Mid-latitudinal Regions Research Articles

Model Estimate of the Acidity of Atmospheric Precipitation Acidity Due to Anthropogenic Sulfur Compounds in the 20th Century

An analysis of the acidity of precipitation is carried out by using the ChAP-1.0 (Chemistry and Aerosol Processes) atmospheric sulfur cycle scheme developed for Earth System Models of Intermediate Complexity (EMICs)These calculations are forced by monthly mean anthropogenic emissions of sulfur dioxide to the atmosphere in 1850–2000 adapted from the CMIP5 (Coupled Models Intercomparison Project, phase 5) database and by long-term means (taking into account annual variations) of meteorological variables adapted from the ERA-Interim reanalysis for 1979–2015. It was revealed that significant acidity of precipitation (minimum pH of hydrometeors) is typical for regions with high anthropogenic loading of sulfur compounds in atmosphere – Europe, southeast Asia, east North America, southern Africa, and western South America. In these regions in the last decades of the 20th century, typical precipitation values of pH amount from 2.5 to 3.5, which agrees well with the available measurements. The maximum acidity of precipitation (the minimum pH of hydrometeors, which is close to 2) due to anthropogenic sulfur are noted in the eastern Mediterranean region. Atmospheric transport leads to regions with pH 3.5 covering almost all of Eurasia in the last decades of the 20th century. The influence of this transport is also noticeable in other midlatitudinal regions – south of North America and western South America. In general, the ChAP scheme can be used in EMICs, but after a refinement to account for the effect of various types of precipitation on the wet deposition of sulfur compounds from the atmosphere and the effects of orography on the transport of chemical species in the atmosphere

Biogeographic and evolutionary history of Crotonoideae based on pollen evidence from Indian Late Cretaceous and Paleogene sediments

AbstractEuphorbiaceae Juss. is a globally distributed angiosperm family with considerable diversity. It is divided into five subfamilies, each of which possesses pollen of distinct morphology. The subfamily Crotonoideae, occurring in evergreen, moist deciduous as well as dry deciduous forests of the world, shows a disjunct distribution pattern in Gondwana and Southeast Asia. This subfamily underwent multiple speciation and extinction events during the northward voyage of the Indian Plate during the Late Cretaceous‐early Paleogene. We studied Crotonoideae fossil pollen from the Late Cretaceous Deccan Intertrappeans (Central India) and early Paleogene sediments from the western Indian lignite mines, and as a result, report 10 pollen fossils showing similarity with the extant genera Blachia, Croton, Endospermum, Jatropha, Klaineanthus, and Tetrorchidium, representing 4 tribes of Crotonoideae viz. Adenoclineae, Codiaeae, Crotoneae, and Jatropheae. Fossil evidences from the present study suggest a Gondwanan origin for Crotonoideae. Based on the extant and fossil species, Africa could be the place of origin for most of the Crotonoideae lineages that later dispersed to India via the Kohistan‐Ladakh Island Arc during the late Maastrichtian‐Paleocene time. The former distribution of Crotonoideae in mid‐high latitudinal regions is due to the dispersal of the subfamily from Africa to mid latitudinal regions of northern hemisphere during the early Paleogene via the boreotropical dispersal route. This study also corroborates the “Out of India” hypothesis for the dispersal of Crotonoideae from India to Southeast Asia. The present deep‐time fossil records from India discussed in this study substantiate the evolutionary and biogeographic history of subfamily Crotonoideae.

Investigating the Tropospheric Chemistry of Acetic Acid Using the Global 3‐D Chemistry Transport Model, STOCHEM‐CRI

