Atmospheric Circulation Features Research Articles

Climatic changes in Estonia during the second half of the 20th century in relationship with changes in large-scale atmospheric circulation

Trends in the time series of air temperature, precipitation, snow cover duration and onset of climatic seasons at ten stations in Estonia during 1951–2000 are analysed. Using the conditional Mann-Kendall test, these trends are compared with trends in the characteristics of large-scale atmospheric circulation: the NAO and AO indices, frequency of circulation forms according to the Vangengeim-Girs’ classification, and the northern hemisphere teleconnection indices. The objective of the study is to estimate the influence of trends in circulation on climate changes in Estonia. Statistically significant increasing trends in air temperature are detected in January, February, March, April and May, in winter (DJF), spring (MAM) and in the cold period (NDJFM). The trends in precipitation, as a rule, differ from station to station. Increasing trends are present during the cold half-year – from October until March – and also in June. Snow cover duration has decreased in Estonia by 17–20 days inland and by 21–36 days on the coast. The onsets of early spring and spring have shifted to an earlier date. Some important changes have occurred in the parameters of atmospheric circulation during 1951–2000. Intensity of zonal circulation, i.e. westerlies, has increased during the cold period, especially in February and March. Results of the conditional Mann-Kendall test indicate that the intensification of westerlies in winter is significantly related to climate changes in winter and also in spring. A negative trend in the East Atlantic Jet (EJ) index, i.e. the weakening of the westerlies in May has caused warming during that month. Decrease in northerly circulation, i.e. in frequency of circulation form C and in East Atlantic/West Russia teleconnection index (EW) is related to an increase in precipitation in October.

The Contrasting Features of Winter Circulation During Surplus and Deficient Precipitation Over Western Himalayas

Contrasting features associated with surplus and deficient precipitation years are studied to examine atmospheric circulation characteristics during the winter season viz., December, January February and March (DJFM) to assess the wintertime synoptic weather system affecting the western Himalayas. Large-scale balances of kinetic energy, vorticity, angular momentum, heat and moisture fields are analyzed. Winter circulations are studied over the domain 15°S–45°S and 30°E–120°E. This domain is considered particularly to illustrate the distribution of precipitation due to wintertime eastward moving synoptic weather systems called western disturbances. Surplus and deficient years of seasonal (DJFM) precipitation are identified using±20% departure from mean from uninitialized daily reanalysis data of fourty (1958–1997) years of the National Center for Environmental Prediction (NCEP), USA. The years 1965–1969, 1973 and 1991 are found to be surplus years and years 1962, 1963, 1971, 1977 and 1985 are found to be deficient years. Composites of these two categories are made. Comparative study is made using Students t-test of significance. Examining the aspects associated with energetics during the two extreme categories of winter seasonal precipitation years, higher heat flux convergence in excess years in the area of study of precipitation takes place. Diabatic heating shows cooling. Higher flux of convergence of kinetic energy and higher dissipation of kinetic energy are observed during surplus years.

Direct correlation of the marine oxygen isotope record with the Chinese Loess Plateau iron oxide and magnetic susceptibility records

Monsoons are a major feature of global atmospheric circulation and derive heat and moisture from the oceans and transports much of that heat and moisture to nearby continents. The east Asian Monsoon derives its heat and moisture from the eastern equatorial Pacific and transfers it to Southeast Asia and the Loess Plateau of central China. In order to examine the phasing of marine and terrestrial climatic events associated with this monsoon we have analyzed a core from the western Pacific, V21-146, downwind from the Loess Plateau. Iron oxides and magnetic susceptibility were analyzed because they have been used as Loess Plateau paleoclimate proxies. Comparison of temporal changes in both iron oxide and susceptibility signals in V21-146, to the well-known Luochuan loess section, reveals an excellent correlation, both in the timing and direction of the variations. Comparison of our Loess Plateau paleoclimatic proxies in V21-146 to previously published oxygen isotope data from this core suggests that through Marine Isotope Stage (MIS) 9, coincident with Loess Plateau interglacial soil level 3 (S3), ocean temperature increases and the onset of monsoonal conditions on the Loess Plateau were synchronous. However, for MIS 11/12 (interglacial/glacial S4/L5) changes in the ocean appear to significantly lead those on land and there is a lack of correlation between MIS 13 and S5. These out of phase events may be a response to a reorganization of the ocean/atmosphere system as indicated by changes in the δ 13C record. It has previously been suggested that changes in eolian flux record eolian transport from the Loess Plateau, but we find little correlation between eolian flux and our Loess Plateau proxies, suggesting that some of the eolian sediments were derived from outside the Loess Plateau.

