Mid-low Latitudes Research Articles

Southern Hemisphere Winter Storm Tracks Respond Differently to Low and High CO2 Forcings

Abstract In the Southern Hemisphere, Earth system models project an intensification of winter storm tracks by the end of the twenty-first century. Previous studies using idealized models showed that storm track intensity saturates with increasing temperatures, suggesting that the intensification of the winter storm tracks might not continue further with increasing greenhouse gases. Here, we examine the response of midlatitude winter storm tracks in the Southern Hemisphere to increasing CO2 from two to eight times preindustrial concentrations in more realistic Earth system models. We find that at high CO2 levels (beyond 4×CO2), winter storm tracks no longer exhibit an intensification across the extratropics. Instead, they shift poleward, weakening the storm tracks at lower midlatitudes and strengthening at higher midlatitudes. By analyzing the eddy kinetic energy (EKE) budget, the nonlinear storm-track response to an increase in CO2 levels in the lower midlatitudes is found to stem from a scale-dependent conversion of eddy available potential energy to EKE. Specifically, in the lower midlatitudes, this energy conversion acts to oppositely change the EKE of long and short scales at low CO2 levels, but at high CO2 levels, it mostly reduces the EKE of shorter scales, resulting in a poleward shift of the storms. Furthermore, we identify a “tug of war” between the upper and lower temperature changes as the primary driver of the nonlinear-scale-dependent EKE response in the lower midlatitudes. Our results suggest that in the highest emission scenarios beyond the twenty-first century, the storm tracks’ response may differ in magnitude and latitudinal distribution from projected changes by 2100. Significance Statement The Southern Hemisphere winter storm track is projected to intensify by the end of the century, with the most significant intensification occurring in the higher midlatitudes. However, we show that the intensification is not a linear function of the radiative forcing associated with increasing CO2 levels. In fact, our study shows a poleward shift at very high CO2 levels, with the storm track moving southward. This suggests that the Southern Hemisphere winter storm track may require time-sensitive adaptation strategies, as the impacts of global warming on the storm track may not be a linear function of CO2 concentration in the atmosphere.

A climate change from icehouse to greenhouse following Huronian glaciation: Evidence from long-term storm deposits of the Paleoproterozoic Hutuo Group in the North China Craton

The Hutuo Group was deposited from 2.14 to 2.0 Ga in Wutai Mountain, North China Craton. This group is composed of the Doucun and Dongye subgroups, which are likely contemporaneous heterotopic facies. The Hutuo Group displays well-known positive to negative drifts of inorganic carbon isotopes, large-scale stromatolitic carbonates, and red beds in epigenetic environments. Twelve storm-deposited lithofacies were identified in the Dongye Subgroup, which changes from sandstones, siltstones, and mudstones of the Qingshicun and lower Wenshan formations to carbonates of the upper Wenshan, Hebiancun, Jian’ancun, Daguanshan, Huaiyincun, Beidaxing, and Tianpengnao formations from bottom to top. The above sedimentary sequence transformation indicates a gradual transformation from terrigenous storm deposits in the Qingshicun and Wenshan formations to endogenous or mixed-source storm deposits in the Hebiancun, Jian’ancun, Daguanshan, and Huaiyincun formations. Additionally, coastal and shallow-marine storm deposits are revealed from sedimentary structures, including hummocky cross-stratification, intraclasts or boulder clays exhibiting radial or chrysanthemum-shaped stacking, and sinuous or torn stromatolites. These storm deposits, occurring with oolitic and stromatolitic carbonates of mid-low latitudes or tropical-subtropical zones, are characterized as tropical storm deposits. Based on reported ages, we propose that such tropical storms started from ca. 2.1 Ga and lasted for over 40 Myr. The long-term storm deposits indicate high temperatures and intense water circulation during the greenhouse climate. A climate change from icehouse to greenhouse is also evident by the extensive distribution of carbonates, evaporates, and organic-rich shales above the glacial diamictites in multi-cratons, and was probably driven by the transformative evolution of the atmosphere.

