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Atmospheric reanalyses are a valuable climate-related resource where in situ data are sparse. However, few studies have investigated the skill of reanalyses to represent extreme climate indices over the North American Arctic, where changes have been rapid and indigenous responses to change are critical. This study investigates temperature and precipitation extremes as defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) over a 17-yr period (2000–16) for regional and global reanalyses, namely the Arctic System Reanalysis, version 2 (ASRv2); North American Regional Reanalysis (NARR); European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis; Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2); and Global Meteorological Forcing Dataset for Land Surface Modeling (GMFD). Results indicate that the best performances are demonstrated by ASRv2 and ERA5. Relative to observations, reanalyses show the weakest performance over far northern basins (e.g., the Arctic and Hudson basins) where observing networks are less dense. Observations and reanalyses show consistent warming with decreased frequency and intensity of cold extremes. Cold days, cold nights, frost days, and ice days have decreased dramatically over the last two decades. Warming can be linked to a simultaneous increase in daily precipitation intensity over several basins in the domain. Moreover, the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) distinctly influence extreme climate indices. Thus, these findings detail the complexity of how the climate of the Arctic is changing, not just in an average sense, but in extreme events that have significant impacts on people and places.

The surface downwelling longwave radiation component (LW↓) is crucial for the determination of the surface energy budget and has significant implications for the resilience of ice surfaces in the polar regions. Accurate model evaluation of this radiation component requires knowledge about the phase, vertical distribution, and associated temperature of water in the atmosphere, all of which control the LW↓ signal measured at the surface. In this study, we examine the LW↓ model errors found in the Antarctic Mesoscale Prediction System (AMPS) operational forecast model and the ERA5 model relative to observations from the ARM West Antarctic Radiation Experiment (AWARE) campaign at McMurdo Station and the West Antarctic Ice Sheet (WAIS) Divide. The errors are calculated separately for observed clear-sky conditions, ice-cloud occurrences, and liquid-bearing cloud-layer (LBCL) occurrences. The analysis results show a tendency in both models at each site to underestimate the LW↓ during clear-sky conditions, high error variability (standard deviations > 20 W m−2) during any type of cloud occurrence, and negative LW↓ biases when LBCLs are observed (bias magnitudes >15 W m−2 in tenuous LBCL cases and >43 W m−2 in optically thick/opaque LBCLs instances). We suggest that a generally dry and liquid-deficient atmosphere responsible for the identified LW↓ biases in both models is the result of excessive ice formation and growth, which could stem from the model initial and lateral boundary conditions, microphysics scheme, aerosol representation, and/or limited vertical resolution.

Temperature trends across Antarctica over the last few decades reveal strong and statistically significant warming in West Antarctica and the Antarctic Peninsula (AP) contrasting with no significant change overall in East Antarctica. However, recent studies have documented cooling in the AP since the late 1990s. This study aims to place temperature changes in the AP and West Antarctica into a larger spatial and temporal perspective by analyzing monthly station-based surface temperature observations since 1957 across the extratropical Southern Hemisphere, along with sea surface temperature (SST) data and mean sea level pressure reanalysis data. The results confirm statistically significant cooling in station observations and SST trends throughout the AP region since 1999. However, the full 60-yr period shows statistically significant, widespread warming across most of the Southern Hemisphere middle and high latitudes. Positive SST trends broadly reflect these warming trends, especially in the midlatitudes. After confirming the importance of the southern annular mode (SAM) on southern high-latitude climate variability, the influence is removed from the station temperature records, revealing statistically significant background warming across all of the extratropical Southern Hemisphere. Antarctic temperature trends in a suite of climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are then investigated. Consistent with previous work the CMIP5 models warm Antarctica at the background temperature rate that is 2 times faster than that observed. However, removing the SAM influence from both CMIP5 and observed temperatures results in Antarctic trends that differ only modestly, perhaps due to natural multidecadal variability remaining in the observations.

