Temperature response to the June 2020 solar eclipse observed by FORMOSAT-7/COSMIC2 in the Tibet sector

This study explores the response of atmospheric temperature to the annular solar eclipse at the summer solstice on 21 June 2020. The radio occultation (RO) technique of the FORMOSAT-7/COSMIC2 (F7/C2) mission observes the temperature in the troposphere and stratosphere. The RO observations show that the temperature decreases significantly (near 4 to 8 °C) between 5 and 8 km altitudes over the Tibetan Plateau area within the 80% obscuration during the eclipse. The tropopause temperature increases by ~ 2 to 5 °C over the same area. By contrast, the tropopause temperature decreases by ~ 4° to 5 °C over the Indian Ocean. The F7/C2 RO technique captured not only the sudden tropospheric cooling and stratospheric warming over Tibet during the eclipse but also the possible response over the Indian Ocean away from the greatest eclipse. RO temperature decreases over the Tibetan Plateau during the eclipse Opposite changes of the tropopause temperatures over Tibet and Indian Ocean Indian summer monsoon circulation was perturbed during the eclipse RO temperature decreases over the Tibetan Plateau during the eclipse Opposite changes of the tropopause temperatures over Tibet and Indian Ocean Indian summer monsoon circulation was perturbed during the eclipse


Introduction
As the Moon is in a position where it blocks the light from the Sun, the quick onset of the darkness induces a sudden drop in temperature of the air and generates local circulation near the Earth's surface (Clayton 1901;Aplin et al. 2016;Clark et al. 2016;Eugster et al. 2017;Turner et al. 2018;Mahmood et al. 2020). Wang and Liu (2010) first examined the FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) radio occultation (RO) temperature profiles during the 22 July 2009 total solar eclipse and found that atmospheric response to the solar eclipse is not as simple as a cooling of the whole atmosphere. The reduction in solar heating results in the cooling in the troposphere near the surface, which is thermally driven. The thermal contraction in the troposphere can induce warming in the upper troposphere and lower stratosphere (between 13 and 23 km altitudes), which is dynamically driven. Wang et al. (2012)  An annular solar eclipse nicknamed the "ring of fire" was visible in Africa and Asia at the summer solstice on 21 June 2020. The moon shadow appears over Africa near 0500 UT and disappears over the Pacific Ocean near 0830 UT (https:// eclip se. gsfc. nasa. gov/). It is another rare opportunity for us to examine the response of the atmosphere to transient obscuration. A solar eclipse is a unique and rare phenomenon that occurs over a particular area at a certain local time in a specific season. This eclipse is the only case occurring over Tibet near noon time in recent decades.

