Subtropical Latitudes Research Articles

A Moments View of Climatology and Variability of the Asian Summer Monsoon Anticyclone

AbstractA comprehensive investigation of the climatology of and interannual variability and trends in the Asian summer monsoon anticyclone (ASMA) is presented, based on a novel area and moments analysis. Moments include centroid location, aspect ratio, angle, and “excess kurtosis” (measuring how far the shape is from elliptical) for an equivalent ellipse with the same area as the ASMA. Key results are robust among the three modern reanalyses studied. The climatological ASMA is nearly elliptical, with its major axis aligned along its centroid latitude and a typical aspect ratio of ~5–8. The ASMA centroid shifts northward with height, northward and westward during development, and in the opposite direction as it weakens. New evidence finding no obvious climatological bimodality in the ASMA reinforces similar suggestions from previous studies using modern reanalyses. Most trends in ASMA moments are not statistically significant. ASMA area and duration, however, increased significantly during 1979–2018; the 1958–2018 record analyzed for one reanalysis suggests that these trends may have accelerated in recent decades. ASMA centroid latitude is significantly positively (negatively) correlated with subtropical jet-core latitude (altitude), and significantly negatively correlated with concurrent ENSO; these results are consistent with and extend previous work relating monsoon intensity, ENSO, and jet shifts. ASMA area is significantly positively correlated with the multivariate ENSO index 2 months previously. These results improve our understanding of the ASMA using consistently defined diagnostics of its size, geometry, interannual variability, and trends that have not previously been analyzed.

Synoptic comparison between two cases of subtropical cyclones in the South coast of Brazil

Subtropical Cyclones are low-pressure systems that occur in tropical or subtropical latitudes and that have characteristics of both tropical cyclones and extratropical cyclones. The subtropical class also has a thermodynamic forcing near the surface resulting from heat and humidity flows from the warm ocean. The objective of this study is to make a comparison between two cases of subtropical cyclones that occurred on the south coast of Brazil during the end of spring and summer periods in the Southern Hemisphere. The chosen cases were SC Cari and SC Eçai, which occurred respectively from March 10th to 13th of 2015 and from December 3rd to 6th of 2016. This study was carried out through the analysis of meteorological fields employing the ERA5 reanalysis data along with GOES-13 satellite imagery. In spite of both systems occurring on a similar spatial domain, their different atmospheric characteristics have an important part in their origin, intensity and trajectory. Their trajectories were of interest because of their uncommonness, where Cari moved at first towards the continent presenting risks to the society and only later moving towards the ocean till its dissipation. And Eçai formed over the land as a thermal low that moved towards the ocean gaining strength as it found warm waters, finding a new and more favorable environment.

Remote climate change propagation across the oceans—the directional swell signature

The energy content of wind-waves is propagated across the oceans in the form of swell waves, the main drivers of long-term changes in coastal morphology and offshore hazards. A state-of-the-art swell tracking algorithm is applied to a global ensemble of CMIP5 dynamic wave climate projections, to assess future changes in remotely originated swell events towards the end of the 21st century, and how they propagate. The contribution of multiple wave generation areas is considered. It is found that the projected climate change signal is effectively propagated from the winds along the extratropical storm tracks to remote locations, in the tropical and subtropical latitudes, through swell waves. The statistically significant projected changes in swell wave heights and swell predominance at the remote swell arrival locations are comparable with the ones at the wave generation areas. Furthermore, different incoming directions for swell events at remote locations are shown to often carry opposite climate change signals, propagated from different remote origins. These results highlight the need for a directional approach on wave climate projections, critical for improved vulnerability assessments and adaptation measures from the climate community.

Effect of temperature on flocking propensity and species interactions in a subtropical mixed-species flocking system

Increased participation in mixed-species flocks is one hypothesized behavioral tactic used by forest birds to adjust to cold temperatures in winter. This phenomenon has been documented in temperate North America, but not at subtropical latitudes where temperatures are less extreme. To understand how temperature structures the size, richness, flocking propensity, and species interactions of subtropical mixed-species flocks, we performed paired mixed-species flock and point count surveys in upland hardwood forests in north-central Florida, USA. We described the composition of 92 mixed-species flocks across a ∼20 °C range of daytime temperatures, using linear mixed models and network analyses to test for effects of temperature. We found that both flock species richness and size were significantly smaller at higher temperatures while statistically controlling for effects of time of day, canopy height, and local site effects. Thirteen of 14 flocking species showed decreases in within-flock abundance with increasing temperature, although this effect was only significant for the nuclear Carolina Chickadee (Poecile carolinensis). Flock composition and species flocking propensities were less plastic than in temperate systems. By comparing metrics from 7 social networks corresponding to temperature ranges, we found that strength of flocking interactions was lower and that species were significantly less likely to co-occur with similarly foraging species at temperatures above 20 °C. Flocking propensities were lower for many species in the warmest temperature range. Thirteen of 14 species joined flocks in all 7 networks, and dissimilarity of social networks was consequently best explained by changes in species co-occurrence patterns rather than species turnover within flocks. In contrast to boreal flocking systems, changes to flocking behavior were prompted by unseasonably warm weather rather than cold spells, which suggests that warming winter temperatures could lead to breakdown of social behaviors in our system.

