A MASCOTTE-based characterization of MCSs over Spain, 2000–2002

A 3-year Mesoscale Convective Systems (MCSs) database, which extends from 2000 to 2002, has been built for the Iberian Peninsula and the Balearic Islands using the objective method MASCOTTE (MAximum Spatial COrrelation Technique). It was originally developed to track the evolution of convective systems over the Amazon region; after modifications, it is able to track MCS evolution even with an hour and a half of missed images and provides essential information of both dynamical and morphological characteristics of MCSs. MASCOTTE is tested against a visual and subjective method, and is found to offer advantages such as automation and a simple and efficient operation that make it very useful for building large MCS databases.Thirty-five MCSs were found between June and October, most of them originated along the Mediterranean coast and near the Pyrenees, showing an increasing occurrence from June to September, when the maximum is found. The regions most influenced by MCS occurrences are Balearic Islands, Valencia, Catalonia, Murcia and the Basque Country. The MCSs tend to be small, short-lived and linear, usually moving eastward or northeastward with low velocities.The MCSs-associated precipitation presents high variability, ranging from 80 to 0 mm h⁻¹ as maximum hourly records. Two different convective regions are identified based on MCS behavior in extreme precipitation events: Northern Spain (the Basque Country) with abundant and continuous precipitation regime but little MCS influence, and the Mediterranean coast, where precipitation is sporadic but much more intense.     

Mesoscale convective systems along the Meiyu frontin a numerical model

Summary  Two organized mesoscale convective systems (MCSs) developed sequentially along the Meiyu front over the Yangzi-Huai River basin and caused severe flooding over eastern China during 12–13 June 1991. In this paper, the structure and evolution of these MCSs are studied with a high-resolution (18 km) numerical simulation using the Fifth Generation Penn-State/NCAR Mesocale Model (MM5). The model reproduced the successive development of these two MCSs along the Meiyu front. The evolution of these MCSs was recorded clearly on satellite-derived cloud-top black body temperature (T _bb ) maps. A mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) were simulated, respectively, to the south and east of each of these two MCSs. Our analyses shows that the mLLJ and mULJ were formed as a responses to the intense convection associated with the MCS. The mLLJs transported warm, moist air with equivalent potential temperature greater than 352 K into the MCSs, and strong low-level convergence can be identified on the left-front end of the mLLJ. This strong convergence was associated with intense upward motion in the MCS with speed up to 80 cm s⁻¹. Much of inflow into the MCSs extends up to the middle and upper troposphere, and ventilated through the mULJ. The development of the MCSs was also associated with substantial increase in potential vorticity (PV). The build up of PV in the lower-level along the Meiyu front was in turn related to a local intensification of the frontal equivalent potential temperature gradient, suggesting a relationship between the MCSs and the local enhancement and cyclogenesis of the front. In a sensitivity experiment without the effect of latent heating, a series of ascent centers with average separation of about 300 km were simulated. This result suggests that the initial formation of the MCSs along the Meiyu front could occur in absence of moist-diabatic process. Since the horizontal velocity gradient across the Meiyu front near the synoptic-scale low-level jet (LLJ) was quite large while the corresponding temperature gradient across the frontal zone was rather weak, we speculate that barotropic process may be responsible for triggering these MCSs along the Meiyu front. Received December 28, 1999 Revised May 11, 2000     

A Case of Convection Development over the Western Mediterranean Sea: A Study through Numerical Simulations

