Extreme variability of the winter-and spring-time stratospheric polar vortex has been shown to affect extratropical tropospheric weather. Therefore, reducing stratospheric forecast error may be one way to improve the skill of tropospheric weather forecasts. In this review, the basis for this idea is examined. A range of studies of different stratospheric extreme vortex events shows that they can be skilfully forecasted beyond 5 days and into the sub-seasonal range (0-30 days) in some cases. Separate studies show that typical errors in forecasting a stratospheric extreme vortex event can alter tropospheric forecast skill by 5-7% in the extratropics on sub-seasonal time-scales. Thus understanding what limits stratospheric predictability is of significant interest to operational forecasting centres. Both limitations in forecasting tropospheric planetary waves and stratospheric model biases have been shown to be important in this context.

This study demonstrates that absorbed solar radiation (ASR) at the top of the atmosphere in early summer (May-July) plays a precursory role in determining the Arctic sea ice concentration (SIC) in late summer (August-October). The monthly ASR anomalies are obtained over the Arctic Ocean (65°N-90°N) from the Clouds and the Earth's Radiant Energy System during 2000-2013. The ASR changes primarily with cloud variation. We found that the ASR anomaly in early summer is significantly correlated with the SIC anomaly in late summer (correlation coefficient, r ≈ À0.8 with a lag of 1 to 4 months). The region exhibiting high (low) ASR anomalies and low (high) SIC anomalies varies yearly. The possible reason is that the solar heat input to ice is most effectively affected by the cloud shielding effect under the maximum TOA solar radiation in June and amplified by the ice-albedo feedback. This intimate delayed ASR-SIC relationship is not represented in most of current climate models. Rather, the models tend to over-emphasize internal sea ice processes in summer.

This study investigates the origin of the interdecadal variability in the warm Arctic and cold Eurasia (WACE) pattern, which is defined as the second empirical orthogonal function of surface air temperature (SAT) variability over the Eurasian continent in Northern Hemisphere winter, by analyzing the Twentieth Century Reanalysis dataset. While previous studies highlight recent enhancement of the WACE pattern, ascribing it to anthropogenic warming, the authors found that the WACE pattern has experienced a seemingly periodic interdecadal variation over the twentieth century. This long-term variation in the Eurasian SAT is attributable to the altered coupling between the Siberian high (SH) and intraseasonal Rossby wave emanating from the North Atlantic, as the local wave branch interacts with the SH and consequentially enhances the continental temperature perturbation. It is further identified that these atmospheric circulation changes in Eurasia are largely controlled by the decadal amplitude modulation of the climatological stationary waves over the North Atlantic region. The altered decadal mean condition of stationary wave components brings changes in local baroclinicity and storm track activity over the North Atlantic, which jointly change the intraseasonal Rossby wave generation and propagation characteristics as well. With simple stationary wave model experiments, the authors confirm how the altered mean flow condition in the North Atlantic acts as a source for the growth of the Rossby wave that leads to the change in the downstream WACE pattern.

Weather and climate extremes exert devastating influence on human society and ecosystem around the world. Recent observations show increase in frequency and intensity of climate extremes around the world including East Asia. In order to assess current status of the observed changes in weather and climate extremes and discuss possible mechanisms, this study provides an overview of recent analyses on such extremes over Korea and East Asia. It is found that the temperature extremes over the Korean Peninsula exhibit long-term warming trends with more frequent hot events and less frequent cold events, along with sizeable interannual and decadal variabilities. The comprehensive review on the previous literature further suggests that the weather and climate extremes over East Asia can be affected by several climate factors of external and internal origins. It has been assessed that greenhouse warming leads to increase in warm extremes and decrease in cold extremes over East Asia, but recent Arctic sea-ice melting and associated warming tends to bring cold snaps to East Asia during winter. Internal climate variability such as tropical intraseasonal oscillation and El Niño-Southern Oscillation can also exert considerable impacts on weather and climate extremes over Korea and East Asia. It is, however, noted that our current understanding is far behind to estimate the effect of these climate factors on local weather and climate extremes in a quantitative sense.

It is known that the western United States (US) precipitation displays a north-south contrast, i.e., the so-called "precipitation dipole," during El Niño and La Niña winters. Furthermore, the Pacific Decadal Oscillation (PDO) has been known to modulate this precipitation dipole. However, the underlying physical mechanism regulating this modulation is not well understood. This study revisits previous studies and suggests a physical mechanism of precipitation dipole modulation based on the PDO-storm track relationship. We found that both jet stream and storm track tend to move northward (southward) over the North Pacific during negative (positive) PDO winters, contributing to the increase of precipitation over the northwestern (southwestern) US, respectively. This relationship is robust regardless of El Niño-Southern Oscillation (ENSO), possibly facilitating modulation of the precipitation dipole. Moreover, changes in oceanic baroclinicity associated with the PDO phase are suggested to be responsible for anchorage of storm tracks over the North Pacific.