Changes In Geopotential Height Research Articles

Changes in the Relationship between ENSO and the Winter Arctic Stratospheric Polar Vortex in Recent Decades

Abstract The impact of El Niño–Southern Oscillation (ENSO) on the winter stratospheric polar vortex intensity (PVI) is re-examined, using JRA-55 datasets and the CESM2 model. Our results show that the negative correlation between ENSO and PVI has weakened in recent decades and is no longer statistically significant after the mid-1990s. This weakening ENSO–PVI relationship after the mid-1990s is associated with wave-1 fluxes entering the stratosphere; that is, the ENSO-induced increasing wave-1 fluxes are weaker after the mid-1990s than before the mid-1990s. Except for the Pacific–North America (PNA)-like wave train, the stratospheric wave flux changes are related to ENSO-induced geopotential height changes over East Asia. The ENSO-induced tropical zonal circulation anomalies are enhanced from the central Pacific to East Asia, leading to more substantial wave flux anomalies propagating westward from the central Pacific to East Asia and a more significant high center over Northeast Asia after the mid-1990s than before the mid-1990s. This ENSO-induced high center after the mid-1990s is out of phase with the climatological center of wave 1, leading to a relatively weaker increase in wave 1. Model results show that changes in the ENSO–PVI relationship and ENSO–Asia circulation are associated with changes in ENSO-related SST combined with global mean SST. Significance Statement In this study, changes in the ENSO–polar vortex relationship and the relevant mechanisms are analyzed from the perspective of the ENSO-induced East Asia circulation. The results can help us better understand and predict variations in the stratospheric polar vortex.

Variability and trends in surface solar spectral ultraviolet irradiance in Italy: on the influence of geopotential height and lower-stratospheric ozone

Abstract. The short- and long-term variability of the surface spectral solar ultraviolet (UV) irradiance is investigated across Italy using high-quality ground-based measurements from three locations: Aosta (45.7∘ N, 7.4∘ E, 570 m a.s.l.), Rome (41.9∘ N, 12.5∘ E, 15 75 m a.s.l.), and Lampedusa (35.5∘ N, 12.6∘ E, 50 m a.s.l.). The three sites are characterized by different environmental conditions and represent almost the full latitudinal extent of the Italian territory. Data of two periods were analysed: 2006–2020 (all sites) and 1996–2020 (Rome only). The main objective of this study is to quantify the effect of the geopotential height (GPH) at 250 hPa on total ozone, and spectral irradiance at 307.5 and 324 nm. We first show that monthly anomalies in GPH, total ozone, and spectral irradiances are correlated amongst the three sites, suggesting that Italy is often affected by the same synoptical weather systems. We further find statistically significant anticorrelations between GPH and monthly anomalies in total ozone for all stations and months. Conversely, we identify positive correlations between GPH and monthly anomalies in spectral irradiance at 307.5 nm for most months. The influence of GPH on short-term variability also hold for long-term trends. For example, long-term changes in total ozone over the period 2006–2020 were associated with changes in GPH for all stations. This suggests that observed negative trends in total ozone were mainly driven by changes in lower-stratospheric ozone as upper-stratospheric ozone was increasing over this period. For several months of the year, positive trends in UV irradiance were observed, and we found that these trends were predominantly caused by changes in clouds and/or aerosols instead of total ozone. For the longer period of 1996–2020, a statistically significant annualized decrease in total ozone of ∼ 0.1 % per year was identified for Rome and could subsequently be attributed to decreasing lower-stratospheric ozone. While positive trends in spectral irradiance at 307.5 nm were observed for several months of this extended period, the negative trend in total ozone did not lead to a positive trend in the spectral irradiance at 307.5 nm in the deseasonalized data. Our study provides evidence that dynamical processes taking place in the troposphere lead to significant variability in total ozone and surface solar UV irradiance.

