Wintertime Atmospheric Research Articles

Relationships Between the Wintertime Atmospheric Blocking and Cold Surges Over Eurasia Revealed by Clustering

AbstractA unified clustering framework based on pattern correlation is used to identify eight blocking regimes over Eurasia and surrounding oceans during the winter months (December–March) of 1948–2021. The regimes are labeled based on their centers of action, which are located over the West Atlantic, Greenland, the East Atlantic, Scandinavia, the Ural Mountains and Siberia, the Okhotsk Sea, the Bering Strait, and the North Pacific. It should be noted that the classification is almost insensitive to the time period but mainly depends on the percentage of different blocking events. The spatial distributions of cold surges differ substantially among these eight regimes. Due mainly to the cold advection downstream of the centers of blocking activities, cold surges can be observed over parts of the Eurasian continent. Possible relationships between the eight blocking regimes and large‐scale modes of climate variability, including the Arctic Oscillation, the North Atlantic Oscillation, and the El Niño‐Southern Oscillation, are explored. Not only contemporary connections but also the predictive value of large‐scale modes is discussed.

Effect of Oceanic Stochastic Forcing on Wintertime Atmospheric Decadal Variability Over Midlatitude North Pacific

AbstractPrevious studies showed that random atmospheric forcing can stimulate oceanic low frequency variability in midlatitudes, since ocean has huge heat capacity that can redden the atmospheric high frequency variability. However, whether the oceanic stochastic forcing can in turn influence atmospheric variability is still unclear. To answer this question, two parallel 63‐year numerical experiments are conducted with an atmospheric general circulation model. In the control experiment, the atmosphere is forced by observed global sea surface temperature (SST). After adding stochastic forcings that are constrained by observed standard deviations of mesoscale SST anomalies (SSTAs) over the North Pacific in a sensitivity experiment, large‐scale atmospheric variability is greatly increased over the midlatitude northwest Pacific, especially on decadal timescale. Moreover, the anomalous atmospheric circulations associated with the Pacific Decadal Oscillation (PDO), like the enhanced westerly wind in the Kuroshio‐Oyashio and its extension (KOE) region, are more consistent with that resolved by a reanalysis model after the stochastic SSTAs are added. Dynamic diagnostics show that under the background of PDO warm phase, stochastic SSTAs can generate larger diabatic heating over the north of KOE region by inducing more moisture condensation in the mid‐lower troposphere, which is balanced by anomalous cold low‐level air temperature advection. Thus, the atmospheric baroclinicity is increased by the cold temperature advection anomaly, and the transient eddy activity and westerly jet are both enhanced above the KOE region accordingly. Our results suggest that to accurately represent the oceanic effect on large‐scale atmospheric variability and anomalies, oceanic stochastic SSTAs should also be considered.

Transport of Water Into the Polar Regions of Mars Through Scavenging by CO2 Snowfall

AbstractThe Martian water cycle is tightly coupled to the polar regions, where water in the residual polar caps exchanges with the atmosphere on seasonal and multi‐annual timescales. It is thus crucial to understand the mechanisms that control water exchange in the polar regions in order to understand the water cycle as a whole. One such mechanism is CO2 snowfall, which previous studies have shown is frequent in the poles and provides a pathway for material to move from the atmosphere to the surface. We hypothesize that water ice particles that form in the wintertime atmosphere above the polar region act as condensation nuclei (CN) for the CO2 ice particles and are deposited onto the polar caps during the CO2 snowfall process. A significant amount of water may thus be contributed to the polar caps in winter through CO2 snowfall. In this study we use atmospheric retrievals of temperature, pressure, and CO2 ice cloud opacity from the Mars Climate Sounder on board NASA's Mars Reconnaissance Orbiter coupled with a simple cloud settling model to determine sedimentation rates of CO2 ice in the polar winter. We then use these snowfall rates to quantify the amount of water ice deposited to the seasonal polar caps as CN and analyze spatial and temporal patterns in its distribution. We find a lower limit on the average mass of water scavenged every winter on the order of 109–1010 kg in the north and 108–109 kg in the south.

