of the main consequences of modern global warming in the Black Sea region is the intensification of droughts ^{[1, 3]}. And not only in terms of reducing the amount of precipitation over a certain period of time and increasing the frequency of droughts, but also in terms of increasing the duration of droughts. Prolonged absence of precipitation leads to depletion of water reserves in reservoirs and lowering of the groundwater level, which affects the debit of wells. Even if such phenomena occur every few years, they pose a serious danger to the water supply of the coastal regions. In spring and early summer, they cause especially great damage to agriculture, and in the second half of summer at the height of the holiday season, when the population of the region increases several times, they lead to serious economic losses for sanatorium organizations and the tourism industry. Therefore, droughts in the south of Russia, their causes, and recurrence have been seriously studied for a long time ^{[7, 8, 9, 10]}.

On the other hand, temperature changes in the coming years are often associated with the manifestation of the ocean-related El Nino phenomenon – the Southern Oscillation ^{[4, 13]}. In this regard, it is of interest to compare the data on droughts in the Crimea over the past decades with the amplitude of the El Nino index. In fact, it is equal to the deviation of the ocean surface temperature in a certain area of the Pacific Ocean from the climatic norm. Therefore, on the shores of the Pacific Ocean, the influence of El Nino on precipitation is especially noticeable. For example, after a strong El Nino in 1997 -1998, 2 million hectares of forests burned out in the Khabarovsk Territory alone in the summer.

Analysis of data on precipitation amounts allows us to judge the frequency of droughts and their duration. Data on air temperature, in turn, allow us to judge the evaporation rate. Abnormally severe droughts are associated with large-scale atmospheric processes and affect at least the entire Black Sea region, as well as the water temperature in the Black Sea.

 As you know, severe droughts include situations when there is no precipitation or very little of it for four decades or more. Moreover, as a rule, it is the growing season that is considered, when the consequences of droughts are especially great. Thus, the task was set to study exactly those years when there was little precipitation during two or three adjacent months of the growing season.

For this purpose, we used data on monthly precipitation amounts and average monthly temperatures. This makes it possible to distinguish very severe droughts as periods of two adjacent dry months. That is, in our case, droughts lasting at least six decades were considered. As it turned out, so far droughts for three months in a row from April to August are very rare, and four-month droughts in this study were not noted at all. However, an increase in the duration and frequency of droughts over time may lead to their overlapping within one or two years, and then there will be a transition to longer droughts. In turn, this will lead to a significant aggravation of the problem under consideration. In particular, the depletion of moisture reserves in the root layer of the earth for two to three years leads to the degradation of green spaces, orchards and vineyards.

We examined data on precipitation and air temperature in Sevastopol in the period from 1950 to 1999, respectively, the length of the series for each month was 50 years. Data on water temperature in the Black Sea for approximately the same period were also used (Fig. 1).

Fig. 1. Change in the average annual temperature of surface waters in the Black Sea according to coastal measurements ^[3].

As a threshold level of precipitation for a month, 10 mm was taken, that is, the months for which no more than 1 cm of precipitation fell were considered dry. The part of the growing season from April to August inclusive was chosen as the most vulnerable period (in terms of the effects of drought). Table 1 shows the results of the analysis for one-month droughts. Some of them are actually at least two months old. The frequency of drought in a given month was calculated as the ratio of the length of the series (50 years) to the number of years with drought in that month. Although, of course, the corresponding frequency of repetition of droughts in Table 1 is clearly not visible.

                                                                                   Table 1

Distribution of droughts by year in different months.

From the table. 1 it can be seen that June is the least arid month (drought is less frequent than once every 6 years), and August is the driest (every 3-4 years). Indeed, in 50 years, June was dry only 8 times: in 1957, 1959, 1970, 1975, from 1980 to 1982 and in 1984. The first half of the 1980s is clearly distinguished here, when June was dry almost every year. The exception is 1983, when a record 193 mm of precipitation fell in June. This is 2.4 times higher than the June precipitation in 1991 (it is the second largest).

It can be assumed that the anomaly of 1983 is associated with the most powerful manifestation of the warm phase of the El Nino phenomenon in the period under review in 1982 - 1983. (Fig. 2), when the El Nino index reached 2.2 three months in a row. A stronger manifestation of El Nino during the period under review was only in 1997-1998. Then the index exceeded 2.2 for four months in a row, and within two months it reached a value of 2.4.

Note that the direct correlation analysis here gives little in connection with the delay in the response of the climate system in the northern hemisphere to temperature changes from the other side of the globe. Therefore, Figure 2 shows the maximum index values for each episode only for the warm phase of the Southern Oscillation. There were about 20 such episodes during this period, the connecting lines between the points are built for clarity. So, from Fig. 2 it can be seen that episodes with the maximum values of the El Nino index after 1980 occurred at least 15 years later. Moreover, this period has been increasing over the years. The maximum values of the index also grew, in 2015 it reached 2.6. Hence, in particular, it follows that, if this trend continues, the next episode of El Nino with an extremely high index value will come no earlier than 2030.

If we consider the area of abnormally warm waters in the tropical Pacific Ocean, according to which the El Nino index is estimated, as an energy-active zone, then the release of heat into the atmosphere there is proportional to the current value of the index, that is, the temperature anomaly in this month of DT. Then the total energy release is proportional to tiT, where ?t is equal to one month and summation is carried out for all months of this El Nino episode. Hence we get that it is proportional to simply Ti, that is, the sum of the El Nino indices for all months of the episode. The corresponding graph of the oscillation envelope is shown in Fig. 3.

