Sudden stratospheric warming (SSW) events have received increased attention since their impacts on the troposphere became evident recently. Studies of SSW usually focus on polar stratospheric conditions; however, understanding the global impact of these events requires studying them from a wider perspective. Case studies are used to clarify the characteristics of the stratosphere-troposphere dynamical coupling, and the meridional extent of the phenomena associated with SSW. Results show that differences in the recovery phase can be used to classify SSW events into two types. The first is the absorbing type of SSW, which has a longer timescale as well as a larger meridional extent due to the persistent incoming planetary waves from the troposphere. The absorbing type of SSW is related to the annular mode on the surface through poleward and downward migration of the deceleration region of the polar night jet. The other is the reflecting type. This is characterized by a quick termination of the warming episode due to the reflection of planetary waves in the stratosphere, which leads to an amplification of tropospheric planetary waves inducing strong westerlies over the North Atlantic and blocking over the North Pacific sector. Differences in the tropospheric impact of the absorbing and reflecting SSWs are also confirmed with composite analysis of 22 major SSWs.

With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20th-century Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean-atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-to-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region. T he Arctic has warmed faster than the global average by a factor of 2 or more since the mid-20th century, a phenomenon known as the Arctic amplification. The recent temperature warming over the Arctic is strongly linked to a drastic reduction in sea ice extent since the 1970s, contributing to the Arctic amplification through positive ice-albedo feedbacks (1-3). A similar rapid warming occurred in the Arctic during the early 20th century (4-8). Compared with the recent warming, the early 20th-century Arctic warming (hereafter referred to as the early Arctic warming) is mysterious as greenhouse gas (GHG) radiative forcing was three to four times weaker than at present (9) and changes in sea ice extent were small (10). The comparison of these two warming epochs suggests that mechanisms other than GHG forcing are important for the early Arctic warming.Several hypotheses have been proposed for the early Arctic warming, including intensified natural forcing due to decreased volcanic aerosols and increased solar radiation (11, 12); increased cloud long-wave emissivity due to sulfate aerosols transported from Central Europe (6, 13); uncertain but possible reduction in the Arctic sea ice extent (4,5,14); variability of the North Atlantic ocean-ice-atmosphere system (15); and atmospheric internal variability (16). Neither coupled ocean-atmosphere models nor atmospheric models driven by historical radiative forcing and observed sea surface tempe...

An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named "the Heavy Rain Event of July 2018", which caused widespread havoc. It was followed by heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of tropical origins confluent persistently into western Japan and largescale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan.

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A reflection of stratospheric planetary waves and its impact on the troposphere during a stratospheric sudden warming of March 2007 are investigated. Zonal propagation and reflection of the planetary waves is clearly seen in the longitude‐height sections of the eddy geopotential height and the vertical and zonal component of the three‐dimensional wave activity flux. A wave packet propagating upward and eastward from Eurasian continent was reflected by a negative wind shear in the upper stratospheric westerly jet caused by stratospheric warming. Waves then propagated downward to the American‐Atlantic sector of the troposphere, which led to the formation of a deep trough over the Atlantic and brought cold weather to the northeastern part of the American continent.

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Predictability and a possible tropospheric precursor of a zonal‐wavenumber (WN) 1 stratospheric sudden warming (SSW) event occurring in December 2001 are examined using all ensemble members of the 1‐month forecasts carried out by the Japan Meteorological Agency (JMA). The SSW is predictable from at least 2 weeks in advance, and high sensitivity to the initial condition for the SSW prediction is observed during the onset period of the SSW. Dipole zonal wind anomaly pattern along 80°N/60°N in the upper troposphere during this period is significantly related to the subsequent warming in polar stratospheric regions through deflecting the WN 1 propagation poleward. The leading EOF of predicted zonal‐mean zonal wind variations among ensemble members for the onset period also shows the similar profile. Thus, this coincidence would be responsible for the high sensitivity to the initial condition observed during the onset period of the SSW.

[1] Case studies are used to elucidate the relationship between stratospheric planetary wave reflection and blocking formation in the troposphere. The enhanced upward propagation of a planetary-scale wave packet from the Eurasian sector, involving a Euro-Atlantic blocking, leads to a stratospheric sudden warming (SSW). Following the weakening of the stratospheric westerly jet due to polar warming, the stratospheric planetary wave packet then propagates downward over the American sector, inducing a ridge over the North Pacific as well as a trough over eastern Canada in the upper troposphere. The ridge promotes the formation of a Pacific blocking. This result explains why Pacific blockings tend to form after SSW, and why they are associated with suppressed upward propagation of planetary waves.Citation: Kodera, K., H. Mukougawa, and A. Fujii (2013), Influence of the vertical and zonal propagation of stratospheric planetary waves on tropospheric blockings,