Large-scale objectively analyzed gridded products and satellite estimates of sensible (H) and latent (LE) heat fluxes over the extratropical Northern Hemisphere are compared to those derived from the regional Arctic System Reanalysis version 2 (ASRv2) and a selection of current-generation global reanalyses. Differences in H and LE among the reanalyses are strongly linked to the wind speed magnitudes and vegetation cover. Specifically, ASRv2 wind speeds match closely with observations over the northern oceans, leading to an improved representation of H compared to the global reanalyses. Comparison of evaporative fraction shows that the global reanalyses are characterized by a similar H and LE partitioning from April through September, and therefore exhibit weak intraseasonal variability. However, the higher horizontal resolution and weekly modification of the vegetation cover based on satellite data in ASRv2 provides an improved snow–albedo feedback related to changes in the leaf area index. Hence, ASRv2 better captures the small-scale processes associated with day-to-day vegetation feedbacks with particular improvements to the H over land. All of the reanalyses provide realistic dominant hemispheric patterns of H and LE and the locations of maximum and minimum fluxes, but they differ greatly with respect to magnitude. This is especially true for LE over oceanic regions. Therefore, uncertainties in heat fluxes remain that may be alleviated in reanalyses through improved representation of physical processes and enhanced assimilation of observations.

Severe flooding occurred in Thailand during the 2011 summer season, which resulted in more than 800 deaths and affected 13.6 million people. The unprecedented nature of this flood in the Chao Phraya River basin (CPRB) was examined and compared with historical flood years. Climate diagnostics were conducted to understand the meteorological conditions and climate forcing that led to the magnitude and duration of this flood. Neither the monsoon rainfall nor the tropical cyclone frequency anomalies alone was sufficient to cause the 2011 flooding event. Instead, a series of abnormal conditions collectively contributed to the intensity of the 2011 flood: anomalously high rainfall in the premonsoon season, especially during March; record-high soil moisture content throughout the year; elevated sea level height in the Gulf of Thailand, which constrained drainage; and other water management factors. In the context of climate change, the substantially increased premonsoon rainfall in CPRB after 1980 and the continual sea level rise in the river outlet have both played a role. The rainfall increase is associated with a strengthening of the premonsoon northeasterly winds that come from East Asia. Attribution analysis using phase 5 of the Coupled Model Intercomparison Project historical experiments pointed to anthropogenic greenhouse gases as the main external climate forcing leading to the rainfall increase. Together, these findings suggest increasing odds for potential flooding of similar intensity to that of the 2011 flood.

The eastern Great Basin (GB) in the western United States is strongly affected by droughts that influence water management decisions. Precipitation that falls in the GB, particularly in the Great Salt Lake (GSL) basin encompassed by the GB, provides water for millions of people living along the Wasatch Front Range. Western U.S. precipitation is known to be influenced by El Niño–Southern Oscillation (ENSO) as well as the Pacific decadal oscillation (PDO) in the North Pacific. Historical connectivity between GB precipitation and Pacific Ocean sea surface temperatures (SSTs) on interannual to multidecadal time scales is evaluated for 20 models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5). While the majority of the models had realistic ENSO and PDO spatial patterns in the SSTs, the simulated influence of these two modes on GB precipitation tended to be too strong for ENSO and too weak for PDO. Few models captured the connectivity at a quasi-decadal period influenced by the transition phase of the Pacific quasi-decadal oscillation (QDO; a recently identified climate mode that influences GB precipitation). Some of the discrepancies appear to stem from models not capturing the observed tendency for the PDO to modulate the sign of the ENSO–GB precipitation teleconnection. Of all of the models, CCSM4 most consistently captured observed connections between Pacific SST variability and GB precipitation on the examined time scales.

