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Abstract and Figures

Over the past decade, anomalously hot summers and persistent droughts frequented over the western United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external forcing and multidecadal modes of variability in sea surface temperature, namely, the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot summers. In this study, 100-member ensemble simulations for 1951–2010 by an atmospheric general circulation model were used to explore relative contributions of anthropogenic warming, atmospheric internal variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing agents, and non-warming counterfactual climate forcing, we found
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DOI 10.1007/s00382-016-3350-x
Clim Dyn (2017) 49:403–417
Forced response and internal variability of summer climate
over western North America
Youichi Kamae1,2 · Hideo Shiogama3 · Yukiko Imada4 · Masato Mori5 · Osamu Arakawa1 ·
Ryo Mizuta4 · Kohei Yoshida4 · Chiharu Takahashi5 · Miki Arai5 · Masayoshi Ishii4 ·
Masahiro Watanabe5 · Masahide Kimoto5 · Shang‑Ping Xie2 · Hiroaki Ueda1
Received: 26 March 2016 / Accepted: 7 September 2016 / Published online: 16 September 2016
© Springer-Verlag Berlin Heidelberg 2016
large portions of recent increases in mean temperature and
frequency of hot summers (66 and 82 %) over the wUS
can be attributed to the anthropogenic global warming. In
contrast, multidecadal change in the wUS precipitation is
explained by a combination of the negative PDO and the
positive AMO after the 2000s. Diagnostics using a linear
baroclinic model indicate that AMO- and PDO-related
diabatic heating anomalies over the tropics contribute to
the anomalous atmospheric circulation associated with
the droughts and hot summers over wUS on multidec-
adal timescale. Those anomalies are not robust during the
periods when PDO and AMO are in phase. The prolonged
PDO–AMO antiphase period since the late twentieth cen-
tury resulted in the substantial component of multidecadal
anomalies in temperature and precipitation over the wUS.
Keywords Global warming hiatus · PDO · AMO · Hot
summers · Linear baroclinic model
1 Introduction
Since the late twentieth century, mean temperature and fre-
quency of warm extremes have both remarkably increased
over land (e.g. Hansen et al. 2012; Perkins et al. 2012).
Anthropogenic influences including human-induced green-
house gases emissions play an essential role in the observed
climate change during the recent six decades (e.g. Jones
et al. 2013; IPCC 2013). In addition, intrinsic variability
in the climate system also influences decadal-to-centennial
climate trends particularly during the winter season (Hawk-
ins and Sutton 2009; Deser et al. 2012). Since the end of
the twentieth century, substantial decadal-to-multidecadal
variations (DMV) in the rate of global-mean temperature
increase have been observed. Particularly, temperature and
Abstract Over the past decade, anomalously hot sum-
mers and persistent droughts frequented over the western
United States (wUS), the condition similar to the 1950s and
1960s. While atmospheric internal variability is important
for mid-latitude interannual climate variability, it has been
suggested that anthropogenic external forcing and multi-
decadal modes of variability in sea surface temperature,
namely, the Pacific Decadal Oscillation (PDO) and Atlantic
Multidecadal Oscillation (AMO), also affect the occurrence
of droughts and hot summers. In this study, 100-member
ensemble simulations for 1951–2010 by an atmospheric
general circulation model were used to explore relative
contributions of anthropogenic warming, atmospheric inter-
nal variability, and atmospheric response to PDO and AMO
to the decadal anomalies over the wUS. By comparing his-
torical and sensitivity simulations driven by observed sea
surface temperature, sea ice, historical forcing agents, and
non-warming counterfactual climate forcing, we found that
Electronic supplementary material The online version of this
article (doi:10.1007/s00382-016-3350-x) contains supplementary
material, which is available to authorized users.