Summary  A convective case producing heavy precipitation in the western Mediterranean region, characterized by pronounced upper level forcing and main rainfall over the sea, is studied. On the day of the event (September 28^th, 1994), more than 140 mm of precipitation were recorded in coastal lands of eastern Spain, and 180 mm were estimated over the sea with radar data. Synoptically, the case appears to combine warm and moist easterly advection at low levels, typically observed in torrential rainfall events of the region, with a less common strong upper level dynamical forcing. A set of mesoscale numerical simulations using the Hirlam model is performed to investigate the mechanisms responsible for the convection development, and to assess the influence of the orography on the rainfall field. Model output diagnosis indicates that in addition to the lower level forcing, a two-jets interaction is decisive for the triggering and driving of the convection during the event. Moreover, a non-topographic simulation reveals a relatively weak influence of the orography on this event when compared with other similar heavy precipitation cases in eastern Spain. Previous studies have shown an orographic influence of more than 90% on the rainfall whereas in this case about 50% of the precipitation over the area is attributed to the orographic forcing. The study is extended with an analysis of the individual effects of the Atlas and Iberian Peninsula, by means of a factor separation technique. It is shown that the Atlas range induces a redistribution of the precipitation over the Mediterranean, whereas local enhancements can be attributed to the Iberian topography. Received March 2, 1999     

Mesoscale Convective System over the Yellow Sea – A Numerical Case Study

Summary  A mesoscale convective system (MCS) case that developed over the Yellow Sea (12–13 July 1993) is studied by using a 23-level, 30 km-mesh Penn State/NCAR mesoscale model MM5. This MCS was generated in northern China, south of the Changma front, in a convectively unstable environment, under the influence of a short-wave trough accompanied by a marked cold vortex aloft. The model with all model physics (refereed to as CNTL) captured the major features of this MCS. A mesoscale low-level jet (mLLJ), with a horizontal scale of a few hundred km, developed within the MCS. Available wind data support the realism of this mLLJ. This mLLJ not only transports convectively unstable air directly toward the MCS but is also responsible for a strong low-level convergence in the MCS. At 200 hPa, an anticyclonic northwesterly flow with a relatively high wind speed core on the east of MCS was simulated. This relatively high-speed flow can be regarded as a mesoscale upper level jet (mULJ), acted as an upper outflow over the MCS. Low-level convergence on the left-front of the mLLJ and upper divergence in the right-rear of the mULJ creates a strong upward motion (≅ 40 cm s⁻¹) in the MCS. Heavy precipitation up to 45 mm between 1800–2100 UTC was observed after this MCS landed on the southern Korean Peninsula. The CNTL run captured this heavy rainfall event. A maximum rainfall of 50 mm 3 h⁻¹ was simulated. In another experiment, with surface sensible and moisture fluxes withheld (NOSF), the 3-h simulated rainfall was decreased to 30 mm. Less latent heat released in the NOSF led to a weaker MCS and mLLJ. The concurrent surface fluxes sustained a high low-level moisture field over the Yellow Sea, which helped the development of the MCS and enhanced its precipitation in this case. Received January 8, 1999     

Monitoring of Mesoscale Precipitation Systems in the Alps and the Northern Alpine Foreland by Radar and Rain Gauges

Summary.  Mesoscale convective precipitation systems in the Alpine region are studied by analyzing radar and rain gauge data. The data from weather radars in Austria, France, Germany, and Switzerland are combined into a composite. Availability of radar data restricts the study mainly to the northern part of the Alpine region. Mesoscale convective systems (MCS) occur often in this region and are comparable to large systems observed in the USA. Seven precipitation events lasting one to six days from the years 1992–1996 are examined in detail. They all moved west to east and showed no diurnal preference in formation or dissipation. They reach sizes of 2 − 6 · 10⁴ km². MCS with leading-line trailing-stratiform structure tended to be larger and more intense. A 25-year set of rain gauge data indicates that a giant MCS (covering more than 4 · 10⁴ km² with more than 30 mm/day) occurs every 6 years in the northern Alpine region. MCS occur more frequently in the southern Alpine region. Received February 25, 1999/Revised June 29, 1999     

Development of the water budget for three extra-tropicalcyclones with intense rainfall over Europe

