Upper-air Circulation Research Articles

Sensitivity of seasonal circulation response to snow reduction in the Northern Hemisphere and Eurasia and its impact on Eurasian climate

In this study, we analyze the impact of reduced snow cover in the Northern Hemisphere on the atmosphere and if the atmospheric response depends on the model resolution. We use the atmospheric component of the global climate model EC-Earth and perform three experiments: in the first experiment, we reduce the snow cover in the entire Northern Hemisphere by reducing the snow albedo to a constant value of 0.3, in the second experiment, we reduce the snow albedo only over Eurasia, and the third experiment is the control run using normal snow conditions. All experiments are integrated over the period 1980–2015 at standard resolution (~ 80 km) and high resolution (~ 40 km). Experiments comprise 11 and 5 ensemble members at standard resolution and high resolution, respectively. Reducing the snow albedo in the Northern Hemisphere leads to 5–10% snow cover reduction in winter and spring. Significant warm responses are found over northern Eurasia in spring and summer with a warm response reaching 3 °C. Similar but weaker warm temperature responses are found in the middle and upper troposphere (up to 2 °C) and reversed temperature responses in the stratosphere (up to – 2 °C), particularly over eastern Eurasia. This is closely associated with westerly jet flow response which is enhanced at high-latitude and weakened at low-latitude in winter and spring over eastern Eurasia. Reduced snow cover leads to warmer surface temperatures that accelerate snow-melting and further lead to different snow-hydrological responses in western and eastern Eurasia and more precipitation occurs over eastern Eurasia (increasing 10–20%), particularly in the Siberian region. When the snow albedo is reduced only in the Eurasian sector, the surface response pattern resembles the results of the Northern Hemisphere experiment. The warm response is slightly weakened about 0.25–0.5 °C over Eurasia and significantly weakened outside of Eurasia. However, the upper air circulation response is much less pronounced over Eurasia. The impact of resolution on the mean surface field response is small yet it is more pronounced on the large-scale circulation response, particularly in spring and winter.

The influence of dust aerosols on solar radiation and near-surface temperature during a severe duststorm transport episode

In the west of China, a rarely seen black storm, with a high intensity of dust aerosols and a large area of influence, occurred from April 26–29, 2015, for the first time, after more than 30 years. Based on the regional climate model (RegCM version 4.6), combined with Moderate Resolution Imaging Spectroradiometer (MODIS) satellite retrieval, meteorological, and environmental data, this work presents the pollution situation, weather background, and backward trajectory during the black storm process. In addition, we analyzed the temporal–spatial distribution of aerosol optical depth (AOD) and the impacts of dust aerosols on solar radiation and near-surface temperature under this extreme weather condition. We discovered that this black storm process was caused by the surface cold high pressure and frontal transit under the background of the upper-air circulation of “two troughs and two ridges.” The pollutants primarily from Xinjiang and the Central Asia region, along with the airflow, entered northern Xinjiang almost simultaneously with the southwest airflow, piled up along the Tianshan Mountains, and then crossed the mountains into southern Xinjiang. In addition, the areas with high AOD were mostly in the desert regions and basins, whereas the low-value areas were mountainous areas with relatively high altitudes due to the effect of geographical and climatic conditions. The AOD from RegCM 4.6 was generally lower, unlike the MODIS AOD. Moreover, the dust aerosols from this black storm caused a significant decline in net short-wave radiation (NSR) both at the top of the atmosphere (TOA) and surface. The cooling effect was more significant in the region with high AOD. For the areas where the AOD was higher than 0.7, the net short-wave radiative forcing of dust aerosols (ADRF) at the surface was above −70 W•m−2; on the other hand, the cooling effect at the TOA was not as significant as that at the surface, with the ADRFTOA being only about a quarter of the ADRFSUR. The ADRFSUR could reduce the near-surface temperature, and the region with a large temperature drop corresponded to the high-value areas of ADRFSUR/AOD. During this black storm, the near-surface temperature response to dust aerosols reached 0.40°C–2.9°C with a significant temperature drop because of cold air.

