Atmospheric circulation as a factor for air temperatures in Bulgaria

The research period is 1950–2012, and includes data for air temperatures in 21 Bulgarian stations. Two circulation indices, covering the same period and showing west–east or south–north directions of transport of air masses, were calculated. Statistical methods were used in the study. The results show a significant positive trend in average annual air temperatures in almost the entire territory of Bulgaria with the exception of its eastern-northeastern part. The warming occurs mainly in March, June and July, with some stations having significant positive values also in January, May and August. The zonal index reveals an insignificant increase of western transport of air masses in the cold half of the year (October–April) and strengthening of the eastern transport in the rest of the year. The meridional index shows an increase of the northern transport of air masses over the entire year and this is particularly visible in March, June, August and September. Correlation coefficients indicate that atmospheric circulation has leading role in determination of air temperatures during the period from November to April. Western transport of air masses leads to higher temperatures in spring, autumn and winter and to lower temperatures in summer. The influence of ENSO on atmospheric circulation over Bulgaria is weak, with a time lag of 2 months. El Niño is associated with increased western and northern transport of air masses, while La Niña is associated with increased eastern and southern transport of air masses over southeastern Europe.     

Spatially explicit modelling of the impact of climate changes on pasture production in the North Island,New Zealand

To assess the potential impact of climate changes on pasture production in the North Island, New Zealand, eight climate scenarios of increased temperature and increased (or decreased) rainfall were investigated by integrating a polynomial regression model for pasture production with a Geographic Information System (GIS). The results indicated that the climate change scenarios assuming an increase in temperature by 1–2°C and a rainfall change by −20 to +20% would have a very significant impact on pasture production with a predicted pasture production variation from −46.2 to +51.9% compared with the normal climate from 1961–1990. Increased temperature would generally have a positive effect on pasture production in the south and southeast of the North Island, and increased rainfall would have a positive effect in the central, south and southeast of the North Island and a negative effect in the north of the North Island. The interaction of decreased rainfall and increased temperature would have a negative impact for the whole North Island except some central areas with high rainfall. Relevant management practices for coping with potential climate change are discussed.     

South China Sea Warm Pool in Boreal Spring

During the boreal spring of 1966, a warm-core eddy is identified in the upper South China Sea (SCS) west of the Philippines through an analysis of the U.S. Navy’s Master Oceanographic Observation Data Set. This eddy occurred before the development of the northern summer monsoon and disappeared afterward. We propose that this eddy is a result of the radiative warming during spring and the downwelling due to the anticyclonic forcing at the surface. Our hypothesis suggests an air-sea feedback scenario that may explain the development and withdrawal of the summer monsoon over the SCS. The development phase of the warm-core eddy in this hypothesis is tested by using the Princeton Ocean model     

Ocean–atmosphere coupling and the boreal winter MJO

The influence of ocean–atmosphere coupling on the simulation and prediction of the boreal winter Madden–Julian Oscillation (MJO) is examined using the Seoul National University coupled general circulation model (CGCM) and atmospheric—only model (AGCM). The AGCM is forced with daily SSTs interpolated from pentad mean CGCM SSTs. Forecast skill is examined using serial extended simulations spanning 26 different winter seasons with 30-day forecasts commencing every 5 days providing a total of 598 30-day simulations. By comparing both sets of experiments, which share the same atmospheric components, the influence of coupled ocean–atmosphere processes on the simulation and prediction of MJO can be studied. The mean MJO intensity possesses more realistic amplitude in the CGCM than in AGCM. In general, the ocean–atmosphere coupling acts to improve the simulation of the spatio-temporal evolution of the eastward propagating MJO and the phase relationship between convection (OLR) and SST over the equatorial Indian Ocean and the western Pacific. Both the CGCM and observations exhibit a near-quadrature relationship between OLR and SST, with the former lagging by about two pentads. However, the AGCM shows a less realistic phase relationship. As the initial conditions are the same in both models, the additional forcing by SST anomalies in the CGCM extends the prediction skill beyond that of the AGCM. To test the applicability of the CGCM to real-time prediction, we compute the Real-time Multivariate MJO (RMM) index and compared it with the index computed from observations. RMM1 (RMM2) falls away rapidly to 0.5 after 17–18 (15–16) days in the AGCM and 18–19 (16–17) days in the CGCM. The prediction skill is phase dependent in both the CGCM and AGCM.     