* Youichi Kamae
1 Faculty of Life and Environmental Sciences, University
of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8506,
2 Scripps Institution of Oceanography, University of California
San Diego, La Jolla, CA, USA
3 Center for Global Environmental Research, National Institute
for Environmental Studies, Tsukuba, Ibaraki, Japan
4 Meteorological Research Institute, Tsukuba, Ibaraki, Japan
5 Atmosphere and Ocean Research Institute, University
of Tokyo, Kashiwa, Chiba, Japan
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... Error bars in Fig. 5b are generally large, indicating the great importance of atmospheric internal variability under the fixed radiative forcing and SST boundary conditions in the d4PDF simulations. Kamae et al. (2017a) showed that the relative importance of atmospheric internal variability compared with the forced atmospheric response to global SST perturbation is larger over the mid-and high-latitudes than the tropics (Figs. 10a, c of Kamae et al. 2017a). ...
... Kamae et al. (2017a) showed that the relative importance of atmospheric internal variability compared with the forced atmospheric response to global SST perturbation is larger over the mid-and high-latitudes than the tropics (Figs. 10a, c of Kamae et al. 2017a). The d4PDF ensemble mean can be considered a forced response to fixed boundary conditions because the effects of atmospheric internal variability cancel out one another (Kamae et al. 2017a, b;Ueda et al. 2018). ...
... Liu et al. 2019). In addition, decadal-to-multidecadal variabilities may affect the AR activity discussed here because of their influence on mid-latitude atmospheric circulations (e.g., Newman et al. 2016;Kamae et al. 2017a;Tokinaga et al. 2017). Environmental factors responsible for the anomalies in the AR activity in individual years will be further examined in future studies. ...
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Atmospheric rivers (ARs), narrow water vapor transport bands over the mid-latitudes, often cause great socio-economic impacts over East Asia. While it has been shown that summertime AR activity over East Asia is strongly induced by preceding-winter El Niño development, it remains unclear the extent to which seasonal transitions of El Niño Southern Oscillation (ENSO) from winter to summer affect the AR activity. Here we examine the relationship between the seasonal transitions of ENSO and the summertime AR activity over East Asia using an atmospheric reanalysis and high-resolution atmospheric general circulation model (AGCM) ensemble simulations. A rapid transition from preceding-winter El Niño to summertime La Niña results in more AR occurrence over northern East Asia via northward expansion of an anomalous low-level anticyclone over the western North Pacific compared to sustained or decayed El Niño cases. The northward expansion of the anticyclone is consistent with a steady response of the atmosphere to the anomalous condensation heating over the Maritime Continent and equatorial Pacific. Meridional positions of the extratropical AR occurrence and circulation anomalies are different between the reanalysis and AGCM simulations, which is possibly contributed by a limited sample size and/or AGCM biases and suggests that seasonal prediction of AR-related natural disaster risk over East Asia on a regional scale remains a challenge.
... An immediate question is how much of the warming trend during 1979-2020 is caused by climate change. Conventional attribution studies (e.g., Dong et al. 2009;Kamae et al. 2017;Christidis et al. 2012), which aimed to address such questions, are confronted with deep uncertainty in identifying the circulation response to climate change (Shepherd 2014), yet circulation exerts a strong control on regional climate. This uncertainty is partly reflected in the diverse model projections of circulation change found even on centennial time scales (Shaw 2019;Wills et al. 2019;Simpson et al. 2014;Vallis et al. 2015). ...
