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This paper presents observational evidence of the atmospheric circulation during the Late Maunder Minimum (LMM, 1685-1715) based on daily wind direction observations from ships in the English Channel. Four wind directional indices and 8-point wind roses are derived at monthly scales to characterize the LMM. The results indicate that the LMM was characterized by a pronounced meridional circulation and a marked reduction in the frequency of westerly days all year round, as compared to the present (1981-2010). The winter circulation contributed the most to the cold conditions. Nevertheless, findings indicate that the LMM in Europe was more heterogeneous than previously thought, displaying contrasting spatial patterns in both circulation and temperature, as well as large decadal variability. In particular, there was an increase of northerly winds favoring colder winters in the first half of the LMM, but enhanced southerlies contributing to milder conditions in the second half of the LMM. The analysis of the atmospheric circulation yields a new and complete classification ofLMMwinters. The temperature inferred from the atmospheric circulation confirms the majority of extremely cold winters well documented in the literature, while uncovering other less documented cold and mild winters. The results also suggest a nonstationarity of the North Atlantic Oscillation (NAO) pattern within the LMM, with extremely cold winters being driven by negative phases of a ''high zonal'' NAO pattern and ''low zonal'' NAO patterns dominating during moderately cold winters. [ABSTRACT FROM AUTHOR]

The early season rainfall (ESR) over southern China, usually occurring from April to June, is a prominent meteorological phenomenon of the East Asian monsoon system. In this paper, output from the 45-day hindcast by the NCEP Climate Forecast System, version 2 (CFSv2), and various observational datasets are analyzed to assess the predictability of the ESR and associated atmospheric circulation. Results show that CFSv2 can successfully predict the ESR and associated circulation patterns over southern China. The lower-tropospheric convergence and upper-tropospheric divergence as well as the local upward motion over southern China lead to the formation of ESR. Analysis of bias shows small differences and close relationships between the predicted and observed ESR values when the forecast lead time is less than 2 weeks. The skill in the ESR predictions by CFSv2 decreases significantly when the lead time is longer than 2 weeks. Overall, CFSv2 has a higher level of skill when predicting the southern China ESR compared to the rainfall over other Asian regions during the same period of time. [ABSTRACT FROM AUTHOR]

Rapid adjustments--the response of meteorology to external forcing while sea surface temperatures (SST) and sea ice are held fixed--can affect the midlatitude circulation and contribute to long-term forced circulation responses in climate simulations. This study examines rapid adjustments in the Southern Hemisphere (SH) circulation using nine models from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), which perform fixed SST and coupled ocean experiments for five perturbations: a doubling of carbon dioxide (2xCO2), a tripling of methane (3xCH4), a fivefold increase in sulfate aerosol (5xSO4), a tenfold increase in black carbon aerosol (10xBC), and a 2%increase in solar constant (2%Sol). In the coupled experiments, the SH eddy-driven jet shifts poleward and strengthens for forcings that produce global warming (and vice versa for 5xSO4), with the strongest response found in austral summer. In austral winter, the responses project more strongly onto a change in jet strength. For 10xBC, which induces strong shortwave absorption, the multimodel mean (MMM) rapid adjustment in DJF jet latitude is ;75% of the change in the coupled simulations. For the other forcings, which induce larger SST changes, the effect of SST-mediated feedbacks on the SH circulation is larger than the rapid adjustment. Nevertheless, for these perturbations the magnitude of the MMM jet shift due to the rapid adjustment is still around 20%-30% of that in the coupled experiments. The results demonstrate the need to understand the mechanisms for rapid adjustments in the midlatitude circulation, in addition to the effect of changing SSTs. [ABSTRACT FROM AUTHOR]