The influence of topographic co-variables on the spatial variability of precipitation over small regions of complex terrain

Precipitation variability results from atmospheric circulation and complex site-specific bio-geoclimatic characteristics; therefore, climatic variables are expected to be correlated in a scale-dependent way. This paper studies the influence of topographic co-variables on the spatial variability of precipitation over small regions of complex terrain. For this purpose, the mutual benefits of an integrated geographic information system (GIS) and a geostatistics approach was used for spatial precipitation interpolation from rainfall observations measured at 51 climatic stations in a mountainous region of southern Italy (Benevento province). As no single method is optimal for all regions, it is important to compare the results obtained using alternative methods applied to the same data set. Therefore, besides ordinary kriging examination, two auxiliary variables were added for ordinary co-kriging of annual and seasonal precipitation: terrain elevation data and a topographic index. Cross-validation indicated that the ordinary kriging yielded the largest prediction errors. The smallest prediction errors were produced by a multivariate geostatistical method. However, the results favour the ordinary co-kriging with inclusion of information on the topographic index. The application of co-kriging is particularly justified in areas where there are nearby stations and where landform is very complex. We conclude that ordinary co-kriging is a very flexible and robust interpolation method because it may take into account several properties (soft and hard data) of the landscape. Copyright © 2005 Royal Meteorological Society

Atmospheric circulation characteristics during some El-Nino years in relation to Indian summer monsoon rainfall

Based on observed rainfall data of India Meteorological Department (IMD), correlation coefficients (CCs) have been computed between Indian summer monsoon rainfall (ISMR) and sea surface temperature (SST) anomalies over different Nino regions and standardised pressure difference between Tahiti and Darwin. Significant positive CCs are found between the Southern Oscillation Index (SOI) in winter and subsequent June rainfall over India. Concurrent with and subsequent to Indian summer monsoon, SOI shows significant positive CC with the mean rainfall of July to September (JAS). Significant negative CCs are found between JAS mean rain and the concurrent and following SST anomalies over Nino-3.4 region. On the basis of these correlations, it is proposed that the entire period of summer monsoon from June to September could be divided into two sub-periods such as: early summer (June) and mid-late summer (July to September) monsoon for prediction of ISMR in the extended range. In order to examine the characteristics of atmospheric circulation during some El-Nino years, divergent flow at 200 hPa and omega field at 500 hPa based on NCEP/NCAR reanalysis have been studied in detail. Major significant southward shift of upper level divergent field from India are related to El-Nino and this shift may be responsible for causing droughts during several El-Nino years over India. Also vertical wind fields at 500 hPa show sinking motion over large parts of India and west Pacific and ascending motion over southern Indian Ocean, central and eastern Pacific during major drought years.

