As can be seen from Table 1, the largest negative anomaly value of the AOI over the past century occurred in February 2010, when the value reached -3.566, while the second-largest negative anomaly occurred in January 1977, when it fell to -3.279. Taking the polar extreme of the anti-phase of the two AO values as an example, the role of volcanic activity in the formation of the AO anti-phase index extremum was examined.
(1) Theformation of the anti-phase maximum of the AO in February 2010 is related to volcanicactivity To observe the relationship between the polar oscillation minimum and volcanic activity, the volcanic activity at high latitudes in the Northern Hemisphere was added to the AOI graph for the years 2008–2011, as indicated by the red arrows in Figure See figure 6,The AOI maximum for this period was 1.68 in October 2008,It is the result of successive eruptions of Kamchatka Bezymianny Volcano (VEI 3) and Alaska Okmok Volcano (VEI 4) and Aleutian bogoslof Volcano (VEI 4) on July 11 solstice August 7, 2008.
After the AO maximum in October 2008, the AO intensity continued to fluctuate downward until February 2010, when it reached -3.566, which was the lowest AOI value since 1950. As can be seen from Figure 6, during the continuous fluctuation of the AO intensity, each fluctuation was associated with a volcanic eruption, indicating that each volcanic eruption could cause an Arctic oscillation peak, followed by a certain amount of
upswing. We will now explore this relationship in detail. The AO index continued to decline after the AO maximum in October 2008. In the mid of the falling AOI, the Alaska Redoubt Volcano (VEI 3) erupted on March 15, 2009. The volcanic dust curtain and aerosol layer then spread to the Arctic region, carried by the polar vortex airflow. In the central region of the polar vortex, the sun umbrella effect of the volcanic dust curtain and aerosol layer resulted in the deepening of the cold polar vortex in the Arctic region, decreases of atmospheric pressure and temperature, and an increase of the AOI. The AOI maximum value was 1.19 in May 2009, which was the first maximum since the AO began to decline in October 2008. After the first maximum,In the process of rapid AOI decline, Kuril Islands Sarychev Peak Volcano (VEI 4) erupted on June 11, 2009, causing AOI to rise again. A second AOI maximum value of 0.87 then occurred in September 2009. After that, the AO index resumed its decline, until another eruption of the Kamchatka Klyuchevskoy Volcano (VEI 2) occurred on August 1, 2009, causing the AOI to rebound and reach a third maximum. Because the Kamchatka Klyuchevskoy Volcano erupted in a zonal flow of air outside the polar vortex, as seen in Figure 7(a), the volcanic dust and aerosol layer were diffused along the periphery of the polar Vortex by the polar vortex flow. Hence, the amount of aerosols entering the Arctic region was small. Plus, the eruption was not very strong, only a magnitude 2,Therefore, the upswing amplitude of the tao Index curve is also very small, and the third maximum value formed is only 0.46. After this maximum, the AOI fell rapidly again. During this descent, the eruption of the Kamchatka Bezymianny Volcano (VEI 3) occurred on December 17, 2009. As a result, the volcanic dust curtain and aerosol layer only spread over the East Asian vortex. Since there was no access to the Arctic region, the resulting AOI fluctuation was small.
As seen in Figure 7(b), in December 2009, the Pacific-North American ridge of high pressure expanded Abnormally toward the Arctic region, and the polar vortex split into three vortices heading south: the North American Great Lakes vortex, the European vortex, and the East Asian vortex, with the East Asian vortex completely removed from the Arctic region. As shown in Figure 7(b), since the Kamchatka Bezymianny Volcano was located in the East Asian low eddy circulation, the volcanic dust curtain and aerosol layer could only spread within the East Asian vortex circulation and thus could not expand into the Arctic region. Moreover, the volcanic dust curtain and aerosol layer promoted further strengthening of the East Asian vortex circulation, and the southwest advection ahead of the vortex was favorable for the Pacific-North American ridge to advance toward the Arctic region, causing the isobaric surface of the Arctic region to rise. As shown in Figure 7(d), the Arctic region was controlled by the positive geopotential height field anomaly, while the middle and high latitudes around the Arctic region formed a negative geopotential height field anomaly zone. As shown in Figure 7(c), a blocking high appeared in the Arctic region, while the Great Lakes, European, and East Asian vortices surrounded the Arctic region. A strong anti-oscillation phenomenon was formed in which the atmospheric pressure in the Northern Hemisphere fell in the middle and high latitudes and rose in the Arctic region. After January 2010, the AOI declined further, reaching a minimum of -3.556 in February 2010, which was the lowest value since 1900.