The South Pacific convergence zone (SPCZ) is the largest rainfall feature in the Southern Hemisphere, and is a critical component of the climate for South Pacific island nations and territories. The small size and isolated nature of these islands leaves them vulnerable to short- and long-term changes in the position of the SPCZ. Its position and strength is strongly modulated by El Niño–Southern Oscillation (ENSO), leading to large interannual variability in rainfall across the southwest Pacific including seasonal droughts and pluvials. Currently much of the analysis about SPCZ activity has been restricted to the satellite observation period starting in 1979. Here, the representation of the SPCZ in the Twentieth Century Reanalysis (20CR), which is a three-dimensional atmospheric reconstruction based only on surface observations, is discussed for the period since 1908. The performance of two versions of the 20CR (version 2 and version 2c) in the satellite era is compared with other reanalyses and climate observation products. The 20CR performs well in the satellite era. Extra surface observations spanning the SPCZ region from the longitude of the Cook Islands has improved the representation of the SPCZ during 1908–57 between 20CRv2 and 20CRv2c. The well-established relationship with ENSO is observed in both the representation of mean SPCZ position and intensity, and this relationship remains consistent through the entire 1908–2011 period. This suggests that the ENSO–SPCZ relationship has remained similar over the course of the past century, and gives further evidence that 20CRv2c performs well back to 1908 over the southwest Pacific region. [ABSTRACT FROM AUTHOR]

The processes governing organized tropical convective systems are not completely understood despite their important influences on the tropical atmosphere and global circulation. In particular, cold pools are known to influence the structure and maintenance of midlatitude systems via Rotunno-Klemp-Weisman (RKW) theory, but cold pools may interact differently with tropical convection because of differences in cold pool strength and environmental shear. In this study, the role of cold pools in organized oceanic tropical convective systems is investigated, including their influence on system intensity, mesoscale structure, and propagation. To accomplish this goal, high-resolution idealized simulations are performed for two different systems that are embedded within a weakly sheared cloud population approaching radiative-convective equilibrium. The cold pools are altered by changing evaporation rates below cloud base in a series of sensitivity tests. The simulations demonstrate surprising findings: when cold pools are weakened, the convective systems become more intense. However, their propagation speeds and mesoscale structure are largely unaffected by the cold pool changes. Passive tracers introduced into the cold pools indicate that the convection intensifies when cold pools are weakened because cold pool air is entrained into updrafts, thereby reducing updraft intensity via the cold pools' initial negative buoyancy. Gravity waves, rather than cold pools, appear to be the important modulators of system propagation and mesoscale structure. These results reconfirm that RKW theory does not fully explain the behavior of tropical oceanic convective systems, even those that otherwise appear consistent with RKW thinking. [ABSTRACT FROM AUTHOR]

Multidecadal internal climate variability centered in the North Atlantic is evident in sea surface temperatures and is assumed to be related to variations in the strength of the Atlantic meridional overturning circulation (AMOC). In this study, the extent to which variations in the AMOC may also alter hemispheric and global surface air temperature trends and ocean heat content during the past century is examined. Forty-seven realizations of the twentieth-century climate change from two large ensembles using the Community Earth System Model (CESM) are analyzed. One of the ensembles shows a much wider spread in global mean surface air temperature between its members. This ensemble simulates diverging trends of the AMOC strength during the twentieth century. The presence and strength of deep convection in the Labrador Sea controls these trends. The AMOC strength influences the air–sea heat flux into the high-latitude ocean, where a strengthening of the AMOC leads to decreased storage of heat in the Atlantic, and a larger fraction of the heat taken up by the global ocean accumulates in the top 300 m, compared to the case of a weakening AMOC. The spread in the amount of heat stored in the global ocean below 300 m is similar across the CESM members as in a set of CMIP5 models, confirming the AMOC as a “control knob” on deep-ocean heat storage. By influencing the ocean heat uptake efficiency and by shifting the pattern of heat uptake, global surface air temperatures are significantly altered on a multidecadal time scale by AMOC variability. [ABSTRACT FROM AUTHOR]