A dynamic and thermodynamic coupling view of the linkages between Eurasian cooling and Arctic warming

Investigating the contrast between wintertime warming in the Arctic and cooling in Eurasia is of great importance for understanding regional climate change. In this study, we propose a dynamic and thermodynamic coupling view of the linkages between wintertime Arctic warming and Eurasian cooling since 1979. The key factors are the energy budget at the Earth’s surface, the diabatic heating and baroclinicity of the atmosphere, and subsurface ocean heat. A summertime origin of wintertime Arctic warming suggests a partial driving role of the Arctic in wintertime Eurasian cooling. The reasons for this finding are as follows. First, there is a dipole pattern in the diabatic heating change in winter over the Arctic Ocean corresponding to the anticyclonic circulation that links Eurasian cooling and Arctic warming. Second, the change in diabatic heating of the atmosphere is determined by sensible heat at the Earth’s surface through vertical diffusion. Third, the positive sensible heat change in the eastern Arctic sector in winter originates from the summertime enhanced absorption of solar radiation by the subsurface ocean over the sea ice loss region. Meanwhile, the negative sensible heat change in the western Arctic sector and wide Arctic warming can be explained by the circulation development triggered by the change in the east. Additionally, the background strong baroclinicity of the atmosphere in mid-high latitudes and corresponding two-way Arctic and mid-latitude interactions are necessary for circulation development in winter. Furthermore, the seasonality of the changes indicates that Eurasian cooling occurs only in winter because the diabatic heating change in the Arctic is strongest in winter. Overall, the comprehensive mechanisms from the summertime Earth’s surface and subsurface ocean to the wintertime atmosphere suggest a driving role of the Arctic. Note that the situation in interannual variability is more complex than the overall trend because the persistence of the influence of summertime sea ice is weakly established in terms of interannual variability.

Pollution Characteristics and Sources of Wintertime Atmospheric Brown Carbon at a Background Site of the Yangtze River Delta Region in China

To investigate the pollution characteristics and sources of atmospheric brown carbon (BrC) in Chongming Island, a background site of the Yangtze River Delta (YRD) region in China, PM2.5 samples collected from December 2018 to January 2019 were analyzed to determine their chemical compositions and optical properties. The results showed that the light absorption coefficient (Abs365,M) of BrC extracted by methanol at 365 nm was (5.39±3.33) M-1·m-1, which was 1.3 times of the water extracted BrC. Both increased significantly with the increase of pH values, suggesting that less acidic conditions can enhance the light absorption ability of BrC. In winter, both Abs365 and MAE365 (mass absorption efficiency) were higher in the nighttime than in the daytime. A strong linear correlation observed between Abs365 and levoglucosan (R2=0.72) indicated that many light absorbing substances in Chongming Island were derived from biomass burning emissions. During the campaign, nitro-aromatic compounds (NACs) and PAHs accounted for (1.5±1.1) ng·m-3 and (8.3±4.7) ng·m-3, respectively, contributing to 0.1% and 0.067% of the absorption of the total BrC at 365 nm, respectively. Positive matrix factorization (PMF) analysis further showed that biomass and fossil fuel combustions were the main sources of BrC in Chongming Island in winter, accounting for 56% of the total BrC, followed by secondary formation, accounting for 24% of the total BrC, with road dust contributing only 6%.

Comparing the effects of solar-related and terrestrial drivers on the northern polar vortex