From October 2008 to February 2010, the AOI curve dropped to the lowest point, forming the maximum of antiphase oscillation.During that time, a total of four eruptions occurred. Although each of these eruptions caused the AOI to temporarily rise, since their locations were farther and farther away from the center of the Arctic vortex, the amplitudes of the associated AOI fluctuations became smaller and smaller, finally reaching the minimum AOI value.
(2) Atmospheric circulation characteristics of the anti-phase Arctic oscillation in February 2010 Figure 7(c) shows the atmospheric circulation characteristics of the Arctic oscillation during the maximum anti-phase in February As can be seen from this figure, the meridional circulation of the mid-high latitudes in the Northern Hemisphere was developing abnormally, as was the North Pacific-North American high pressure ridge. Another ridge,The ridge of high pressure in the North Atlantic also developed abnormally,Purple ridge line 2 is shown in the figure,The ridge passes through Greenland and goes deep into the arctic region, forming a closed high pressure center in the Arctic region,Blocking high pressure is formed. Thus, the polar vortex was split into the Eurasian vortex and the North American Great Lakes vortex, while the Eurasian vortex moved southward,completely out of the Arctic region. The Great Lakes vortex also moved south, mostly out of the Arctic region. There was also a high pressure ridge in the Caucasus region of Eurasia moving toward the North Pole, as indicated by ridge line 3. The Eurasian vortex was split into the European vortex and the Asian vortex, and since the center of Asian vortex was in East Asia, it was also referred to as the East Asian vortex.
As a result, the arctic region is controlled by the Arctic vortex circulation arctic oscillation peak period,It gradually evolved into a period of anti oscillation under the influence of high pressure ridge or high pressure circulation. Correspondingly, the Arctic region also evolves from the negative anomaly area of potential height field in the high period of the Arctic oscillation to the positive anomaly area of potential height field in the anti-oscillation period,See Fig 2 d, Fig 3 d, Fig 4 d, and Fig 7 d.On the contrary, the positive anomalous zone of geopotential height field around the Arctic region at middle and high latitudes during the high AO period,As shown in fig 2 d, fig 3 d and fig 4 d.A positive anomaly belt composed of several positive anomaly centers connected by a white circle,It becomes the negative anomaly zone of geopotential height field in the period of anti oscillation, In fig 7d, the negative anomaly belt is composed of several negative anomaly centers connected by the white circle.
Throughout this period (2008–2010), as shown in Figure 6, each significant fluctuation of the AOI was associated with a volcanic eruption, and each volcanic eruption caused AOI increases to varying degrees.
(3) The formation of the anti-phase maximum of the AO in January 1977 is related to volcanic activity As seen in Figure 8, from 1974 to 1977, the AOI maximum was 1.66, occurring in February 1976. This was related to three previous consecutive volcanic eruptions that of the Kamchatka Tolbachik Volcano (VEI 4) on June 28, 1975, the Alaska Pavlof Volcano (VEI 2) on September 13, and the Alaska Ustine Volcano (VEI 4) on January 23, 1976. After the AO maximum in February 1976, the AO intensity fluctuated continuously. By January 1977, the AOI had dropped to -3.279, which was the second-largest AOI index of the anti-phase Arctic Oscillation since 1950. As can be seen from Figure 8, during the continuous fluctuation of the AO intensity, each fluctuation was associated with a volcanic eruption. In other words,each volcanic eruption appeared to cause an Arctic Oscillation peak, followed by a certain amount of upswing. As an example, during the AOI decrease following the February 1976 AO maximum, the Aleutian Shishaldin Volcano (VEI 2) erupted on April 6, 1976.Because the Aleutian Shishaldin Volcano is small and erupts outside the Arctic vortex circulation,The amount of volcanic dust curtain and volcanic aerosol layer spreading to the Arctic region is small,only caused AOI index curve a small rise.By August 1976, the AOI maximum value was only 0.56,It is the first maximum since the AO began its decline in February 1976;After the first maximum,In the rapid AOI decline process,The Kuril Islands Sarychev Peak Volcano (VEI 2) erupted on 23 September 1976.Because the Kuril Islands Sarychev Peak Volcano are small in intensity, as shown in Figure 9 d,And the eruption occurred outside the Arctic vortex circulation,The amount of volcanic dust curtain and volcanic aerosol layer spreading to the Arctic region is small,AOI index curve only caused a small rise,The second AOI maximum value -0.09 was formed in November 1976;Since then the AOI has continued to decline,In January 1977, it reached the second anti-phase maximum -3.279 since 1900 .