Summary  Three cyclones developing between 28 August and 6 September 1995 were studied with respect to the temporal evolution of their water budget components. The cyclones were simulated with the regional model REMO. Water budget values were determined from hourly model output for circle areas with 500 km radius around the pressure minimum. The results show a maximum liquid water path of about 0.12 kg m⁻² and a maximum ice water path of 0.16 kg m⁻². In the vertical cloud structure the medium cloud layer disappears at the end of the life cycle for all three cyclones. The release of precipitation onto the Baltic Sea drainage basin is different for each cyclone. It lies between 13 and 22 · 10¹² kg. This is about 50% of the total precipitation in the whole area for the strongest cyclone and 65% for the others. The P — E (precipitation minus evaporation) is 15 · 10¹² kg for two of the cyclones and 10 · 10¹² kg for the third one. Received August 7, 2000 Revised March 19, 2001     

Rain-aerosol coupling in the tropical atmosphere of Southeast Asia: distribution and scavenging ratios of major ionic species

Both aerosol and rainwater samples were collected and analyzed for ionic species at a coastal site in Southeast Asia over a period of 9 months (January–September 2006) covering different monsoons. In general, the occurrence and distribution of ionic species showed a distinct seasonal variation in response to changes in air mass origins. Real-time physical characterization of aerosol particles during rain events showed changes in particle number distributions which were used to assess particle removal processes associated with precipitation, or scavenging. The mean scavenging coefficients for particles in the range 10–500 nm and 500–10 μm were 7.0 × 10⁻⁵ ± 2.8 × 10⁻⁵ s⁻¹ and 1.9 × 10⁻⁴ ± 1.6 × 10⁻⁵ s⁻¹, respectively. A critical analysis of the scavenging coefficients obtained from this study suggested that the wet removal of aerosol particles was greatly influenced by rain intensity, and was particle size-dependent as well. The scavenging ratios, another parameter used to characterize particle removal processes by precipitation, for NH₄ ⁺, Cl⁻, SO₄ ²⁻, and NO₃ ⁻ were found to be higher than those of Na⁺, K⁺, and Ca²⁺ of oceanic and crustal origins. This enrichment implied that gaseous species NH₃, HCl, and HNO₃ could also be washed out readily. These additional sources of ions in precipitation presumably counter-balanced the dilution effect caused by high total precipitation volume in the marine and tropical area.     

一次寒潮降温过程的等熵位涡和热力学分析

通过等熵位涡和热力学能量方程的各项诊断对2018年1月上旬我国东部一次寒潮天气过程进行分析,重点给出垂直运动在寒潮降温中的作用。结果表明：此次寒潮天气过程主要受蒙古国南部的横槽转竖影响,巴尔喀什湖东部和西伯利亚地区及其北部为引起这次寒潮的主要冷空气源地。欧亚大陆北部和极区对流层高层和平流层低层的高位涡强冷空气沿着等熵面向南向下平流,引导强冷空气侵袭我国东部。等熵位涡大值区的东侧对应上升运动区,有利于降水的产生。寒潮期间风场平流引起的850 hPa强降温区主要位于东南沿海地区,降温幅度最高可达6×10^-4 K·s^-1,而东北地区在整个寒潮期间冷平流强度较弱,最大降温幅度仅约为1×10^-4 K·s^-1。通过计算东南沿海和东北地区区域平均风场平流和垂直运动引起850 hPa温度变化,得出寒潮期间两地的温度总降幅约为1×10^-4 K·s^-1。东南沿海地区的寒潮主要由风场的冷平流引起,而东北地区则是由冷平流和垂直上升运动的共同作用引起。垂直方向上,东北地区冷空气能影响的高度要远高于东南沿海地区。     

A subtropical rainstorm associated with a tropical plume overAfrica and the Middle-East