Clustering of different atmospheric patterns leading to an increase in different types of atmospheric pollutants in the greater Tehran area during the last 17 years (2004-2020)

Introduction: Air pollution is one of the main problems of industrial and large cities like Tehran, due to its harmful effects, especially on human health. Materials and methods: Based on the intensity and continuity of the standard air pollution index during 2004-2020 for 112 cases in Tehran, the principal component analysis method was applied in T-mode and clustering of days for similar 500hPa geopotential height changes were provided. The synoptic patterns leading to these different pollution cases were obtained. Results: The study of the mentioned days shows the activity of five atmospheric patterns with the frequency of 47, 28, 12, 13, and 4%, respectively. Conclusion: The first pattern happens during the cold season, associated with the simultaneous placement of the atmospheric ridge along with Siberian high-pressure system. The second group, which occurs mostly in summer and during late spring, is controlled with the predominant ozone pollutant. The main activity of the third pattern occurrences in summer and late spring, which is associated with the presence of monsoon Indian low-pressure on the southeastern regions in Iran. The fourth pattern, which occurs mostly in spring and autumn, associates with the establishment of dynamic low pressure in the western regions of Iran and Iraq, as well as the establishment of the thermal low-pressure system in the central part of the country. The last group is a local pattern that occurs due to the passage of the gust front and the shear of the wind field dust rise in the Tehran.

Diagnosis of meteorological factors associated with recent extreme rainfall events over Burundi

This study aims to identify the main causes of extreme rainfall events specifically recent ones that occurred in Central East Africa, which seriously affected the western part of Burundi causing 77 deaths, destruction of infrastructures, loss of properties and hectares of lands in February, 2014. To understand underlying issues about extreme rainfall occurrence and its key drivers, the present study investigated two distinct extreme rainfall events that caused massive losses in the western part of Burundi on November 11th, 2009 and February 9th, 2014. Different datasets have been used to depict which one captured reliable information of the two extreme rainfall events investigated. Analysis of different weather parameters was conducted using the European Centre for Medium-Range Weather Forecast (ECMWF) datasets. The Weather Research and Forecasting (WRF) Model was also employed to analyse different parameters over different periods, thus prior, during, and after the occurrence of an extreme rainfall event. It was found that the two distinct rainfall events were induced by the combination of local convection enhanced by moist air from the Congo basin and abnormal conditions in the mid-upper layer. Instabilities in the lower and mid-upper layers mainly emanated from the changes in geopotential height and wind circulation. Local physical factors especially the topography and the presence of the Lake Tanganyika localized in the Western Rift Valley of Africa enhanced local convection which triggered rapid condensation that induced a heavy rainfall over the western part of Burundi. Weather conditions vary depending on the time or season of occurrence of the extreme rainfall events. This study will propel future studies on historical diagnosis and forecasting of extreme weather events over Burundi. It will also guide policy-makers to mitigate negative impacts expected from future occurrences of extreme weather events.

Zonally asymmetric trends of winter total column ozone in the northern middle latitudes

Using various satellite-based observations, a linear ozone transport model (LOTM), a chemistry-climate model (WACCM3) and an offline chemical transport model (SLIMCAT), zonally asymmetric trends of the total column ozone (TCO) in the northern middle latitudes during winter for the period 1979–2015 are analyzed and factors responsible for the trends are diagnosed. The results reveal that there are significant negative TCO trends over the North Pacific and positive TCO trends over the northwestern North America. The zonally asymmetric TCO trends are mainly contributed by the trends in partial column ozone in the upper troposphere and lower stratosphere (UTLS) which are closely related to the long-term changes of geopotential height in the troposphere. Furthermore, the trends of geopontential height in the UTLS are mainly modulated by pattern changes in the Arctic Oscillation (AO), the Cold Ocean–Warm Land (COWL) and the North Pacific (NP) index. Accordingly, the zonally asymmetric TCO trends can be largely reconstructed by the trends of the above three teleconnection patterns. Sea surface temperature (SST) changes over the Pacific Ocean and the Atlantic Ocean can also exert a significant contribution to the zonally asymmetric TCO trends through their influence on the COWL and NP patterns. In addition, chemical ozone loss partially offsets the positive trends in zonal TCO anomalies over Central Siberia and enhances the positive TCO trends over northwestern North America. However, the contribution of chemical processes to the zonally asymmetric TCO trends is relatively smaller than that of dynamical transport effects. Interpreting the zonally asymmetric TCO trends and their responsible factors would be helpful for accurately predicting the stratospheric ozone return date in the northern middle latitudes.