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Significant surface air temperature warming during summer 1979-2020 is not uniformly distributed in the northern midlatitude over land, but rather confined to several longitudinal sectors including Europe, Central Siberia-Mongolia and both coasts of North America. These hot spots are accompanied by a chain of high-pressure ridges from an anomalous, circumglobal, Rossby wave train in the upper troposphere. From reanalysis data and several Coupled Model Intercomparison Project Phase 6 (CMIP6) baseline experiments, we find that the circulation trend pattern is associated with fluctuations of the Atlantic Multidecadal Variability (AMV) and the Interdecadal Pacific Oscillation (IPO). The phase shift of AMV in the 1990s is particularly noteworthy for accelerating warming averaged over the northern midlatitude land. The amplitude of the observed trend in both surface air temperature and the upper-level geopotential height generally falls beyond the range of multidecadal trends simulated by the CMIP6 preindustrial control runs, supporting the likelihood that anthropogenic forcing played a critical role in the observed trend. On the other hand, the fidelity of the simulated low-frequency modes of variability and their teleconnections, especially on multidecadal timescales, is difficult to assess because of the relatively short observational records. Our mechanistic modeling results indicate that synoptic eddy-mean flow interaction is a key to the formation of the anomalous wavetrain but how the multidecadal modes can modulate the synoptic eddies through atmosphere-ocean and atmosphere-land interactions remains poorly understood. This gap in our knowledge makes it challenging to quantify the roles of the low-frequency modes and external forcings in causing the observed multidecadal trends.
... This threshold obtained from the PAST simulations was also used to define extreme rainfall events in the PLUS4K simulations and evaluate changes in frequency of events under global warming (see Figure 1c of Schär et al., 2016). We calculated a 60-year climatology for each simulation, then averaged across ensemble members (10 members for PAST and 30 members for PLUS4K simulations; 5 members for each of the six-type PLUS4K simulations) to minimize the effects of atmospheric internal variability Kamae, Shiogama, et al., 2017). ...
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Portions of East Asia often experienced extremely heavy rainfall events over the last decade. Intense atmospheric rivers (ARs), eddy transports of moisture over the middle latitudes, contributed significantly to these events. Although previous studies pointed out that landfalling ARs will become more frequent under global warming, the extent to which ARs produce extreme rainfall over East Asia in a warmer climate remains unclear. Here we evaluate changes in the frequency and intensity of AR‐related extreme heavy rainfall under global warming using a set of high‐resolution global and regional atmospheric simulations. We find that both the AR‐related water vapor transport and rainfall intensify over the southern and western slopes of mountains over East Asia in a warmer climate. ARs are responsible for a large fraction of the increase in the occurrence of extreme rainfall in boreal spring and summer. ARs will bring unprecedented extreme rainfall over East Asia under global warming.
... Here we use the 30-member ensemble simulations to quantify the total variability and its two components: SST-forced variability and internal variability, following Mei et al. (2014Mei et al. ( , 2015Mei et al. ( , 2019 and Kamae et al. (2017c). Specifically, the total variability T at each grid point is calculated as: ...
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The sea surface temperature (SST)-forced and internal variability in cold-season (December–March) atmospheric river (AR) occurrence frequency during 1951–2010 over the North Atlantic (NA) basin are examined using a 30-member ensemble of high-resolution atmospheric simulations. The first leading mode of the forced variability features a north–south wobbling pattern modulated by an out-of-phase combination of El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). Co-existing El Niño and negative NAO act to shift ARs equatorward, whereas concurrent La Niña and positive NAO tend to displace ARs poleward. The second leading mode is characterized by a meridional concentration and dispersion of AR occurrence at a basin scale and can be linked to the Scandinavian pattern and the SST difference between the central and easternmost tropical Pacific. The third leading mode is dominated by an oscillation of AR occurrence north and south of 40°N in the eastern NA basin, and modulated by an in-phase combination of ENSO and the NAO. Its time series exhibits a significant upward trend, which can be linked to the SST warming in the Indo-western Pacific since the 1970s. The internal variability in cold-season NA AR occurrence frequency is then quantified by means of the signal-to-noise ratio. The calculations show that the internal variability is relatively weak over the Great Antilles and central-to-eastern US but extremely strong over Northwestern Europe, which can be attributed to the strong SST control associated with ENSO and the chaotic variations of the NAO, respectively.
... Large ensemble simulations indicate potential seasonal predictability The large ensemble simulations (d4PDF) incorporate climate boundary forcings (e.g., SST and greenhouse gases) and produce realistic atmospheric circulation and weather events [41][42][43][44] . With respect to the distributions of baroclinic waves (see "Classification of baroclinic wave life cycle"), a comparison between the observation (Fig. 2a-d) and model climatologies (contours in Fig. 2e-l) suggests that the simulation generates too few LC2 events at high latitudes (e.g., the North Atlantic during January-March). ...