The present study applies the empirical orthogonal function (EOF) method to investigate the large-scale pattern and the plausible dynamic processes of the boreal winter diurnal temperature range (DTR) changes in the East Asia (EA)–Australia (AUS) region based on the CRU Time Series version 4.00 (TS4.00) and NCEP–NCAR reanalysis datasets. Results show that the DTR changes during 1948–2015 are dominated by two distinct modes. The first mode, characterized by a same-sign variation over most regions of EA–AUS, represents a declining trend of DTR. The second mode, featuring an opposite-sign variation, represents the interannual variations in DTR. The two modes are both closely associated with the changes in cloud cover (CLT) caused by atmospheric circulation anomalies in EA–AUS. For the trend mode, anomalous southerly and northerly winds over EA and AUS, respectively, bring warm and wet air from low latitudes to EA–AUS, inducing an increase in CLT and thereby reducing DTR in most areas of EA–AUS. The changes of circulation are mainly due to the thermodynamic responses of atmosphere to the nonuniform warming in EA–AUS. In addition, the second mode of DTR is largely forced by the ENSO variability. The weakened Walker circulation associated with warm ENSO events triggers a pair of anomalous low-level anticyclones (south and north of the equator) over the western Pacific. The AUS region is under control of the southern anticyclone, thereby reducing the CLT and increasing the DTR in AUS as a result of anomalous descending motion. In contrast, the EA region is controlled by anomalous southerlies to the west of the northern anticyclone. The northward transports of moistures from the warm ocean increase the CLT, reducing DTR in EA. [ABSTRACT FROM AUTHOR]

Climate indicators related to Long Island Sound (LIS) water and air temperature variability were investigated. The Pacific decadal oscillation (PDO) and east Pacific/North Pacific (EP/NP) patterns are found to be strongly correlated with LIS air temperature anomalies during most seasons, especially during the winter. Additionally, the winter EP/NP index is strongly correlated with subsequent spring and summer LIS water temperature anomalies, potentially rendering the EP/NP index useful in extended LIS water temperature outlooks. Such lagged relationships are found to be related largely to the decorrelation time scale of seasonal water temperature anomalies. The atmospheric circulation pattern associated with anomalous LIS water temperature conditions is consistent with atmospheric Rossby wave trains emanating from the western equatorial Pacific. The EP/NP index has a characteristic time scale of approximately 5 to 10 years and such fluctuations are termed the quasi-decadal mode, the mode identified as varying coherently with LIS air and water temperature anomalies. Apparent PDO and EP/NP regime shifts in 1997 are found to coincide with a LIS water temperature regime shift. This result suggests that not all LIS warming experienced during recent decades is solely due to anthropogenic causes but rather is to some extent a result of natural variability. The results from this study provide a useful framework for both seasonal and decadal prediction of LIS water temperature variability. [ABSTRACT FROM AUTHOR]

The impact of future greenhouse gas forcing on the North Atlantic and North Pacific tropospheric jets remains uncertain. Opposing changes in the latitudinal temperature gradient—forced by amplified lower-atmospheric Arctic warming versus upper-atmospheric tropical warming—make robust predictions a challenge. Despite some models simulating more realistic jets than others, it remains the prevailing approach to treat each model as equally probable (i.e., democratic weighting). This study compares democratically weighted projections to an alternative Bayesian-weighting method based on the ability of models to simulate historical wintertime jet climatology. The novel Bayesian technique is developed to be broadly applicable to high-dimensional fields. Results show the Bayesian weighting can reduce systematic bias and suggest the wintertime jet response to greenhouse gas forcing is largely independent of this historical bias (i.e., not state dependent). A future strengthening and narrowing is seen in both winter jets, particularly at the upper levels. The widely reported poleward shift at the level of the eddy-driven jet does not appear statistically robust, particularly over the North Atlantic, indicating sensitivity to current model deficiencies. [ABSTRACT FROM AUTHOR]

The salient features and drivers of wintertime warm and cold spells in the high Arctic are investigated. The analysis is based on the European Centre for Medium-Range Weather Forecasts interim reanalysis dataset. It is found that the warm spells are systematically associated with an intense sea level pressure and geopotential height anomaly dipole, displaying a low over the Arctic basin and a high over northern Eurasia. This configuration creates a natural pathway for extreme moisture influx episodes from the Atlantic sector into the Arctic (herein termed moisture intrusions). Anomalous cyclone frequency at the pole (largely attributable to local cyclogenesis) then favors a deep penetration of these intrusions across the Arctic basin. The large-scale circulation pattern associated with the warm spells further favors the advection of cold air across Siberia, leading to the so-called warm Arctic-cold Eurasia pattern previously discussed in the literature. On the contrary, cold Arctic extremes are associated with a severely reduced frequency of moisture intrusions and a persistent low pressure system over the pole. This effectively isolates the high latitudes from midlatitude air masses, favoring an intense radiative cooling of the polar region. [ABSTRACT FROM AUTHOR]