Illuminating an uncatalogued micropalaeontological collection at the Lapworth Museum of Geology, University of Birmingham

The Lapworth Museum of Geology (LMoG) at the University of Birmingham (UoB) holds >300,000 specimens, including a sizable micropalaeontology collection. However, ~50% of the total LMoG collection is not digitised (e.g., there is no digital record) and thus inaccessible to external audiences. We contributed to the LMoG’s ongoing digitisation efforts by assessing the curation, historic and scientific significance of an uncatalogued assortment of “large” micropalaeontological specimens. We digitised and curated 219 specimens (comprising 307 parts) made up of hand specimens, containers and vials of loose fossils and sediment, and thin sections. The collection contained organisms from ten different phyla but was dominated by Larger Benthic Foraminifera (LBF). This dominance is reflected in the predominantly low-mid latitude geographic distribution of the collection, reflecting LBFs subtropical ecological preferences. The completeness of specimen metadata varied considerably with generally good geographic, chronostratigraphic, and taxonomic information at higher levels but poorer information with increasing detail and in relation to acquisition. Despite these limitations, we find that the studied collection was primarily assimilated for teaching purposes with donations from UoB staff and other specialist museums, and purchase from commercial dealers. There is little evidence for research on these specimens to date. Future work at the LMoG will digitise the collections and contribute to the ongoing efforts of international bodies to create large, open-access natural history datasets.

Seasonal variation in nighttime NO radiative cooling as observed by TIMED/SABER in lower thermosphere during solar maximum and solar minimum

Both composition and temperature play a crucial role in determining the NO radiative cooling in lower thermosphere as observed by TIMED/SABER. In this work, we present a detailed investigation of seasonal variation in thermospheric NO radiative cooling. We have carried forward the investigation of Li et al. (2018) regarding the variations in local nighttime peak NO radiative cooling and its altitude during solar maximum and solar minimum conditions. By analyzing latitudinal changes over quiet times for each month in year 2018, it is evident that both the investigated parameters exhibit summer-winter variability. The qualitative contribution of different species (i.e., NO, and O), and temperatures in determining the vertical profile of NO radiative cooling for different latitudes is investigated by utilizing the NRLMSISE-00 estimated parameters, and SNOE observed NO density. The temperature, NO density, and compositional variations due to asymmetrical solar heating and associated summer-to-winter circulation in both the hemispheres during solar minimum conditions seem to be the dominating factor in controlling the NO radiative cooling during different seasons. The altitudes at which maximum cooling by NO occurs exhibits an inverse correlation with the amount of radiative cooling. The region of enhanced NO densities (polar and summer hemispheric low-mid latitude regions) have larger NO radiative cooling with lower peak altitudes in comparison to other regions (equatorial to winter hemispheric low-mid latitude regions), where NO radiative cooling is low with higher peak altitude values.

Radiative Heating of High‐Level Clouds and Its Impacts on Climate

AbstractThe interactions of clouds with radiation influence climate. Many of these impacts appear to be related to the radiative heating and cooling from high‐level clouds, but few studies have explicitly tested this. Here, we use simulations with the ICON‐ESM model to understand how high‐level clouds, through their radiative heating and cooling, influence the large‐scale atmospheric circulation and precipitation in the present‐day climate. We introduce a new method to diagnose the radiative heating of high‐level clouds: instead of defining high‐level clouds as all clouds at temperatures colder than −35°C, we define them as all clouds with a cloud top at temperatures colder than −35°C. The inclusion of the lower cloud parts at temperatures warmer than −35°C circumvents the creation of artificial cloud boundaries and strong artificial radiative heating at the temperature threshold. To isolate the impact of high‐level clouds, we analyze simulations with active cloud‐radiative heating, with the radiative heating from high‐level clouds set to zero, and with the radiative heating from all clouds set to zero. We show that the radiative interactions of high‐level clouds warm the troposphere and strengthen the eddy‐driven jet streams, but have no impact on the Hadley circulation strength and the latitude of the Intertropical Convergence Zone. Consistent with their positive radiative heating and energetic arguments, high‐level clouds reduce precipitation throughout the tropics and lower midlatitudes. Overall, our results confirm that the radiative interactions of high‐level clouds have important impacts on climate and highlight the need for better representing their radiative interactions in models.

