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The subseasonal variability of the low-cloud fraction (LCF) over the southeastern North Pacific (SENP) and northwestern North Pacific (NWNP) was studied using satellite observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. It is found that subseasonal variability of the LCF was closely related to variations in the estimated inversion strength (EIS), sea surface wind speed (SSW), sensible heat flux (SHF), sea surface temperature (SST), surface temperature advection (Tadv), relative humidity (RH), surface level pressure (SLP) and surface air temperature (SAT). An increase in the LCF over the SENP is associated with the development of an anomalous anticyclonic circulation, which is located on the west coast of America. The cold advection, together with the subsidence warming associated with the anticyclonic circulation, strengthens the temperature inversion, favoring the development of the LCF. In the NWNP, the maximum LCF anomaly was also correlated with the stable boundary layer. The southerly wind blows airflow over the Kuroshio Extension from the subtropics, which brings warm and moist air. When air flows to the colder sea surface, it is cooled and condensed by the intensified heat exchange. A lead-lag composite analysis indicates that the mechanisms are different between the SENP and the NWNP, possibly due to the different types of low-level clouds over these two regions. In the SENP, the trade cumulus dominates under a strong capping inversion over the subtropics, whereas fog and stratus often occur under a shallow capping inversion in the NWNP. The effects of atmospheric circulation are also discussed. [ABSTRACT FROM AUTHOR]

Year‐to‐year forecasts of the Arctic climate conditions have drawn much interest in recent decades. This study investigates the primary drivers responsible for interannual variations (without multi‐decadal trends) of air temperature at 2 m (T2m) and sea ice concentration (SIC) over the Barents‐Kara Seas (BKS) during September from 1950 to 2021. The Arctic Dipole (AD), a leading factor in the BKS variability, plays an important role via influencing large‐scale atmospheric circulations and local thermodynamical feedbacks (i.e., vapor and albedo feedbacks). During a negative AD phase, the positive surface air temperature advection anomalies warm up the BKS region and promote sea ice melting, which in turn maintains the below‐normal sea ice and warmer‐than‐normal conditions via positive vapor and albedo feedbacks. Moreover, our results suggest that the sea surface temperature (SST) variability in the midlatitudes of the North Atlantic in the preceding month could also be an important driver of the SIC variability over the BKS area. The anomalous temperature advections from the surface to the upper troposphere are accompanied by a Rossby wave‐type atmospheric response, which intensifies the energy exchanges between the ocean and the atmosphere. Local SST anomalies associated with the surface heat flux anomalies further persist in the following month and influence sea ice variations due to their long‐lasting memory. A better understanding of the mechanisms of interannual climate variabilities over the BKS region in boreal early autumn is helpful to improving the Arctic regional climate predictions. Key Points: Understanding primary drivers of the interannual climate variabilities over the Arctic is essential to improve the regional prediction skillsThe Arctic Dipole is mainly responsible for driving the Barents‐Kara Seas (BKS) surface air temperature interannual variability in SeptemberThe North Atlantic sea surface temperature variability at 1‐month lead is associated with the early autumn sea ice cover variability in the BKS region [ABSTRACT FROM AUTHOR]

The Siberian High (SH) over central Asia can affect East Asian weather and climate, and its formation is primarily due to the uplift of the Tibetan and Mongolian Plateaus (TP and MP). However, it is unclear whether these plateaus can modulate the synoptic (2–7 days) evolution of the SH and the subsequent temperature over East Asia during the boreal winter. Using model simulations with and without the plateau topography north of the Himalaya, we show that the synoptic evolution of the winter SH is accompanied by an anomalous tropospheric transient wave train that migrates eastward to the MP over central Siberia before the SH intensification, which is then forced to propagate southeastward along the northeastern flank of the TP after the SH peaks. This results in subsequent cold anomalies from the north to the south of East Asia and thus increases synoptic variability of winter surface air temperature (Tas) over South China. However, the plateau uplift weakens this synoptic wave train due to the inefficiency of energy conversion from the mean flow to the transient flow, thus partly limiting the increase of the synoptic variability over South China. Further analyses showed that the synoptic evolution of the SH and its associated temperature anomalies is mainly caused and maintained by the anomalous cold advection due to the southeastward propagating northerly wind anomaly and the increased winter mean temperature gradient in the lower troposphere due to the northern plateau uplift. Our findings suggest that the TP and MP uplifts north of the Himalaya can affect both the winter mean state and synoptic-scale variability over East Asia and thus play a crucial role in the emergence of frequent cold air outbreaks from the higher latitudes toward South China. [ABSTRACT FROM AUTHOR]