Northern polar vortex experiences significant variability during Arctic winter. Solar activity contributes to this variability via solar irradiance and energetic particle precipitation. Recent studies have found that energetic electron precipitation (EEP) affects the polar vortex by forming ozone depleting NOxcompounds. However, it is still unknown how the EEP effect compares to variabilities caused by, e.g., solar irradiance or terrestrial drivers. In this study we examine the effects of EEP, solar irradiance, El-Niño-Southern Oscillation (ENSO), volcanic aerosols and quasi-biennial oscillation (QBO) on the northern wintertime atmosphere. We use geomagneticAp-index to quantify EEP activity, sunspot numbers to quantify solar irradiance, Niño 3.4 index for ENSO and aerosol optical depth for the amount of volcanic aerosols. We use a new composite dataset including ERA-40 and ERA-Interim reanalysis of zonal wind and temperature and multilinear regression analysis to estimate atmospheric responses to the above mentioned explaining variables in winter months of 1957–2017. We confirm the earlier results showing that EEP and QBO strengthen the polar vortex. We find here that the EEP effect on polar vortex is stronger and more significant than the effects of the other drivers in almost all winter months in most conditions. During 1957–2017 the considered drivers together explain about 25–35% of polar vortex variability while the EEP effect alone explains about 10–20% of it. Thus, a major part of variability is not due to the linear effect by the studied explaining variables. The positive EEP effect is particularly strong if QBO-wind at 30 hPa has been easterly during the preceding summer, while for a westerly QBO the EEP effect is weaker and less significant.

Anthropogenic control over wintertime oxidation of atmospheric pollutants.

During winter in the mid-latitudes, photochemical oxidation is significantly slower than in summer and the main radical oxidants driving formation of secondary pollutants, such as fine particulate matter and ozone, remain uncertain, owing to a lack of observations in this season. Using airborne observations, we quantify the contribution of various oxidants on a regional basis during winter, enabling improved chemical descriptions of wintertime air pollution transformations. We show that 25-60% of NOx is converted to N2O5 via multiphase reactions between gas-phase nitrogen oxide reservoirs and aerosol particles, with ~93% reacting in the marine boundary layer to form >2.5 ppbv ClNO2. This results in >70% of the oxidizing capacity of polluted air during winter being controlled, not by typical photochemical reactions, but from these multiphase reactions and emissions of volatile organic compounds, such as HCHO, highlighting the control local anthropogenic emissions have on the oxidizing capacity of the polluted wintertime atmosphere.

Impact of Atmospheric Circulation and Meteorological Parameters on Wintertime Atmospheric Extinction in Chengdu and Chongqing of Southwest China during 2001–2016

ABSTRACT This study investigated the effect of atmospheric circulation and meteorological parameters on the surface atmospheric extinction coefficient (SEC) in Chengdu and Chongqing of southwest China during the winters of 2001–2016. Four predominant circulation types (CT) were first identified, representing a high-pressure system in the north (CT 1), a low-pressure system in the north (CT 2), a weak high-pressure system (CT 3) and a local low-pressure system (CT 4) at a geopotential height of 850 hPa. In Chengdu, variation in the SEC between the CTs was dominated by meteorological factors, such as wind, subsidence, inversion and planetary boundary layer height (PBLH), whereas the variation within each CT was mainly controlled by dispersion-related parameters (such as wind speed and PBLH for CTs 1, 2 and 4, which were associated with strong weather systems) or chemistry-related parameters (such as RH and T for CT 3, which was associated with a weak system). In Chongqing, dispersion and chemical processes were equally important due to the weaker impact of the atmospheric circulation. A new method for discriminating between synoptic and non-synoptic signals in SEC time series was proposed to estimate the effect of the circulation intensity by considering multiple influential meteorological parameters. Atmospheric circulation heightened the interannual SEC variations by ~23% in both Chengdu and Chongqing in more than half of the years but reduced them by 44.79% and 8.02% in Chengdu and Chongqing, respectively, in the remaining years. Synoptic changes were estimated to contribute 5–10% of the decreasing SEC trend but less than 1% of the increasing trend.

2004–2016 Wintertime Atmospheric Blocking Events over Western Siberia and Their Effect on Surface Temperature Anomalies

Western Siberia is a large area in Northern Eurasia, which lies between the Urals and the Yenisei River. The atmospheric blocking events are not a frequent phenomenon in this region. Nevertheless, they noticeably affect the weather and living conditions of people there. We have investigated 14 winter blocking events, identified over Western Siberia, over 2004–2016, and have studied their effect on the surface temperature in this region. We have compared each of the 14 blocking events to the corresponding surface temperature anomalies in the north and in the south of Western Siberia. As a result, the temperature anomalies were separated into two groups: (1) dipole, with a positive surface temperature anomaly (or close to the norm) in the north, and with a negative anomaly (or close to the norm) in the south, and (2) non-dipole. Ten events were attributed to Group 1, four events were referred to Group 2. Analyzing the potential temperature on the dynamic tropopause (advection characteristic) showed that the Group 1 events feature strong advection over the investigated territory. In the non-dipole situations from Group 2 Western Siberia are away from strong blocking events.