Eocene–Oligocene vegetation and climate changes in southeastern Brazil

The Eocene–Oligocene Transition (EOT) marks the onset of a major phase of global cooling with significant consequences to the vegetation worldwide. Here, we present palynological analyses from a site in southern São Paulo basin, southeastern Brazil (23.67°S; 46.58°W). An Oligocene age was constrained by the abundance of Dacrydiumites florinii and Podocarpidites spp., alongside the presence of taxa such as Cicatrocosisporites dorogensis and Polypodiisporites usmensis. Autochthonous elements including Paleoazolla, Corsinipollenites spp., Monoporopollenites annulatus, and Zygnemataceae algae spores are indicative of a low-energy freshwater depositional environment. In addition, we performed a comparison of Eocene and Oligocene temperature and precipitation estimates from records in southeastern Brazil using the weighted Mutual Climate Range approach. Results support global trends with a cooling of ca. 3–4 °C from the Eocene to the Oligocene accompanied by slightly drier regional conditions. Oligocene environmental changes drove a vegetation turnover by local extinction of warm-tropical taxa or a significant retreat of species unable to stand comparatively colder and drier climates under lower CO2atm concentrations. Our data are consistent with previous estimates and verify the impact of global cooling on the vegetation of mid-low latitudes of the Southern Hemisphere.

Vegetation greening amplifies shallow soil temperature warming on the Tibetan Plateau

Vegetation changes are expected to alter soil thermal regimes, consequently modifying climate feedbacks related to frozen ground thawing and carbon cycling in cold regions. The Tibetan Plateau (TP) contains diverse alpine ecosystems and the largest area of frozen ground in low–mid latitude regions. Evidence suggests ongoing vegetation greening and permafrost degradation during the past several decades on the TP. However, the effect of vegetation changes on soil thermal regimes on the TP is not well understood. Here, we quantify the response of shallow soil temperature change to vegetation greening on the TP using remote–sensing data, in–situ observations, and physics–based modelling. Our results show that over the past 20 years, vegetation greening on the TP was accompanied a notable decrease in the area of bare land by approximately 0.7% (5000 km2). Annual mean soil temperature showed a significant warming trend of 0.57 °C decade–1 (p < 0.05) during the period 1983–2019, exceeding the warming rate of surface air temperature. Changes in vegetation resulted in a warming effect on annual shallow soil temperature of 0.15 ± 0.33 °C across the TP during the period 2000–2019. The warming effect varies with frozen soil types: 0.24 ± 0.48 °C in permafrost, 0.18 ± 0.36 °C in seasonally frozen ground, and 0.11 ± 0.32 °C in unfrozen ground. The net warming effect was caused by a decrease in albedo and increase in radiation penetrating the canopy, outweighing the cooling effect related to a limited increase in evapotranspiration.

Arctic summer precipitation variability and its association with Indian summer monsoon anomalies

In recent years, there has been an increased focus on changes in Arctic precipitation，however, dominant characteristics of Arctic summer precipitation variability and their relationship to atmospheric circulation anomalies in the mid-low latitudes remain debatable. Spatially, the leading mode of Arctic summer precipitation variability is positive in most regions of the Arctic, corresponding to a cold anomaly in the middle and lower troposphere of the Arctic. As the temperature gradient between the Arctic and high latitudes increases, the zonal westerlies over the Arctic enhanced and lead to increased local baroclinicity and moisture convergence over the Arctic. This creats favorable conditions for increased summer precipitation. Additionally, it is found that there is a robust correlation between the first mode of Arctic summer precipitation and the Indian summer monsoon. Specifically, the positive Arctic summer precipitation anomaly is accompanied by a systematic northward shift of the zonal westerlies in the mid-latitude. The northward shift of the Asian westerly jet stream is associated with the intensified tropical easterly jet in the upper-troposphere and low-stratosphere, resulting in the strengthening of the monsoon circulation over the Indian Peninsula and then facilitating the precipitation anomaly over India. Numerical experiments reproduce the major features of the observed Arctic-Indian summer monsoon association, implying that Arctic summer precipitation variability is dynamically linked to tropical atmospheric circulation anomalies.

