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The sea‐ice extent (SIE) in the Weddell Sea plays a crucial role in the Antarctic climate system. Many studies have examined its long‐term trend, however whether its year‐to‐year variation has changed remains unclear. We found an amplified year‐to‐year variance of the Weddell Sea SIE in austral summer since 1998/1999 in observational datasets. Analyses of sea‐ice concentration budget and surface fluxes indicate that it is the thermodynamic process that drives the amplification of SIE variations, rather than the sea‐ice‐drift‐related dynamic process. Compared to 1979–1998, the Southern Annular Mode in the preceding spring shows a closer linkage with the Weddell Sea SIE in 1999–2021 through a stronger and more prolonged impact on sea surface temperature, which thermodynamically modulates local sea ice via changing surface heat and radiation fluxes. Our study helps advance the understanding of extreme low Antarctic‐SIE records occurring in recent decades and improve future projections of the Antarctic sea‐ice variability. Plain Language Summary: Unlike Arctic sea ice, which has significantly declined in recent decades, Antarctic sea ice possesses a moderate increase with large spatial and seasonal discrepancies. The amount of austral‐summer sea ice extent (SIE) around the Antarctica exhibits more pronounced year‐to‐year variations in the 21st century. Interestingly, we found that the amplified SIE variation is most significant in the Weddell Sea, east of the Antarctic Peninsula. This amplified variations in Weddell Sea SIE apparently cannot be fully explained by anthropogenic forcing, suggesting an important role of internal variability in the Antarctic climate system. Our analyses showed that the late‐1990s changes in the Southern Annular Mode (SAM)—the predominant mode of climate variability in the extratropical Southern Hemisphere—might cause the interdecadal amplification of the summer sea‐ice variation in the Weddell Sea via the thermodynamic process. Key Points: The year‐to‐year variability of summer sea‐ice extent in the Weddell Sea, Antarctica, has significantly increased since 1998/1999This interdecadal change in sea‐ice extent is dominated by changes in thermodynamic process rather than dynamic and mechanical processesThe Southern Annular Mode contributes to such a sea‐ice change via altering sea surface temperature and thus local thermodynamic processes [ABSTRACT FROM AUTHOR]

Different from other upper‐tropospheric jet stream that owns the single jet axis, the tropical easterly jet (TEJ) generally splits into two branches over the tropical western Pacific where the TEJ inflows from June to September, with the north branch locating near 20°N and the south branch appearing near the equator. However, how these double easterly jets change from year to year and whether they have effects on the tropical cyclones (TCs) over the tropical western Pacific are still unknown. Observational evidence has demonstrated that the double easterly jets in the TEJ's inflow region exhibit an out‐of‐phase relationship in the interannual time scale and is significantly related to the TCs genesis. Corresponding to a stronger south branch and a weaker north branch, the accelerated (decelerated) easterly wind for the south (north) branch would induce a northerly (southerly) ageostrophic wind, thus triggering an upper‐tropospheric divergence between the double easterly jets. This divergence can further lead to anomalous ascending flows over the western North Pacific, which turns out to be the primary cause that increases the TCs number over there. The results highlight that the out‐of‐phase variation of the double easterly jets could dynamically modulate the occurrence of the TCs over the western North Pacific by inducing the secondary vertical circulation. [ABSTRACT FROM AUTHOR]

This study investigated the influence of the stratospheric quasi‐biennial oscillation (QBO) on the tropical easterly jet (TEJ), using the ERA5 and QBO series of Singapore and Freie Universität Berlin. Since the 1980s, the easterlies of the TEJ over the Maritime Continent are weaker during easterly QBO (EQBO) than westerly QBO. QBO influences TEJ by affecting convection on the east and west sides of the Maritime Continent. The climatological rising movement on the west side of the Maritime Continent can reach a higher altitude than that on the east side, which implies the convections on the west side are likely to be more effectively promoted by the reduced stability near the tropopause during EQBO. The zonal asymmetry in the stability near the tropopause is also a possible cause for QBO modulating convection. QBO has a significant influence on the Asian summer monsoon and tropical/subtropical precipitation through modulating the TEJ. Plain Language Summary: The tropical easterly jet (TEJ) is an essential component of the Asian monsoon system. Quasi‐biennial oscillation (QBO) is a phenomenon characterized by quasi‐periodic oscillation in the tropical stratospheric wind field. The opposite east‐west wind directions in the lower stratosphere during different phases of QBO are accompanied by opposite temperature and stability anomalies around the tropopause. Due to the different heights of the rising movements on the east and west sides of the Maritime Continent, their responses to the change of upper tropospheric stability caused by QBO are different. Responding to the change of the rising movement, the TEJ over the Maritime Continent during easterly QBO is significantly weakened compared with westerly QBO, and the decreased magnitude of easterly QBO can reach 11% of the climatological TEJ. The change of TEJ has a significant influence on the Asian summer monsoon and the precipitation in densely populated agricultural areas. The relationship between QBO and TEJ has recently emerged since the 1980s possibly due to climate change. Considering that QBO has a relatively robust periodicity, the relationship between QBO and TEJ has potential merit for long‐range prediction of TEJ and weather systems related to the Asian monsoon system. Key Points: Tropical easterly jet (TEJ) is weakened over the Maritime Continent during easterly quasi‐biennial oscillation (QBO) since the 1980sThe zonal asymmetries in the convection, stability, and Kelvin waves are possible causes for QBO modulating TEJQBO influences the Asian summer monsoon and tropical/subtropical precipitation via TEJ over the Maritime Continent [ABSTRACT FROM AUTHOR]