Mongolian Mountains Matter Most: Impacts of the Latitude and Height of Asian Orography on Pacific Wintertime Atmospheric Circulation

Abstract The impacts of Asian orography on the wintertime atmospheric circulation over the Pacific are explored using altered-orography, semi-idealized, general circulation model experiments. The latitude of orography is found to be far more important than height. The Mongolian Plateau and nearby mountain ranges, centered at ~48°N, have an impact on the upper-level wintertime jet stream that is approximately 4 times greater than that of the larger and taller Tibetan Plateau and Himalayas to the south. Key contributing factors to the importance of the Mongolian mountains are latitudinal variations in the meridional potential vorticity gradient and the strength of the impinging wind—both of which determine the amplitude of the atmospheric response—and the structure of the atmosphere, which influences the spatial pattern of the downstream response. Interestingly, while the Mongolian mountains produce a larger response than the Tibetan Plateau in Northern Hemisphere winter, in April–June the response from the Tibetan Plateau predominates. This result holds in two different general circulation models. In experiments with idealized orography, varying the plateau latitude by 20°, from 43° to 63°N, changes the response amplitude by a factor of 2, with a maximum response for orography between 48° and 53°N, comparable to the Mongolian mountains. In these idealized experiments, the latitude of the maximum wintertime jet increase changes by only ~6°. It is proposed that this nearly invariant spatial response pattern is due to variations in the stationary wavenumber with latitude leading to differences in the zonal versus meridional wave propagation.

Interannual Modulations of Oceanic Imprints on the Wintertime Atmospheric Boundary Layer under the Changing Dynamical Regimes of the Kuroshio Extension

Abstract The Kuroshio Extension (KE) fluctuates between its different dynamic regimes on (quasi) decadal time scales. In its stable (unstable) regime, the KE jet is strengthened (weakened) and less (more) meandering. The present study investigates wintertime mesoscale atmospheric structures modulated under the changing KE regimes, as revealed in high-resolution satellite data and data from a particular atmospheric reanalysis (ERA-Interim). In the unstable KE regime, a positive anomaly in sea surface temperature (SST) to the north of the climatological KE jet accompanies positive anomalies in upward heat fluxes from the ocean, surface wind convergence, and cloudiness. As revealed in the atmospheric reanalysis, these positive anomalies coincide with local lowering of sea level pressure, weaker vertical wind shear, warming and thickening of the marine atmospheric boundary layer (MABL), anomalous ascent, and convective precipitation. In the stable KE regime, by contrast, the corresponding imprints of sharp SST gradients across the KE and Oyashio fronts on the wintertime MABL are separated more distinctly, and so are the surface baroclinic zones along those two SST fronts. In the ERA-Interim data, such mesoscale imprints of the KE variability as above are not well represented in a period during which the resolution of SST data prescribed is relatively low. The present study thus elucidates the importance of high-resolution SST data prescribed for atmospheric reanalysis in representing modulations of the MABL structure and air–sea fluxes by the variability of oceanic fronts and/or jets, including the modulations occurring with the changing KE regimes through the hydrostatic pressure adjustment and vertical mixing mechanisms.

Variation of Urban Atmospheric Ammonia Pollution and its Relation with PM2.5 Chemical Property in Winter of Beijing, China