Exploring the link between convectively generated gravity waves and ionospheric anomalies: Insights from observations near the low-mid latitude geomagnetic transition region

In this paper, we examine the role of gravity waves (GWs) generated from tropospheric convection in initiating multiple nighttime ionospheric anomalies over Srinagar, located at 34.1°N, 74.8°E, and 25.91°N MLAT. Optical airglow observations during a geomagnetic quiet night (Ap = 4) of 29–30 June 2021, show the presence of plasma depletion tilted by an angle of ∼14.9° from the geographic North, quasi-periodic south eastward moving wave (QPSEMW), onset of two electrified MSTIDs and generation of plasma depletion channel between two MSTID phase fronts at ∼250 km altitude. GWs originating from deep convection in troposphere are detected at ∼85 and ∼97 km altitudes in OH and 557 nm filters respectively. The GWs in both the filters are propagating northward with almost similar characteristics. The intrinsic time period and vertical wavelength of GWs is estimated to be ∼11 min and ∼53 km respectively. We also present the observational indications of localised upliftment of F-layer ionosphere by ∼38 km, possibly initiated by polarization electric field generated through secondary GWs. The penetration of secondary GWs, generated by the dissipation of convective GWs, beyond the altitude of 500 km is also seen. FORMOSAT-7/COSMIC-2 RO data and SAMI3 model electron density suggest the occurrence of Sporadic-E (Es) layer at around 100 km altitude, caused by the negative gradient in eastward wind.

Impacts of global biogenic isoprene emissions on surface ozone during 2000–2019

Biogenic isoprene is an important precursor of tropospheric ozone (O3). Here, a coupled chemistry–vegetation model was used to quantify the contributions of isoprene emissions to surface O3 pollution on the global scale during 2000–2019. The biogenic isoprene emissions showed high values in mid–low latitudes and seasonal peaks in the summer hemispheres. They promote global surface O3 concentrations by 1.75 ppbv annually with regional hotspots of 4.39 ppbv (8.8%) in China and 5.36 ppbv (11.1%) in the U.S. in boreal summer. In the past two decades, isoprene emissions increased by 1.32 TgC yr−1 (0.67% yr−1) in the Northern Hemisphere but decreased by 0.71 TgC yr−1 (0.44% yr−1) in the Southern Hemisphere. Such changes of isoprene made opposite contributions to the surface O3 trend, with 0.26 ppbv yr−1 in eastern China but −0.32 ppbv yr−1 in the southeastern U.S. due to the changes in the background regime of chemical reactions. The impact of anthropogenic changes on the O3 trend is consistent with that of biogenic isoprene, but two to four times stronger in magnitude. This study revealed that the effective control of anthropogenic NOx emissions could mitigate regional O3 pollution even with the increased isoprene emissions under global warming.

Subtropical SST improved the understanding of the subseasonal reversal of surface air temperature in winter over the mid-high latitudes of Asia

The mid-high latitudes of Asia are cold air gathering places and dust source areas, whose subseasonal reversal of surface air temperature (SAT) between early winter (December 1 to January 14) and late winter (January 15 to February 28) could trigger extreme temperature at mid-low latitudes. Based on the observed interannual variation of SAT over mid-high latitudes of Asia (SATMH-A) in early and late winter after detrending from 1979 to 2021, the across-latitude linkage between the SATMH-A subseasonal reversal and subtropical sea surface temperature (SST) was revealed. The southern Indian Ocean SST anomalies caused the Ural high weakened through meridional circulations and led to a warm SATMH-A in early winter, but had little effect on SATMH-A in later winter. Whereas the North Atlantic SST anomalies enhanced the cold air intrusion in late winter by exciting Rossby wave to strengthen the Ural blocking, while the SATMH-A in early winter was largely unresponsive. The SST factors in Indian Ocean and North Atlantic primarily impacted the SATMH-A in early and late winter respectively. Their synergies effectively modulated the subseasonal reversal, which was conducive to improving the explanation and prediction for the SATMH-A subseasonal reversal. Especially in the winter of 2020/21 and 2021/22, the mean prediction bias was only 0.03 °C and 0.20 °C in early and late winter contributed by the two SST factors. Moreover, the improved prediction ability on the subseasonal scale was also helpful for the accurate predictions of SATMH-A on winter-mean scale. Findings of this study also have the potential to be applied to more accurately predict temperature extremes in mid-low latitudes.