Two El Niño flavors have been defined based on whether warm sea surface temperature (SST) anomalies are located in the central or eastern tropical Pacific (CP or EP). This study further characterizes the impacts on atmospheric circulation in the high latitudes of the Southern Hemisphere associated with these types of El Niño events though a series of numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM). Comparing results with the Interim ECMWF Re-Analysis (ERA-Interim), CAM simulates well the known changes to blocking over Australia and a southward shift in the subtropical jet stream across the eastern Pacific basin during CP events. More importantly for the high southern latitudes, CAM simulates a westward shift in upper-level divergence in the tropical Pacific, which causes the Pacific–South American stationary wave pattern to shift toward the west across the entire South Pacific. These changes to the Rossby wave source region impact the South Pacific convergence zone and jet streams and weaken the high-latitude blocking that is typically present in the Amundsen-Bellingshausen Seas during EP events. Anticyclonic flow becomes established farther west in the south central Pacific, modifying high-latitude heat and momentum fluxes across the South Pacific and South Atlantic associated with the ENSO–Antarctic dipole.

Western Nepal has experienced consecutive and worsening winter drought conditions since 2000, culminating in a severe drought episode during 2008/09. In this study, the meteorological conditions and a historical perspective of the winter droughts in western Nepal were analyzed using instrumental records, satellite observations, and climate model simulations. Meteorological diagnosis using atmospheric reanalysis revealed that 1) winter drought in western Nepal is linked to the Arctic Oscillation and its decadal variability, which initiates a tropospheric short-wave train across Eurasia and South Asia; and that 2) the persistent warming of the Indian Ocean likely contributes to the suppression of rainfall through enhanced local Hadley circulation. Simulations from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) sets of historical single-forcing experiments indicated that the increased loading of anthropogenic aerosols is also a compounding factor in the precipitation decline during the later decades. It is therefore conceivable that the recent spells of decadal drought in Nepal are symptomatic of both natural variability and anthropogenic influences. Given the observations that winter precipitation has declined to near zero while groundwater has hardly been replenished, appropriate management of western Nepal's water resources is both critical and necessary.

Using multiple observational and model datasets, the authors document a strengthening relationship between boreal winter sea surface temperature anomalies (SSTAs) in the western North Pacific (WNP) and the development of the El Niño–Southern Oscillation (ENSO) in the following year. The increased WNP–ENSO association emerged in the mid-twentieth century and has grown through the present, reaching correlation coefficients as high as ~0.70 in recent decades. Fully coupled climate experiments with the Community Earth System Model, version 1 (CESM1), replicate the WNP–ENSO association and indicate that greenhouse gases (GHGs) are largely responsible for this observed increase. The authors speculate that shifts in the location of the largest positive SST trends between the subtropical and tropical western Pacific impact the low-level circulation in a manner that reinforces the link between the WNP and the development of ENSO. A strengthened GHG-driven relationship with the WNP provides an example of how anthropogenic climate change may directly influence one of the most prominent patterns of natural climate variability, ENSO, and potentially improve the skill of intraseasonal-to-interannual climate prediction.

A majority of tropical cyclones in the North Atlantic develop from African easterly waves (AEWs), which originate along both the southern and northern flanks of the midtropospheric African easterly jet (AEWS and AEWn, respectively). The purpose of this note is to identify the contribution of AEWSs and AEWns to North Atlantic tropical cyclones that develop from AEWs. Applying a manual backtracking approach to identify the genesis locations of AEWS, it was found that the population ratio of tropical cyclones formed from AEWSs to those formed from AEWns is 1:1.2. Because the population ratio of AEWSs to AEWns is 1:2.5, the conversion rate of the former AEWS to tropical cyclones is twice as effective as the latter waves. In addition, it was found that AEWns travel farther and take longer to transform into tropical cyclones than AEWSs, which is likely because the AEWns are drier and shallower than AEWSs. An analysis of various terms in the moisture and vorticity budgets reveals that the monsoon trough over West Africa provides moisture and enhances low-level vorticity for both AEWns and AEWSs as they move off the West African coast. The monsoon trough appears to be of particular importance in supplying AEWns with enough moisture so that they have similar properties to AEWSs after they have traveled a considerable westward distance across the tropical Atlantic.