Effects on winter circulation of short and long term solar wind changes

Indices of the North Atlantic Oscillation and the Arctic Oscillation show correlations on the day-to-day timescale with the solar wind speed (SWS). Minima in the indices were found on days of SWS minima during years of high stratospheric aerosol loading. The spatial distribution of surface pressure changes during 1963–2011 with day-to-day changes in SWS shows a pattern resembling the NAO. Such a pattern was noted for year-to-year variations by Boberg and Lundstedt (2002), who compared NAO variations with the geo-effective solar wind electric field (the monthly average SWS multiplied by the average southward component, i.e., negative Bz component, of the interplanetary magnetic field). The spatial distribution of the correlations of geopotential height changes in the troposphere and stratosphere with the SWS; the geo-effective electric field (SWS∗Bz); and the solar 10.7cm flux suggests that solar wind inputs connected to the troposphere via the global electric circuit, together with solar ultraviolet irradiance acting on the stratosphere, affect regional atmospheric dynamics.

Future trend of extreme value distributions of wintertime surface air temperatures over Korea and the associated physical changes

Daily winter temperatures in Korea have been analyzed via CSEOF analysis. Then, each PC time series was detrended and was fitted to an AR (autoregressive) model. Based on the identified AR model, an artificial time series of arbitrary length can be generated by using an arbitrary white-noise time series. In this way, one hundred new sets of PC time series were generated over the period of 1973–2058. Then, the trend for each PC time series was added back to the artificial PC time series extending the trend until 2058. Ultimately, artificial daily winter temperatures in Korea have been constructed by using the artificial PC time series and the original loading vectors derived from the observational data. The 100 new data sets have been investigated in order to understand the winter temperature variability 50 years into the future. Regression analysis in CSEOF space shows that temperature increase in Korea is associated with increased 850-hPa air temperature over most of the Asian domain (97°-153°E × 22°-73°N) and increased 850-hPa geopotential height in the southern part of the domain. As a result, southerly and southeasterly wind anomalies develop carrying positive temperature anomalies northward and northwestward. Both the 200-hPa air temperature and geopotential height changes indicate that there will be fairly significant northward shift of the jet stream in future. The standard deviation of the 200-hPa potential vorticity increases implying that shortwave trough and henceforth baroclinic instability will increase in future. Finally, GEV (Generalized Extreme Value) distribution and GPD (Generalized Pareto distribution) distribution have been compared between the observational records and the future records of the same length. The extreme value distributions based on the synthetic datasets show that warm extreme events will be more extreme in future and cold extreme events, on the other hand, will be less extreme. This study provides an estimate of future temperatures based on the observational data and serves as an independent baseline solution for comparisons with numerical model solutions.

Impact of a Reduced Arctic Sea Ice Cover on Ocean and Atmospheric Properties

Abstract The Arctic sea ice cover declined over the last few decades and reached a record minimum in 2007, with a slight recovery thereafter. Inspired by this the authors investigate the response of atmospheric and oceanic properties to a 1-yr period of reduced sea ice cover. Two ensembles of equilibrium and transient simulations are produced with the Community Climate System Model. A sea ice change is induced through an albedo change of 1 yr. The sea ice area and thickness recover in both ensembles after 3 and 5 yr, respectively. The sea ice anomaly leads to changes in ocean temperature and salinity to a depth of about 200 m in the Arctic Basin. Further, the salinity and temperature changes in the surface layer trigger a “Great Salinity Anomaly” in the North Atlantic that takes roughly 8 yr to travel across the North Atlantic back to high latitudes. In the atmosphere the changes induced by the sea ice anomaly do not last as long as in the ocean. The response in the transient and equilibrium simulations, while similar overall, differs in specific regional and temporal details. The surface air temperature increases over the Arctic Basin and the anomaly extends through the whole atmospheric column, changing the geopotential height fields and thus the storm tracks. The patterns of warming and thus the position of the geopotential height changes vary in the two ensembles. While the equilibrium simulation shifts the storm tracks to the south over the eastern North Atlantic and Europe, the transient simulation shifts the storm tracks south over the western North Atlantic and North America. The authors propose that the overall reduction in sea ice cover is important for producing ocean anomalies; however, for atmospheric anomalies the regional location of the sea ice anomalies is more important. While observed trends in Arctic sea ice are large and exceed those simulated by comprehensive climate models, there is little evidence based on this particular model that the seasonal loss of sea ice (e.g., as occurred in 2007) would constitute a threshold after which the Arctic would exhibit nonlinear, irreversible, or strongly accelerated sea ice loss. Caution should be exerted when extrapolating short-term trends to future sea ice behavior.