Investigations of tornadogenesis in supercells attempt to find the origin of the tornado's large vorticity by determining vorticity generation and amplification along trajectories that enter the tornado from a horizontally uniform unstable environment. Insights into tornadogenesis are provided by finding analytical formulas for vorticity variations along streamlines in idealized, steady, inviscid, isentropic inflows of dry air imported from the environment. The streamlines and vortex lines lie in the stationary isentropic surfaces so the vorticity is 2D. The transverse vorticity component (positive leftward of the streamlines) arises from imported transverse vorticity and from baroclinic vorticity accumulated in streamwise temperature gradients. The streamwise component stems from imported streamwise vorticity, from baroclinic vorticity accrued in transverse temperature gradients, and from positive transverse vorticity that is turned streamwise in cyclonically curved flow by a 'river-bend process.' It is amplified in subsiding air as it approaches the ground. Streamwise stretching propagates a parcel's streamwise vorticity forward in time. In steady flow, vorticity decomposes into baroclinic vorticity and two barotropic parts ωBTIS and ωBTIC arising from imported storm-relative streamwise vorticity (directional shear) and storm-relative crosswise vorticity (speed shear), respectively. The Beltrami vorticity ωBTIS is purely streamwise. It explains why abundant environmental storm-relative streamwise vorticity close to ground favors tornadic supercells. It flows directly into the updraft base unmodified apart from streamwise stretching, establishing mesocyclonic rotation and strong vortex suction at low altitudes. Increase (decrease) in storm-relative environmental wind speed with height near the ground accelerates (delays) tornadogenesis as positive (negative) ωBTIC is turned into streamwise (antistreamwise) vorticity within cyclonically curved flow around the mesocyclone. [ABSTRACT FROM AUTHOR]

With the Weather Research and Forecasting (WRF) Model specifically configured to simulate the intensification and evolution of an extratropical baroclinic wave, this study first investigates why cold fronts are characteristically longer, narrower, and more intense than warm fronts in the extratropical atmosphere. It is found that the differential thermal advection by the geostrophic and ageostrophic wind components in the two frontal regions results in a greater thermal contrast across the cold front. The length of the cold front is essentially the length scale of the intensifying baroclinic wave (i.e., on the order of radius of deformation). The frontal system as a whole moves eastward under the influence of a steering flow. In addition, the cold front outpaces the warm front eastward, making the western portion of the warm front progressively occluded and the eastern portion of the warm front shorter. The dynamical processes tend to move the cold front eastward, whereas the diabatic heating processes tend to move it westward, contributing to the narrowness of the cold front. This study also investigates whether, how, and why an upper-level front (ULF) would synergistically interact with a surface front (SF). It is found that a favorable circumstance for such interaction to occur in an observed extratropical cyclone and in the WRF Model simulation is when the ULF and SF are roughly parallel to one another with the ULF aloft located a few hundred kilometers to the west of the SF. The relative importance of 'forcing' for the ageostrophic circulation associated with the geostrophic circulation, diabatic heating, and friction are diagnosed in such interaction. [ABSTRACT FROM AUTHOR]

The question of how rotation arises in sheared updrafts is analyzed using the shear and curvature vorticity framework. Local rotation exists where the shear and curvature vorticity have a similar magnitude and the same sign, such that parcels are in near-solid-body rotation. It is shown that the tilting terms of the vertical vorticity equation cannot explain the development of local rotation in the canonical cases where the horizontal vorticity is either purely streamwise or purely crosswise. Rather, vertical shear vorticity develops if crosswise vorticity is tilted, and vertical curvature vorticity develops if streamwise vorticity is tilted. To analyze how local rotation develops, two simulations of updrafts in an environment with crosswise and mostly streamwise vorticity, respectively, are discussed. A trajectory analysis is performed and shear and curvature vorticity budgets are analyzed. It is found that much of the horizontal vorticity near the updraft becomes streamwise, which results from pressure gradient accelerations in the vicinity of the updraft. Consequently, in the analyzed scenarios, the tilting mechanism results primarily in vertical curvature vorticity. Local rotation is achieved via an interchange process that facilitates a partial conversion of vertical curvature vorticity to vertical shear vorticity. Updraft rotation in supercells thus does not result from tilting of horizontal vorticity alone, but partial conversion of curvature to shear vorticity is also required. [ABSTRACT FROM AUTHOR]