To understand the air pollution problem in megacities such as Beijing, field measurement investigating the variation of NH_3 and its association with PM_(2.5) chemical property was conducted from 25 November to 24 December 2013. The results indicated that the daily concentration of wintertime NH_3 tended to be high on the days with relatively high temperatures and low wind speeds. Affected by the synoptic condition, NH3 concentration showed a bimodal diurnal variation pattern, which tended to peak at around 09:00 and 22:00 of the day. As the sole precursor for NH_4^+, NH_3 exerted a significant impact on the ion chemistry of PM_(2.5) through enhancing the nighttime NH_4Cl formation and promoting both homogeneous and heterogeneous formation of NO_3^-. During heavy pollution episodes with PM_(2.5) concentrations over 200 μg m^(-3), the NH_3 levels and NH_4^+/NH_3 ratios grew simultaneously with the increase of PM_(2.5) levels, indicating that NH3 is one of the key reasons for heavy pollution events. Revealed by the features of measured ionic species in PM_(2.5), in conjunction with the acidity analysis using thermodynamic model, our results suggested that NH_3 was frequently sufficient in wintertime atmosphere of urban Beijing and the fine particulates were neutralized nearly fully by NH_3.

Wintertime atmospheric response to Atlantic multidecadal variability: effect of stratospheric representation and ocean–atmosphere coupling

The impact of the Atlantic multidecadal variability (AMV) on the wintertime atmosphere circulation is investigated using three different configurations of the Community Atmospheric Model version 5 (CAM5). Realistic SST and sea ice anomalies associated with the AMV in observations are prescribed in CAM5 (low-top model) and WACCM5 (high-top model) to assess the dependence of the results on the representation of the stratosphere. In a third experiment, the role of ocean–atmosphere feedback is investigated by coupling CAM5 to a slab-ocean model in which the AMV forcing is prescribed through oceanic heat flux anomalies. The three experiments give consistent results concerning the response of the NAO in winter, with a negative NAO signal in response to a warming of the North Atlantic ocean. This response is found in early winter when the high-top model is used, and in late winter with the low-top model. With the slab-ocean, the negative NAO response is more persistent in winter and shifted eastward over the continent due to the damping of the atmospheric response over the North Atlantic ocean. Additional experiments suggest that both tropical and extratropical SST anomalies are needed to obtain a significant modulation of the NAO, with small influence of sea ice anomalies. Warm tropical SST anomalies induce a northward shift of the ITCZ and a Rossby-wave response that is reinforced in the mid-latitudes by the extratropical SST anomalies through eddy–mean flow interactions. This modeling study supports that the positive phase of the AMV promotes the negative NAO in winter, while illustrating the impacts of the stratosphere and of the ocean–atmosphere feedbacks in the spatial pattern and timing of this response.

Wintertime Atmospheric Response to North Atlantic Ocean Circulation Variability in a Climate Model

Abstract Maximum covariance analysis of a preindustrial control simulation of the NCAR Community Climate System Model, version 4 (CCSM4), shows that a barotropic signal in winter broadly resembling a negative phase of the North Atlantic Oscillation (NAO) follows an intensification of the Atlantic meridional overturning circulation (AMOC) by about 7 yr. The delay is due to the cyclonic propagation along the North Atlantic Current (NAC) and the subpolar gyre of a SST warming linked to a northward shift and intensification of the NAC, together with an increasing SST cooling linked to increasing southward advection of subpolar water along the western boundary and a southward shift of the Gulf Stream (GS). These changes result in a meridional SST dipole, which follows the AMOC intensification after 6 or 7 yr. The SST changes were initiated by the strengthening of the western subpolar gyre and by bottom torque at the crossover of the deep branches of the AMOC with the NAC on the western flank of the Mid-Atlantic Ridge and the GS near the Tail of the Grand Banks, respectively. The heat flux damping of the SST dipole shifts the region of maximum atmospheric transient eddy growth southward, leading to a negative NAO-like response. No significant atmospheric response is found to the Atlantic multidecadal oscillation (AMO), which is broadly realistic but shifted south and associated with a much weaker meridional SST gradient than the AMOC fingerprint. Nonetheless, the wintertime atmospheric response to the AMOC shows some similarity with the observed response to the AMO, suggesting that the ocean–atmosphere interactions are broadly realistic in CCSM4.