AbstractAcetic acid (CH3COOH) is one of the most abundant carboxylic acids in the troposphere. In the study, the tropospheric chemistry of CH3COOH is investigated using the 3‐D global chemistry transport model, STOCHEM‐CRI. The highest mixing ratios of surface CH3COOH are found in the tropics by as much as 1.6 ppb in South America. The model predicts the seasonality of CH3COOH reasonably well and correlates with some surface and flight measurement sites, but the model drastically underpredicts levels in urban and midlatitudinal regions. The possible reasons for the underprediction are discussed. The simulations show that the lifetime and global burden of CH3COOH are 1.6–1.8 days and 0.45–0.61 Tg, respectively. The reactions of the peroxyacetyl radical (CH3CO3) with the hydroperoxyl radical (HO2) and other organic peroxy radicals (RO2) are found to be the principal sources of tropospheric CH3COOH in the model, but the model‐measurement discrepancies suggest the possible unknown or underestimated sources which can contribute large fractions of the CH3COOH burden. The major sinks of CH3COOH in the troposphere are wet deposition, dry deposition, and OH loss. However, the reaction of CH3COOH with Criegee intermediates is proposed to be a potentially significant chemical loss process of tropospheric CH3COOH that has not been previously accounted for in global modeling studies. Inclusion of this loss process reduces the tropospheric CH3COOH level significantly which can give even larger discrepancies between model and measurement data, suggesting that the emissions inventory and the chemical production sources of CH3COOH are underpredicted even more so in current global models.

Ground-based observation of the cyclic nature and temporal variability of planetary-scale UV features at the Venus cloud top level

A planetary-scale bright and dark UV feature, known as the “Y-feature,” rotates around Venus with a period of 4–5 days and has been long-time interpreted as planetary waves. When assuming this, its rotation period and spatial structure might help to understand the propagation of the planetary-scale waves and find out their role in the acceleration-deceleration of the zonal wind speed, which is essential for understanding the super-rotation of the planet. The rotation period of the UV feature varied over the course of observation by the Pioneer Venus orbiter. However, in previous explorations of Venus such as Pioneer Venus and Venus Express, the spacecraft were operated in nearly fixed inertial space. As a result, the periodicity variations on sub-yearly timescales (one Venusian year is ∼224 Earth days) were obscured by the limitations of continuous dayside observations. We newly conducted six periods of ground-based Venus imaging observations at 365nm from mid-August 2013 to the end of June 2014. Each observation period spanned over half or one month, enabling long-term monitoring of Venus’ atmosphere above the equator region. Distributions of the relative brightness were obtained from the equatorial (EQ) to mid-latitudinal regions in both hemispheres, and from the cyclical variations of these distributions we deduced the rotation periods of the UV features of the cloud tops brightness. The relative brightness exhibited periods of 5.2 and 3.5 days above 90% of significance. The relative intensities of these two significant components also seemed subject to temporal variations. Although the 3.5-day component considered persists throughout the observation periods, its dominance over the longer period varied in a cyclic fashion. The prevailing first significant mode seems to change from 5.2-day waves to 3.5-day waves in about nine months, which is clearly inconsistent with the Venusian year. Clear periodic perturbations, indicating stability of the planetary-scale UV-feature, were observed only in the presence of single longer or shorter periodic waves. During the transition periods of dominant-wave changing, the amplitude of the relative brightness was largely changed. This can be explained by the deformation of the Y-shaped UV feature as observed by Pioneer Venus in 1979.

Low diversity of planktonic bacteria in the tropical ocean.

The diversity of macro-organisms increases towards the equator, with almost no exceptions. It is the most conserved biogeographical pattern on earth and is thought to be related to the increase of temperature and productivity in the tropics. The extent and orientation of a latitudinal gradient of marine bacterioplankton diversity is controversial. Here we studied the euphotic zone of the Atlantic Ocean based on a transect covering ~12.000 km from 51°S to 47 °N. Water samples were collected at 26 stations at five depths between 20 and 200 m and sequentially filtered through 8 μm, 3 μm and 0,22 μm filters, resulting in a total of 359 samples. Illumina sequencing of the V5–V6 region of the 16S rRNA gene revealed a clear biogeographic pattern with a double inverted latitudinal gradient. Diversity was higher in mid-latitudinal regions of the Atlantic Ocean and decreased towards the equator. This pattern was conserved for bacteria from all three planktonic size fractions. Diversity showed a non-linear relationship with temperature and was negatively correlated with bacterial cell numbers in the upper depth layers (<100 m). The latitudinal gradients of marine bacterial diversity and the mechanisms that govern them are distinct from those found in macro-organisms.