Synoptic-scale climate patterns associated with rainfall δ18O in southern Australia

A detailed examination of the relationship between synoptic patterns and the oxygen isotopic composition ( δ 18O) of rainfall is made using a 5-year long dataset of daily-event based rainfall samples from Tasmania, southern Australia. Rainfall δ 18O varies inversely with rainfall amount and has only a weak relationship with site surface temperature. Events with extreme rainfall and δ 18O values were identified from monthly anomalies. The synoptic patterns for these events were averaged using NCEP/DOE reanalyses-2 mean sea-level pressure data to define the major atmospheric circulation features. Large amounts of rainfall with low δ 18O values occur when mid-latitude Southern Ocean low pressure systems pass close to the southeast of Tasmania in a circulation pattern with a strong meridional component. By contrast, small amounts of rainfall with high δ 18O values occur when the centre of these mid-latitude cyclonic systems pass much further south of Tasmania. In the latter, only the distal limits of the fronts associated with these low pressure systems cross Tasmania and airflow has a stronger zonal component. In addition, synoptic patterns causing advection of air from lower latitudes towards Tasmania may raise the δ 18O value of precipitation. Oxygen isotope records from southern Australia should not be interpreted exclusively as proxies for temperature changes.

Specification and prediction of monthly and seasonal rainfall over the south‐west peninsula of England

AbstractAn assessment of the extent to which atmospheric variables add to the specification and predictability of rainfall over the south‐west peninsula of England (SWPE), as provided by North Atlantic sea surface temperatures (SSTs), is undertaken. To this end multivariate specification and prediction models are developed, describing rainfall variations over the SWPE at monthly and seasonal time‐scales. Results reveal that synchronous variations in atmospheric circulation characteristics are the main specifiers of monthly and seasonal rainfall variability, chiefly mean monthly sea‐level pressure, vorticity and atmospheric water vapour flux. In contrast, the predictability of rainfall for one‐ and two‐months, and one‐ and two‐seasons ahead is more dependent on SST than atmospheric circulation variability. Both the tropical Atlantic and the Greenland–Iceland (Norwegian Sea) regions of the North Atlantic appear important as centres of action in terms of SST–SWPE rainfall links. Although SSTs account for the majority of variance in monthly and seasonal rainfall, a moderate improvement of rainfall prediction is achieved with the inclusion of atmospheric water vapour flux in prediction models for one‐month to two‐seasons ahead, as it acts as an atmospheric bridge between the ocean and land in terms of precipitation delivery. The empirical relationships revealed between SWPE rainfall, and atmospheric and SST variability are discussed in terms of the main components of the North Atlantic climate system. Copyright © 2004 Royal Meteorological Society

A Global-Scale Examination of Monsoon-Related Precipitation

Abstract A pentad version of the Global Precipitation Climatology Project global precipitation dataset is used to document the annual and interannual variations in precipitation over monsoon regions around the globe. An algorithm is described that determines objectively wet season onset and withdrawal for individual years, and this tool is used to examine the behavior of various characteristics of the major monsoon systems. The definition of onset and withdrawal are determined by examining the ramp-up and diminution of rainfall within the context of the climatological rainfall at each location. Also examined are interannual variations in onset and withdrawal and their relationship to rainy season precipitation accumulations. Changes in the distribution of “heavy” and “light” precipitation events are examined for years in which “abundant” and “poor” wet seasons are observed, and associations with variations in large-scale atmospheric general circulation features are also examined. In particular, some regio...

Coupled Climate Simulation of the Evolution of Global Monsoons in the Holocene*

Abstract Evolution of global monsoons in the Holocene is simulated in a coupled climate model—the Fast Ocean Atmosphere Model—and is also compared with the simulations in another coupled climate model—the NCAR Climate System Model. Holocene climates are simulated under the insolation forcing at 3000, 6000, 8000, and 11 000 years before present. The evolution of six major regional summer monsoons is investigated: the Asian monsoon, the North African monsoon, the North American monsoon, the Australian monsoon, the South American monsoon, and the South African monsoon. Special attention has been paid to the relative roles of the direct insolation forcing and oceanic feedback. It is found that the responses of the monsoons to the insolation forcing and oceanic feedback differ substantially among regions, because of regional features of atmospheric and oceanic circulation and ocean–atmosphere interaction. In the Northern Hemisphere, the coupled models show a significant enhancement of all of the monsoons in th...