The present study reveals a close relationship between the leading mode of continental U.S. (CONUS) summer rainfall and the East Asian subtropical monsoon rainfall (viz., mei-yu in China, baiu in Japan, and changma in the Korean peninsula). The East Asian subtropical monsoon rainfall and the CONUS dipole rainfall patterns are connected by an upper-level Asia-North America (ANA) teleconnection. The Rossby wave energy propagates along the path of the westerly jet stream (WJS) from East Asia to North America, affecting the CONUS summer rainfall. Mechanisms through which East Asian summer monsoon heating influence North American rainfall are illustrated by idealized anomaly atmospheric general circulation model experiments. In boreal winter, because of the southward shift of the WJS, the Pacific-North American (PNA) pattern can be excited by the tropical central/eastern Pacific heating associated with El Niño, affecting the rainfall over CONUS. In boreal summer, because the WJS is weaker and locates farther to the north, an equatorial heating anomaly cannot directly perturb the WJS. A perturbation heating over subtropical East Asia, however, can trigger an ANA pattern along the path of the WJS, affecting the rainfall over North America. The season-dependent teleconnection scenario illustrates that the predictability source of CONUS rainfall variability is different between winter and summer. While the PNA pattern generated by El Niño is critical for CONUS rainfall in northern winter, the CONUS dipole rainfall variation in boreal summer is mainly governed by the remote forcing over subtropical East Asia via the ANA teleconnection. [ABSTRACT FROM AUTHOR]

A series of stationary wave model (SWM) experiments are performed in which the boreal summer atmosphere is forced, over a number of locations in the continental United States, with an idealized diabatic heating anomaly that mimics the atmospheric heating associated with a dry land surface. For localized heating within a large portion of the continental interior, regardless of the specific location of this heating, the spatial pattern of the forced atmospheric circulation anomaly (in terms of 250-hPa eddy streamfunction) is largely the same: a high anomaly forms over west-central North America and a low anomaly forms to the east. In supplemental atmospheric general circulation model (AGCM) experiments, similar results are found; imposing soil moisture dryness in the AGCM in different locations within the U.S. interior tends to produce the aforementioned pattern, along with an associated near-surface warming and precipitation deficit in the center of the continent. The SWM-based and AGCM-based patterns generally agree with composites generated using reanalysis and precipitation gauge data. The AGCM experiments also suggest that dry anomalies imposed in the lower Mississippi River valley have remote surface impacts of particularly large spatial extent, and a region along the eastern half of the U.S.-Canadian border is particularly sensitive to dry anomalies in a number of remote areas. Overall, the SWM and AGCM experiments support the idea of a positive feedback loop operating over the continent: dry surface conditions in many interior locations lead to changes in atmospheric circulation that act to enhance further the overall dryness of the continental interior. [ABSTRACT FROM AUTHOR]

Multidecadal variability of observed near-surface wind speed from 24 stations across Sweden has been analyzed for 1956-2013, with a focus on 1979-2008 (incorporating an additional 9 stations) for comparison with previous studies. Wind speed data have been subjected to a robust data processing protocol, consisting of quality control, reconstruction, and homogenization, by using geostrophic wind speed series as reference. The homogenized dataset displays a significant (at p < 0.05) downward trend for 1956-2013 (−0.06 m s−1 decade−1) and an even larger decreasing trend for 1979-2008 (−0.14 m s−1 decade−1). However, differences have been observed seasonally, with significant decreasing values in spring, summer, and autumn and a small downward trend in winter for 1956-2013. Most interestingly, a nonsignificant wind speed increase has been detected in winter for 1979-2008, which contrasts with the marked 'stilling' reported for this season in much of midlatitude regions. The decreasing rate in wind speed is larger for coastal stations and in the southern part of Sweden. Decreasing trends were found at 91.7% of the stations during summer, whereas 58.3% of the stations displayed decreasing trends in winter. On the contrary, increasing trends occurred in 41.7% of the stations for winter and in only 8.3% for summer. The possible impact of the North Atlantic Oscillation (NAO) index has also been investigated, showing evidence that the small increasing trend in winter for 1979-2008 is hypothetically associated with the positive tendency of the NAO index during the last decades. These results reveal the influence of large-scale atmospheric circulation on wind speed variability across Sweden. [ABSTRACT FROM AUTHOR]