The Cenozoic Cooling – continental signals from the Atlantic and Pacific side of Eurasia

The evolution of Cenozoic continental climate signals from the Atlantic and Pacific side of Eurasia can be assessed for the first time by comparing climate records obtained for two mid-latitudinal regions. For the West, a detailed climate record over the past 45 Ma, based on palaeofloras from two Northern German Cenozoic basins (Mosbrugger et al., 2005) revealed major trends and shorter-term events throughout the Cenozoic Cooling, thus testifying the close correlation of continental and marine temperature evolution as derived from oxygen isotopes (Zachos et al., 2008). Using the same methodology, we analyze a total of 14 floral horizons originating from continental strata of Southern Primory'e (Russia) in order to study the evolution at the eastern side of the continent. The Primory'e record spans the middle Eocene to early Pleistocene. As the coeval record for the Atlantic side, it reflects major global signals of Cenozoic climate change such as the temperature decline throughout the late Eocene, coinciding with the growth of Antarctic Ice-sheets, warming during the Mid-Miocene Climatic Optimum, and step-wise cooling throughout the later Neogene. The comparison of both records reveals differing regional patterns. The considerable longitudinal temperature gradient, currently existing between both study areas, already began to evolve during the Aquitanian, and was very significant during the Mid-Miocene Climatic Optimum. The temperature offset between East and West is likely attributable to an effective North Atlantic Current, already operational from the late early Miocene onwards bringing about mild winters and low seasonality in Western Europe, while in Primory'e, seasonality steadily increased from the late Oligocene on. The strong late Pliocene decline of cold month mean temperatures recorded in Primory'e is supposed to coincide with the establishment of the Siberian High as semi-permanent structure of the Northern Hemisphere circulation pattern. When comparing the precipitation records obtained for both study areas, an unexpected co-variability at the longer-term (in the order of 5–20 Ma) is noted, pointing to continent-wide hydrological changes. The steady decline of mean annual precipitation in the Primory'e record, beginning in the Bartonian and culminating in the Aquitanian, coincides with an aridity increase reported from coeval Chinese inland localities of the mid-latitudes. The seasonality patterns of rainfall point to progressive intensification of the East Asian Summer Monsoon in Primory'e since the later Tortonian while the post-Zanclean decline of the precipitation of the dry season can be related to an increasing impact of the winter monsoon.

Effectiveness of heliotherapy for psoriasis clearance in low and mid-latitudinal regions: A theoretical approach

The action spectrum for psoriasis clearance is reconstructed taking into account the results obtained in the early 1980s. The antipsoriatic action spectrum is used for weighting the medical cabinet UV spectra, and the solar spectra measured in San Diego (USA) and Belsk (Poland). The mean cumulative antipsoriatic effective dose of 450mJcm−2, due to TL-01 (UVB narrowband) tubes, is taken by a patient with skin phototype II during routine 20 phototherapy sessions carried out in a phototherapy cabinet in the Medical University of Łódz. Thus, the daily mean dose of value 22.5mJcm−2 is proposed as the threshold for daily solar dose for numbers of out-door exposures to clear psoriasis. We assume that the heliotherapy will last a whole month with every day 2h exposition to the direct sunlight around local noon. The heliotherapy will be successful if weather conditions permit at least 20days with the daily exposure over the threshold. The minimum cumulative ambient erythemal dose, necessary for psoriasis clearance, is estimated as 144 standard erythema dose (SED) for the whole heliotherapy period. We find that heliotherapy could be effectively used in March through October (San Diego) and in June through August (Belsk). Thus, the heliotherapy against psoriasis is possible not only at southern resorts but even at the mid-latitude sites.

Ice-rafting from the British–Irish ice sheet since the earliest Pleistocene (2.6 million years ago): implications for long-term mid-latitudinal ice-sheet growth in the North Atlantic region