A 124 (1882–2005) summer record of total surface energy content consisting of time series of surface equivalent temperature (TE) and its components T (mean air temperature) and Lq/cp (moist enthalpy, denoted Lq) is developed, quality controlled, and analyzed for Columbus, Ohio, where long records of monthly dewpoint temperature are available. The analysis shows that the highest TE occurs during the summer of 1995 when both T and Lq were very high, associated with a severe midwestern heat wave. That year contrasts with the hot summers of 1930–36, when Lq and TE had relatively low or negative anomalies (low humidity) compared to those of T. Following the 1930–36 summers, T and Lq departures are much more typically the same sign in individual summers, and the two parameters develop a statistically significant high positive correlation into the twenty-first century. Mean T and Lq departures from the long-term normal have opposite signs, however, when summers are stratified either by seasonal total rainfall amounts or by the Palmer drought severity soil moisture index. Normalized trends of T, Lq, and TE are downward from 1940 to 1964 with those of TE exceeding T. Since 1965, however, significant positive T trends slightly exceed TE in magnitude and those of dewpoint temperature and Lq are comparatively lower. A highly significant upward trend in minimum temperatures especially dominates the T variability, creating a significant downward trend in the temperature range that dominates recent summer climate variability more than moisture trends. Regional moisture flux variations are largest away from Columbus, over the upper Midwest and western Atlantic Ocean, during its seasonal extremes in total surface energy.

An effort was made to search for relationships between interannual variations of population, lifetime, genesis locations, and intensity of named typhoons and numbered tropical depressions in the western North Pacific during the 1979–2002 period. To support this research task, climatological relationships of tropical cyclone characteristics were also investigated for these cyclones. Major findings of this study are summarized as follows:Climatology: Measured by the intensity scale of the Japan Meteorological Agency, three groups of tropical cyclones were identified in terms of population versus intensity: Group 1 [tropical depression (TD) + typhoon (TY)], Group 2 (strong + very strong TY), and Group 3 (catastrophic TY). This group division coincides with that formed in terms of lifetime of tropical cyclones versus intensity. Weak cyclones (Group 1) have a larger population than strong cyclones (Group 3), while the former group has shorter lifetime than the latter group. For genesis locations, the monsoon trough is established as a favorable region of tropical cyclone genesis because it provides an environment of large vorticity. Therefore, the northward latitudinal displacement of the maximum genesis frequency in the three groups of tropical cyclones follows that of the monsoon trough.Interannual variation: Any mechanism that can modulate the location and intensity of the monsoon trough affects the genesis location and frequency of tropical cyclones. In response to tropical Pacific sea surface temperature anomalies, a short wave train consisting of east–west oriented cells emanates from the Tropics and progresses along the western North Pacific rim. Population of the Group-1 tropical cyclones varies interannually in phase with the oscillation of the anomalous circulation cell northeast of Taiwan and south of Japan in this short wave train, while that of Group 3 fluctuates coherently with the tropical cell of this short wave train. Because these two anomalous circulation cells exhibit opposite polarity, the out-of-phase interannual oscillation between these two cells results in the opposite interannual variation of genesis frequency between tropical cyclones of Groups 1 and 3.

Rainfall variation of the East Asian summer monsoon has long been believed to be caused by the transition of weather regimes in company with the evolution of monsoon circulation. However, this claim was neither comprehensively analyzed nor convincingly demonstrated. Four datasets [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall, NCEP–NCAR reanalysis data, Japan Meteorological Agency (JMA) 6-h surface analysis maps, and Joint Typhoon Warning Center (JTWC) 6-h tropical cyclone tracks] were used to depict the climatology of the monsoon rainfall variation and to explore the cause of this variation, particularly through the relationship between the seasonal evolution of the monsoon circulation and the associated synoptic disturbance activity. The monsoon life cycle in the southern part of East Asia is basically developed by the sequential passages of the mei-yu rainband in early summer, the western Pacific subtropical high in midsummer, and the tropical cyclone activit...