Biome partitioning of the global ocean based on phytoplankton biogeography

Biomes are geographical units that can be defined based on biological communities sharing specific environmental and climatic requirements. Contemporary ocean biomes have been constructed based on various approaches. These included the biogeographic patterns of higher trophic level organisms, physical and biogeochemical properties, or bulk biological properties such as chlorophyll-a, but none considered the biogeographic patterns of the first trophic level explicitly, i.e. phytoplankton biogeography. A global description of marine biomes based on phytoplankton and defined in analogy to terrestrial vegetation biomes is still lacking. A bioregionalization based on phytoplankton appears particularly timely, as phytoplankton have a high sensitivity to climatic changes and fuel marine productivity. Here, we partition the global ocean into biomes by using self-organizing maps and hierarchical clustering, drawing on the biogeographic patterns of 536 phytoplankton species predicted from empirical evidence. Our approach reveals eight different biomes at the seasonal scale, and seven at the annual scale. The biomes host characteristic phytoplankton species compositions, and differ in their prevailing environmental conditions. The largest differences in phytoplankton composition are found between a Pacific equatorial biome and other tropical biomes, and between subtropical and high latitude biomes. The Pacific equatorial biome is characterized by species with narrower ecological niches, the tropical and subtropical biomes by cosmopolitan generalists, and the high latitudes by species with a heterogeneous biogeography. The strongest differences between biomes are found along gradients of temperature and macronutrient availability, associated with latitude. We test whether our biomes can be reproduced based on indicator species, or potential co-occurrence networks of species determined from the predicted species distributions that are wide-spread in some but rare in other biomes. We find that our biomes can be reproduced by the 51 species identified, which together form significant species co-occurrences. This suggests that species co-occurrences, rather than individual indicator species drive oceanic biome partitioning at the first trophic level. Our biome partitioning may be especially useful for comparative analyses on the functional implications of phytoplankton organization, and impacts on zoogeographical partitionings. Furthermore, it provides a framework for predicting large-scale changes in phytoplankton community structure due to anthropogenic climate and environmental change.

Enhancement of the North Atlantic CO<sub>2</sub> sink by Arctic Waters

Abstract. The North Atlantic north of 50∘ N is one of the most intense ocean sink areas for atmospheric CO2 considering the flux per unit area, 0.27 Pg-C yr−1, equivalent to −2.5 mol C m−2 yr−1. The northwest Atlantic Ocean is a region with high anthropogenic carbon inventories. This is on account of processes which sustain CO2 air–sea fluxes, in particular strong seasonal winds, ocean heat loss, deep convective mixing, and CO2 drawdown by primary production. The region is in the northern limb of the global thermohaline circulation, a path for the long-term deep-sea sequestration of carbon dioxide. The surface water masses in the North Atlantic are of contrasting origins and character, with the northward-flowing North Atlantic Drift, a Gulf Stream offspring, on the one hand and on the other hand the cold southward-moving low-salinity Polar and Arctic waters with signatures from Arctic freshwater sources. We have studied by observation the CO2 air–sea flux of the relevant water masses in the vicinity of Iceland in all seasons and in different years. Here we show that the highest ocean CO2 influx is to the Arctic and Polar waters, respectively, -3.8±0.4 and -4.4±0.3 mol C m−2 yr−1. These waters are CO2 undersaturated in all seasons. The Atlantic Water is a weak or neutral sink, near CO2 saturation, after poleward drift from subtropical latitudes. These characteristics of the three water masses are confirmed by data from observations covering 30 years. We relate the Polar Water and Arctic Water persistent undersaturation and CO2 influx to the excess alkalinity derived from Arctic sources. Carbonate chemistry equilibrium calculations clearly indicate that the excess alkalinity may support at least 0.058 Pg-C yr−1, a significant portion of the North Atlantic CO2 sink. The Arctic contribution to the North Atlantic CO2 sink which we reveal was previously unrecognized. However, we point out that there are gaps and conflicts in the knowledge about the Arctic alkalinity and carbonate budgets and that future trends in the North Atlantic CO2 sink are connected to developments in the rapidly warming and changing Arctic. The results we present need to be taken into consideration for the following question: will the North Atlantic continue to absorb CO2 in the future as it has in the past?

