Northern Hemisphere Winter Months Research Articles

The Influence of Dust‐Smoke Mixtures on Boundary Layer Processes and Nocturnal Warming in the Sahel

AbstractUsing a single column model with ground‐based, aircraft, and satellite data sets we assess the combined role of smoke and dust aerosols, land degradation/aridization (LDA), and their impact on the planetary boundary layer (PBL) in influencing near‐surface air temperature over the Sahel. Our study is unique because it assesses the combined role of smoke and dust aerosols on PBL evolution and near‐surface air temperatures during both day and nighttime. More importantly, using a theoretical framework, we provide a careful explanation of the geophysical processes responsible for the changes in PBL and near‐surface air temperature. Our results indicate that during northern hemisphere winter months, dust, and smoke over Sahel radiatively combine to impact the PBL. We show that aerosol mixtures dominated by dust modify PBL height in a manner that minimizes/maximizes surface layer cooling/warming at times when daytime maximum/nocturnal minimum temperatures occur. Furthermore, we find that increasing smoke contribution to total column aerosol optical extinction counteracts nighttime warming through daytime cooling. When smoke constitutes half or more of to the total column aerosol optical extinction, the ratio of longwave to shortwave radiative forcing is less than 10%, and nighttime cooling ensues. Minimum temperature is most sensitive to changes in mid‐visible aerosol optical depth (AOD) values <1 and doubling of dust AOD within this range during the 1950–1980 Sahelian LDA event is estimated to have a nocturnal warming potential of 0.6°C.

Historical and future changes in air pollutants from CMIP6 models

Abstract. Poor air quality is currently responsible for large impacts on human health across the world. In addition, the air pollutants ozone (O3) and particulate matter less than 2.5 µm in diameter (PM2.5) are also radiatively active in the atmosphere and can influence Earth's climate. It is important to understand the effect of air quality and climate mitigation measures over the historical period and in different future scenarios to ascertain any impacts from air pollutants on both climate and human health. The Coupled Model Intercomparison Project Phase 6 (CMIP6) presents an opportunity to analyse the change in air pollutants simulated by the current generation of climate and Earth system models that include a representation of chemistry and aerosols (particulate matter). The shared socio-economic pathways (SSPs) used within CMIP6 encompass a wide range of trajectories in precursor emissions and climate change, allowing for an improved analysis of future changes to air pollutants. Firstly, we conduct an evaluation of the available CMIP6 models against surface observations of O3 and PM2.5. CMIP6 models consistently overestimate observed surface O3 concentrations across most regions and in most seasons by up to 16 ppb, with a large diversity in simulated values over Northern Hemisphere continental regions. Conversely, observed surface PM2.5 concentrations are consistently underestimated in CMIP6 models by up to 10 µg m−3, particularly for the Northern Hemisphere winter months, with the largest model diversity near natural emission source regions. The biases in CMIP6 models when compared to observations of O3 and PM2.5 are similar to those found in previous studies. Over the historical period (1850–2014) large increases in both surface O3 and PM2.5 are simulated by the CMIP6 models across all regions, particularly over the mid to late 20th century, when anthropogenic emissions increase markedly. Large regional historical changes are simulated for both pollutants across East and South Asia with an annual mean increase of up to 40 ppb for O3 and 12 µg m−3 for PM2.5. In future scenarios containing strong air quality and climate mitigation measures (ssp126), annual mean concentrations of air pollutants are substantially reduced across all regions by up to 15 ppb for O3 and 12 µg m−3 for PM2.5. However, for scenarios that encompass weak action on mitigating climate and reducing air pollutant emissions (ssp370), annual mean increases in both surface O3 (up 10 ppb) and PM2.5 (up to 8 µg m−3) are simulated across most regions, although, for regions like North America and Europe small reductions in PM2.5 are simulated due to the regional reduction in precursor emissions in this scenario. A comparison of simulated regional changes in both surface O3 and PM2.5 from individual CMIP6 models highlights important regional differences due to the simulated interaction of aerosols, chemistry, climate and natural emission sources within models. The projection of regional air pollutant concentrations from the latest climate and Earth system models used within CMIP6 shows that the particular future trajectory of climate and air quality mitigation measures could have important consequences for regional air quality, human health and near-term climate. Differences between individual models emphasise the importance of understanding how future Earth system feedbacks influence natural emission sources, e.g. response of biogenic emissions under climate change.