The transboundary transport of air pollutants and their environmental impacts on Tibetan Plateau

The Tibetan Plateau is the largest high altitude landform on Earth, with an area of over 2.5×106 km2 and an average elevation of ~4000 m above sea level. With a unique multisphere environmental system, the Tibetan Plateau provides an important ecological sheltering function for China and other parts of Asia. The Tibetan Plateau is one of the world’s most pristine regions, benefiting from a sparse population with negligible local influence on its environment. However, it is surrounded by some of the most polluted areas in the world, such as South Asia, East Asia, and Southeast Asia. With the atmospheric circulation, such pollutants may impact the Tibetan Plateau through long-range transport. Clearly, the scientific research on the transboundary transport of pollutants is not only important for the understanding of multisphere interactions on the earth surface, but also could meet the national strategic needs for ecological and environmental protection. Long-term monitoring combined with short-term intensive observation campaigns, were used to comprehensively summarize the latest research progress regarding the spatial-temporal distribution and transport mechanism of air pollutants, as well as their climate and ecological impacts, which were achieved during the Second Tibetan Plateau Scientific Expedition. With respect of historical trends reconstructed from environmental archives, e.g., glacial ice cores and lake sediments, the black carbon and heavy metals like mercury show a dramatic increase since 1950s, which reflect the enhanced emission of air pollutants in Asia. On-line observation data and WRF-Chem modeling indicate that upper air circulation and local mountain-valley breeze system are the main drivers of trans Himalaya air pollution from South Asia. A regional climate-chemistry model coupled with an aerosol-snow/ice feedback module was used to reveal the natural and anthropogenic light-absorbing aerosols’ radiative effects over the Tibetan Plateau. Results indicated that the mineral dust both in the atmosphere and snow induced 0.1−0.5°C warming over the western Tibetan Plateau and Kunlun Mountains in spring. Meanwhile, dust aerosols caused snow water equivalent to decrease by 5−25 mm over the western TP, Himalayas and Pamir Mountains in winter and spring. The radiative effects of BC-in-snow induced surface temperature increased by 0.1− 1.5°C and snow water equivalent decreased by 5−25 mm over the western Tibetan Plateau and Himalayas. According to the observations the black carbon and dust found in the snow and ice on the surfaces of glaciers were responsible for on average 20% of the albedo reduction within the TP region. Those atmospherically transported pollutants also have obvious negative impacts on the ecosystem in Tibetan Plateau. For example, bioaccumulation of DDTs have been found in Tibetan terrestrial and aquatic food chains, and newly emerging compounds such as polyfluoroalkyl substances and hexabromocyclododecanes have been widely detected in wild fish species. Therefore, the corresponding ecological risks are of great concern. In the future, it is necessary to quantify the extent of atmospherically transported pollution and model the pollutant fate under the future environmental scenarios as well as establish environmental and health risk.

Widespread fog over the Indo-Gangetic Plains and possible links to boreal winter teleconnections

Boreal wintertime planetary-scale atmospheric circulations and their possible consequences to widespread fog occurrences over the Indo-Gangetic Plains (IGP) region of the Himalayan valley are investigated in this study. Among the different fog types, radiation fog type seen at night or early morning hours favored by large-scale subsidence aloft and strong near-surface inversion is focused in this study. A composite analysis reveals that upper air circulation associated with 105 fog days over the IGP region show a trail linked to circulation anomalies over the Eurasian continents and the Arctic Circle. The findings suggest that there is a footprint of the Arctic Oscillation (AO) and conventional Eurasian (EU) circulation patterns linked to anticyclonic circulation aloft over the IGP region. Although widespread IGP fog occurrences under the large-scale subsidence environment are seen to occur during both phases of AO, the negative AO phase (high pressure environment over the Arctic Circle) portends a greater likelihood for fog occurrences in the IGP region. A coupling of positive mid-tropospheric height anomalies over western Eurasia and the anticyclonic circulation anomalies over the IGP region is evident during the IGP fog periods concomitant with EU positive (height excess over Siberia) phase. Further, anomalous circulation over the IGP region during the fog periods appears to rely more on the strength of the AO negative phase than the circulation strengths over Eurasia. On the contrary, the Eurasian circulation largely appears to influence the subsidence aloft over the IGP region irrespective of the strength of the AO positive phase. It is also noted that upper-air circulation during non-foggy periods over the IGP region has conformity with positive AO phase and rapidly progressing EU pattern. These planetary-scale teleconnection pathways offer new dynamical insights into comprehending widespread IGP fog scenario, which have been hitherto perceived mostly from a regional context.