Atmospheric Climate Change Detection by Radio Occultation Data Using a Fingerprinting Method

Abstract The detection of climate change signals in rather short satellite datasets is a challenging task in climate research and requires high-quality data with good error characterization. Global Navigation Satellite System (GNSS) radio occultation (RO) provides a novel record of high-quality measurements of atmospheric parameters of the upper-troposphere–lower-stratosphere (UTLS) region. Because of characteristics such as long-term stability, self calibration, and a very good height resolution, RO data are well suited to investigate atmospheric climate change. This study describes the signals of ENSO and the quasi-biennial oscillation (QBO) in the data and investigates whether the data already show evidence of a forced climate change signal, using an optimal-fingerprint technique. RO refractivity, geopotential height, and temperature within two trend periods (1995–2010 intermittently and 2001–10 continuously) are investigated. The data show that an emerging climate change signal consistent with the projections of three global climate models from the Coupled Model Intercomparison Project cycle 3 (CMIP3) archive is detected for geopotential height of pressure levels at a 90% confidence level both for the intermittent and continuous period, for the latter so far in a broad 50°S–50°N band only. Such UTLS geopotential height changes reflect an overall tropospheric warming. 90% confidence is not achieved for the temperature record when only large-scale aspects of the pattern are resolved. When resolving smaller-scale aspects, RO temperature trends appear stronger than GCM-projected trends, the difference stemming mainly from the tropical lower stratosphere, allowing for climate change detection at a 95% confidence level. Overall, an emerging trend signal is thus detected in the RO climate record, which is expected to increase further in significance as the record grows over the coming years. Small natural changes during the period suggest that the detected change is mainly caused by anthropogenic influence on climate.

Snow and ice facies variability and ice layer formation on Canadian Arctic ice caps, 1999–2005

Time series of enhanced resolution data from the SeaWinds scatterometer aboard QuikScat were used to map the distribution of snow and ice surface facies and ice layer formation in the percolation zone on ice caps in the Queen Elizabeth Islands during the period 1999–2005. Iterative Self‐Organizing Data Analysis classification of the mean postfreeze‐up biweekly average σ0 signal for the 7‐year period resulted in the delineation of four snow and ice surface facies (interpreted as the percolation, saturation, superimposed ice, and glacier ice zones). Analysis with National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis reveals that changes in geopotential height in the troposphere (700, 500, and 300 hPa) and air temperature (700 hPa level) are positively (negatively) correlated with area changes in the glacier ice (percolation and saturation) zones and changes in facies boundary elevation. The change in biweekly‐averaged backscatter following the freeze‐up periods between successive autumns was used to map changes in the distribution of ice layers formed by meltwater percolation and refreezing in the snowpack within the percolation zone. Strongly positive air temperature anomalies at the 700 hPa level in 2001 and 2005 are consistent with extensive ice layer formation in the percolation zones of all ice caps. Such large interannual changes in ice layer formation are likely associated with large changes to the density profile of the snowpack and may be associated with surface elevation changes that are unrelated to changes in surface mass balance.

Moisture budget variations in the Yangtze River Basin, China, and possible associations with large-scale circulation

We analyzed seasonal and annual variations of the whole layer atmospheric moisture budget and precipitation during 1961–2005 and their associations with large-scale circulation in the Yangtze River basin, China. The results indicated increasing moisture budget in summer and winter, but decreasing moisture budget in spring and autumn. Positive correlations between moisture budget and precipitation illustrate tremendous impacts the moisture budget has on the precipitation changes across the Yangtze River basin. In terms of seasonal variations, significant correlations were observed between precipitation and moisture budget in spring and autumn in the upper Yangtze River basin. Besides, we also analyzed changes of geopotential height. The positive trends of the geopotential height (850 hPa) were observed in the East Asia and the negative trends in the middle and west Pacific Ocean, indicating increasing geopotential height from south to north in east Asia which largely limited the moisture propagation to north China. While decreasing meridional geopotential height from west to east along the Yangtze River basin caused more moisture propagation from the west to the east parts of the study region, which may benefit more precipitation in the middle and lower Yangtze River basin.