Summary  Cloud bands that extend from the ITCZ along the subtropical jet toward the subtropics are known as ‘tropical plumes’. At times rainstorms develop at their subtropical edges. One such rainstorm swept eastern North Africa and the Middle East on 23–24 December 1988, with rainfall comparable with the annual averages there. This study examines the storm using the ECMWF initialized data together with surface observations and satellite imageries. The analysis indicates that the storm developed at the inflection region ahead of a pronounced trough in the subtropical jet, with which a mid-latitude trough was merged. Two ageostrophic effects taking place along the jet ahead of the trough contributed to the intensity of the rainstorm. One was associated with acceleration at the jet entrance, located at tropical latitudes, which contributed to the enhancement of both tropical convection and the southerly wind component, which enhanced the moisture tropical transport toward the subtropics. The second was the enhanced near-tropospheric divergence associated with positive vorticity advection at the inflection region itself. Since both effects have a quadratic dependence on wind speed, the observed jet speed, 50% larger than its average value, explains the observed divergence at the inflection point at the 200 hPa level, over 6 × 10⁻⁵ s⁻¹, and the vertical velocity at the 700 hPa level, about 10⁻¹ ms⁻¹. It is suggested here that the merging of a mid-latitude with the trough in the subtropical jet, with which the tropical plume is associated, is the cause for the intensification of the subtropical jet and hence of its related rainstorms. Received May 24, 2000     

Self-similarity patterns of precipitation in the Iberian Peninsula

Summary An objective classification of the precipitation field over the Iberian Peninsula and the Balearic Islands is obtained. Data are derived from a high-resolution daily precipitation dataset obtained from in-situ measurements. The dataset, Iberian monthly Precipitation Dataset (IPD), consists of monthly precipitation data over a 25 km × 25 km grid from 1^st January 1961 to 31^st December 2003. Therefore, 960 data series over the Iberian Peninsula and the Balearic Islands are disposed over the grid for 43-year period. Multi-resolution wavelet analysis is used to extract similar information in the precipitation field at different timescales. An objective classification of the obtained wavelet coefficient series is carried out by means of the Kohonen’s neural network, also named Self-Organizing Map (SOM). SOM is formed by an unsupervised learning algorithm that may be used to find clusters of similar events in the input data and is able to identify some underlying dynamic structures of the multi-dimensional datasets. SOM is applied to the wavelet coefficients for intramonthly, intermonthly and interannual oscillations, obtaining self-organised maps which objectively identify similar zones of precipitation behaviour over the Iberian Peninsula. The homogeneity of the patterns is also studied by means of non-parametric correlations, energy scalograms and tests of significance. The intramonthly, intermonthly and interannual waves resulted in seven, five and three SOM patterns, respectively. As timescale increases, the wavelet series coefficients tend to be highly clustered. The results indicate that as the oscillation frequencies decrease, the Iberian precipitation behaves more linearly.     

The contribution of mesoscale convective systems to intense hourly precipitation events during the warm seasons over central East China

Central East China is an area where both intense hourly precipitation(IHP) events and mesoscale convection systems(MCSs) occur frequently in the warm seasons. Based on mosaics of composite Doppler radar reflectivity and hourly precipitation data during the warm seasons(May to September) from 1 July 2007 to 30 June 2011, the contribution of MCSs to IHP events exceeding 20 mm h~(-1) over central East China was evaluated. An MCS was defined as a continuous or quasicontinuous band of 40d BZ reflectivity that extended for at least 100 km in at least one direction and lasted for at least 3h. It was found that the contribution of MCSs to IHP events was 45% on average over central East China. The largest contribution,more than 80%, was observed along the lower reaches of the Yellow River and in the Yangtze River–Huaihe River valleys.These regions were the source regions of MCSs, or along the frequent tracks of MCSs. There were two daily peaks in the numbers of IHP events: one in the late afternoon and one in the early morning. These peaks were more pronounced in July than in other months. MCSs contributed more to the early-morning IHP event peaks than to the late-afternoon peaks. The contributions of MCSs to IHP events with different intensities exhibited no significant difference, which fluctuated around 50% on average over central East China.     