The Permian Monos Formation: Stratigraphic and detrital zircon evidence for Permian Cordilleran arc development along the southwestern margin of Laurentia (northwestern Sonora, Mexico)

Abstract The southwestern margin of Laurentia transitioned from a left-lateral transform margin to a convergent margin by middle Permian time, which initiated the development of a subduction zone and subsequent Cordilleran arc along western Laurentia. The displaced Caborca block was translated several hundred kilometers from southern California, USA, to modern Sonora, Mexico, beginning in Pennsylvanian time (ca. 305 Ma). The Monos Formation, a ∼600-m-thick assemblage of mixed bioclastic and volcaniclastic units exposed in northwestern Sonora, provides lithostratigraphic, petrographic, and geochronologic evidence for magmatic arc development associated with subduction by middle Permian time (ca. 275 Ma). The Monos Formation was deposited in a forearc basin adjacent to a magmatic arc forming along the southwestern Laurentian margin. Detrital zircon U-Pb geochronology suggests that Permian volcanic centers were the primary source for the Monos Formation. These grains mixed with far-traveled zircons from both Laurentia and Gondwana. Zircon age spectra in the Monos Formation are dominated by a ca. 274 Ma population that makes up 65% of all analyzed grains. The remaining 35% of grains range from 3.3 Ga to 0.3 Ma, similar to age spectra from Permian strata deposited in the Paleozoic sequences in the western continental interior. An abundance of Paleozoic through early Neoproterozoic ages suggests that marginal Gondwanan sources from Mexico and Central America also supplied material to the basin. The Monos Formation was deposited within tropical to subtropical latitudes, yet faunal assemblages are biosiliceous and heterotrophic. The lack of photozoan assemblages suggests that cold-water coastal upwelling combined with sedimentation from the Cordilleran arc and Laurentian continent promoted conditions more suitable for fauna resilient to biogeochemically stressed environments. We propose that transform faulting and displacement of the Caborca block ceased by middle Permian time and a subduction zone developed along the southwestern margin of Laurentia as early as early Permian time. The Monos basin developed along the leading edge of the continent as a magmatic arc developed, and facies indicate a consistent shoaling trend over the span of deposition.

Extreme Drought in the Brazilian Pantanal in 2019–2020: Characterization, Causes, and Impacts

The Pantanal region in South America is one of the world's largest wetlands. Since 2019, the Pantanal has suffered a prolonged drought that has spelled disaster for the region, and subsequent fires have engulfed hundreds of thousands of hectares. The lack of rainfall during the summers of 2019 and 2020 was caused by reduced transport of warm and humid summer air from Amazonia into the Pantanal. Instead, a predominance of warmer and drier air masses from subtropical latitudes contributed to a scarcity of summer rainfall at the peak of the monsoon season. This led to prolonged extreme drought conditions across the region. This drought had severe impacts on the hydrology of the Pantanal. Hydrometric levels fell all along the Paraguay River. In 2020, river levels reached extremely low values, and in some sections of this river, transportation had to be restricted. Very low river levels affected the mobility of people and shipping of soybeans and minerals to the Atlantic Ocean by the Hidrovia -Paraná-Paraguai (Paraná-Paraguay Waterway). This study is directed to better understand the hydroclimatic aspects of the current drought in the Brazilian Pantanal and their impacts on natural and human systems. As a consequence of the drought, fires spread and affected natural biodiversity as well as the agribusiness and cattle ranching sectors. While fires had serious socioecological and economic consequences, we do not intend to investigate the effect of the downstream low-level waters on the Pantanal ecosystems or the drought in the risk of fire.

Future Changes in the Hadley Circulation: The Role of Ocean Heat Transport

AbstractThe projected widening and weakening of the Hadley circulation hold large societal impacts at low and subtropical latitudes. Previous studies suggested that ocean heat transport (OHT) might play a central role in future circulation's changes. Here, using ensembles of model integrations, we quantify the role of OHT in the evolution of the Hadley cell over the 20th and 21st centuries. We find that by the end of the current century OHT reduces the widening of the circulation by ∼35% (0.42°) and its weakening by ∼60% (1.3 × 1010 kg s−1). As a result, OHT delays the emergence from internal variability of the widening by 30 years and of the weakening by 20 years. Lastly, while oceanic heat uptake accounts for most of the reduced widening, and thus merely delays it by reducing surface warming, horizontal heat transport and net heat uptake have comparable impacts to reduce the weakening of the circulation.