Simulation of an extreme heavy rainfall event over Chennai, India using WRF: Sensitivity to grid resolution and boundary layer physics

In this study, the heavy precipitation event on 01 December 2015 over Chennai located on the southeast coast of India was simulated using the Weather Research and Forecast (WRF) model. A series of simulations were conducted using explicit convection and varying the planetary boundary layer (PBL) parameterization schemes. The model results were compared with available surface, satellite and Doppler Weather Radar observations. Simulations indicate strong, sustained moist convection associated with development of a mesoscale upper air cyclonic circulation, during the passage of a synoptic scale low-pressure trough caused heavy rainfall over Chennai and its surroundings. Results suggest that veering of wind with height associated with strong wind shear in the layer 800–400 hPa together with dry air advection facilitated development of instability and initiation of convection. The 1-km domain using explicit convection improved the prediction of rainfall intensity of about 450 mm and its distribution. The PBL physics strongly influenced the rainfall prediction by changing the location of upper air circulation, energy transport, moisture convergence and intensity of convection in the schemes YSU, MYJ and MYNN. All the simulations underestimated the first spell of the heavy rainfall. While YSU and MYJ schemes grossly underestimated the rainfall and dislocated the area of maximum rainfall, the higher order MYNN scheme simulated the rainfall pattern in better agreement with observations. The MYNN showed lesser mixing and simulated more humid boundary layer, higher convective available potential energy (CAPE) and stronger winds at mid-troposphere than did the other schemes. The MYNN also realistically simulated the location of upper air cyclonic flow and various dynamic and thermodynamic features. Consequently it simulated stronger moisture convergence and higher precipitation.

Empirical and modeling analyses of the circulation influences on California precipitation deficits

AbstractAmplified and persistent ridges in western North America are recurring features associated with drought conditions in California. The recent drought event (2012–2016) lasted through both La Niña and El Niño episodes, suggesting additional climate drivers are important in addition to the commonly perceived El Niño‐Southern Oscillation. Diagnostic analyses presented here suggest that, while the Pacific North American (PNA) and North Pacific Oscillation (NPO) do not directly cause drought in California, the relationships between them and with the upper air circulation pattern do modulate the spatial drought pattern. The positive PNA relative circulation leads drier northern California, and (‐NPO) relative circulation leads southern California to be drier. The types of drought in this region emerge mostly from the combination of two PNA and NPO relative oceanic and atmospheric oscillations. At present, climate model projections do not indicate any significant change in these particular drought‐modulating processes.

Role of the Himalayan Orography in Simulation of the Indian Summer Monsoon using RegCM3