Timing of fatal BASE jumping incidents: 1981–2018

BASE jumping, or fixed object parachuting, is a high-risk sport that has evolved significantly since its inception in the 1960s. Differences in parachute technology, teaching methods, jumping locations, practices, and popularity, among other factors, have qualitatively transformed the sport, particularly since 2000. The current study seeks to better understand how the rate of BASE jumping fatalities have changed as BASE jumping has grown. Using data from the BASE Fatality List, a resource continuously compiled by the BASE community with information on all known BASE fatalities, temporal trends in BASE fatalities that accompanied these changes over time are explored. The number of annual fatalities stayed between 0 and 5 from 1981 to 1999, but around the year 2000 began an upward trend that has continued through 2018. There appears to be large seasonal variation in monthly fatality rate, with the most deaths occurring in the northern hemisphere's summer months, the peak being in August, and the fewest deaths occurring in the northern hemisphere's winter months. The years 2013–2018 have continued the increasing fatality trend, but have otherwise deviated through increased variability in short-term and seasonal trends. Potential causes of these short-term trends, changing trends in the causes of fatal accidents, factors potentially unique to anomalous events, and directions for future research are also discussed.

Applying the tourist area life cycle within sport tourism: the case of Stellenbosch and German athletes

ABSTRACTThe popular tourist town of Stellenbosch in South Africa has long been a favoured training destination for European athletes during the northern hemisphere winter months. This paper considers the case study of German athletes – one of the most prominent groups to train in Stellenbosch since the early 1990s. The analysis involves the application of the concept of the Tourist Area Life Cycle (TALC) to the town’s sport tourism offering. In this regard Stellenbosch continues to experience notable demographic changes, not all of which are related to tourism. The impact of these changes is considered in the context of the factors which have historically made the town a sought-after destination for high performance athletics training camps. Consequently this paper offers an analysis of Stellenbosch’s current and future attractiveness as far as this particular dimension of sport tourism is concerned. This is an important consideration given the availability of alternative training destinations both in South Africa and internationally.

Characterizing heat stress on livestock using the temperature humidity index (THI)\u2014prospects for a warmer Caribbean

There is an urgent need to mitigate climate change-induced heat stress in livestock and poultry in the Caribbean, given the deleterious effects it has on food and nutrition security. The temperature humidity index (THI) was used to assess the potential for heat stress on four types of livestock and poultry (broiler and layer chickens, pigs and ruminants) for three different agro-ecological locations in Jamaica. The THI was formulated specifically to each livestock type and was examined for 2001–2012 for seasonal and annual patterns of variability. Differences in THI were observed between summer (July to September) and winter (December to February) with some moderation due to agro-ecological location. Our results suggest that animals in ambient field conditions in Jamaica may already be experiencing considerable periods of heat stress even during the relatively cooler northern hemisphere winter months. Future patterns of heat stress relative to a 1961–1990 baseline were derived from a regional climate model when mean global surface air temperature is 1.5, 2.0 and 2.5 °C above pre-industrial levels. At 1.5 °C, marked increases were noted in THI and almost persistent year-round heat stress is projected for Caribbean livestock. Conditions will be exacerbated at the higher global warming states. Possible response strategies such as cooling technologies are discussed.

Evaluating biases in simulated land surface albedo from CMIP5 global climate models

AbstractLand surface albedo is a key parameter affecting energy balance and near‐surface climate. In this study, we used satellite data to evaluate simulated surface albedo in 37 models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). There was a systematic overestimation in the simulated seasonal cycle of albedo with the highest bias occurring during the Northern Hemisphere's winter months. The bias in surface albedo during the snow‐covered season was classified into that in snow cover fraction (SCF) and albedo contrast (β1). There was a general overestimation of β1 due to the simulated snow‐covered albedo being brighter than the observed value; negative biases in SCF were not always related to negative albedo biases, highlighting the need for realistic representation of snow‐covered albedo in models. In addition, models with a lower leaf area index (LAI) tend to produce a higher surface albedo over the boreal forests during the winter, which emphasizes the necessity of improving LAI simulations in CMIP5 models. Insolation weighting showed that spring albedo biases were of greater importance for climate. The removal of albedo biases is expected to improve temperature simulations particularly over high‐elevation regions.