North Sea – Caspian Pattern (NCP) – an upper level atmospheric teleconnection affecting the eastern Mediterranean – implications on the regional climate

Summary ?A calendar of the negative and positive phases of the North Sea – Caspian Pattern (NCP) for the period 1958–1998 was used to analyse the implication of the NCP upper level teleconnections on the regional climate of the eastern Mediterranean basin. Series of monthly mean air temperature and monthly total rainfall from 33 stations across Greece, Turkey and Israel, for the same period, were used. For each month, from October to April, averages of the monthly mean temperatures and the monthly rainfall totals as well as the standardized values of both parameters were calculated separately for the negative (NCP (−)) and the positive (NCP (+)) phases of the NCP. At all stations and in all months, temperature values were significantly higher during the NCP (−) as compared with the NCP (+). Furthermore, apart from very few exceptions, the absolute monthly mean maximum and monthly mean minimum values were obtained during the NCP (−) and the NCP (+) phases, respectively. The maximum impact of the NCP on mean air temperature was detected in the continental Anatolian Plateau, where the mean seasonal differences are around 3.5 °C. This influence decreases westwards and southwards. The influence on the rainfall regime is more complex. Regions exposed to the southern maritime trajectories, in Greece and in Turkey, receive more rainfall during the NCP (−) phase, whereas in the regions exposed to the northern maritime trajectories, such as Crete in Greece, the Black Sea region in Turkey, and in all regions of Israel, there is more rainfall during the NCP (+) phase. The accumulated rainfall differences between the two phases are over 50% of the seasonal average for some stations. A comparison of the capabilities of the NCP, the North Atlantic Oscillation (NAO) and the Southern Oscillation (SO) indices to differentiate between below and above normal temperatures was made. The results have placed the NCP, as the best by far of all three teleconnections in its ability to differentiate between below or above normal temperatures and as the main teleconnection affecting the climate of the Balkans, the Anatolian Peninsula and the Middle East. These results may serve to downscale General Circulation Model (GCM) scenarios to a regional scale and provide forecasts regarding eventual temperature and/or precipitation changes. Received June 25, 2001; revised February 25, 2002; accepted March 3, 2002     

Modelling the response of glaciers to climate change by applying volume-area scaling in combination with a high resolution GCM

 A seasonally and regionally differentiated glacier model is used to estimate the contribution that glaciers are likely to make to global sea level rise over a period of 70 years. A high resolution general circulation model (ECHAM4 T106) is used to estimate temperature and precipitation changes for a doubled CO₂ climate and serves as input for the glacier model. Volume-area relations are used to take into account the reduction of glacier area resulting from greenhouse warming. Each glacieriated region has a specified glacier size distribution, defined by the number of glaciers in a size class and a mean area. Changes in glacier volume are calculated by a precipitation dependent mass balance sensitivity. The model predicts a global sea level rise of 57 mm over a period of 70 years. This corresponds to a sensitivity of 0.86 mm yr⁻¹K⁻¹. Assuming a constant glacier area as done in earlier work leads to an overestimation of 19% for the contribution to sea level rise. Received: 16 August 2000 / Accepted: 21 May 2001     

Microclimatological characteristics of a miscanthus (Miscanthus cv. giganteus) stand during stable conditionsat night in the nonvegetative winter period

Summary ?Microclimatological data obtained during a field experiment in the nongrowing winter period were used to study the microclimatologically stable night conditions of a 200 × 150 m miscanthus (Miscanthus cv. giganteus) stand and compared to open field conditions. The microclimatological pattern within the miscanthus canopy was characterized by long-wave radiative cooling of the plant stand and by an established temperature inversion within the canopy at calm nights. The results show that there are significant differences in air temperature and energy balance components between the open field and the miscanthus field during calm and clear nights. In general, net radiation difference during the cold and calm nights was relatively constant and about 20 W m⁻² less negative in miscanthus (because of lower surface temperatures) than at the open field. Air temperature differences also remained fairly constant and were up to 3 °C lower than at the open field (at the height of 1 m). Through thermal inversion cold air accumulated in the lower parts of the canopy as shown by the vertical air temperature profiles. They showed a greater amplitude within the diurnal cycle in the miscanthus stand than in the open field. Through the onset of wind, temperature profiles changed rapidly and differences diminished. Vertical katabatic air drainage into the canopy layers was estimated indirectly by using the energy balance approach. It was calculated from the significant energy balance closure gap and showed a mean air exchange rate of up to 22 m³ m⁻² h⁻¹, related to a stand volume of 1 m² area and 4 m height, during the mostly calm and clear nights, depending on the canopy net radiation and turbulent heat exchange forced by slight wind spells. Quantitative uncertainties in calculated cold air drainage which are introduced by the measurement method and certain assumptions in the calculations, were considered in a sensitivity analysis. In spite of these uncertainties evidence of katabatic cold air flow is given. Received July 29, 1999; revised June 11, 2001; accepted March 14, 2002     