The Plio-Pleistocene intensification of Northern Hemisphere continental ice-sheet development is known to have profoundly affected the global climate system. Evidence for early continental glaciation is preserved in sediments throughout the North Atlantic Ocean, where ice-rafted detritus (IRD) layers attest to the calving of sediment-loaded icebergs from circum-Atlantic ice sheets. So far, Early-Pleistocene IRD deposition has been attributed to the presence of high-latitudinal ice sheets, whereas the existence and extent of ice accumulation in more temperate, mid-latitudinal regions remains enigmatic.Here we present results from the multiproxy provenance analysis of a unique, Pleistocene-Holocene IRD sequence from the Irish NE Atlantic continental margin. There, the Challenger coral carbonate mound (IODP Expedition 307 site U1317) preserved an Early-Pleistocene record of 16 distinctive IRD events, deposited between ca 2.6 and 1.7 Ma. Strong and complex IRD signals are also identified during the mid-Pleistocene climate transition (ca 1.2 to 0.65 Ma) and throughout the Middle-Late Pleistocene interval. Radiogenic isotope source-fingerprinting, in combination with coarse lithic component analysis, indicates a dominant sediment source in the nearby British–Irish Isles, even for the oldest, Early-Pleistocene IRD deposits. Hence, our findings demonstrate, for the first time, repeated and substantial (i.e. marine-terminating) ice accumulation on the British–Irish Isles since the beginning of the Pleistocene. Contemporaneous expansion of both high- and mid-latitudinal ice sheets in the North Atlantic region is therefore implied at the onset of the Pleistocene. Moreover, it suggests the recurrent establishment of (climatically) favourable conditions for ice sheet inception, growth and instability in mid-latitudinal regions, even in the earliest stages of Northern Hemisphere glacial expansion and in an obliquity-driven climate system.

Global distribution and variability of formic acid as observed by MIPAS‐ENVISAT

Formic acid (HCOOH) vertical profiles have been retrieved from MIPAS‐ENVISAT satellite data in the upper troposphere for the first time. Based on new spectroscopic line‐strength measurements by Vander Auwera et al. (2007) of HCOOH, a retrieval setup was developed and optimized to study its global distribution between 2002 and 2008. A strong seasonality, directly associated to plant growth and corresponding biogenic emissions, is observed and dominates in the middle latitudes of the Northern Hemisphere. Here, the mean monthly volume mixing ratios (VMR) at 8 km altitude typically reach 100–110 parts per trillion in volume (pptv) during the summer and decrease to about 45 pptv in the early winter. At 16 km and higher altitudes, the VMRs remain under the 20 pptv level and have a much smaller amplitude (&lt;10 pptv). In the Southern Hemisphere, strong signals (up to 1 ppbv at 10 km in a single measurement) are detected from biomass burning during the August–October time period and can enhance the monthly mean background levels above specific tropical and midlatitudinal regions by a factor of 2 or more. In‐plume production of HCOOH through photochemical processes has been identified during an extreme event in September 2006, although it is not likely to contribute significantly to the overall upper tropospheric abundances of formic acid.

Relationships of surface air temperature anomalies over Europe to persistence of atmospheric circulation patterns conducive to heat waves

Abstract. Heat waves are among natural hazards with the most severe consequences for human society, including pronounced mortality impacts in mid-latitudes. Recent studies have hypothesized that the enhanced persistence of atmospheric circulation may affect surface climatic extremes, mainly the frequency and severity of heat waves. In this paper we examine relationships between the persistence of the Hess-Brezowsky circulation types conducive to summer heat waves and air temperature anomalies at stations over most of the European continent. We also evaluate differences between temperature anomalies during late and early stages of warm circulation types in all seasons. Results show that more persistent circulation patterns tend to enhance the severity of heat waves and support more pronounced temperature anomalies. Recent sharply rising trends in positive temperature extremes over Europe may be related to the greater persistence of the circulation types, and if similar changes towards enhanced persistence affect other mid-latitudinal regions, analogous consequences and implications for temperature extremes may be expected.

Water vapor distributions measured with the Michelson Interferometer for Passive Atmospheric Sounding on board Envisat (MIPAS/Envisat)

We present water vapor profiles obtained from infrared limb emission measurements recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the European Environmental Satellite (Envisat). These retrievals are based on constrained nonlinear least squares fitting. The retrievals are very sensitive to the radiative signals of thin transparent clouds and measurements showing any signature of cloud contamination have been rigorously excluded. The vertical resolution of the retrieved water vapor profiles is 4.5 to 6.5 km up to an altitude of approximately 42 km. The resulting total error of the retrieved water vapor profiles, including measurement noise, systematic and random parameter uncertainties like interfering species or preretrieved temperature or spectroscopic data, is in the range of 6 to 9% in the stratosphere. Towards the tropopause, the error increases up to 30% due to the exponential gradient of the tropospheric water vapor profile, where small line of sight uncertainties lead to strong absolute variations in the water vapor profile below the hygropause. Averaged water vapor distributions obtained from measurements taken during 11 days in June, July, and August 2003 show the expected distributions with low water vapor volume mixing ratios (VMRs) above the tropopause, comparatively dry air inside the tropical stratospheric updraft region and indications for strong dehydration above the Antarctic continent inside the polar vortex. Additionally, in the transition from tropics to subtropics, a latitude band was observed where, in higher altitudes, large water vapor VMRs were measured compared to adjacent tropical and midlatitudinal regions. Especially over the Arabic peninsula, a moist region at 18 km altitude was observed, which was probably related to the South Asian monsoon circulation.