In this study, sensitivity of the Indian summer monsoon simulation to the Himalayan orography representation in a regional climate model (RegCM) is examined. The prescribed height of the Himalayan orography is less in the RegCM model than the actual height of the Himalayas. Therefore, in order to understand the impact of the Himalayan orography representation on the Indian summer monsoon, the height of the Himalayan orography is increased (decreased) by 10 % from its control height in the RegCM model. Three distinct monsoon years such as deficit (1987), excess (1988) and normal rainfall years are considered for this study. The performance of the RegCM model is tested with the use of a driving force from the reanalysis data and a global model output. IMD gridded rainfall and the reanalysis-2 data are used as verification analysis to validate the model results. The RegCM model has the potential to represent mean rainfall distribution over India as well as the upper air circulation patterns and some of the semi-permanent features during the Indian summer monsoon season. The skill of RegCM is reasonable in representing the variation in circulation and precipitation pattern and intensity during two contrasting rainfall years. The simulated seasonal mean rainfall over many parts of India especially, the foothills of the Himalaya, west coast of India and over the north east India along with the whole of India are more when the orography height is increased. The low level southwesterly wind including the Somali jet stream as well as upper air circulation associated with the tropical easterly jet stream become stronger with the enhancement of the Himalayan orography. Statistical analysis suggests that the distribution and intensity of rainfall is represented better with the increased orography of RegCM by 10 % from its control height. Thus, representation of the Himalayan orography in the model is close to actual and may enhance the skill in seasonal scale simulation of the Indian summer monsoon.

Validating a regional climate model’s downscaling ability for East Asian summer monsoonal interannual variability

Performance of a regional climate model (RCM), WRF, for downscaling East Asian summer season climate is investigated based on 11-summer integrations associated with different climate conditions with reanalysis data as the lateral boundary conditions. It is found that while the RCM is essentially unable to improve large-scale circulation patterns in the upper troposphere for most years, it is able to simulate better lower-level meridional moisture transport in the East Asian summer monsoon. For precipitation downscaling, the RCM produces more realistic magnitude of the interannual variation in most areas of East Asia than that in the reanalysis. Furthermore, the RCM significantly improves the spatial pattern of summer rainfall over dry inland areas and mountainous areas, such as Mongolia and the Tibetan Plateau. Meanwhile, it reduces the wet bias over southeast China. Over Mongolia, however, the performance of precipitation downscaling strongly depends on the year: the WRF is skillful for normal and wet years, but not for dry years, which suggests that land surface processes play an important role in downscaling ability. Over the dry area of North China, the WRF shows the worst performance. Additional sensitivity experiments testing land effects in downscaling suggest the initial soil moisture condition and representation of land surface processes with different schemes are sources of uncertainty for precipitation downscaling. Correction of initial soil moisture using the climatology dataset from GSWP-2 is a useful approach to robustly reducing wet bias in inland areas as well as to improve spatial distribution of precipitation. Despite the improvement on RCM downscaling, regional analyses reveal that accurate simulation of precipitation over East China, where the precipitation pattern is strongly influenced by the activity of the Meiyu/Baiu rainfall band, is difficult. Since the location of the rainfall band is closely associated with both lower-level meridional moisture transport and upper-level circulation structures, it is necessary to have realistic upper-air circulation patterns in the RCM as well as lower-level moisture transport in order to improve the circulation-associated convective rainfall band in East Asia.

QPF model for Sabarmati basin based on Synoptic analogue method

The paper formulates a synoptic analogue model for issuing Quantitative Precipitation Forecast (QPF) for Sabarmati basin based on 10 years data (2000-2009) during southwest monsoon period. The model was verified with the actual Average Areal Precipitation (AAP) for the corresponding synoptic situations during 2010.The performance of the model were observed Percentage Correct (PC) up to 71%. The cases out by one or two stage were due to variation in the intensity of the system especially upper air circulation (S3) over the basin. The synoptic analogue model was able to generate accurate QPF 24 hrs in advance to facilitate flood forecasters of Central Water Commission.

