Yuanyuan Guo

The minimum Arctic sea ice extent has drawn considerable attention due to its global climate impacts and implications for Arctic shipping routes. In August 2012, an exceptionally strong Arctic cyclone (AC12) contributed to the September sea ice extent reaching its lowest level since 1979. However, the cause of the extreme intensity of AC12 has not been fully understood. In this study, we propose that a tropical–Arctic teleconnection is responsible for the rapid intensification of AC12, which is triggered by extremely strong convection over the western North Pacific (WNP) and elongated by the extremely northward‐displaced North Pacific Jet (NPJ). AC12 reinforces the preexisting Arctic tropopause polar vortex by anomalous poleward heat and moisture transport and enhanced baroclinicity. Observations and simulations show that WNP convection alone typically cannot trigger Rossby waves into the Arctic; rather, idealized experiments demonstrate that the northward‐shifted NPJ plays an essential role in establishing a continuous waveguide by strengthening the meridional potential vorticity gradient near the Bering Strait. Further model simulations driven by observed sea surface temperature anomalies reveal that the extremely negative phase of the Pacific Meridional Mode in late summer 2012 plays a crucial role in establishing the tropical–Arctic teleconnection. Its tropical component, the cold tongue over the northeastern tropical Pacific, favors more frequent strong WNP convection, while its extratropical component displaces NPJ via changing meridional temperature gradients. These findings have important implications for the tropical–Arctic intraseasonal interaction and its effect on Arctic extremes.

As one of the key components of the Asian summer monsoon system in boreal summer, the upper‐tropospheric tropical easterly jet (TEJ) is significantly modulated by the El Niño‐Southern Oscillation (ENSO) on the interannual time scale. In this study, we report how the El Niño decay rate prominently controls the TEJ's behaviours from the post‐El Niño spring to summer. For the slow‐decaying (SD) El Niño cases, a warmer‐than‐usual sea surface temperature anomaly (SSTA) pattern in the tropical central‐eastern Pacific sustains from the mature winter to decaying summer. Consequently, the associated equatorial Kelvin wave response will decelerate the TEJ in its outflow and core regions in early summer. Moreover, the SD El Niño SSTA pattern induces a pair of anomalous upper‐tropospheric anticyclones over the western Pacific, where the TEJ usually inflows in the shape of double easterly jets. These anticyclone responses will strengthen the south branch of double easterly jets and weaken the north one branch. The fast‐decaying (FD) El Niño cases experience a phase transition into a La Niña‐like cooling pattern in the decaying phase. The FD El Niño‐induced atmospheric circulation is almost a mirror to the counterpart induced by the SD El Niño cases in early summer. That is, the FD El Niño cases aid in a weaker‐than‐normal south branch of double easterly jets and a stronger‐than‐normal north branch in the western Pacific, as well as an acceleration of the entire TEJ. These results foreground the significant influences of the El Niño decaying pace on the TEJ in early summer and indicate that the temporal diversity in the ENSO events should be considered for further understanding of the ENSO impacts on the Asian summer monsoon system.

During cold wave movements, land‐atmosphere interaction functions as a “switch” mechanism, regulating energy and moisture exchange. However, existing methods have limitations in capturing this process. The dielectric contrast between frozen and thawed soil induces diurnal amplitude variation (DAV) in brightness temperature at L‐band, providing a basis for tracking land‐atmosphere interactions. Our study shows that DAV effectively captures cold air masses' origin and seasonal movement. During a cold wave, its phase relationship with air and soil temperatures falls into four categories: precedes (−1.1 to −0.6 days), lags (0.71–0.98 days), in between (−0.62 to 0.34 days), and synchronized. Moreover, DAV lags air temperature in 63% of cases and precedes soil temperature in 59%, consistently fluctuating within their overlap region. This suggests DAV is a more direct indicator of freeze–thaw, reflecting the conditions at the atmosphere–land interface.

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.

Despite extratropical forcing being recognized as an important factor that can modulate El Niño-Southern oscillation (ENSO) properties on the interannual time scale, little is known about whether and how Arctic forcing changes the tropical sea surface temperature (SST). This current study reveals a significant link between the net surface sensible heat flux (SHF) in the Arctic and the SST anomalies in the tropical eastern Pacific (TEP). Specifically, anomalous upward SHF into the Arctic atmosphere in February leads to a warmer TEP in the subsequent spring and summer. A northeast-southwest-tilted North Pacific Oscillation-like atmospheric pattern associated with the upward Arctic SHF anomaly induces SST cooling in the subtropical North Pacific via positive Wind-Evaporation-SST feedback, which further promotes TEP SST warming via meridional heat advection, thermocline feedback, and nonlinear processes. The spring-to-summer TEP SST anomalies driven by the preceding anomalous Arctic SHF can potentially modulate the seasonal evolution of ENSO. Our findings imply that we should take into account the Arctic-tropics linkages when comprehensively understanding the ENSO variability and improving ENSO projection skills.

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.

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.