Under global warming, surface air temperature has risen rapidly and sea ice has decreased markedly in the Arctic. These drastic climate changes have brought about various severe impacts on the vulnerable environment and ecosystem there. Thus, accurate prediction of Arctic climate becomes more important than before. Here we examine the seasonal to interannual predictive skills of 2-m air temperature (2-m T) and sea ice cover (SIC) over the Arctic region (70°–90°N) during 1980–2014 with a high-resolution global coupled model called the Met Office Decadal Prediction System, version 3 (DePreSys3). The model captures well both the climatology and interannual variability of the Arctic 2-m T and SIC. Moreover, the anomaly correlation coefficient of Arctic-averaged 2-m T and SIC shows statistically significant skills at lead times up to 16 months. This is mainly due to the contribution of strong decadal trends. In addition, it is found that the peak warming trend of Arctic 2-m T lags the maximum decrease trend of SIC by 1 month, in association with the heat flux forcing from the ocean surface to lower atmosphere. While the predictive skill is generally much lower for the detrended variations, we find a close relationship between the tropical Pacific El Niño–Southern Oscillation and the Arctic detrended 2-m T anomalies. This indicates potential seasonal to interannual predictability of the Arctic natural variations. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 and anthropogenic aerosols (AA) have increased simultaneously. Because of their opposite radiative effects, these increases may offset each other, which may lead to some nonlinear effects. Here the seasonal and regional characteristics of this nonlinear effect from the CO2 and AA forcings are investigated using the fully coupled Community Earth System Model. Results show that nonlinear effects are small in the global mean of the top-of-the-atmosphere radiative fluxes, surface air temperature, and precipitation. However, significant nonlinear effects exist over the Arctic and other extratropical regions during certain seasons. When both forcings are included, Arctic sea ice in September–November decreases less than the linear combination of the responses to the individual forcings due to a higher sea ice sensitivity to the CO2-induced warming than the sensitivity to the AA-induced cooling. This leads to less Arctic warming in the combined-forcing experiment due to reduced energy release from the Arctic Ocean to the atmosphere. Some nonlinear effects on precipitation in June–August are found over East Asia, with the northward-shifted East Asian summer rain belt to oppose the CO2 effect. In December–February, the aerosol loading over Europe in the combined-forcing experiment is higher than that due to the AA forcing, resulting from CO2-induced circulation changes. The changed aerosol loading results in regional thermal responses due to aerosol direct and indirect effects, weakening the combined changes of temperature and circulation. This study highlights the need to consider nonlinear effects from historical CO2 and AA forcings in seasonal and regional climate attribution analyses. [ABSTRACT FROM AUTHOR]

This study concerns atmospheric responses to the North Pacific subtropical front (NPSTF) in boreal spring over the period 1982-2014. Statistical results show that a strong NPSTF in spring can significantly enhance the East Asian jet stream (EAJS). Both transient eddy activity and the atmospheric heat source play important roles in this process. The enhanced atmospheric temperature gradient due to a strong NPSTF increases atmospheric baroclinicity, resulting in an intensification of transient eddy and convection activities. On the one hand, the enhanced transient eddy activities can excite an anomalous cyclonic circulation with a quasi-baraotropical structure in the troposphere to the north of the NPSTF. Accordingly, the related westerly wind anomalies around 30°N can intensify the component of the EAJS over the Northeast Pacific. On the other hand, an enhanced atmospheric heat source over the NPSTF, which is related to increased rainfall, acts to excite an anomalous cyclonic circulation system in the troposphere to the northwest of the NPSTF, which can explain the enhanced component of the EAJS over the Northwest Pacific. The two mechanisms may combine to enhance the EAJS. [ABSTRACT FROM AUTHOR]