Fast atmospheric response to a sudden thinning of Arctic sea ice

In order to understand the influence of a thinner Arctic sea ice on the wintertime atmosphere, idealized ensemble experiments with increased sea ice surface temperature have been carried out with the Integrated Forecast System of the European Centre for Medium-Range Weather Forecasts. The focus is on the fast atmospheric response to a sudden “thinning” of Arctic sea ice to disentangle the role of various different processes. We found that boundary layer turbulence is the most important process that distributes anomalous heat vertically. Anomalous longwave radiation plays an important role within the first few days before temperatures in the lower troposphere had time to adjust. The dynamic response tends to balance that due to boundary layer turbulence while cloud processes and convection play only a minor role. Overall the response of the atmospheric large-scale circulation is relatively small with up to 2 hPa in the mean sea level pressure during the first 15 days; the quasi-equilibrium response reached in the second and third month of the integration is about twice as large. During the first few days the response tends to be baroclinic in the whole Arctic. Already after a few days an anti-cyclonic equivalent-barotropic response develops over north-western Siberia and north-eastern Europe. The structure resembles very much that of the atmospheric equilibrium response indicating that fast tropospheric processes such as fewer quasi-barotropic cyclones entering this continental area are key opposed to slower processes such as those involving, for example, stratosphere-troposphere interaction.

Separation of Climatological Imprints of the Kuroshio Extension and Oyashio Fronts on the Wintertime Atmospheric Boundary Layer: Their Sensitivity to SST Resolution Prescribed for Atmospheric Reanalysis

Abstract Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as climatological imprints of oceanic fronts within the Kuroshio–Oyashio Extension (KOE) region east of Japan are investigated by taking advantage of high horizontal resolution of the ERA-Interim global atmospheric reanalysis data, for which the resolution of sea surface temperature (SST) data has been improved. These imprints, including locally enhanced sensible and latent heat fluxes and local maxima in cloudiness and precipitation in association with locally strengthened surface-wind convergence in the vicinities of SST fronts along the warm Kuroshio Extension and cool Oyashio to its north, are also identified in high-resolution satellite data. In addition to these mesoscale MABL features, meridionally confined near-surface baroclinic zones and zonally oriented sea level pressure (SLP) minima associated with the dual SST fronts are represented in ERA-Interim only in the period of high-resolution SST, but those imprints of the Oyashio front are missing in the low-resolution SST period. In the presence of the prevailing monsoonal northerlies, latitudinal displacements of the SLP trough, baroclinic zone, and the peak meridional gradient of the turbulent heat fluxes from each of the corresponding SST fronts are also found to be sensitive to the frontal width that depends on the SST resolution. The analysis herein suggests that the converging surface northerlies into the SLP minima can contribute positively to the formation of a surface baroclinic zone along the Kuroshio Extension, while a stronger baroclinic zone along the Oyashio front is maintained primarily through the pronounced cross-frontal contrast in sensible heat release from the ocean.

Does the Madden–Julian Oscillation Influence Wintertime Atmospheric Rivers and Snowpack in the Sierra Nevada?

The relationships between the Madden–Julian oscillation (MJO), activities of atmospheric rivers (ARs), and the resulting snowpack accumulation in the California Sierra Nevada, are analyzed based on 13 yr of observations for water years 1998–2010 inclusive. The AR activity, as measured by the number of high-impact ARs, mean per event snow water equivalent (SWE) changes, and the cumulative SWE changes, is shown to be significantly augmented when MJO convection is active over the far western tropical Pacific (phase 6 on the Wheeler–Hendon diagram). The timing of high-impact ARs (early- versus late-winter occurrences) also appears to be regulated by the MJO. Total snow accumulation in the Sierra Nevada (i.e., AR and non-AR accumulation combined) is most significantly increased when MJO convection is active over the eastern Indian Ocean (phase 3), and reduced when MJO convection is active over the Western Hemisphere (phase 8), with the magnitude of the daily anomaly being roughly half the cold-season mean daily snow accumulation over many snow sensor sites. The positive (negative) SWE anomaly is accompanied by a cold (warm) surface air temperature (SAT) anomaly and an onshore (offshore) water vapor flux anomaly. The contrasting SAT anomaly patterns associated with MJO phases 3 and 8, revealed by the in situ observations, are more realistically represented in the Atmospheric Infrared Sounder retrievals than in the European Centre for Medium-Range Weather Forecasts Interim reanalysis.