Climatology of stratospheric gravity waves and their interaction with zonal mean wind over the tropics using GPS RO and ground-based measurements in the two phases of QBO

The present manuscript deals with the spatial distribution of gravity wave activity over the tropics using ten years (2001–2010) of CHAllenging Mini Payloads (CHAMP) and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Global Positioning System (GPS) Radio Occultation (RO) data and ground-based radiosonde measurements over an equatorial station Singapore (1.36°N, 103.98°E) and four tropical stations, Guam (13.48°N, 144.80°E), Palau (7.33°N, 134.48°E) in the northern hemisphere, Darwin (12.41°S, 130.88°E) and Pago-Pago (14.33°S, 170.71°W) in the southern hemisphere from January 2001–December 2010. It also aims to quantify the difference in wave activity in the two phases of QBO, climatologically. Space-time spectra have been constructed over a latitude band of ±10° and decomposing the CHAMP/COSMIC temperature perturbations into symmetric and antisymmetric modes about the equator. Clear signature of equatorial waves with higher wavelength and a constant background of gravity waves (GW) with inertial frequency are prominent in the spectra. Strong GW and mean flow interaction can be seen in the lower stratosphere potential energy density (EP) and momentum flux with enhanced wave activity during the westerly (eastward wind) phase of quasi-biennial oscillation (QBO) (WQBO) over the equatorial and tropical stations like Singapore and Palau/Darwin, respectively. From the latitudinal distribution of energy density, the occurrence of two-peak structure in energy density can be seen in the middle and lower latitudes with an enhancement during the WQBO phase. The EP associated with GWs are calculated at lower stratospheric (19–26 km) heights and are compared with outgoing longwave radiation (OLR) to correlate the wave events with tropical deep convection during the easterly, i.e. westward wind (EQBO) and WQBO phases of QBO. Clear coherence of convection due to Asian summer monsoon with localized enhancement of wave activity over Western Pacific, South America and African region during the WQBO phase is observed at the lower stratospheric heights. Significant enhancement is observed during Northern Hemisphere winter months and minimum during summer. The longitudinally elongated portion of EP over the equator is partially affected by Kelvin wave (KW) like disturbances with short vertical scales and also by inertia GW.

‘Motherbirth or childbirth’? A prospective analysis of vaginal birth after caesarean blogs

Objectivevaginal birth after caesarean (VBAC) is a controversial issue with strong opinions for and against. The means in which women work through the different opinions and options using the internet should be of interest to midwives, obstetricians and policy makers. The aim of this study was to examine how women use English language internet blog sites to discuss the option of VBAC and what factors influence these women's decision to have a VBAC or repeat caesarean section. Designa qualitative study using internet blog sites as the source of data was undertaken. Google alerts were created to search for the term VBAC in internet blogs. These alerts were sent to the first author's email account daily for a one-year period (November 2007 to October 2008) and downloaded. The content was analysed using thematic analysis. Findingsthere were 311 blogs mentioning the word VBAC in the one-year period. Most of the blog sites and discussion originated from the USA. There were more blogs written during the Northern Hemisphere winter months than during other seasons. The main theme identified was a dichotomy in philosophical framework women held about birth; that is a ‘motherbirth’ or ‘childbirth’ framework. Whether women eventually wrote that they chose a VBAC or repeat caesarean or the extent to which they pursued their birth choice depended on whether they came from a perspective that a ‘good parent sacrifices themselves for their baby (prioritises the baby) and takes no risks’ (childbirth) or that ‘giving birth matters to the woman and a happy, healthy mother is a happy healthy baby (mother and baby have equal priority)’ (motherbirth). Several themes were identified including: surviving the damage; inadequate bodies; choice and control; fearing and trusting birth; negotiating the system; and minimising or overestimating risk. Key conclusionwomen filtered their decision making regarding VBAC through a belief system that prioritises according to their personal approaches. Implications for clinical practiceblogging may be providing a valuable insight into factors that inform decision making and may provide a forum of information and support for women who have experienced a caesarean section.