Two anthropogenic forcing factors dominate in eastern China: aerosols and urban land cover. Usually, aerosols induce surface cooling while urban land cover causes surface warming. It is important to explore whether or not a nonlinear effect may result from the coexistence of these two opposing effects, and to what extent such nonlinear effect may become significant in affecting the climate change in East Asia. In this study, the Community Atmosphere Model version 5.1 (CAM5.1) coupled with the Community Land Model version 4 (CLM4) is employed to investigate the nonlinear effect on the East Asian summer monsoon due to the coexistence of aerosols and urban land cover. The anthropogenic forcing can be studied by including only aerosol emissions, only urban land cover, or a combination of the two in eastern China. The nonlinear effect obtained in CAM5.1 is evident in eastern China to offset the urbanization effect. Large-scale atmospheric response produces anomalous upward motion and increases total cloud amount and precipitation. This increased total cloud amount and its associated negative shortwave cloud forcing in turn significantly decrease surface air temperature and cool the troposphere, especially in northern China, resulting in a reduced land-sea thermal contrast, which acts to weaken the prevailing southwesterly wind over the Yangtze River Valley and southwestern China and to enhance the wind over the northern South China Sea. The nonlinear effect also indirectly excites strong convection over southern China, leading to a pronounced increase in summer precipitation. [ABSTRACT FROM AUTHOR]

Based on the daily NCEP/DOE reanalysis II data, dates of the boreal spring Stratospheric Final Warming (SFW) events during 1979-2010 are defined as the time when the zonal-mean zonal wind at the central latitudes (65°-75°N) of the westerly polar jet drops below zero and never recovers until the subsequent autumn. It is found that the SFW events occur successively from the mid to the lower stratosphere and averagely from the mid to late April with a temporal lag of about 13 days from 10 to 50 hPa. Over the past 32 years, the earliest SFW occurs in mid March whereas the latest SFW happens in late May, showing a clear interannual variability of the time of SFW. Accompanying the SFW onset, the stratospheric circulation transits from a winter dynamical regime to a summertime state, and the maximum negative tendency of zonal wind and the strongest convergence of planetary-wave are observed. Composite results show that the early/late SFW events in boreal spring correspond to a quicker/ slower transition of the stratospheric circulation, with the zonal-mean zonal wind reducing about 20/5 m s at 30 hPa within 10 days around the onset date. Meanwhile, the planetary wave activities are relatively strong/weak associating with an out-of-/in-phase circumpolar circulation anomaly before and after the SFW events in the stratosphere. All these results indicate that, the earlier breakdown of the stratospheric polar vortex (SPV), as for the winter stratospheric sudden warming (SSW) events is driven mainly by wave forcing; and in contrast, the later breakdown of the SPV exhibits more characteristics of its seasonal evolution. Nevertheless, after the breakdown of SPV, the polar temperature anomalies always exhibit an out-of-phase relationship between the stratosphere and the troposphere for both the early and late SFW events, which implies an intimate stratosphere-troposphere dynamical coupling in spring. In addition, there exists a remarkable interdecadal change of the onset time of SFW in the mid 1990s. On average, the SFW onset time before the mid 1990s is 11 days earlier than that afterwards, corresponding to the increased/decreased planetary wave activities in late winter-early spring before/after the 1990s. [ABSTRACT FROM AUTHOR]

Previous studies have documented the significant influences of the North Pacific subtropical sea surface temperature (SST) front (NPSTF) on the overlying atmospheric circulation. However, whether and how the NPSTF could affect remote circulations has not been well addressed. Here, we demonstrate that the enhanced NPSTF can induce a southward shift of the downstream North Atlantic westerly jet stream (NAWJS) in winter. A strong NPSTF with an increased meridional SST gradient induces a positive (negative) air temperature anomaly in its south (north) flank through the heat flux, leading to a direct thermal wind response and an indirect transient eddy feedback. The direct thermal wind response benefits a negative height anomaly south of Gulf of Alaska below 250 hPa and a positive one above 200 hPa, while the transient eddy activity could locally enhance and tilt northward with height. The kinetic energy significantly grows from the baroclinic energy conversion below 250 hPa over the NPSTF and from the barotropic energy conversion north of the NPSTF at the upper levels. This enhanced transient eddy activity gives rise to a negative height anomaly south of Gulf of Alaska through high-frequency transient eddy feedback forcing at all levels dominated by transient eddy vorticity flux. The anomalous low south of Gulf of Alaska favors an anomalous high over Canada and an anomalous low over the United States through Rossby energy propagation, causing the southward shift of the NAWJS. The numerical model further verifies the downstream influences of the NPSTF on the NAWJS in winter. • In addition to local effects, NPSTF can deliver its effect further downstream and modulate the meridional shift in NAWJS. • The height response south of Alaska Bay fosters downstream wind and height anomalies via Rossby energy propagation. • The transient eddy activity jointly induced by BTEC and BCEC dominates the local atmospheric response of NPSTF. [ABSTRACT FROM AUTHOR]