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Midlatitude baroclinic waves drive extratropical weather and climate variations, but their predictability beyond 2 weeks has been deemed low. Here we analyze a large ensemble of climate simulations forced by observed sea surface temperatures (SSTs) and demonstrate that seasonal variations of baroclinic wave activity (BWA) are potentially predictable. This potential seasonal predictability is denoted by robust BWA responses to SST forcings. To probe regional sources of the potential predictability, a regression analysis is applied to the SST-forced large ensemble simulations. By filtering out variability internal to the atmosphere and land, this analysis identifies both well-known and unfamiliar BWA responses to SST forcings across latitudes. Finally, we confirm the model-indicated predictability by showing that an operational seasonal prediction system can leverage some of the identified SST-BWA relationships to achieve skillful predictions of BWA. Our findings help to extend long-range predictions of the statistics of extratropical weather events and their impacts.
... Moreover, the availability of this dataset is also useful for climate researchers who intend to understand or confirm a specific climate phenomenon or mechanism, if the experimental setup is suitable for their purposes. Since this dataset became available in the public, many studies that make use of this dataset for the abovementioned purposes exist in the literature (Kamae et al., 2017a;Kamae et al., 2017b;Ueda et al., 2018;Xie et al., 2018). ...
Statistically significant time‐lagged relationship of pre‐monsoon precipitation in Indochina Peninsula (ICP) with large‐scale feature over the Pacific and Indian Oceans, which was recently found in observational datasets by the authors, is investigated with a large‐ensemble (100 ensembles) simulation database called “Database for Policy Decision‐Making for Future Climate Change” (d4PDF). Two different strategies were taken to perform Empirical Orthogonal Function (EOF) analysis on the large‐ensemble dataset, namely statistics of the analysis results for each ensemble member separately and analysis on the 6000‐year data (100 times 60 years). d4PDF can reproduce well the climatological characteristics of the observed pre‐monsoon precipitation. Significant time‐lagged correlations of several climate indices related to sea surface temperature (SST) with the pre‐monsoon precipitation in ICP as obtained in the observation are confirmed in d4PDF for both strategies of EOF analyses on each ensemble member and on 6000‐year data. The climate simulation in d4PDF can capture the lag‐to‐lag variations of the time‐lagged correlation patterns very well, though it is weaker. The time‐lagged regressions of SST and zonal wind at 850 hPa upon the first principal component (PC1) time‐series of the EOF analysis of the pre‐monsoon precipitation over ICP are investigated in low latitudes over the Pacific and Indian Oceans. The observed time‐lagged regression features are also confirmed in d4PDF dataset; the significant regression areas expand larger and significant time lags become longer than the observation, especially in the case of 6000‐year data, due to enough number of samples. Cluster analysis on the regression maps shows that the obtained groups with larger numbers of ensemble members are closer to the observation than the other groups. The randomness of time‐lagged regression feature among the 100 ensemble members is not directly related to the randomness of the SST perturbation introduced in the lower boundary condition for the ensemble simulation of d4PDF. This article is protected by copyright. All rights reserved.
... of Asia has been previously reported (Yu et al., 2018). Previous studies also suggest that the anomalies over the northeastern and tropical Pacific have a significant impact on the occurrence of CDHW events in the conterminous U.S. (Dulière et al., 2013;Kamae et al., 2017;McCabe et al., 2004;Peterson et al., 2013). Figure 7 presents the significant Poisson GLM regression coefficients for NAO during the four seasons. ...