Sensitivity of the GCM driven summer monsoon simulations to cumulus parameterization schemes in nested RegCM3

The regional climate model (RegCM3) from the Abdus Salam International Centre for Theoretical Physics has been used to simulate the Indian summer monsoon for three different monsoon seasons such as deficit (1987), excess (1988) and normal (1989). Sensitivity to various cumulus parameterization and closure schemes of RegCM3 driven by the National Centre for Medium Range Weather Forecasting global spectral model products has been tested. The model integration of the nested RegCM3 is conducted using 90 and 30-km horizontal resolutions for outer and inner domains, respectively. The India Meteorological Department gridded rainfall (1° × 1°) and National Centre for Environment Prediction (NCEP)–Department of Energy (DOE) reanalysis-2 of 2.5° × 2.5° horizontal resolution data has been used for verification. The RegCM3 forced by NCEP–DOE reanalysis-2 data simulates monsoon seasons of 1987 and 1988 reasonably well, but the monsoon season of 1989 is not represented well in the model simulations. The RegCM3 runs driven by the global model are able to bring out seasonal mean rainfall and circulations well with the use of the Grell and Anthes–Kuo cumulus scheme at 90-km resolution. While the rainfall intensity and distribution is brought out well with the Anthes–Kuo scheme, upper air circulation features are brought out better by the Grell scheme. The simulated rainfall distribution is better with RegCM3 using the MIT-Emanuel cumulus scheme for 30-km resolution. Several statistical analyses, such as correlation coefficient, root mean square error, equitable threat score, confirm that the performance of MIT-Emanuel scheme at 30-km resolution is better in simulating all-India summer monsoon rainfall. The RegCM3 simulated rainfall amount is more and closer to observations than that from the global model. The RegCM3 has corrected its driven GCM in terms of rainfall distribution and magnitude over some parts of India during extreme years. This study brings out several weaknesses of the RegCM model which are documented in this paper.

Future changes in the relationship of precipitation intensity in Eastern Mediterranean with large scale circulation

Abstract. The objective of this study is to investigate the linkage of large-scale upper air circulation over the greater European area with intense precipitation events over Eastern Mediterranean and then to estimate potential changes in the atmospheric patterns in the future, under global warming conditions. For this purpose, results from the regional climate model HadRM3P and Global Circulation Model HadAM3P have been used for the present period 1960–1990 (control run) and the future period 2070–2100 based on the B2a IPCC emission scenario. For the identification of the precipitation extremes the Simple Daily Intensity Index (SDII) was employed. Our analysis has shown a notable relation of extreme events with the East Atlantic and Scandinavia teleconnection patterns, as well as the Eastern Mediterranean Pattern (EMP) during the wet period. In the future, similar patterns are found, with different magnitude and position, following the projected changes in atmospheric circulation over Europe.

ENSO Modoki impact on the Southern Hemisphere storm track activity during extended austral winter

Impacts of the recently discovered ENSO Modoki phenomena on extended winter storm track activity in the Southern Hemisphere are examined using the observed rainfall, sea surface temperature, and reanalyzed upper air circulation data for the period 1979–2004. The partial correlation technique is utilized to distinguish the impact of ENSO Modoki events from those of the ENSO and Indian Ocean Dipole (IOD). El Niño Modoki events introduce an anomalous blocking over central eastern Australia, which suppresses the storm track activity from southwest till centraleast, reducing the storm‐associated rainfall in southeastern tip and portions of the southeast. On the other hand, the storm track activity in central Argentina is enhanced owing to the strengthened upper air westerlies in this region. The impacts from the ENSO Modoki events are apparently stronger than the individual impacts from the ENSO and IOD events.

Statistical downscaling of precipitation through nonhomogeneous stochastic weather typing