Results
Figure 1a displays that the F7/C2 RO recorded twelve temperature profiles within and near the 80% obscuration area in the Tibetan Plateau longitudinal sector (75° to 100°E) from 05:00 to 10:00 UT. Figure 1b shows the temperature profiles within the areas of 20° to 28°N (over North India), 28° to 35°N (over the Tibetan Plateau area and within the 80% obscuration), 35° to 40°N (beyond Tibet). The temperatures below 10 km over Tibet are lower than those over North India. By contrast, temperature near the tropopause over Tibet is higher than that over India. In this study, tropopause is defined as the minimum temperature of the vertical temperature profile. The average of the temperature within the entire path (from Africa to Pacific Ocean longitudes) of 80% obscuration did not change prominently during the eclipse (not shown here). The temperature changes occurred mainly over the Tibetan sector. Figure 2 is the comparison of the temperature profiles during the eclipse period and the reference that is the average of the profiles collected over the three areas during seven days before and after the eclipse day (two weeks in total). Figure 2a and b display no significant change in temperature over North India. However, Fig. 2c-d illustrate the temperature changes over the Tibetan Plateau area within the 80% obscuration. Figure 2d shows that the temperature decreases by ~ 4 to 8 °C between 5 and 8 km. By contrast, the temperature increases by ~ 2° to 5 °C near the tropopause. In Fig. 2e and f, the two temperature profiles recorded over the northern side (35° to 40°N) of Tibet yield the similar changes as those between 28° and 35°N. Sun et al. (2014) examined the F3/C RO soundings and showed that the latitudinal variation in the tropopause temperature is significant near the latitudes of 35° to 40°N in the Tibetan sector in summer of the Northern Hemisphere. Accordingly, the temperature changes over the northern side of Tibet should contain both the effects of obscuration and latitudinal variation in the tropospheric temperature profiles.
Most of the studies analyzed the ground-based observations on the land to examine the eclipse effect on local circulation and weather conditions (Aplin et al. 2016). However, the RO technique sounds the atmosphere worldwide, which benefits us to examine the effect over the ocean. Therefore, this study further examined the temperature changes over the Indian Ocean away from the greatest eclipse. Figure 3a and b show no obvious response of the temperature to the eclipse over the South India area (10° to 20°N, 75° to 100°E). Nevertheless, in Fig. 3c and d, the temperature near the tropopause decreases by ~ 3 to 5 °C over the area of Indian Ocean (0° to 10° N, 75° to 100° E). Figure 4 shows the lapse rate profiles over the Tibetan Plateau and the Indian Ocean areas. The lapse rate ranges from − 6 to − 9 °C/km in the upper troposphere from 10 to 15 km over Tibet during the eclipse (Fig. 4a). The lapse rate values during the eclipse vary mainly within one standard deviation of the reference values of lapse rate. The lapse rate increases significantly and exceeds the standard deviation above 15 km altitude. By contrast, over the Indian Ocean area, the lapse rate is lower than one standard deviation of the reference values near 15 km altitude and drastically increases above it (Fig. 4b). The variations in the lapse rates reveal that the structures and dynamics are different near the tropopause in both the Tibet and Indian Ocean areas during the eclipse. Figure 5 displays the temporal variations in the temperatures over the areas of Tibetan Plateau and Indian Ocean. Figure 5a shows the sudden drop in temperature averaged within 5 to 8 km over the Tibetan Plateau area during the obscuration. The local minimum occurs near the end of the obscuration. On the other hand, the tropopause temperature reaches its maximum near the end (Fig. 5b). By contrast, the tropopause temperature decreases over the Indian Ocean area and reaches the minimum near the end of the eclipse (Fig. 5c). Page 5 of 7 Sun et al. Terrestrial, Atmospheric and Oceanic Sciences (2022) 33:2 3 Discussion Wang and Liu (2010) reported that the maximum daily mean difference of the RO temperature can reach -6 ºC at ~ 5-10 km altitudes within the obscuration path of the 22 July 2009 total solar eclipse. The magnitudes of the temperature decreasing in this study and Wang and Liu (2010) are comparable. The temperature decreased by near − 4° to − 8 °C at ~ 5 to 8 km altitudes above the surface of Tibetan Plateau rather than the entire obscuration path during the annular solar eclipse on 21 June 2020. The heat capacity of air over the Tibetan Plateau is smaller than that over India, because the precipitation and humidity over the plateau are much less than those in the surrounding low-elevation region. It would explain why the temperature changes prominently in the lower troposphere over Tibet during the obscuration. On the other hand, Wang and Liu (2010) showed that the RO temperature increased by ~ 7 °C near the tropopause in the obscuration path during the 2009 total solar eclipse. The tropopause temperature also increases near tropopause above Tibet (Figs. 2 and 5). By contrast, the RO soundings show that the temperature near the tropopause deceases over the Indian Ocean away from the obscuration path.
The classical Hadley circulation, characterized by the air rising from the equator and descending over midlatitudes, is reversed in the summer of the Northern Hemisphere. Heating the Tibetan Plateau (mean altitude of ~ 5 km) due to solar radiation results in the reversed circulation in summer, so-called the Indian monsoon circulation (Webster and Fasullo 2003). The Tibet Plateau is an intense heat source that leads the air rising near and above the plateau and then flowing toward and descending near the equator. The descending flow can suppress the convection over the Indian Ocean in summer (Sun et al. 2014).
The annular solar eclipse occurred on the June solstice of 2020, the beginning of the season of the Indian summer monsoon. The thermal contraction in the lower troposphere induces downward movement in the upper troposphere and lower stratosphere (Wang and Liu 2010). The change of the dynamical process can disturb the convection over the Tibetan Plateau, which further weaken the descending flow and allow the convection to become deeper over the Indian Ocean. Therefore, the tropopause would become colder over the Indian Ocean during and after the obscuration (Figs. 3c, 5c). The obscuration of solar eclipses can induce mesoscale-or synoptic-scale weather conditions (Clayton 1901;Aplin et al. 2016;Eugster et al. 2017;Turner et al. 2018). The results in this study reveal that a solar eclipse may be able to change the monsoon circulation. Utilizing a numerical model analysis in the future can better describe the dynamic process.
It has been speculated that solar eclipses can induce atmospheric gravity waves (AGWs) in the lower atmosphere. The AGWs can propagate upward and generate bow and stern waves (Liu et al. 2011) that consist of plasma density disturbances and sporadic E in the ionosphere near 80% obscuration over Asia particularly in the Northern Hemispheric summer Wang et al. 2021). The stratopause and mesopause with high cooling rates could be the sources of the eclipse-generated AGWs (Brasseur and Solomon 2005). However, the exact origin of the AGWs in the lower atmosphere remains obscure. The prominent changes of the temperature in the troposphere above Tibet suggest that the plateau can be considered as a candidate of the source of the AGWs trigged by the eclipse. The vertical propagation of atmospheric waves from the surface to the ionosphere over and around Tibet can be further examined (Chen et al. 2021a, b).

Conclusion
The FORMOSAT-7/COSMIC2 (F7/C2) radio occultation (RO) technique vertically scanned the atmosphere during the annular solar eclipse at the summer solstice on 21 June 2020. The moon shadow obscuration provides us a rare and unique chance to depict the response of the lower atmosphere to a space weather phenomenon. The obscuration induced transient temperature changes Fig. 5 Time series of the temperature variations over the areas of Tibetan Plateau (28° to 35° N, 75° to 100° E) and Indian Ocean (0° to 10° N, 75° to 100° E). a The mean temperature between 5 and 8 km and b the tropopause temperature over the Tibetan Plateau area. c The variation in the tropopause temperature over the Indian Ocean area. To avoid the contamination of the long-term variability, the daily mean was removed from the observation over the Indian Ocean area. The gray dots are the temperatures from each RO profile. The black curves are the 4 h moving average of the temperature observations. The dash red lines indicate the beginning and the end of the obscuration over Tibet mainly over the Tibetan Plateau area. The thermal contraction of the troposphere appears especially within the 80% obscuration area over Tibet. The RO observations show that a solar eclipse can not only change weather conditions within the obscuration area, but also possibly disturb the monsoon circulation over Indian Ocean away from the greatest obscuration path.