Seasonal carbon dioxide balance and respiration of a high-arctic fen ecosystem in NE-Greenland

Summary  Turbulent fluxes of CO₂ were continuously measured by eddy correlation for three months in 1997 over a gramineous fen in a high-arctic environment at Zackenberg (74°28′12″N, 20°34′23″W) in NE-Greenland. The measurements started on 1 June, when there was still a 1–2 m cover of dry snow, and ended 26 August at a time that corresponds to late autumn at this high-arctic site. During the 20-day period with snow cover, fluxes of CO₂ to the atmosphere were small, typically 0.005 mg CO₂ m⁻² s⁻¹ (0.41 g CO₂ m⁻² d⁻¹), wheres during the thawed period, the fluxes displayed a clear diurnal variation. During the snow-free period, before the onset of vegetation growth, fluxes of CO₂ to the atmosphere were typically 0.1 mg CO₂ m⁻² s⁻¹ in the afternoon, and daily sums reached values up to almost 9 g CO₂ m⁻² d⁻¹. After 4 July, downward fluxes of CO₂ increased, and on sunny days in the middle of the growing season, the net ecosystem exchange rates attained typical values of about −0.23 mg m⁻² s⁻¹ at midday and max values of daily sums of −12 g CO₂ m⁻² d⁻¹. Throughout the measured period the fen ecosystem acted as a net-sink of 130 g CO₂ m⁻². Modelling the ecosystem respiration during the season corresponded well with eddy correlation and chamber measurements. On the basis of the eddy correlation data and the predicted respiration effluxes, an estimate of the annual CO₂ balance the calender year 1997 was calculated to be a net-sink of 20 g CO₂ m⁻² yr⁻¹. Received October 6, 1999 Revised May 2, 2000     

Variations in the Temperature Regime Across the Mediterranean During the Last Century and their Relationship with Circulation Indices

Summary  Circulation types were identified by means of zonal and meridional indices calculated separately over ten different regions of 20°×20° over the Mediterranean and Europe. Seasonal temperature trends in 22 grid boxes of 5°×5° covering the entire Mediterranean, and at six stations Lisbon, Madrid, Florence, Luqa (Malta), Athens and Jerusalem, were calculated. A warming trend in the period 1873–1989 was detected. The warming is more evident in the western Mediterranean with an average rate of about 0.4 [°C/100 yr], than in the eastern Mediterranean with an increase of only 0.2 [°C/100 yr]. A cooling trend in autumn in the eastern Mediterranean with an average rate of −0.5 [°C/100 yr] was detected and attributed to an increase in northerly meridional circulation in that region. Warming trends at Lisbon, Madrid, Florence, Athens and Jerusalem, were more important than the trends in the grid boxes containing these stations. This rapid warming was attributed to urban effects. No such effects were found in Luqa due to its location and the lack of urban effects there. Temperatures at Luqa, Athens and Jerusalem are highly positively correlated. Likewise, temperatures at Lisbon and Madrid. Temperatures at Florence are either correlated with Madrid or with Luqa. Negative or no correlations were found between Lisbon or Madrid with Athens or Jerusalem, except during the winter. This was attributed to the fact that favourable circulation for high temperatures in the eastern stations was opposite to the favourable circulation for high temperatures in the western stations and vice versa. Finally, the above reinforces the concept of a Mediterranean Oscillation between the western and eastern basins. Received November 14, 1997 Revised June 2, 1998     

Influence of vegetation changes during the Last Glacial Maximum using the BMRC atmospheric general circulation model