Role of circulation parameters in long range aerosol transport: Evidence from Winter-ICARB

This study explores the inevitable role of wind parameters such as wind speed, wind convergence and wind vorticity in the long range transport and distribution of aerosols in the winter time atmosphere over the Bay of Bengal (BoB) during the campaign Winter ICARB. MODIS observed aerosol optical depth (AOD), with an excellent agreement with ship borne Microtops AOD, was found to increase over the BoB particularly, in the eastern parts during the course of the campaign. The influence of atmospheric circulation on this increase is examined using the wind field from NCEP reanalysis and computed wind convergence and vorticity for first and second halves (FH and SH) of the campaign along with a back trajectory analysis using HYSPLIT transport and dispersion model. While surface winds over the BoB remained nearly the same throughout the campaign denying the possibility of enhancement in marine aerosol generation, the higher altitude winds altered significantly in SH providing a channel for aerosol transport from the Indian landmass to the BoB in addition to the increased forest fire contribution from south Asia. This suggested mechanism is supported by CALIPSO aerosol extinction profiles over the eastern BoB and the surrounding land masses. Fine particle dominance in MODIS AOD and diminished correlation between ship borne AOD and surface aerosol mass measurements during SH corroborate this inference by indicating the presence of elevated aerosol layers, which can contribute substantially to the radiative effects of the earth–atmosphere system. This study throws light on the importance of wind convergence and vorticity in the investigations on the long range transport and spatial distribution of aerosols.

On the Relationship between Synoptic Wintertime Atmospheric Variability and Path Shifts in the Gulf Stream and the Kuroshio Extension

Abstract Coherent, large-scale shifts in the paths of the Gulf Stream (GS) and the Kuroshio Extension (KE) occur on interannual to decadal time scales. Attention has usually been drawn to causes for these shifts in the overlying atmosphere, with some built-in delay of up to a few years resulting from propagation of wind-forced variability within the ocean. However, these shifts in the latitudes of separated western boundary currents can cause substantial changes in SST, which may influence the synoptic atmospheric variability with little or no time delay. Various measures of wintertime atmospheric variability in the synoptic band (2–8 days) are examined using a relatively new dataset for air–sea exchange [Objectively Analyzed Air–Sea Fluxes (OAFlux)] and subsurface temperature indices of the Gulf Stream and Kuroshio path that are insulated from direct air–sea exchange, and therefore are preferable to SST. Significant changes are found in the atmospheric variability following changes in the paths of these currents, sometimes in a local fashion such as meridional shifts in measures of local storm tracks, and sometimes in nonlocal, broad regions coincident with and downstream of the oceanic forcing. Differences between the North Pacific (KE) and North Atlantic (GS) may be partly related to the more zonal orientation of the KE and the stronger SST signals of the GS, but could also be due to differences in mean storm-track characteristics over the North Pacific and North Atlantic.

Inversion of extinction data for irregularly shaped particles

The retrieval of extinction data in the highly urbanized region of Bratislava city is analyzed for both, strictly non-spherical particles and volume equivalent particles. Two most typical situations are discussed in more details: the summer time dusty atmosphere consisting of strongly scattering particles with refractive index about 1.59, and winter time atmosphere with usual occurrence of ice-phase aerosols. The attention is paid to more frequent continental polar air mass, with 54% occurrence probability in the Bratislava region. The root-mean-square error of measured optical data varies from 4% to 8%. When processing extinction data it is shown that there are no significant differences between modal radii computed under assumption of randomly oriented irregularly shaped particles and for Mie equivalents. However, the differences can occur in case on non-randomly oriented particles. The modal radius of equally oriented particles in the ice cloud can be about two times larger than a modal radius of the system volume of volume identical spheres. Particle irregularity and the roughness of its surface eliminate the interference structure and ripple typical for monodisperse scattering patterns. As a consequence the subsidiary mode within size distribution function disappears. Such a mode usually occurs with Mie particles. Assuming spherical particles for the computation yields a narrower size distribution compared to assuming irregularly shaped particles.