Synoptic‐based evaluation of climatic response to vegetation change over southern Africa

AbstractThe results of regional climate model (RCM) simulations of the effects of vegetation change in southern Africa are analysed to asses the role of synoptic forcing in land–atmosphere interactions. A self‐organizing map (SOM) is used to identify the dominant large‐scale features in the atmospheric boundary conditions used to force the RCM. The fields used to characterize the large‐scale circulation are geopotential height at 850 and 500 hPa and total precipitable water between these two levels. For each of the patterns (nodes) identified in these variables by the SOM, the mean RCM‐simulated response to vegetation change is evaluated. Notable differences are seen in the response of precipitation, near‐surface temperature and geopotential heights to the land surface change between different nodes. Conditions characterized by strong sub‐tropical anticyclones and low atmospheric moisture show the greatest temperature and geopotential height changes and are most sensitive to changes in radiative fluxes, whereas precipitation and surface hydrological processes are more sensitive under conditions of weak subsidence and high levels of atmospheric moisture. Copyright © 2009 Royal Meteorological Society

Impacts of coastal SST variability on the East Asian summer monsoon

The impacts of the seasonal and interannual SST variability in the East Asia coastal regions (EACRSST) on the East Asian summer monsoon (EASM) have been examined using a regional climate model (PσRCM9) in this paper. The simulation results show that the correlation between the EACRSST and the EASM is strengthened after the mid-1970s and also the variability of the EACRSST forcing becomes much more important to the EASM interannual variability after the mid-1970s. The impacts of the EACRSST on the summer precipitation over each sub-region in the EASM region become weak gradually from south to north, and the temporal evolution features of the summer precipitation differences over North and Northeast China agree well with those of the index of EASM (IEASM) differences. The mechanism analyses show that different EACRSST forcings result in the differences of sensible and latent heat flux exchanges at the air-sea interface, which alter the heating rate of the atmosphere. The heating rate differences induce low level air temperature differences over East Asia, resulting in the differences of the land-sea thermal contrast (LSTC) which lead to 850 hPa geopotential height changes. When the 850 hPa geopotential height increases over the East Asian continent and decreases over the coast of East China and the adjacent oceans during the weakening period of weakens consequently. On the contrary, the EASM enhances during the strengthening period of the LSTC.

Summertime tropospheric ozone columns from Aura OMI/MLS measurements versus regional model results over the United States

Ozone columns below 147 hPa are derived over the United States from September 2004 to August 2005 from the differences between clear‐sky Aura OMI columns and coincident MLS columns. The mean difference from coincident ozonesonde measurements at four USA sites is 0.3 DU with an rms difference of 10.1 DU and a correlation coefficient of 0.67. Semimonthly patterns of the columns over the USA for the summer of 2005 have been produced. The observed columns, as well as Regional Air Quality Forecast (RAQAST) model columns, show high values over the southeastern USA and its surrounding oceans. Changes of these columns exceeding 6 DU in many places were observed between June 17–30 and July 1–16 and the changes reversed in the following two‐week period. Comparisons against calculations from the RAQAST model, as well as correlations with geopotential height changes at 147 hPa, indicate that these changes were primarily related to dynamics.

Systematic errors of IMD Operational NWP model

The systematic errors of IMD (India Meteorological Department) limited area model over Indian region using 48 hours forecast fields for winter and summer seasons are examined in this paper. The study reveals that the core of the sub tropical westerly jet and tropical easterly jet are well simulated in the zonal component of wind. In case of meridional component of wind significant difference is noticed between forecast and analysis. In the upper tropospheric levels (above 100 hPa) northerly biases are noticed. Thermal structure of the model exhibits cool bias in middle and lower tropospheric levels over the northern latitudes. The nature of model biases of the geopotential height changes with the season. Model drying over the tropical belt and moistening towards the northern latitudes are found in all the seasons studied.