 The influence of different vegetation distributions on the atmospheric circulation during the Last Glacial Maximum (LGM, 21 000 years before present) is investigated. The atmospheric general circulation model of the Bureau of Meteorology Research Center was run using a modern vegetation and in a second experiment with a vegetation reconstruction for the LGM. It is found that a change from conifer to desert and tundra causes an additional LGM cooling of 1–2 °C in Western Europe, up to −4 °C in North America and −6 °C in Siberia. An expansion of dryland vegetation causes an additional annual cooling of 1–2 °C for Australia and northern Africa. On the other hand, an increase of temperature (2 °C) is found in Alaska due to changes in circulation. In the equatorial region the LGM vegetation leads to an increased modelled temperature of 0.5–1.5 °C and decreased precipitation (30%) over land due to a reduction of the tropical rainforest, mainly in Indonesia, where the reduction of precipitation over land is associated with an increase of precipitation of 30% over the western Pacific. Received: 15 December 1999 / Accepted: 10 January 2001     

Temperature and surface pressure anomalies in Israel and the North Atlantic Oscillation

Summary Teleconnections associated with changing patterns of temperature and pressure anomalies over Israel during the second half of the 20th century are investigated. Relatively high, statistically significant, correlation coefficients of −0.8 and +0.9 were found between the North Atlantic Oscillation (NAO) Index anomalies and smoothed (5 year running mean) cool season temperature and surface pressure anomalies in Israel, respectively. A relatively high positive correlation, (r = 0.8) was also found between the NAO Index anomalies and smoothed geopotential height of the 1000 hPa pressure level, during the cool season at Bet Dagan radiosonde station located on the Israel Mediterranean coastal plain. Correlation coefficients between NAO Index anomalies and the higher standard pressure levels, 850 and 700 hPa, decrease gradually and become negative (not statistically significant) for the 500 hPa level. Received January 25, 2000/Revised March 6, 2001     

The Siberian High and climate change over middle to high latitude Asia

Summary The Siberian High is the most important atmospheric centre of action in Eurasia during the winter months. Here its variability and relationship with temperature and precipitation is investigated for the period 1922 to 2000. The pronounced weakening of the Siberian High during the last ∼ 20 years is its most remarkable feature. Mean temperature, averaged over middle to high latitude Asia (30° E–140° E, 30° N–70° N), is correlated with the Siberian High central intensity (SHCI) with correlation coefficient of − 0.58 (1922–1999), and for precipitation, the correlation coefficient is − 0.44 (1922–1998). Taking the Arctic Oscillation (AO), the SHCI, the Eurasian teleconnection pattern (EU), and the Southern Oscillation (SO) index into account, 72 percent of the variance in temperature can be explained for the period 1949–1997 (for precipitation the variance is 26 percent), with the AO alone explaining 30 percent of the variance, and the Siberian High contributing 24 percent. The precipitation variance explained by the Siberian High is only 9.8 percent of the total. Received January 2, 2001 Revised November 24, 2001     

Temperature and snow-melt controls on interannual variability in carbon exchange in the high Arctic

Summary Net Ecosystem CO₂ Exchange (NEE) was studied during the summer season (June–August) at a high Arctic heath ecosystem for 5 years in Zackenberg, NE Greenland. Integrated over the 80 day summer season, the heath is presently a sink ranging from −1.4 g C m⁻² in 1997 to −23.3 g C m⁻² in 2003. The results indicate that photosynthesis might be more variable than ecosystem respiration on the seasonal timescale. The years focused on in this paper differ climatically, which is reflected in the measured fluxes. The environmental conditions during the five years strongly indicated that time of snow-melt and air temperature during the growing season are closely related to the interannual variation in the measured fluxes of CO₂ at the heath. Our estimates suggest that net ecosystem CO₂ uptake is enhanced by 0.16 g C m⁻² per increase in growing degree-days during the period of growth. This study emphasises that increased summer time air temperatures are favourable for this particular ecosystem in terms of carbon accumulation.