Enhanced Seasonal Rainfall in Northern Venezuela and the Extreme Events of December 1999

Abstract Torrential rainfall during December 1999 resulted in devastating floods and landslides along the northern coast of Venezuela. These events occurred in an area with a predominantly dry climate, took place during what is regionally the dry season, and were preceded by unusually heavy seasonal rainfall. An observational study was undertaken in an attempt to identify anomalous features of the climate system associated with both the enhanced seasonal rainfall prior to the landslides as well as the extreme December rainfall events that triggered them. Observational data for the period 1950–99 are used to provide historical context. Results indicate that the copious seasonal rainfall prior to the floods was associated with anomalous conditions in both the tropical Pacific and Atlantic basins similar to past events, although some features were unusually strong in 1999. The extreme daily rainfall in December 1999 was associated with atmospheric circulation features originating in the extratropical Northern Hemisphere that penetrated unusually far into the Tropics. Possible physical mechanisms acting to enhance rainfall on both timescales are discussed.

A case study of the anomalous Arctic sea ice conditions during 1990: Insights from coupled and uncoupled regional climate model simulations

The regional coupled atmosphere‐ice‐ocean model HIRHAM‐MOM has been developed to simulate the Arctic climate. The model has been applied for the year 1990, which was characterized by anomalous ice retreat along the eastern Arctic coasts. Many observed features of the anomalous atmospheric circulation and sea ice concentration pattern during spring to early summer are reproduced by the model. It was even able to generate the extreme summer sea ice conditions in the Eurasian Arctic and the associated high surface air temperatures. The model shows less success in simulating the large ice retreat in the Siberian Arctic in late summer. This is explained mainly by deviations in the simulated atmospheric circulations. Results from an ocean‐ice‐alone model confirm that the atmospheric forcing dominates the simulated ice retreat. On the other hand, a sensitivity study with the atmosphere‐alone model shows the impact of the sea ice conditions on the atmospheric circulation. Only with the accurate satellite‐derived sea ice data is the model able to reproduce the anomalous late summer atmospheric pressure patterns.

The 850 hPa relative vorticity centres of action for winter precipitation in the Greek area

AbstractIn this work, the relationship between atmospheric circulation over Europe and precipitation in Greece is studied during high winter (January–February). Specifically, the 850 hPa relative vorticity centres, affecting or related to precipitation in the various areas of Greece, are defined and the corresponding temporal variability of both parameters is examined. Factor analysis is first applied on both fields, in order to reduce the dimensionality of the original data sets. Canonical correlation analysis is then applied on the factor analysis results, in order to interrelate the two parameters. It is found that: (i) precipitation over western and northern Greece, the eastern Aegean islands and the western Turkish coasts is mainly controlled by vorticity over Italy and the Gulf of Genoa; (ii) southern Aegean Sea and Crete precipitation is controlled by relative vorticity west of Crete; and (iii) eastern mainland areas precipitation is significantly affected by a vorticity seesaw between Tunisia and the northern Aegean. The temporal variations of precipitation and relative vorticity parameters over the above areas show a precipitation reduction over most Greek stations, combined with enhanced anticyclonic activity over almost the whole Mediterranean during the late 1980s and the early 1990s, a period characterized by high North Atlantic oscillation index values. Finally, cluster analysis is applied on the factor scores time series of precipitation, in order to classify the winter precipitation anomaly patterns into objectively defined clusters. Six main precipitation patterns were revealed and the corresponding relative vorticity mean anomaly regimes over southern Europe indicate the atmospheric circulation characteristics causing these specific patterns. Copyright © 2003 Royal Meteorological Society

Wintertime regional aerosol distribution and the influence of continental transport over the Indian Ocean

During the Asian winter monsoon period, the Indian Ocean region is affected by large scale transport of aerosols from the dry Asian continent, which can influence the radiation budget of the region. The regional distribution of aerosols over the Indian Ocean and the influence of long-range transport during the Asian dry period of November–April of 1996–1999 are studied, based on satellite derived aerosol optical depth (AOD) and the wind data obtained from the NCEP-NCAR reanalysis. AOD over the Indian Ocean region is observed to be significantly influenced by the transport of aerosols from Indian subcontinent, Southeast Asia and Arabia. AOD as well as its latitude gradient in the Northern Hemisphere generally increase from November to April. High aerosol loading observed over the Indian Ocean during February–April period of 1999 is closely associated with the changes in atmospheric circulation features and transport from the Asian continent.