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 34:169-184 (2007) - DOI: https://doi.org/10.3354/cr00696 Statistical downscaling of precipitation through nonhomogeneous stochastic weather typing M. Vrac1,5,*, M. Stein2, K. Hayhoe3,4 1Center for Integrating Statistical and Environmental Science, The University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637, USA 2Department of Statistics, The University of Chicago, 5734 S. University Avenue, Chicago, Illinois 60637, USA 3Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, 105 S. Gregory Street, Urbana, Illinois 61801, USA 4Department of Geosciences, Texas Tech University, Lubbock, Texas, USA 5Present address: Laboratoire des Sciences du Climat et de L’Environnement, Centre d’Etudes de Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France *Email: mathieu.vrac@cea.fr ABSTRACT: We present a novel statistical downscaling method that provides accurate and relatively transparent simulations of local-scale precipitation characteristics. The method combines large-scale upper-air circulation with surface precipitation fields, and is based on a nonhomogeneous stochastic weather typing approach. Here we applay the method to downscale precipitation at 37 rain gauges in the state of Illinois, USA. Regional climate conditions are categorized in terms of 2 different types of weather states: (1) ’precipitation patterns‘ developed by a hierarchical ascending clustering (HAC) method with an original metric applied directly to the observed rainfall characteristics in Illinois, and (2) ’circulation patterns‘ developed by a mixture model applied to large-scale NCEP reanalysis fields. We modeled the transition probabilities from one pattern to another by a nonhomogeneous Markov model that is influenced by large-scale atmospheric variables such as geopotential height, humidity and dew point temperature depression. Our results indicate that including the precipitation states in the statistical model allows us to simulate important precipitation features such as conditional distributions of local simulated rainfall intensities and wet/dry spell behavior more accurately than with a traditional approach based on upper-air circulation patterns alone. KEY WORDS: Weather states · Weather typing · Nonhomogeous Markov model · Hierarchical clustering · EM algorithm · Statistical downscaling Full text in pdf format NextCite this article as: Vrac M, Stein M, Hayhoe K (2007) Statistical downscaling of precipitation through nonhomogeneous stochastic weather typing. Clim Res 34:169-184. https://doi.org/10.3354/cr00696 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 34, No. 3. Online publication date: September 18, 2007 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2007 Inter-Research.

The role of terrestrial snow cover in the climate system

Snow cover is known to exert a strong influence on climate, but quantifying its impact is difficult. This study investigates the global impact of terrestrial snow cover through a pair of GCM simulations run with prognostic snow cover and with all snow cover on land eliminated (NOSNOWCOVER). In this experiment all snowfall over land was converted into its liquid–water equivalent upon reaching the surface. Compared with the control run, NOSNOWCOVER produces mean-annual surface air temperatures up to 5 K higher over northern North America and Eurasia and 8–10 K greater during winter. The globally averaged warming of 0.8 K is one-third as large as the model’s response to 2 × CO2 forcing. The pronounced surface heating propagates throughout the troposphere, causing changes in surface and upper-air circulation patterns. Despite the large atmospheric warming, the absence of an insulating snow pack causes soil temperatures in NOSNOWCOVER to fall throughout northern Asia and Canada, including extreme wintertime cooling of over 20 K in Siberia and a 70% increase in permafrost area. The absence of snow melt water also affects extratropical surface hydrology, causing significantly drier upper-layer soils and dramatic changes in the annual cycle of runoff. Removing snow cover also drastically affects extreme weather. Extreme cold-air outbreaks (CAOs)—defined relative to the control climatology—essentially disappear in NOSNOWCOVER. The loss of CAOs appears to stem from both the local effects of eliminating snow cover in mid-latitudes and a remote effect over source regions in the Arctic, where −40°C air masses are no longer able to form.

Surface and Upper Air Fields During Extreme Winter Precipitation Over the Western Himalayas

Surface and upper air circulation features associated with extreme precipitation years are demonstrated during winter season viz., December, January, February and March (DJFM) to examine winter weather affecting the western Himalayas. These circulations are studied over the domain 15°S–45°N and 30°E–120°E. This domain is considered particularly to illustrate the distribution of precipitation due to a wintertime eastward moving synoptic weather system called western disturbances. Surplus and deficient years of seasonal (DJFM) precipitation are identified using ± 20% departure from mean from uninitialized daily reanalysis data of forty (1958–1997) years of the National Center For Environmental Prediction (NCEP), US. The years 1965–1969, 1973 and 1991 are found to be surplus years and the years 1962, 1963, 1971, 1977, and 1985 are found to be deficient years. Comparative study between composites of these two categories is made using students t-test of significance. Significant differences in sea-level pressure, zonal and meridional component of wind at surface and upper levels, total precipitable water content, geopotential height and temperature are observed in the two contrasting seasons.