Poleward intensification of midlatitude extreme winds under warmer climate

Our study investigates the global impact of midlatitude cyclones on extreme wind speed events in both hemispheres under a warmer climate. Using the latest version of the high-resolution ≈ 50 km grid-spacing atmospheric climate model AM4, developed by the Geophysical Fluid Dynamics Laboratory, we conducted simulations covering the 71-years period 1949–2019 for both the present-day climate and an idealised future global warming climate scenario with a homogeneous Sea Surface Temperature (SST) increase by 2 K. Our findings reveal that extreme near-surface wind speeds increase by up to 3% K−1 towards the poles while decrease by a similar amount in the lower midlatitudes. When considering only extreme wind speed events objectively attributed to midlatitude cyclones, we observe a migration by the same amount towards higher latitudes both in percentage per degree SST warming and absolute value. The total number of midlatitude cyclones decreases by roughly 4%, but the proportion of cyclone-associated extreme wind speed events increases by 10% in a warmer climate. Finally, Northwestern Europe, the British Isles, and the West Coast of North America are identified as hot spots with the greatest socio-economic impacts from increased cyclone-associated extreme winds.

Characteristics of IDL and Es layers and the impact of increasing solar activity on their descent at the Arabian Peninsula

Intermediate descending layers (IDLs) are often referred to enhanced plasma regions that are developed near lower-F/upper-E layers in the ionosphere. These layers often move down to the lower E-layers and merge with sporadic-E (Es) layers within a time span of several minutes to hours. The current study is focused on the investigation of the variability of IDL and Es layers’ diurnally and seasonally at a low-mid latitude region (Sharjah: 25.28oN, 55.46oE) close to the northern crest of equatorial ionization anomaly. We have used the very well-known technique of height-time-intensity (HTI) based on raw ionosonde data. It seems conclusive from our data that the diurnal, semidiurnal, and terdiurnal tides impact Es layer indirectly through the formation of IDLs in the upper-E region and their subsequent descent in the lower-E region. Also, the presence of descending IDL and Es layers follow a triannual pattern; the first one is a single layer diurnal pattern of descending early morning IDL during winter solstice and continued to be visible until spring equinox. The second one is the semidiurnal pattern prevails in autumnal equinox and the third one is a terdiurnal pattern in summer solstice. An important observation is the demonstration of the presence of terdiurnal tides being more prominent during lower solar activity period compared to higher solar activity.

Variation of Electron Density in the D-Region Using Kunming MF Radar under Low Solar Activity

So far, the least is known about the D-region ionosphere out of the entire ionosphere due to the lack of a conventional detecting method and continuous data accumulation. Medium frequency (MF) radar is an important conventional tool for understanding the D-region ionosphere by measuring the electron density (Ne) within the height range of 60–90 km. To investigate the statistical variation of the D-region, especially at the mid-low latitude area, this study presents the statistical variations in the D-region Ne with the solar zenith angle (SZA), season, and altitude observed by Kunming MF radar (25.6° N, 103.8° E) under low solar activity (2008–2009). The diurnal variation of Ne behaves like typical diurnal changes, which are closely consistent with the SZA. The outstanding feature, the diurnal asymmetry phenomenon, significantly appears in different seasons and at different altitudes. The Ne has obvious semi-annual characteristics, and is larger in summer and fall and the smallest in winter. Compared to other seasons, the variation in the Ne with altitude is the most stable in summer. Due to the impacts of the highest SZA, the value of Ne in winter is the smallest, with a maximum value of less than 300 electrons/cm3, and the largest in summer and fall, with a maximum of 472 electrons/cm3. Particularly, the peaks of Ne above 76 km do not always appear at the time when the SZA is the smallest (at noon). Both the simulations by the International Reference Ionosphere (IRI2016) and observations using MF radar present a strong positive correlation with solar radiation. Meanwhile, it cannot be ignored that there were still large differences between the simulations and observations. To quantitatively analyze the differences between the observations and simulations, the observed value was subtracted from the simulated value. The results show that the maximum value between them was up to 350 electrons/cm3, and the minimum difference appeared at around 72 km, with a value less than 100 electrons/cm3. However, below 66 km, the observations were larger than the simulations, which were, on the contrary, above 76 km.