Validation and data characteristics of water vapor profiles observed by the Improved Limb Atmospheric Spectrometer (ILAS) and processed with the version 5.20 algorithm

Vertical profiles of water vapor concentration at high latitudes (57–72°N; 64–89°S) were observed by the Improved Limb Atmospheric Spectrometer (ILAS) solar occultation sensor aboard the Advanced Earth Observing Satellite (ADEOS). These measurements were made continuously from November 1996 through June 1997 with some additional periods in September to October 1996. A validation study of the water vapor data processed with the version 5.20 ILAS retrieval algorithm is presented in this paper. Uncertainty and general characteristics of the ILAS water vapor measurements are briefly reviewed. Comparisons are made with data obtained by (1) the ILAS validation balloon campaigns at Kiruna, Sweden and at Fairbanks, Alaska; (2) the aircraft measurements under the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign; and (3) available satellite measurements of the version 19 Halogen Occultation Experiment (HALOE) and the version 6 Stratospheric Aerosol and Gas Experiment II (SAGE II). The agreement between ILAS water vapor and all independent reliable correlative measurements in the altitude region of 15–60 km is better than 10% for the majority of cases and better than 20% for all comparisons, with the exception of some isolated cases detailed in this paper. Climatological comparisons of ILAS data with Upper Atmosphere Research Satellite (UARS) climatology and HALOE data show the overall consistency of ILAS water vapor data considering the known features of atmospheric circulation. The characteristics of ILAS measurements, i.e., high sampling frequency in polar latitudes with high vertical resolution along with the good data quality, make the ILAS water vapor data set valuable for various polar stratospheric research applications.

Universal spectrum of two-dimensional turbulence on a rotating sphere and some basic features of atmospheric circulation on giant planets.

The Kolmogorov-Batchelor-Kraichnan (KBK) theory of two-dimensional turbulence is generalized for turbulence on the surface of a rotating sphere. The energy spectrum develops considerable anisotropy; a steep -5 slope emerges in the zonal direction, while in all others the classical KBK scaling prevails. This flow regime in robust steady state is reproduced in simulations with linear drag. The conditions favorable for this regime may be common for giant planets' atmospheric circulations; the same steep spectra are found in their observed zonal velocity profiles and utilized to explain their basic characteristics.

Relative influences of the Interdecadal Pacific Oscillation and ENSO on the South Pacific Convergence Zone

The South Pacific Convergence Zone (SPCZ), one of the most extensive features of the global atmospheric circulation, is shown to vary its location according to both the polarity of the El Niño/Southern Oscillation (ENSO), and of the Interdecadal Pacific Oscillation (IPO). We first demonstrate that the IPO can be regarded as the quasi‐symmetric Pacific‐wide manifestation of the Pacific Decadal Oscillation that has been described for the North Pacific. Shifts in the position of the SPCZ related to ENSO on interannual time scales and to the IPO on decadal time scales appear to be of similar magnitude and are largely linearly independent. A station pressure‐based index of variations in SPCZ latitude is shown to be significantly related to the polarity of the IPO when ENSO influences are accounted for. Movements of this sensitive section of the SPCZ have occurred in phase with those of the IPO since the 1890s.