Feasibility of a 100-Year Reanalysis Using Only Surface Pressure Data

Climate variability and global change studies are increasingly focused on understanding and predicting regional changes of daily weather statistics. Assessing the evidence for such variations over the last 100 yr requires a daily tropospheric circulation dataset. The only dataset available for the early twentieth century consists of error-ridden hand-drawn analyses of the mean sea level pressure field over the Northern Hemisphere. Modern data assimilation systems have the potential to improve upon these maps, but prior to 1948, few digitized upper-air sounding observations are available for such a “reanalysis.” We investigate the possibility that the additional number of newly recovered surface pressure observations is sufficient to generate useful weather maps of the lower-tropospheric extratropical circulation back to 1890 over the Northern Hemisphere, and back to 1930 over the Southern Hemisphere. Surprisingly, we find that by using an advanced data assimilation system based on an ensemble Kalman filter, it would be feasible to produce high-quality maps of even the upper troposphere using only surface pressure observations. For the beginning of the twentieth century, the errors of such upper-air circulation maps over the Northern Hemisphere in winter would be comparable to the 2-3-day errors of modern weather forecasts.

Surface air temperature variability over turkey and its connection to large-scale upper air circulation via multivariate techniques

The problem of statistical linkages between large-scale and local-scale processes is investigated through noise reduction by singular spectrum analysis (SSA) and spatial principal component analysis in order to construct appropriate statistical models for estimating the local-scale climate variables from large-scale climate processes. This paper presents an approach for downscaling monthly temperature series over Turkey by upper air circulations derived from the National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis data sets (500 hPa geopotential heights and 500–1000 hPa thicknesses). The proposed approach consists of three stages. First, the available data sets are separated into deterministic, statistical components and random components by SSA. Second, the deterministic components are saved and the random components are eliminated by spatial principal component analysis. Subsequently, the statistical components are combined with the deterministic components constituting a noise-free data set. Furthermore, so-called Sampson correlation patterns are determined between the noise-free large-scale and the local-scale variables for interpreting the large-scale process impacts on local-scale features. Third, the significant redundancy variates based on canonical correlation analysis are extracted in order to identify the statistical downscaling model for temperature series of 62 stations in Turkey. The results show that the interpretation of the local-scale processes with the noise-free data sets is more significant than with the raw data sets. Copyright © 2005 Royal Meteorological Society

Investigation of a Large-Scale Mode of Ocean–Atmosphere Variability and Its Relation to Tropical Pacific Sea Surface Temperature Anomalies

By examining the linearly coupled atmospheric and oceanic signals related to interannual variability in sea surface temperatures and upper-air wind fields, a hemispheric-scale ocean–atmosphere teleconnection mode is isolated that is significantly correlated with equatorial Pacific SSTs 12 months later. The interannual component of this teleconnection mode is related to a basin-scale dipole in the upper-air wind fields stretching across the extratropical Pacific, with additional anomalies extending from the eastern tropical Pacific over North America and into the Atlantic basin. In addition, it is related to variability in the SST field with warm anomalies found over the tropical/subtropical western Pacific as well as the equatorial eastern Pacific; also, there are related cold anomalies over the extratropical central North Pacific that extend down into the central subtropical/tropical Pacific. Diagnostic studies investigating the ocean–atmosphere structure for this mode of variability indicate that the large-scale variations in the upper-air circulation patterns are associated with anomalous equatorward propagation of transient and stationary wave activity over the North Pacific. In addition, they are characterized by vertical circulation patterns over both the subtropical and extratropical Pacific, which are collocated with variations in surface pressure and wind stress fields over the central subtropical and tropical North Pacific. Previous research has shown that modifications of these two fields are significantly related to the evolution of equatorial Pacific SSTs and may provide the dynamic mechanism whereby the ocean–atmosphere teleconnection mode described here influences the development of the ENSO system. This influence appears to be related to a modification of the basin-scale heat content over the central and eastern tropical Pacific; however, significant discussion is provided concerning alternative hypotheses.