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Compound drought and heatwave (CDHW) events have garnered much attention in recent studies. However, thus far, the identification of such events is oversimplified and their association with natural climate variability is not fully explored. Here, we derive anomalies in the weekly self‐calibrated Palmer Drought Severity Index (sc_PDSI) and daily maximum temperatures to identify CDHW events from 1982 to 2016 over 26 climate regions across the globe. Using a Poisson Generalized Linear Model (GLM), we analyze yearly occurrences of seasonal CDHW events and their association with the warm and cold phases of El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO). ENSO exhibits robust association with CDHW events over the Southern Hemisphere during the austral summer and fall, while PDO influences their occurrences over the Western North America in the Northern Hemisphere during the boreal summer, which is supported by the composites of anomalies in the atmospheric circulations and surface energy budget. However, NAO association with CDHW events is relatively weak. The CDHW occurrence over other regions is driven by a combination of these large‐scale natural forcing. Our analyses also highlight that the co‐occurrence of weekly to sub‐monthly scale anomalies in the observed temperature and precipitation may not be always aligned between the observations and the reanalysis. Therefore, caution must be exercised while explaining such observed anomalies on the basis of reanalysis‐based circulations and surface energy budget. Overall, our analyses provide a new insight towards concurrent extremes and should help foster research efforts in this area.
The variability and predictability of tropical cyclone genesis frequency (TCGF) during 1973–2010 at both basinwide and sub-basin scales in the northwest Pacific are investigated using a 100-member ensemble of 60-km-resolution atmospheric simulations that are forced with observed sea surface temperatures (SSTs). The sub-basin regions include the South China Sea (SCS) and the four quadrants of the open ocean. The ensemble-mean results well reproduce the observed interannual-to-decadal variability of TCGF in the southeast (SE), northeast (NE), and northwest (NW) quadrants, but show limited skill in the SCS and the southwest (SW) quadrant. The skill in the SE and NE quadrants is responsible for the model’s ability to replicate the observed variability in basinwide TCGF. Above-normal TCGF is tied to enhanced relative SST (i.e., local SST minus tropical-mean SST) either locally or to the southeast of the corresponding regions in both the observations and ensemble mean for the SE, NE, and NW quadrants, but only in the ensemble mean for the SCS and the SW quadrant. These results demonstrate the strong SST control of TCGF in the SE, NE, and NW quadrants; both empirical and theoretical analyses suggest that ensembles of ∼10, 20, 35, and 15 members can capture the SST-forced TCGF variability in these three sub-basin regions and the entire basin, respectively. In the SW quadrant and the SCS, TCGF contains excessive noise, particularly in the observations, and thus shows low predictability. The variability and predictability of the large-scale atmospheric environment and synoptic-scale disturbances and their contributions to those of TCGF are also discussed.
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A large-ensemble climate simulation database, which is known as the database for policy decision-making for future climate changes (d4PDF), was designed for climate change risk assessments. Since the completion of the first set of climate simulations in 2015, the database has been growing continuously. It contains the results of ensemble simulations conducted over a total of thousands years respectively for past and future climates using high-resolution global (60 km horizontal mesh) and regional (20 km mesh) atmospheric models. Several sets of future climate simulations are available, in which global mean surface air temperatures are forced to be higher by 4 K, 2 K, and 1.5 K relative to preindustrial levels. Nonwarming past climate simulations are incorporated in d4PDF along with the past climate simulations. The total data volume is approximately 2 petabytes. The atmospheric models satisfactorily simulate the past climate in terms of climatology, natural variations, and extreme events such as heavy precipitation and tropical cyclones. In addition, data users can obtain statistically significant changes in mean states or weather and climate extremes of interest between the past and future climates via a simple arithmetic computation without any statistical assumptions. The database is helpful in understanding future changes in climate states and in attributing past climate events to global warming. Impact assessment studies for climate changes have concurrently been performed in various research areas such as natural hazard, hydrology, civil engineering, agriculture, health, and insurance. The database has now become essential for promoting climate and risk assessment studies and for devising climate adaptation policies. Moreover, it has helped in establishing an interdisciplinary research community on global warming across Japan.