Seasonality change and the third millennium BP rainforest crisis in southern Cameroon (Central Africa)

The third millennium BP crisis of the central African rainforest is not sufficiently understood. The low resolution of most pollen profiles and a large plateau of the calibration curve aggravate the exact dating of the event, and its causal climatic parameters are debated. We present a high-resolution pollen profile from the swamp site Nyabessan in the southern Cameroonian rainforest, covering the period 3100-2300 cal yr BP. Between 3100 and 2500 cal yr BP, the climate was favourable for a regional evergreen forest with Caesalpiniaceae and Lophira and a local Raphia swamp forest. Around 2500/2400 cal yr BP, a significant decrease of mature forest and swamp forest taxa and an increase of pioneers indicate that the rain forest was seriously disturbed and replaced by secondary formations. The dominance of Trema orientalis, a pioneer well adapted to seasonal desiccation, points to a much more accentuated seasonality after 2500 cal yr BP, which seems to be linked to a southwards shift of the ITCZ during the northern hemisphere winter months. We propose that the rain forest crisis between 2500 and 2200 cal BP created favourable conditions for farming and paved the way for a major expansion of Bantu speaking populations.

Interannual variability of Greenland winter precipitation sources: 2. Effects of North Atlantic Oscillation variability on stable isotopes in precipitation

A new Lagrangian moisture source diagnostic is applied to identify the atmospheric conditions relevant for the fractionation of stable water isotopes during evaporation over the ocean and subsequent transport to Greenland. Northern Hemisphere winter months with positive and negative North Atlantic Oscillation (NAO) index are studied on the basis of ERA‐40 reanalysis data. Diagnosed moisture transport conditions are supplied to a Rayleigh‐type isotope fractionation model to derive estimates for the isotopic composition of stable isotopes in winter precipitation on the Greenland plateau for the two NAO phases. Because of changes in atmospheric circulation, moisture source locations for precipitation in Greenland vary strongly for different phases of the NAO. The mean source SST is ∼5.0 K warmer during negative NAO months compared to the positive phase. This signal is considerably stronger than what would result from interannual SST variability at a spatially fixed moisture source. Furthermore, moisture transport takes place at warmer temperatures during NAO negative conditions. Simulated average isotopic depletion of Greenland precipitation is less negative by 3.8 ± 6.8‰ for δ18O during the negative compared to the positive NAO phase. Comparison with ice core data from central Greenland for three winters shows good agreement between observed and simulated variability. The synoptic interplay of the initial conditions at the moisture sources and of the atmospheric transport conditions leads to enhanced NAO‐related interannual variability of stable isotopes. This could be important for understanding rapid shifts in stable isotopes during past climates. The isotope modeling applied here, however, considerably underestimates the absolute level of isotopic depletion. The offset is attributed to approximations in the model and uncertainties in the comparison with observational data. The high spatial resolution of the Lagrangian method reveals the nonhomogeneous structure of isotope NAO variability over the Greenland ice sheet. The results are therefore potentially useful for selecting new ice core drilling sites with maximum NAO variability.

Statistical study of inverted-V structures in FAST data

Abstract. Since the 1970s the inverted-V structures have been observed in the satellite-borne particle data. These Λ-shaped enhancements of the differential energy flux on the high energy channels on the top of low number flux on the lower energies are traces of auroral particle acceleration by U-shaped potential drops along the field lines within the auroral zones. In this study, we show the results of a statistical study of inverted-V events in five years (1997–2001) of the Fast Auroral SnapshoT (FAST) electron spectrograms. We found 588 well-defined events in total, but the statistical results are also shown for a subset of 97 very best events. The statistical analysis shows that the inverted-V's mainly occur within the auroral oval latitudes (65°–75° invariant latitude), in the pre-midnight sector (21–23 magnetic local time) and during the Northern Hemisphere winter months (November to February). The typical maximum energies of the inverted-V's are 2–4 keV, when a threshold value of 0.3 keV is used as a selection criterion. This corresponds to an acceleration potential drop of a few kV. The typical peak energy tends to increase towards the magnetic midnight. Most of the inverted-V events occur during magnetically quiet or moderate activity, Kp 1–4. The usual width of an inverted-V varies from 20 to 40 km and is comparable to the peak value of the typical meso-scale auroral arc width of 10–20 km. Furthermore, the polar cap potential values estimated from the SuperDARN radar data are always at least three times larger than the inverted-V acceleration potential drop.

AO, COWL, and Observed Climate Trends

Abstract The linear trends for a number of fields obtained from the reanalyses of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) are calculated for the Northern Hemisphere winter months (January–March) from the 55-yr period of 1948–2002. The fields include sea level pressure (SLP); geopotential height at 500 and 50 hPa; temperature at 500 and 50 hPa; zonally averaged height; temperature; zonal, meridional, and vertical velocities from 1000 to 50 hPa; and surface air temperature (SAT). The trend fields are expressed in terms of two alternate expansions: (i) contributions from the Arctic Oscillation (AO) and cold ocean–warm land (COWL) patterns, as defined from the leading modes of an empirical orthogonal function (EOF) analysis of sea level pressure; or (ii) contributions from the modified AO (AO*) and modified COWL (COWL*) patterns, defined from the leading EOFs of 500-hPa height. The residuals in each expansion are considered, and the completeness p...