Effects of clouds on the surface erythemal UV-B irradiance at northern midlatitudes: estimation from the observations taken at Belsk, Poland (1999–2001)

Results of 2-year measurements at Belsk, Poland, have been analyzed to investigate the effects of clouds on ultraviolet (UV) radiation. Data were collected using a Solar Light UV-biometer 501A (erythemally weighed UV irradiances), Kipp&Zonnen CM11 pyranometer (total solar radiation), and SCI-TEC Brewer Spectrophotometer (total ozone, aerosol and thin cloud optical depth). Hourly reports on total cloud cover and cloud type were also used in this study. It is found that overcast conditions prevail over central Poland and the sky is covered mainly by low-level clouds. The averaged cloud properties derived from visual observations correspond with those estimated from satellite measurements. An empirical relationship has been postulated for the cloud reduction factor of the erythemally weighted UV radiation as a function of the cloud reduction factor for total solar radiation. The relation may be used to reconstruct the past UV-time series. It is established that information of cloud amount and type to estimate the UV-B attenuation provides a substantial uncertainty. Stronger attenuation of the UV radiation by real clouds was found than that estimated by a radiative transfer model when examining results of the cloud optical depth at 320 nm and the cloud attenuation of the UV radiation measurements. The reduction of the erythemally weighted UV irradiance by thin clouds seems to be independent of the cloud type. It is reported that 1.4% of UV-B irradiance measurements were cloud enhanced in the snowless period of the year. The maximum UV-B enhancement of ∼20% greater than an equivalent clear-sky value was found. Numerous enhancements occurred when high-, mid-, and low- (broken Cu) clouds appeared simultaneously. Similar range of the observed enhancements and cloud composition as in previous cloud enhancement studies (over low latitudinal regions) suggest that common mechanisms may govern this phenomenon in low- and mid-latitudinal regions.

Climatic Impact on Small Grain Production in the Subarctic Region of the United States

The Subarctic comprises the higher mid-latitudinal regions with short, cool, moist summers and long, cold, dry winters. Indeed, the short, cool growing season is often thought of as a barrier to crop growth and diversity in these regions. Little is known, however, concerning the impact of the Subarctic climate on crop production. This study aimed to identify the climatic factors that are most important to the production of small grains in the Subarctic region of Alaska. The impact of climate on 'Galt' and 'Weal' barley (Hordeum vulgare L.), 'Nip' and 'Toral' oat (Avena sativa L.), and 'Gasser' and 'Park' wheat (Triticum aestivum L.) was assessed using climate and grain yield data collected from 1972 to 1989 at Fairbanks. Multiple regression analysis was used to identify the climatic factors that most influence yield. Different factors accounted for the largest proportion of variability in yield across years for the different grains. 1) For barley, variations in precipitation deficit (pan evaporation minus precipitation) and distribution of precipitation events within a growing season accounted for 41% of the variability across years in yield of Galt and Weal cultivars. 2) For oat, variations in the precipitation deficit ratio (ratio between precipitation deficit and pan evaporation) accounted for 44% of the variability across years in yield of Nip and 58% in yield of Toral oat. 3) For wheat, variations in number of days between precipitation events within a growing season, precipitation deficit, and temperature explained 70% of the variability across years in yield of Gasser and Park wheat. Results from our analysis further indicated that small grain production was bolstered in seasons with greater precipitation, more frequent precipitation, or lower evaporative demand (pan evaporation). Only wheat production appeared to be favored by higher minimum air temperatures. This study suggests that, despite the cool growing season in interior Alaska, the primary climatic limitation to crop production is water stress, associated with low precipitation or high evaporative demand. Therefore, land management practices aimed at conserving soil water will likely bolster crop production in the Subarctic.

