The WNP tropical cyclone (TC) activity peaks in July-August-September, accounting for about 60% of the annual TC count (Chan, 2005;Lander, 1994). TCs in this study are defined as those in the data sets with maximum wind speed exceeding the tropical storm intensity (17.2 m s −1 or 34 knots). Climatologically, about four TCs form in July and it usually takes a northward-recurved track and exerts great impacts on subtropical East Asia. However, no TC formation in July 2020 over the WNP was recorded in July 2020 (Figure 1a), which is the first time in the available historical records since 1945. The unprecedented absence of TC formation suggests its possible unique driving factors.It is well known that El Niño-Southern Oscillation (ENSO) is an important factor modulating TC activity over the WNP (Chan, 1985;Lander, 1994;Wang & Chan, 2002). During the El Niño decaying summer, an anomalous anticyclonic circulation is a recurrent pattern in the WNP, which acts to suppress TC genesis over the WNP (
This paper reports some interesting equatorial plasma bubbles (EPBs) on 4 and 5 October 2013 and 29 and 30 September 2013 observed by two all‐sky imagers located in the magnetically equatorial region of Hainan (19.5°N; 9.5° geomagnetic latitude) and Guiping (23.8°N; 13.9° geomagnetic latitude), China. The case of 4 and 5 October 2013 shows the following: (1) EPBs had planar wave‐like structures and their wavefronts were parallel to each other before midnight. (2) The angle between the wavefronts of those EPBs and the geomagnetic meridian were ~30°. (3) Near midnight, the higher‐latitude part of one EPB suddenly began to rotate with a large eastward zonal drift, resulting in an unusual C‐shaped EPB that was observed for the first time by an all‐sky imager. (4) After midnight, one EPB merged into another EPB and formed an integrated EPB. The other EPB case on 29 and 30 September 2013 shows the following: (1) The images captured the full evolution processes of an “I” shaped EPB with an angle of ~30° with respect to the geomagnetic meridian. The imagers also observed “S” and “Y” shaped EPBs. (2) The change of EPBs' shape was directly related to the change of EPBs' zonal drift velocities. (3) The angle between the I‐shaped EPBs and the geomagnetic meridian was likely caused by the change of the latitudinal gradient of zonal neutral wind velocities and ionospheric conductivity. (4) The zonal velocity of each branch of the “Y” shape EPB was different, which could be related to the polarization electric fields within each branch.
The lower pair satellites of Swarm mission, flying side‐by‐side and separated by 1.4° in longitude (about 150 km), usually observed equatorial plasma depletions (EPDs) showing quite different structures, and sometime even only one satellite observed EPD. In this study, we provided 6‐h continuous observations of EPDs on the night of 23–24 September 2014, from an all‐sky imager located at Fuke (geographic:19.5°N,109.1°E), south of China. From the airglow images the EPDs were found with longitudinal extensions of about 50 km and all tilted from northwest to southeast direction. We further checked the in situ electron density simultaneously measured by the Swarm lower pair satellites and found the differences of Swarm in situ electron densities explained well by the longitudinally thin structure of EPDs observed from the all‐sky imager. During later periods the bifurcation and merging were observed by the airglow images, and it was the first time to report both processes in the evolution of one EPD. The bifurcation was first observed at the higher‐latitude part, and then observed at lower latitudes of EPD. The subbranches generated through bifurcation showed even thinner longitudinal extension of about 20–30 km, and later the subbranches started to merge with each other, forming a really complicated mesh of depleted regions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.