Variability of diurnal tides and planetary waves during November 1978–May 1979

Nonlinear interactions between stationary waves and the migrating tides have been proposed as possible sources of nonmigrating tides in the middle and upper atmosphere. The objective of this study is to increase observational support for these processes. We examine the evolution of stationary planetary waves and nonmigrating diurnal tides in the lower mesosphere during November 1978–May 1979, based on a newly released, version 6 of the Nimbus 7 LIMS dataset. Planetary wavenumber one is large and variable during the Northern hemisphere winter months, reaching peak amplitude in the lower mesosphere between 20 and 30 January. This behavior is accompanied by rapid amplification of nonmigrating diurnal tides with zonal wavenumbers zero and two. These components correspond to product waves generated by interaction between the migrating diurnal tide and the stationary wave. The westward traveling zonal wavenumber two diurnal tide is dominant at tropical latitudes, in accordance with theoretical studies. The correlation between the nonmigrating tide and stationary wavenumber one is highest when the stationary wave penetrates to subtropical latitudes.

On the annual variation of tropospheric zonal winds observed above Christmas Island in the central equatorial Pacific

Observations of tropospheric winds have been made nearly continuously since early 1986 in the altitude range 2–18 km using a 50 MHz VHF wind profiler at Christmas Island (2°N, 157°W). Tropospheric zonal winds observed at Christmas Island show a pronounced annual variation in the middle to upper troposphere with westerly winds typically observed during northern winter. The annual variation is strongest during La Niña and weakest during El Niño. The amplitude of the cycle has a broad peak in the upper troposphere. Empirical orthogonal function (EOF) analysis of the vertical structure of the annual variation shows that the principal mode explains nearly 90% of the variance in the 90‐ to 365‐day frequency band. We attribute the annual variation in zonal winds to the annual variation in deep tropical heating associated with deep convection. The annual variation in tropical convection gives rise to annual variations in the surface pressure and pressure difference between Darwin and Tahiti. The pressure difference is greatest during northern hemisphere winter months. Other measures of the annual variation in tropical convection are provided by the tropical tropopause height and by highly reflective clouds (HRC). The tropical tropopause is highest during the northern hemisphere winter months when convection is most active over the maritime continent/western Pacific warm pool region and lowest in northern summer when convection is most active over the land masses of the northern hemisphere. The HRC show an annual migration in the centers of active convection. The annual latitudinal migration of the active centers of convection over the western Pacific with respect to the equatorial waveguide appears to be an important factor in determining the annual variation evident in the remote response of the zonal winds observed over Christmas Island.

Contribution to the ozone trend of heterogeneous reactions of ClONO2 on the sulfate aerosol layer

A photochemical two‐dimensional model including a grid‐point temperature distribution calculated from long‐term circulation statistics has been used to assess the relative role of heterogeneous reactions of chlorine nitrate on background sulfate aerosols. The sulfuric acid weight percentage is calculated explicitly as a function of temperature and water vapor content. It is shown that the contribution of these reactions to the ozone trend in the 1980‧s accounts for about only 10% the ozone loss due to N2O5 + H2O during the Northern Hemisphere winter months at mid‐latitude and between 50–200 mb. The weight of these reactions, however, may substantially increase in the presence of larger aerosol loading. The reaction ClONO2 + HCl on sulfate aerosols is found to be of comparable importance with respect to ClONO2 + H2O, and it is found to represent a significant source of active chlorine in the Antarctic spring.

A Worldwide Study of theH,DandZVariation On Abnormal Quiet Days (Aqds)

Summary An extensive analysis, using 80 stations, has been made of the latitudinal and longitudinal variation of the additional fields in H, D and Z that occurred on Abnormal Quiet Days (AQDs) during the Northern Hemisphere winter months in the years 1964-65. This analysis shows that the additional fields occurring on AQDs may be represented by a single-current vortex (SCV) that has a focus in the latitude range 15-20°N for the Northern Hemisphere winter AQDs. the evidence strongly suggests that the most likely location of the SCV is the ionosphere, although no further insight into what drives it is forthcoming.