A discussion of upper stratospheric ozone asymmetries and SAGE trends

A chemical box model constrained by Upper Atmosphere Research Satellite (UARS) measurements of CH4, H2O, NOx, and temperature in 1992–1994 has been used in an attempt to simulate ozone trend measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) at 43 km, 45° latitude and their hemispheric differences. The model is successful in simulating the seasonal cycle in ozone mixing ratios observed by the Microwave Limb Sounder (MLS) and the Solar Backscattered Ultraviolet (SBUV) instrument including the 15% larger values in the Southern Hemisphere at the wintertime ozone maximum. The model indicates that this hemispheric asymmetry is associated with hemispheric temperature differences of approximately 10°K; in contrast, hemispheric differences in methane and the resulting effects on ClOx make only a small contribution to this ozone asymmetry. The model predicts that, based on observed Cly increases, the ozone decrease in the Southern Hemisphere should have exceeded that in the Northern Hemisphere by approximately 1%/decade. This is consistent with an observed ozone hemispheric trend difference of 1.7 ± 2.1%/decade (1σ) for 1979–1997 and −0.8 ± 2.8%/decade for 1985–1997 from SAGE I and SAGE II (version 6.1) measurements. These trend differences are caused by hemispheric differences in temperature in early winter and in CH4 in late winter. The model, based on changes in Cly but not in other specified parameters, calculates a larger downtrend of ozone than is observed by SAGE II in 1985–1997 by approximately 3.1 ± 1.4%/decade (1σ), but it qualitatively simulates the observed ozone trend variations over the period 1979–1997. The amplitude of the model calculated downtrend would be reduced by approximately 1%/decade for a methane increase of 1%/yr. Observed temperature decreases of approximately 0.1°K/yr would have decreased the magnitudes of the calculated ozone downtrends on pressure surfaces by approximately 1%/decade but could have affected SAGE trends by a small amount in the opposite direction because of associated geopotential height changes.

Tropospheric response to stratospheric ozone loss

The response to realistic total column ozone trends on the troposphere and the stratosphere as simulated by the ARPEGE General Circulation Model (GCM) has been investigated. In both hemispheres, the lower stratosphere cooled and the polar vortex strengthened significantly during spring/early summer. The cooling trend was weaker than the observed trend in the Northern Hemisphere (NH), but stronger than the observed trend in the Southern Hemisphere (SH). In the troposphere, the changes in geopotential height resembled the positive phase of the Arctic Oscillation (AO) in the NH in March and the positive phase of the Antarctic Oscillation (AAO) in the SH during summer (December–February).

夏季中国大陸の気候変動と大気循環, 熱帯対流活動との関係

This paper is concernd with interannual and interdecadal variabilities of summer rainfall and temperature patterns in China and their association with 500 hPa height in the Northern Hemisphere (NH), tropical convective activities and global sea surface temperature anomaly (SSTA). The temporal evolutions and spatial structures of interannual variation of summer (JJA) rainfall and temperature from 1951 to 1994 over China are revealed through EOF analysis. The spatial pattern of EOF1 for rainfall (EOF1.R) is dominated by a maximum over the middle-lower reaches of the Yangtze River, and a large negative value region in the middle reach of the Yellow River is also obvious. The spatial pattern of EOF1 for temperature (EOF1.T) reflects coherent variations over most regions of China, and it is dominated by a maximum over the middle-lower reaches of the Yangtze River. Linear increase and decrease trends are found in the time coefficients of EOF1.R and EOF1.T, respectively. The quasi-biennial oscillation (QBO) signal is also strong after the middle of the 1970's in repect of their time coefficients. The coupled patterns of rainfall and temperature are picked up through the singular value decomposition (SVD) analysis. The spatial patterns and their temporal evolutions of SVD1 for rainfall (SVD1.R) and SVD1 for temperature (SVD1.T) are quite similar to those of EOF1.R and EOF1.T. There is an abrupt change in the middle 1970's in the time coefficients of SVD2.R and SVD2.T. The variations of summer rainfall and temperature coupled patterns in China are closely connected with the 500 hPa height anomaly over the Northern Hemisphere (NH). The Pacific-Japan (PJ) and Eurasia (EU) teleconnection patterns play a very important role in the spatial patterns of SVD1.R and SVD1.T, especially in the East Asia monsoon region along the middle-lower reaches of the Yangtze River. The abrupt change of China summer climate in the middle 1970's is related with the intensification and southerly location of the western Pacific subtropical high and also the geopotential height changes over Eurasia and in the regions to the north of the Japan Sea in 1977 or 1978. Correlations between the summer rainfall and temperature coupled patterns and monthly-averaged outgoing longwave radiation (OLR) and high-cloud amount (HCA) data are significant with the PJ teleconnection pattern. There exist positive correlations between the coupled patterns and sea surface temperature anomaly (SSTA) in the North Pacific and the tropical western Pacific. A comparison study shows that there are coherent variations between summer rainfall in the middle-lower reaches of the Yangtze River and in the western part of Japan. It is also demonstrated that there are close correlations between the summer temperature variations in China and in Japan.