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Atmospheric rivers (ARs), intense water vapor transports associated with extra-tropical cyclones, frequently bring heavy rainfalls over mid-latitudes. Over East Asia, landfalling ARs result in major socio-economic impacts including widespread floods and landslides; for example, western Japan heavy rainfall in July 2018 killed more than 200 people. Using results of high-resolution atmospheric model ensemble simulations, we examine projected future change in summertime AR frequency over East Asia. Different sea surface temperature (SST) warming patterns derived from six atmosphere–ocean coupled model simulations were assumed to represent uncertainty in future SST projections. The rate of increase in the frequency of landfalling ARs over summertime East Asia is on average 0.9% K-1 and is dependent on SST warming patterns. Stronger warming over the North Indian Ocean and South China Sea or weaker warming over the tropical central Pacific produce more frequent landfalling ARs over East Asia. These patterns are similar to the co-variability of SST, atmospheric circulation, and ARs over the western North Pacific found on the interannual time scale. The results of this study suggest that the natural disaster risk related to landfalling ARs should increase over East Asia under global warming and SSTs over the Indo-Pacific region holds the key for a quantitative projection.
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An unprecedentedly large ensemble of climate simulations with a 60 km atmospheric general circulation model and dynamical downscaling with a 20 km regional climate model have been performed to obtain probabilistic future projections of low-frequency local-scale events. The climate of the latter half of the 20th century, the climate 4 K warmer than the pre-industrial climate, and the climate of the latter half of the 20th century without historical trends associated with the anthropogenic effect are simulated respectively more than 5000 years. From large ensemble simulations, probabilistic future changes in extreme events are available directly without using any statistical models. The atmospheric models are highly skillful in representing localized extreme events such as heavy precipitation and tropical cyclones. Moreover, mean climate changes in the models are consistent with those in the CMIP5 model ensembles. Therefore, the results enable the assessment of probabilistic change in localized severe events that have large uncertainty from internal variability. The simulation outputs are open to the public as a database called “Database for Policy Decision-Making for Future Climate Change” (d4PDF), which is intended to be utilized for impact assessment studies and adaptation planning for global warming.
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We describe two unprecedented large (100-member), long-term (61-year) ensembles based on MRI-AGCM3.2, which were driven by historical and non-warming climate forcing. These ensembles comprise the “Database for Policy Decision making for Future climate change (d4PDF)”. We compare these ensembles to large ensembles based on another climate model, as well as to observed data, to investigate the influence of anthropogenic activities on historical changes in the numbers of record-breaking events, including: the annual coldest daily minimum temperature (TNn), the annual warmest daily maximum temperature (TXx) and the annual most intense daily precipitation event (Rx1day). These two climate model ensembles indicate that human activity has already had statistically significant impacts on the number of record-breaking extreme events worldwide mainly in the Northern Hemisphere land. Specifically, human activities have altered the likelihood that a wider area globally would suffer record-breaking TNn, TXx and Rx1day events than that observed over the 2001-2010 period by a factor of at least 0.6, 5.4 and 1.3, respectively. However, we also find that the estimated spatial patterns and amplitudes of anthropogenic impacts on the probabilities of record-breaking events are sensitive to the climate model and/or natural-world boundary conditions used in the attribution studies.
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The Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities, and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last fifteen years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere/ocean interactions, which operate on different timescales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of processes often more independent of the Tropics than is observed. Finally, it is suggested that assessment of PDO-related regional climate impacts, reconstruction of PDO-related variability into the past with proxy records, and diagnosis of Pacific variability within coupled GCMs should all account for the effects of these different processes, which only partly represent direct forcing of the atmosphere by North Pacific Ocean SSTs.
Severe winters have occurred frequently in mid-latitude Eurasia during the past decade. Simulations with a 100-member ensemble of an atmospheric model detect an influence of declining Arctic sea-ice cover.