Impact of stratospheric variability on tropospheric climate change

An improved stratospheric representation has been included in simulations with the Hadley Centre HadGEM1 coupled ocean atmosphere model with natural and anthropogenic forcings for the period 1979–2003. An improved stratospheric ozone dataset is employed that includes natural variations in ozone as well as the usual anthropogenic trends. In addition, in a second set of simulations the quasi biennial oscillation (QBO) of stratospheric equatorial zonal wind is also imposed using a relaxation towards ERA-40 zonal wind values. The resulting impact on tropospheric variability and trends is described. We show that the modelled cooling rate at the tropopause is enhanced by the improved ozone dataset and this improvement is even more marked when the QBO is also included. The same applies to warming trends in the upper tropical troposphere which are slightly reduced. Our stratospheric improvements produce a significant increase of internal variability but no change in the positive trend of annual mean global mean near-surface temperature. Warming rates are increased significantly over a large portion of the Arctic Ocean. The improved stratospheric representation, especially the QBO relaxation, causes a substantial reduction in near-surface temperature and precipitation response to the El Chichón eruption, especially in the tropical region. The winter increase in the phase of the northern annular mode observed in the aftermath of the two major recent volcanic eruptions is partly captured, especially after the El Chichón eruption. The positive trend in the southern annular mode (SAM) is increased and becomes statistically significant which demonstrates that the observed increase in the SAM is largely subject to internal variability in the stratosphere. The possible inclusion in simulations for future assessments of full ozone chemistry and a gravity wave scheme to internally generate a QBO is discussed.     

An approach to modeling climate based on feedback relationships

Feedback occurs between many components of the climate system, and makes the study of climate very difficult. A modeling approach is presented in which feedbacks are represented specifically. Analysis of very simple models shows how feedback between two components affects their behavior; positive feedback increases persistence, and can produce climatic changes even without changes in external forcing. In any quantitative study, the magnitudes of all relevant feedbacks must be known accurately. As an example, it is shown how the effect of CO₂ on global temperature must depend greatly on the feedback between global temperature and ice extent.     

Arctic hazes in summer over Greenland and the North American Arctic: II. Nature and concentrations of accumulation-mode and giant particles

Airborne measurements made during August 1985 over Greenland and its environs show that both accumulation-mode (0.1 m D2.0 m) and giant (D2 m) particles were present in relatively high concentrations in arctic haze layers and that the accumulation-mode particles dominated light scattering. Particles with diameters (D) between 1 and 4 m consisted predominately of mixed materials, small and dense inclusions, and probably organic compounds containing sulfur. Many of the particles from 0.1 to 1 m in diameter were also of mixed composition, with sulfuric acid, ammonium sulfate and organics probably the dominant constituents.     

Effects of Mesoscale Surface Thermal Heterogeneity on Low-Level Horizontal Wind Speeds

Using large-eddy simulation, we investigate characteristics of horizontal wind speed at 100 m above the ground, with surface heat-flux variations that are sinusoidal with amplitudes of 0, 50, and 200 W m⁻² and wavelengths of 16, 32, and 128 km, and no background flow. When the amplitude is 200 W m⁻², wind speeds induced by the surface-flux variations on scales of 16 and/or 32 km have multiple temporal oscillations from 0600 to 1800 local standard time. The positive peaks first appear before noon. In contrast, for wind speeds induced by the 128-km surface heterogeneity, a single oscillation occurs in the late afternoon, which is much larger than those generated by the 16- and 32-km surface heterogeneity. In addition, at the oscillation onset the kurtosis of the velocity increment over a distance of 1 km significantly increases, which implies intermittency in the generation of 1-km scale eddies. The spatially intermittent energy cascade generated by surface heterogeneity scaled down to 1-km eddies is analogous to the well-known intermittent energy cascade in the inertial subrange. The kurtosis of the 1-km eddies is much larger with the 128-km surface heterogeneity than with the 16- and 32-km heterogeneities. Thus we conclude that localized rapid changes of low-level horizontal wind speed may be caused by significant local surface heterogeneity on scales between a few tens and a few hundreds of kilometres.     

Analysis of Astex-Stratocumulus Observational Data Using a Mass-Flux Approach

A mass-flux approach is applied to observational data obtained in a convective boundary layer topped with stratocumulus clouds. The observational data were obtained from aircraft measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX). A conditional sampling method is used to calculate average updraft and downdraft values. The vertical fluxes calculated with the mass-flux approach are found to be proportional to the real (measured) fluxes, with a proportionality factor being about 0.6. This value is predicted by theory for two variables having a joint Gaussian distribution function; proportionality factor = 2^-1 0.637. The horizontal fractional entrainment and detrainment rates calculated from the data ( 1–2 × 10^-2 m^-1) are an order of magnitude higher than the rates obtained by large eddy simulations for cumulus convection ( 2–3 × 10^-3 m^-1) and two orders of magnitude higher than those used in modelling cumulus convection with a mass-flux scheme in an operational weather forecast model ( 3 × 10^-4 m^-1). A numerical mass-flux model for the thermodynamics was developed and showed that results are in good agreement when compared with measured profiles of the liquid water content.