Climate simulations based on a different-grid nested and coupled model

An atmosphere-vegetation interaction model (A VIM) has been coupled with a nine-layer General Cir-culation Model (GCM) of Institute of Atmospheic Physics/State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (IAP/LASG), which is rhomboidally truncated at zonal wave number 15, to simulate global climatic mean states. A VIM is a model having inter-feedback between land surface processes and eco-physiological processes on land. As the first step to couple land with atmosphere completely, the physiological processes are fixed and only the physical part (generally named the SVAT (soil-vegetation-atmosphere-transfer scheme) model) of AVIM is nested into IAP/LASG L9R15 GCM. The ocean part of GCM is prescribed and its monthly sea surface temperature (SST) is the climatic mean value. With respect to the low resolution of GCM, i.e., each grid cell having lon-gitude 7.5° and latitude 4.5°, the vegetation is given a high resolution of 1.5° by 1.5° to nest and couple the fine grid cells of land with the coarse grid cells of atmosphere. The coupling model has been integrated for 15 years and its last ten-year mean of outputs was chosen for analysis. Compared with observed data and NCEP reanalysis, the coupled model simulates the main characteris-tics of global atmospheric circulation and the fields of temperature and moisture. In particular, the simu-lated precipitation and surface air temperature have sound results. The work creates a solid base on coupling climate models with the biosphere.

A Comparative Review of the Weather and Climate of the Southern Alps of New Zealand and the European Alps

Abstract A comparative review is provided of the weather and climate processes and phenomena that characterize the New Zealand Southern Alps and European Alps. The general climate conditions and atmospheric circulation features that affect the 2 mountain regions are assessed. Interaction of the mountains with synoptic weather systems is described, including their dynamic and thermal effects on airflow, such as the foehn and nor'wester. The different orientations of the mountain barriers are seen as creating differences between the 2 regions, including the high frequency of cyclogenesis south of the European Alps and the marked orographic effect on fronts along the east coast of New Zealand. Other distinctive features of the regional wind field are described and explained. Mountain effects on rainfall amounts and distribution are also briefly covered, including the very high precipitation received on the west coast of the New Zealand Southern Alps and the more even spread of precipitation north and south o...

Atmospheric Circulation Features Associated with Rainfall Variability over Southern Northeast Brazil

Abstract The atmospheric circulation features associated with rainfall variability during the main rainy period of the region of southern Northeast Brazil (sNEB) are determined through principal components analysis and cluster analysis. Daily rainfall, NCEP reanalysis, and OLR data are used from 1979 to 1997. The analyses are performed considering a large area including South America and the Pacific and Atlantic Oceans and also considering a regional area over tropical South America. The influence of synoptic systems, as well as the phenomena associated with their variability, such as El Nino–Southern Oscillation (ENSO) and baroclinic waves over the Pacific Ocean and South America are discussed. Positive (negative) rainfall anomalies over sNEB are related to cold (warm) episodes in the tropical Pacific. On the intraseasonal timescale there are interrelationships among the South Atlantic convergence zone (SACZ), Bolivian high, and the northeast upper-cyclonic vortex. It is noted that rainy periods in sNEB ...

Spatial patterns of pollen deposition in arctic snow

The pollen content of 77 snow samples, collected at 41 sites in the Canadian Arctic, the adjacent Arctic Ocean and Greenland can be used to identify source regions that produced the assemblages. The major vegetation zones of northern Canada produce distinctive pollen assemblages, and principal components analysis (PCA) indicate that these assemblages are retained even in snow on the sea ice surface. It is shown that pollen percentages and concentrations are related to the density of the regional vegetation and to the distance to the source of more productive regions. Because the pollen grains may be transported for great distances (even to the central regions of the Arctic Ocean), they may be used to indicate the source of that pollen and the trajectory of the air masses that carried and deposited them. Pine is particularly valuable in this sense because it has longer trajectories than other tree pollen. For example, there are indications of “over‐the‐pole” transport of pollen from higher pine pollen concentrations at the North Pole than on northern Ellesmere Island. Pollen concentrations of certain taxa change significantly at ∼75°N, north of which the concentrations become lower, thereby suggesting that there is a climatic boundary at that latitude. Therefore it would appear that studies of the concentration of pollen in snow have the potential for determining past and present characteristics of atmospheric circulation and also for helping in the development and interpretation of paleoenvironmental records in regions without vegetation, such as ice caps.