Fig. 2. Change over time of the maximum values of the El Nino index per episode

Fig. 3. Changes in energy release in individual El Nino episodes

Figure 3 shows six peaks more clearly, and it also unexpectedly resembles a graph of changes in the number of sunspots on the Sun ^{[2, 6]}. Moreover, there were also six cycles of activity on the Sun during the same period. However, other extravagant hypotheses are also associated with El Nino ^[5].

Returning to the analysis of precipitation in Sevastopol, we note that July was dry 11 times during the period under review, in connection with which we can talk about the average recurrence of droughts once every 4-5 years. Attention is also drawn to the fact that the coincidence of droughts in June and July took place only once – in 1984. Moreover, May was also dry this year. In Fig. 3, an abnormally wide peak corresponds to this period.

May droughts coincided with June droughts only in 1982, although they have happened 9 times in 50 years. This once again underlines the special nature of atmospheric processes in the early 1980s.  This feature is also indicated by the change in the sign of the linear trend of changes in the average annual water temperature in the Black Sea in those years (Fig. 1). As can be seen from Fig. 2, the absence of El Nino episodes with maximum index values of less than 1.7 is also characteristic for the period1982 – 1993.

The most acute drought is felt in Sevastopol in August. It accounts for the peak of the tourist season, in addition, in June and July, water reserves in reservoirs are usually already significantly reduced. From the table. 1 it can be seen that it is in August that drought occurs more often than in other months. During the period under review, this happened 15 times, that is, almost twice as often as in June or July. Accordingly, on average, a drought in August occurred once every 3.3 years.  At the same time, there were five years when the August droughts were superimposed on the July ones: in 1958, 1961, 1963, 1967 and 1990.

In the table.2 shows data on two-month droughts. For the April-May period, this took place only in 1968, that is, very rarely, once every 50 years. For the period May-June – in 1982 and 1984, once every 25 years. For June-July – only in 1984. Finally, for July-August – much more often, in 1958, 1961, 1963, 1967 and in 1990, that is, once every 10 years.

At the same time, in accordance with the climatic norm for the period from March to September inclusive, all months are characterized by almost the same monthly precipitation at the level of 30 mm (from 25 to 32 mm). In dry years, however, this is not the case. For example, immediately after the episode of a strong El Nino manifestation in 1997-1998, a characteristic three-peak sequence of precipitation with two one-month dry periods was observed for two consecutive years (Fig. 4).

With the exception of the July-August period, two-month droughts occurred only in 1968, 1982 and 1984. 1 shows that these years correspond to the increase in water temperature in the Black Sea. In particular, after the drought of 1968, starting in December 1969, Novorossiysk had to carry water from Sochi and Tuapse by tankers for four months after all local water supply sources were depleted. However, the corresponding El Nino episode was weak at that time, the index values did not exceed 1.1. It can be assumed that then the drought was associated with the blocking of the western transfer, as evidenced by the large negative values of the North Atlantic Oscillation index ^[12]. A similar situation took place later in the summer of 2010.

As for three-month droughts, for the period from April to June inclusive, they have not been once in 50 years, for the period May–July - only in 1984 and for the period June–August - there was no.

Fig. 4. Precipitation in 1998 (solid line) and in 1999 (dashed line).

                                                                                   Table 2

Distribution of two-month droughts by year.

Thus, there was a three-month drought in general only once and it was in the period from May to July (Fig. 5). A comparison of Fig. 4 and Fig. 5 shows that in 1984 there were also spring and July precipitation minimums, but the June maximum did not manifest itself. In this sense, two one-month droughts of increased duration were superimposed on each other.

Fig. 5. The structure of the three-month drought in 1984

Summing up, we can say that so far the main danger in the Crimea is represented by two-month droughts. During the considered 50 years, droughts lasting more than six consecutive decades in the period from April to August occurred 8 times. That is, their repeatability is about once every 6 years. Moreover, in 6 out of 8 cases, these droughts ended after August. So, in 1958, only 4 mm of precipitation fell in September, that is, it was actually a three-month drought.                   

To predict long-term droughts, you can also use the features of changes in average monthly temperatures. These data are shown in Table 3 along with the climatic norm for the base period of 1961 – 1991.

From the table. 3 it can be seen that during the years of prolonged droughts, the temperature in May, with the exception of 1990, always exceeded the norm. So, in 1968 – by 3.2oS. As already noted, this year also falls on the peak of water temperature in the Black Sea (Fig. 1). And in July, the temperature during droughts was mostly below normal (in 1982 – by 2oC), although in 1963 it was 1.5 degrees above normal. With the exception of the same anomalous year of 1963 (it also corresponds to one of the peaks in Fig. 1), the January temperature was usually higher than the climatic norm during years of prolonged droughts.  This is especially true for the only year with a three–month drought - 1984. Then the temperature in January exceeded the norm by 3.3 oC.

Table 3.

Average monthly temperatures during two-month droughts

It should be noted that in July and August, the maximum average monthly air temperatures are observed in Crimea (Table 3). This corresponds to increased evaporation in these months, which, in turn, leads to increased drought. On the other hand, according to measurements over the past 30 years ^[11], July and August account for the largest increase in average monthly temperatures in the Crimea (up to 2oC).

Thus, it is in August in the near future that we should expect the greatest aggravation of the problem of water scarcity in Sevastopol and in the Crimea. Up to the delivery of fresh water by tankers to individual coastal settlements in an emergency, as it was in Novorossiysk in 1970, and as it is now being done in a drought in a number of Southern European countries. Shallow-sitting river-sea tankers, bunkering tankers and river tankers are suitable for Crimea in this regard ^[14].