Interdecadal variations in the relationship between the winter North Atlantic Oscillation (NAO) and surface air temperature in China are investigated using observational and reanalysis data. Focus is on south-central China, in which temperature variability is strongly related to the NAO. It is revealed that the relationship shows clear interdecadal variations in midwinter during 1951-2015. A relatively weak in-phase relationship occurs before the early 1970s (P1), but a significant out-of-phase relationship dominates in the last two decades of the twentieth century (P2), though it is clearly weaker from the late 1990s onward. Observational evidence shows that such interdecadal variations are related mainly to variations in the spatial pattern and amplitude of the NAO. The northern center of the NAO shifted eastward over the second half of the twentieth century. In addition, the amplitude of the center strengthened from P1 to P2, resulting in a perturbation in the atmospheric circulation response pattern over Eurasian mid-to-high latitudes. During P2, the eastward shift and amplitude intensification of the NAO favored a north-south dipole structure in circulation anomalies over the Asian continent, which tended to produce cold temperature anomalies in south-central China during the positive NAO phase and warm anomalies during the negative phase. However, in the past two decades the northern center of the NAO has weakened and retreated westward. This was concurrent with a weakening relationship between the NAO and temperature anomalies in south-central China and northern Eurasia, indicating weaker downstream impacts of the NAO in midwinter. [ABSTRACT FROM AUTHOR]

The sea surface temperature gradient (SSTG) between the southwest Pacific (SWP; 40°-20°S, 160°E-170°W) and the western Pacific warm pool (WWP; 0°-16°N, 125°-165°E) in boreal spring was revealed to contribute significantly to the interannual variability of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) since 1980. The present study extends the analysis back to 1951. Results show that the interannual relationship between the SSTG and WNP TC genesis frequency is statistically significant only after the mid-1970s while insignificant prior to 1974. Further analyses show that the SSTG is strongly negatively correlated with the SST anomaly (SSTA) in the central Pacific (CP) in the recent period while with the SSTA in the equatorial eastern Pacific (EP) in the prior period. In the prior period, the spring positive (negative) EP SSTA induces an anticyclonic (cyclonic) circulation over the WNP. This partly offsets the impact of the SSTG anomaly on the atmospheric circulation over the WNP. In addition, the SSTG anomaly in spring shows poor persistence through summer. As a result, the relationship between the spring SSTG anomaly and WNP TC genesis frequency is largely weakened in the prior period. In the recent period, the response of the atmospheric circulation over the WNP to the CP SSTAs enhances the atmospheric circulation response to the spring SSTG anomaly, thus strengthening the relationship between the SSTG and WNP TC genesis frequency. These results are further confirmed by the atmospheric general circulation model (AGCM) simulations. [ABSTRACT FROM AUTHOR]

The potential of recent Arctic changes to influence hemispheric weather is a complex and controversial topic with considerable uncertainty, as time series of potential linkages are short (<10 yr) and understanding involves the relative contribution of direct forcing by Arctic changes on a chaotic climatic system. A way forward is through further investigation of atmospheric dynamic mechanisms. During several exceptionally warm Arctic winters since 2007, sea ice loss in the Barents and Kara Seas initiated eastward-propagating wave trains of high and low pressure. Anomalous high pressure east of the Ural Mountains advected Arctic air over central and eastern Asia, resulting in persistent cold spells. Blocking near Greenland related to low-level temperature anomalies led to northerly flow into eastern North America, inducing persistent cold periods. Potential Arctic connections in Europe are less clear. Variability in the North Pacific can reinforce downstream Arctic changes, and Arctic amplification can accentuate the impact of Pacific variability. The authors emphasize multiple linkage mechanisms that are regional, episodic, and based on amplification of existing jet stream wave patterns, which are the result of a combination of internal variability, lower-tropospheric temperature anomalies, and midlatitude teleconnections. The quantitative impact of Arctic change on midlatitude weather may not be resolved within the foreseeable future, yet new studies of the changing Arctic and subarctic low-frequency dynamics, together with additional Arctic observations, can contribute to improved skill in extended-range forecasts, as planned by the WMO Polar Prediction Project (PPP). [ABSTRACT FROM AUTHOR]