A new pseudoscorpion genus (Garypinoidea: Garypinidae) from the Eocene supports extinction and range contraction in the European paleobiota.

During the Paleogene, the Holarctic experienced drastic climatic oscillations, including periods of extensive glaciation. These changes had a severe impact on both the flora and fauna causing widespread extinction and range shifts with some taxa retreating to refugia in the Mediterranean Basin. Here we provide evidence for this hypothesis using fossils from the pseudoscorpion family Garypinidae Daday, 1889 (Arachnida: Pseudoscorpiones). This family comprises 21 extant genera from all continents except Antarctica but is restricted to low mid-latitudes (<44°N) in the Northern Hemisphere. We provide the second record of garypinids from the European succinite ambers of the Eocene by describing the first extinct genus in Garypinidae, Baltamblyolpium gen. nov., which includes two species: Baltamblyolpium gizmotum sp. nov. from Baltic amber and Baltamblyolpium grabenhorsti sp. nov. from Bitterfeld amber. The new genus exhibits a morphology that closely resembles Neoamblyolpium Hoff, 1956 from western North America and the genus Amblyolpium Simon, 1898, which is widespread but includes taxa restricted to Mediterranean refugia in Europe. The discovery of a new fossil genus of Garypinidae from Europe confirms that the family was found at more northerly latitudes during the Eocene, however, extinction and range contraction resulted in their present-day relictual distribution in southern Europe like many other lineages that once thrived in the European "Baltic amber forest" of the Eocene.

Unique observations and interactions over the low-mid latitude transition region: Simultaneous study of plasma blobs, MSTIDs and plasma irregularities

This paper presents the novel simultaneous observations of plasma blobs, MSTIDs and plasma depletions in OI 630 nm all sky airglow images over Srinagar, India (34.1 °N, 74.8 °E) during the night of July 29, 2019, obtained using an airglow imager. Notably, the MSTID bands that followed the plasma blob interacted with a plasma depletion structure and suddenly disappeared, resulting in a reduction in the intensity of the plasma depletion. The SWARM satellite observations during the airglow observations indicated the occurrence of the plasma blob event and showed the conjugate appearance of MSTIDs in both hemispheres. The ROTI maps indicated the absence of EPBs but the occurrence of irregularities near the transition zone just before the presence of plasma blobs in airglow images and at the time of plasma irregularity detection. These observations suggest potential interactions between these phenomena, indicating that the presence of one impacts the other. This paper discusses in detail the observed ionospheric features and time evolution of the occurrences of these events, in addition to their interactions.Utilizingthe satellite observations, including SABER, INSAT-3DR and AIRS, the presence of deep convective clouds and upward propagating gravity waves near the location of the plasma blob is demonstrated, suggesting that convective-generated gravity waves could possibly trigger the formation of plasma blobs.

Receiving energy analysis and optimal design of crystalline silicon solar cell array on solar airship

The energy of solar airships comes from solar array, and the amount of solar radiation received by solar array determines airship endurance. Many studies optimize the solar array layout to receive more solar radiation. However, most of them are for flexible thin-film array. The influence of the size of the currently commonly used crystalline silicon panels on received energy is not considered. This paper establishes a solar irradiation model and a receiving energy model suitable for crystalline silicon arrays. The effects of array shape, array location, and module size on the energy received by crystalline silicon solar arrays are analyzed at different flight latitudes. Then, an improved genetic algorithm is designed to optimize the array geometric parameters. The results show that altering the size and layout of solar modules significantly influences energy capture, with opposing trends between regions of mid-low latitudes and high latitudes. Airships flying at mid-low latitudes should adopt larger solar modules, while airships flying at high latitudes should adopt smaller ones. After the optimization using the proposed algorithm, the received energy of solar array is further enhanced. The optimization resulted in a 2% improvement at mid-low latitudes and an 18% improvement at high latitudes over conventional optimization.