On the definition and persistence of blocking

In this paper, we have investigated under what conditions positive geopotential height anomalies can be interpreted as blocking structures. Furthermore, we have studied the persistence of blocking. The data used here are the observed daily 500 hPa geopotential heights for 10 winter seasons from 1982/83 to 1991/92. The objective blocking criteria of Dole (1978) have been compared with the subjective blocking criteria established by Rex (1950a, b). Our study indicates that if we want to relate positive geopotential height anomalies to blocking as defined by Rex, not only the amplitude, but also the latitudinal position of the anomalies is important. Positive anomalies centered at about 45°N represent a northward extension of the subtropical anticyclone. Large positive anomalies centered at about 60°N are accompanied with negative anomalies at low latitudes and blocking anticyclones centered at about 50°N. We found that blocking and strong zonal flow have the same anomaly pattern but with opposite sign. The day-to-day changes in geopotential height associated with blocking are computed and the relevance of these day-to-day changes to the persistence of the blocking pattern is examined. We find that the day-to-day changes are smaller than normal over the blocking area, but larger than normal north of it. The anomalous day-to-day changes are mainly caused by the northward shift of the jet stream and the corresponding shift in the track of the transient weather disturbances. The strength of the day-to-day changes depends very much on the exact latitudinal location where the changes are computed. These results indicate that the day-to-day changes in geopotential height do not contain information about the persistence of the blocking pattern itself, but merely about the shift of the transient eddies caused by the blocking pattern. Hence, we can not simply interpret the persistence of blocking in terms of the day-to-day changes in positive anomalies. The persistence of blocking and strong zonal flow has been examined. As blocking and strong zonal flow are associated with approximately the same anomaly pattern but with opposite sign, the persistence of blocking and strong zonal flow can be expressed in terms of the duration of the corresponding anomaly patterns. We have shown that the average duration of the blocking and strong zonal flow patterns is approximately the same. This leads us to conclude that although the daily changes in geopotential height are small near the center of the blocking high, the blocking regime itself is not more persistent than the strong zonal flow regime. The persistence of blocking and strong zonal flow has also been compared with a first-order Markov process. Generally speaking, we find no significant differences in persistence between the blocking and strong zonal flow regimes and the first-order Markov process. This leads us to conclude that blocking is not more persistent than other atmospheric anomalies of the same geographical scale. This is in agreement with Lindzen (1986). DOI: 10.1034/j.1600-0870.1994.t01-2-00004.x

The Madden-Julian Oscillation in an Idealized General Circulation Model

Abstract The structure of the intraseasonal oscillations in the tropics of an idealized general circulation model with a zonally symmetric climate is described. Space-time spectra show a peak in zonal winds and velocity potential at the equator in zonal wavenumbers 1 and 2, corresponding to eastward-propagating power at phase speeds of ≈18 m s−1. This speed is significantly greater than that of the observed oscillation but comparable to that obtained in similar models by Hayashi and Sumi and Swinbank et al. The corresponding eastward-propagating power in the precipitation spectrum is concentrated in wavenumbers 2–5. A composite procedure is used to describe the three-dimensional structure of the model's oscillation. The oscillation is characterized by circulation cells oriented along the equatorial zonal plane, with enhanced precipitation in the region of rising motion. Zonal wind changes tend to be positively correlated with geopotential height changes at the same level. Positive perturbations in the wat...