Quantification of large-scale climate drivers of drought is necessary to understand better and manage these spatially extensive and often prolonged natural hazards. Here, this issue is advanced at the continental scale for Europe. Drought events are identified using two indices-the 6-month cumulative standardized precipitation and standardized precipitation evapotranspiration indices (SPI-6 and SPEI-6, respectively)-both calculated using the gridded Water and Global Change (WATCH) Forcing Dataset for 1958-2001. Correlation of monthly time series of the percentage of European area in drought with geopotential height for 1958-2001 indicates that a weakening of the prevailing westerly circulation is associated with drought onset. Such conditions are linked to variations in the eastern Atlantic/western Russia (EA/WR) and North Atlantic Oscillation (NAO) atmospheric circulation patterns. Event-based analysis of the most widespread European droughts reveals that a higher number are identified by the SPEI-6 than the SPI-6, with SPEI-6 drought events showing a greater variety of spatial locations and start dates. Atmospheric circulation drivers also vary between the two types of events, with EA/WR-type variation associated most frequently with SPEI-6 drought, and the NAO associated with SPI-6. This distinction reflects the sensitivity of these drought indices to the underlying drought type (meteorological water balance versus precipitation, respectively) and associated differences in their timing and location (Europe-wide year round versus northern Europe winter). As such, this study provides new insight into both the identification of Europe-wide drought and patterns of large-scale climate variation associated with two different drought indices. [ABSTRACT FROM AUTHOR]

A simple analytical model including both diurnal thermal forcing over sloping terrain (the 'Holton' mechanism) and diurnally varying boundary layer friction (the 'Blackadar' mechanism) is developed to account for the observed amplitude and phase of the low-level jet (LLJ) over the Great Plains and to understand better the role of each mechanism. The present model indicates that, for the pure Holton mechanism (time-independent friction coefficient), the maximum southerly wind speed occurs (depending on the assumed friction coefficient) between sunset and midnight local standard time, which is earlier than the observed after-midnight maximum. For the pure Blackadar mechanism (time-independent thermal forcing), the present model shows that generally occurs later (closer to sunrise) than observed and has a strong latitudinal dependence. For both mechanisms combined, the present model indicates that occurs near to the observed time, which lies between the time obtained in the pure Holton mechanism and the time obtained in the pure Blackadar mechanism; furthermore, is larger (and closer to that observed) than in each one considered individually. The amplitude and phase of the LLJ as a function of latitude can be obtained by the combined model by allowing for the observed latitude-dependent mean and diurnally varying thermal forcing. [ABSTRACT FROM AUTHOR]

Friction is an important parameter in atmospheric modeling that may affect internal variability in a number of ways. It directly damps the annular-mode variability, but it also helps to maintain it through baroclinic feedbacks. Also, by determining the mean strength and position of the midlatitude jet, friction affects the internal dynamics that drive this variability. This work investigates the relevance of all of these factors for the sensitivity of the persistence of annular variability to changes in the zonal-mean friction using an idealized quasigeostrophic two-layer model. This model produces realistic variability, yet it is so simple that one can cleanly separate the different effects. It is found that the sensitivity of persistence to friction is dominated by the direct damping effect, while changes in the eddy momentum forcing and in the mean jet are not as important. As in more complex models, the persistence of the jet anomalies in this model decreases at all lags with increasing friction, but the long-time decorrelation decay rate of these anomalies is remarkably insensitive to friction. Although this implies that the eddy feedback must increase with friction to maintain the anomalies against the enhanced damping, it is shown that this is not due to baroclinic effects. A model that assumes that the eddy forcing does not change with friction can reproduce reasonably well the numerical results. The crucial factor that determines the model's sensitivity to friction is the spectral structure of the eddy momentum forcing. [ABSTRACT FROM AUTHOR]