Snow Cover on the Tibetan Plateau and Topographic Controls

Snow cover plays a critical role in global energy and water cycles. Snow cover on the Tibetan Plateau (TP) provides vital water sources in western China and Himalayan regions, in addition to its weather and climate significance. The massive high mountain topography of the TP is the main condition for the presence and persistence of snow cover on the plateau at the mid-low latitudes of the Northern Hemisphere (NH). However, how the mountain topography controls snow-cover distribution on the TP remains largely unclear, and the relationship is not well quantified. Here, the spatial distribution and the topographic controls of snow cover on the TP are examined based on snow cover frequency (SCF) derived from MODIS snow cover product (MOD10A2 v005) and digital elevation model (DEM) data. The results show that snow cover on the TP is spatially unevenly distributed, and that it is characterized by rich snow and high SCF on the interior and the surrounding high mountain ranges, with less snow and low SCF in inland basins and river valleys. Snow cover on the TP presents elevation dependence: the higher the altitude, the higher the SCF, the longer the snow cover duration, and the more stable the intra-annual variation. The annual mean SCF below 3000 m above sea level (m a.s.l) is less than 4%, and it reaches 77% above 6000 m a.s.l. The intra-annual snow cover variation below 4000 m a.s.l features a unimodal distribution, while above 4000 m a.s.l it presents a bimodal distribution. The mean minimum SCF below 6000 m a.s.l occurs in summer, while above 6000 m a.s.l it occurs in winter. Due to differences in solar radiation and moisture condition caused by the mountain slope and aspect, the mean SCF generally increases with mountain slopes, and it is the highest on the north-facing aspect and the lowest on the south-facing aspect.

Interannual variation of the number of winter extreme low temperature days in southern China and influencing factors

The interannual variation of the number of winter extreme low temperature days (NELTD) in southern China from 1980 to 2019 is analyzed, and the individual and combined effects of the autumn sea surface temperature anomaly (SSTA) in the central equatorial Pacific and Siberian snow cover extent anomaly (SCEA) are investigated. The analysis shows that the anomalous atmospheric circulation in Eurasia is dominated by the meridional dipole pattern with the characteristic of “high north and low south” during the winter with large NELTD in southern China. The autumn SSTA in the central equatorial Pacific influences the winter NELTD mainly through the anomalous atmospheric circulation at the mid–low latitudes while the Siberian SCEA in October acts by affecting that at the mid–high latitudes. A cold SSTA is conducive to strengthening the winter East Asian trough and subtropical jet, as well as abnormal northeasterly wind in most parts of southern China. However, the central equatorial Pacific SSTA has a limited influence on the Ural ridge, polar front jet, and Siberian High at the mid–high latitudes. On the other hand, the Siberian SCEA could result in anomalous atmospheric circulation at the mid–high latitudes, which has a strengthening and supplementing effect on that of SSTA in the central equatorial Pacific. During the winter following the negative SCEA in October, the Ural ridge and Siberian High are strengthened while the polar front jet is weakened. The anomalous atmospheric circulation in Eurasia is dominated by a meridional dipole pattern of “high north and low south”. The comparative analysis reveals that both SSTA and SCEA play an important role in the NELTD in southern China, but they are independent factors and act in different ways. Therefore, the combined effect of autumn SSTA in the central equatorial Pacific and Siberian SCEA could contribute to the prediction of the winter NELTD in southern China.

Evaluation of Five Global Top-of-Atmosphere Outgoing Longwave Radiation Products

Five global monthly top-of-atmosphere (TOA) outgoing longwave radiation (OLR) products are evaluated in this study, including the products derived from the High-Resolution Infrared Radiation Sounder (HIRS), Clouds and the Earth’s Radiant Energy System (CERES), Advanced Very High Resolution Radiometer (AVHRR), the CM SAF cLoud, Albedo and surface RAdiation dataset from AVHRR data (CLARA), and the Global Energy and Water Cycle EXchanges (GEWEX) project. Results show that overall there is good consistency among these five products. Larger differences are found between GEWEX and CERES (HIRS) after (before) 2000 (RMSE ~ 5 W/m2), particularly in the tropical regions. In terms of global mean values, GEWEX shows large differences with the other products from the year 1992 to 2002, and CLARA shows large differences from the year 1979 to 1981, which are more obvious in the global ocean values. Large discrepancies among these products exist at low latitudinal bands, particularly before the year 2000. Australia and Asia (mid–low latitude part) are two typical regions in which larger differences are found.