Interannual Variability of Summertime Convective Cloudiness and Precipitation in the Central Andes Derived from ISCCP-B3 Data

The interannual variability of austral summer [December‐January‐February‐March (DJFM)] convective activity and precipitation in the central Andes (158‐308S) is investigated between 1983 and 1999 based on in situ rain gauge measurements, International Satellite Cloud Climatology Project (ISCCP) reduced radiance satellite data (the B3 dataset), and National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) reanalysis data. Twice-daily ISCCP-B3 calibrated infrared data, corrected for limb-darkening effects and representing equivalent blackbody temperatures Tb emitted by clouds are used to derive seasonal composites of fractional cold cloud coverage F*. Comparison of in situ rain gauge measurements with F* show a good correlation when a temperature threshold Tb 5 240 K is used to derive F*. A rotated empirical orthogonal function (REOF) applied to the seasonal estimates of F* yielded three spatially separated modes of convective activity in the south, northwest, and northeast of the central Andes. Results indicate that precipitation variability in the central Andes shows less spatial coherence than previously thought, with many years showing an antiphasing of wet/dry conditions between the northern and southern part of the study area. Regression analyses confirm the crucial role of both intensity and location of upper-air circulation anomalies with easterly wind anomalies favoring wet conditions, and westerly winds producing dry conditions. Two different forcing mechanisms are identified as main causes of upper-air zonal wind anomalies in the northern and southern part of the central Andes, respectively. Easterly wind anomalies during wet summers in the northern part are in geostrophic balance with reduced meridional baroclinicity due to low-latitude (midlatitude) cooling (warming), consistent with earlier studies. Farther to the south, easterly wind anomalies during wet summers are the result of an upper-air anticyclonic anomaly centered over southeastern South America, leading to a relaxation of the upper-air westerly winds and episodic easterly transport of humid air toward the subtropical Andes. This pattern is similar to one of the leading modes of intraseasonal variability, related to extratropical Rossby wave dispersion and modulation of the position of the Bolivian high. Correlation analysis of F* with near-surface specific humidity reveals that humidity variations in the lowlands to the east are not relevant on interannual time scales for the more humid northern part of the Altiplano. In the southern Altiplano, however, there is a significant correlation between convective activity and precipitation at high elevation and the low-level humidity content to the southeast of the Andes.

The climate of the Altiplano: observed current conditions and mechanisms of past changes

The large-scale controls on the climate of the South American Altiplano are investigated using local observations, reanalysis data and general circulation model experiments. The objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial–interglacial. Our results suggest that, on all timescales, the climatic conditions on the Altiplano are closely related to the upper-air circulation, with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions. Different factors influence the upper-air circulation on the different timescales. Intraseasonal variability is a reflection of the position and intensity of the Bolivian High, which is modulated by Rossby waves emanating from the midlatitude South Pacific. The annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere, rather than direct insolation forcing over the Altiplano or moisture changes in the source area. Interannual variability is primarily related to changes in the mean zonal flow over the Altiplano, reflecting changes in meridional baroclinicity between tropical and subtropical latitudes, which in turn is a response to sea-surface temperature changes in the tropical Pacific. Orbitally forced changes in the land–sea contrast drive continental-scale circulation changes, which significantly alter the zonal flow over the Altiplano. On glacial–interglacial timescales, the contrast in heating between northern and southern hemispheres during the glacial leads to upper-air easterly anomalies throughout the tropics. On modern timescales the marked submonthly, seasonal and interannual changes of moisture over the Altiplano cannot be accounted for by moisture changes in the humid tropical lowlands. However, the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands, which counteracts the effect of the upper-level circulation, resulting in little overall change in precipitation. This observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the Altiplano on a wide range of timescales.