El Niño signals contained in precipitation at the East China Sea

In general, El Nino signals can be easily identified in the tropical Pacific Ocean but not in the East China Sea, due to the complexity of the monsoon system and insufficient knowledge of El Nino effects in mid-latitudinal regions. In this study interannual variability of precipitation at the East China Sea is analyzed. Regional correlation and the spatial distribution of interannual variation of precipitation are also analyzed in order to confirm the teleconnection pattern. Most of the rainfall stations located in the southern area of Korea show significant biennial and interannual cycles with 2 to 8 yr periodicity. As for the wavelet analysis, it was found that the periods of significant maximum power spectrum corresponded with historical El Nino episodes, especially when a strong El Nino/La Nina was repeated. Maximum positive correlation appears between the Nino3 region (equatorial eastern Pacific) and the East China Sea at a time lag of about 4 mo. This correlation tendency can be confirmed in the second mode of Empirical Orthogonal Function analysis explaining the strong El Nino event. These results also suggest variation in upper wind fields such as the Hadley circula- tion and East Asia jet stream.

Northern Hemisphere Snow Cover Variability and Change, 1915–97

Abstract Historical and reconstructed snow cover data from stations in Canada, the United States, the former Soviet Union, and the People’s Republic of China were used to reconstruct monthly snow cover extent (SCE) fluctuations over midlatitudinal (∼40°–60°N) regions of North America (NA) and Eurasia back to the early 1900s using an areal snow index approach. The station distribution over NA allowed SCE to be reconstructed back to 1915 for 6 months (November–April), along with estimates of monthly mean snow water equivalent (SWE) from gridded daily snow depth data. Over Eurasia, SCE was able to be reconstructed back to 1922, but major gaps in the station network limited the approach to 3 months (October, March, and April). The reconstruction provided evidence of a general twentieth century increase in NA SCE, with significant increases in winter (December–February) SWE averaging 3.9% per decade. The results are consistent with an observed increasing trend in winter snow depth over Russia and provide furth...

Characterization of Drought across Climatic Spectrum

A conceptual model of drought characterization across the climatic spectrum is formulated. The model is particularly suited to subtropical and midlatitudinal regions. Drought duration, intensity, and recurrence interval are expressed in terms of the ratio of mean annual precipitation to annual global terrestrial precipitation. The model is useful as a framework for the systematic analysis of droughts and the assessment of changes in drought characteristics due to climatic changes.

Surface UV radiation over Australia, 1979–1992: Effects of ozone and cloud cover changes on variations of UV radiation

Time series of daily erythemal ultraviolet radiation (UVR) exposure, ozone, and cloud cover were analyzed over the Australian continent using data sets from the NASA Total Ozone Mapping Spectrometer (TOMS) from 1979–1992. The TOMS UVR exposures showed good agreement with data from surface observations. Using a relationship derived from comparisons of the TOMS partial cloud reflectivities with surface cloud cover observations, the TOMS reflectivities were converted into estimates of cloud cover for several Australian regions. It was shown that the deseasonalized time series of UVR exposures can be statistically described as a linear function of ozone and cloud cover anomalies. Results of a trend analysis indicated statistically significant increases in UVR exposures of 10% decade−1 in the summer months in the tropics. These were associated with a simultaneous depletion of ozone and a decrease in cloud cover. Midlatitudinal regions showed no significant trends of UVR. It was found that variations of ozone and UVR over Australia were significantly influenced by the quasi‐biennial oscillation (QBO). An increase in zonal wind strength of 20 m s−1 was correlative with reductions of ozone of 1.7% and enhancements of UVR exposures of 2.2%. An increase in solar radio flux of 100×10−22 W m−2 (Hz)−1 was associated with significant reductions of UVR of 5–10% in the tropical and subtropical regions in summer. The results suggested that enhancements in summer UVR exposures of about 10–20% above the climatological average might be expected in years in which the QBO is in its westerly phase and the solar cycle is at its minimum.

Airborne Raman lidar

We designed and tested an airborne lidar system using Raman scattering to make simultaneous measurements of methane, water vapor, and temperature in a series of flights on a NASA-operated C-130 aircraft. We present the results for methane detection, which show that the instrument has the requisite sensitivity to atmospheric trace gases. Ultimately these measurements can be used to examine the transport of chemically processed air from within the polar vortex to mid-latitudinal regions and the exchange of stratospheric air between tropical and mid-latitudinal regions.