(4)Influence of volcanic eruption location on AOI As mentioned above, the Aleutian Shishaldin Volcano erupted in the north on April 6, 1976. Since it was on the periphery of the polar vortex circulation, the size of the volcanic dust curtain and aerosol layer spreading to the Arctic region was small, causing only a small rise in the AOI. Later that year, on October 15, the Aleutian Akutan Volcano erupted, outside the Arctic vortex. As shown in Figure 9 (d), a new vortex had formed off the Aleutian Islands, and the Aleutian Akutan Volcano was within the Aleutian Low eddy circulation. Hence, the volcanic dust curtain and aerosol layer could only diffuse within the Aleutian Low vortex circulation and could not enter the Arctic region. Moreover, the volcanic dust curtain and aerosol layer promoted further strengthening of the Aleutian vortex circulation, The enhanced southwest advection in front of the vortex is conducive to the further strengthening of the Pacific-North American ridge toward the
Arctic region,This causes the isobaric surface of the Arctic region to rise.As shown in Figure 9(b),the Arctic region was controlled by a positive geopotential height anomaly, while a negative geopotential height anomaly zone existed in the middle and high latitudes around the Arctic region.
A blocking high was present in the Arctic region, with the Great Lakes vortex and East Asian-Aleutian vortex surrounding the Arctic region. A strong anti-oscillation event was generated in which the atmospheric pressure in the Northern Hemisphere fell in the middle and high latitudes and rose in the Arctic region. In addition, the Kamchatka Klyuchevskoy Volcano erupted on April 8, 1974, although this eruption occurred outside the Arctic vortex circulation and under the subsiding diffuse flow ahead of the high pressure ridge, as shown in Figure 9(c). It is also apparent that a vortex had formed near the Aleutian Islands, and the volcanic dust and aerosol layer from the eruption circulated within the vortex. Hence, the amount of outward diffusion was small, and the effect on the center of the Arctic vortex was small.
(5) Atmospheric circulation characteristics of the anti-phase Arctic oscillation in January 1977 Figure 9(a) Shows the atmospheric circulation characteristics of the Arctic Oscillation during the anti-phase period in January 1977.As can be seen from this figure, a meridional circulation anomaly at middle and high latitudes in the Northern Hemisphere had developed, and the North Pacific-North American ridge of high pressure had developed abnormally. This ridge of high pressure extended toward the Arctic region, as depicted by ridge line 1 in the figure, forming a closed high pressure center in the Arctic region, which was a blocking feature. Thus, the polar vortex was split into the Asian vortex and the North American Great Lakes vortex, with the Asian vortex pushed southward and almost entirely out of the Arctic region. Since the center of the Asian vortex is in East Asia, it is also referred to as the East Asian vortex. In addition, since the bulk of the East Asian vortex covers the Aleutian Sea area, it is also known as the Aleutian Low. The Great Lakes vortex also shifted south, mostly out of the Arctic region. Another high pressure ridge, the North Atlantic high pressure ridge, was also developing abnormally during this period, as depicted by ridge line 2 in Figure 9(a). The high pressure ridge bulged toward Greenland, making the Greenland low pressure area shallower. There was also a high pressure ridge in the Eurasian Caucasus heading toward the North Pole, as depicted by ridge 3 in the figure. Thus, the peak of the Arctic Oscillation controlled by the Arctic vortex circulation evolved into a period of anti-phase Arctic Oscillation influenced by a ridge or circulation. Corresponding to this, the Arctic region also evolved from the negative anomaly area of the potential height field during the peak period of the Arctic oscillation, as shown in Figures 2(d), 3(d), and 4(d), to the positive anomaly area of the potential height field during the anti-oscillation period, as shown in Figure 9(b). Contrary to this, the positive anomaly zone of the potential height field around the Arctic region at middle and high latitudes during the peak period of the Arctic oscillation, as shown by the positive anomaly zone composed of several positive anomaly centers connected by the white circle in Figures 2(d), 3(d), and 4(d), became the negative anomaly zone composed of several negative anomaly centers connected by the white circle in Figure 9(b).
Throughout this period (1974-1977), as shown in fig 8, each significant fluctuation of AOI curve of the Arctic Oscillation Index is associated with a volcanic eruption, or each volcanic eruption can cause a rise of AOI curve to different degrees.