La profesion –formacion- docente es un tema crucial en los actuales debates educativos. La existencia de dos decretos y el desplazamiento del verdadero sentido del ser maestro reclaman de los analisis un ejercicio de comprension del orden discursivo oficial. La calidad es el sustrato de la sociedad de control. En este marco se agencia nuevas practicas de subjetivacion del maestro los cuales podriamos situar en la calidad, flexibilidad, adaptabilidad, eficiencia, eficacia. En cualquier caso, el esfuerzo por hacer del maestro un intelectual de la educacion fue borrado. La gran cuestion consiste en saber que discursos regula el saber del docente a la luz de la sociedad de control.
Spontaneous, internally-generated variability of the climate system is pervasive. On the multidecadal time scale it dominates the variability of surface air temperature averaged over extratropical land areas as large as the contiguous United States, and it modulates the rate of rise of global mean temperature in response to the buildup of greenhouse gases. Unforced variability is one of the factors that imposes limitations on the degree of confidence that can be attached to assessments and predictions of human-induced climate change. This chapter summarizes results of some recent studies based on the analysis of large ensembles of numerical integrations run with a suite of different atmospheric initial conditions but with the same prescribed external forcing scenario. The future trajectory of the real climate system is, in some sense, like the trajectory of an individual member of such an ensemble. The diversity of the trends among the different ensemble members is a part of the irreducible uncertainty inherent in projections of future climate change. It is shown how statistical methods can be used to diagnose the causes of this diversity, most of which is in response to member-to-member diversity in the atmospheric circulation trends, as reflected in the associated patterns of the sea-level pressure trends. Interactions between the atmosphere, oceans, and land also contribute to the variability of surface air temperature trends on the multidecadal time scale, as discussed in several chapters of this book. It is argued that in the face of such large uncertainties in the attribution of climate change in the extratropics, more attention should be focused on climate change in the tropics, where the greenhouse warming signal stands out more clearly, and on the broader suite of environmental issues that impact food security and the viability of ecosystems.
It has been claimed that the early-2000s global warming slowdown or hiatus, characterized by a reduced rate of global surface warming, has been overstated, lacks sound scientific basis, or is unsupported by observations. The evidence presented here contradicts these claims.
A number of studies have suggested that heat waves will increase in frequency, intensity, and duration under anthropogenic global warming. However, changes to heat extremes and temperature distributions in general are less understood in regions of complex terrain and substantial land–water contrasts. Such surface variations are important in the northwest United States, where synoptic- and mesoscale circulations interact with local topography and land–water interfaces to produce heat waves that have substantial impacts on fire weather, air quality, wind energy, and the population in general. Thus it is crucial to identify how the synoptic- and mesoscale circulations that drive such local extremes will change in a warming world. This paper analyzes changes to the conditions associated with heat extremes over the northwest United States by utilizing global and regional climate models. A maximum in 700-hPa warming is projected over the northwest United States and southwest Canada, with low-level warming attenuated near the coast. Soil moisture declines are projected over the region, which further enhances future extremes. It is found that low-level zonal wind distributions over the northwest United States become narrower, leading to fewer days with strong offshore flow. This is important since extreme warming events are associated with the strongest offshore/downslope flow. Historical and future regional temperature distributions are described and it is shown that although CMIP5 models predict increases in heat extremes for west Oregon and Washington, these increases are less than for inland areas.
Identifying predictability sources of heat wave variations is a scientific challenge and of practical importance. This study investigates the summertime heat wave frequency (HWF) over Eurasia for 1950–2014. The Eurasian HWF is dominated by two distinct modes: the interdecadal (ID) mode featured by an increasing pattern overall, centered around eastern Europe-central Asia and Mongolia-southwestern China; the interannual (IA) mode resembling a tri-pole anomaly pattern with three centers over western-northern Europe, northeastern Asia and East Asia. The ID mode is found to be influenced by mega-El Niño/Southern Oscillation (mega-ENSO) and the Atlantic multidecadal oscillation (AMO), and the latter has far more effect, whereas the IA mode is connected with mega-ENSO.