Monitoring of mercury in the mesopelagic domain of the Pacific and Atlantic oceans using body feathers of Bulwer's petrel as a bioindicator

Global mercury pollution has markedly and consistently grown over the past 70 years (although with regional variations in trends) and is a source of major concern. Mercury contamination is particularly prevalent in biota of the mesopelagic layers of the open ocean, but these realms are little studied, and we lack a large scale picture of contamination in living organisms of this region. The Bulwer's petrel Bulweria bulwerii, a species of migratory seabird, is a highly specialised predator of mesopelagic fish and squid, and therefore can be used as a bioindicator for the mesopelagic domain. Mercury accumulated by the birds through diet is excreted into feathers during the moulting process in adults and feather growth in chicks, reflecting contamination in the non-breeding and breeding periods, respectively, and hence the influence of different, largely non-overlapping breeding and non-breeding ranges. We studied mercury in feathers and the trophic position in two colonies from the Atlantic Ocean (Portugal and Cape Verde) and two colonies from the Pacific Ocean (Japan and Hawaii). We found significantly lower levels of mercury in adult and chick samples from the Pacific Ocean compared with samples from the Atlantic Ocean. However, we did not detect differences in trophic position of chicks among colonies and oceans, suggesting that differences in mercury measured in feathers reflect levels of environmental contamination, rather than differences in the structure of the trophic chain in different oceans. We conclude that despite a reduction in mercury levels in the Atlantic in recent decades, mesopelagic organisms in this ocean remain more heavily contaminated than in the Pacific at tropical and subtropical latitudes. We suggest that Bulwer's petrel is a highly suitable species to monitor the global contamination of mercury in the mesopelagic domain.

Geographical and environmental contributions to genomic divergence in mangrove forests

Abstract Assessing the relative importance of geographical and environmental factors to the spatial distribution of genetic variation can provide information about the processes that maintain genetic variation in natural populations. With a globally wide but very restricted habitat distribution, mangrove trees are a useful model for studies aiming to understand the contributions of these factors. Mangroves occur along the continent–ocean interface of tropical and subtropical latitudes, regions considered inhospitable to many other types of plants. Here, we used landscape genomics approaches to investigate the relative contributions of geographical and environmental variables to the genetic variation of two black mangrove species, Avicennia schaueriana and Avicennia germinans, along the South American coast. Using single nucleotide polymorphisms, our results revealed an important role of ocean currents and geographical distance in the gene flow of A. schaueriana and an isolation-by-environment pattern in the organization of the genetic diversity of A. germinans. Additionally, for A. germinans, we observed significant correlations between genetic variation with evidence of selection and the influence of precipitation regimens, solar radiation and temperature patterns. These discoveries expand our knowledge about the evolution of mangrove trees and provide important information to predict future responses of coastal species to the expected global changes during this century.

VLF radio signal anomaly associated with geomagnetic storm followed by an earthquake at a subtropical low latitude station in northeastern part of India

This paper presents a case study of very low frequency (VLF) radio signal anomaly during a moderate geomagnetic storm (Kp = 6, Ap = 80 and Dst = − 60 nT) on September 10, 2018 followed by a moderate earthquake (M = 5.3) on September 12, 2018 which occurred near to (~ 80 km) a subtropical low latitude station Cooch Behar (CHB), India (26.345°N, 89.448°E). Signal amplitudes from the two transmitters, VTX (18.2 kHz) and NWC (19.8 kHz), are analyzed for this study. Seven VLF parameters, such as nighttime amplitude fluctuation (dAn), amplitude difference between pre-sunrise signal and sunrise minimum signal (dAm), amplitude difference between sunset minimum signal and post-sunset signal (dAe), time (Tm) and amplitude (Am) during sunrise minimum, time (Te) and amplitude (Ae) during sunset minimum have been examined. Anomalous behaviors of all the parameters have been observed during September 10–12, 2018. D-region electron density is estimated using the long wave propagation capability code which revealed a maximum increment of ~ 20 electron/cm3 at altitude 85 km over the VTX-CHB path ~ 2 days before the earthquake event. The anomalous behaviors of the VLF signals ~ 1–2 days before the earthquake are identified as possible precursor of the earthquake which could be linked with various pre-earthquake seismo-electromagnetic processes.

Mangroves of Sri Lanka: Distribution, status and conservation requirements

Mangroves are a diverse group of highly salt-tolerant woody plants, which grow in the inter-tidal zones in tropical and subtropical latitudes. Despite its unique services to the people, coastal and marine systems, mangroves have become one of the most rapidly disappearing ecosystems in the world. This paper reviews the available information on distribution, current status and challenges of mangroves in Sri Lanka. Today, around 160 km2 of mangrove vegetation is available in Sri Lanka and distributed mainly in Jaffna, Batticaloa, Kalpitiya, Rekawa and Trincomalee and is composed of 21 species of true mangroves and 24 species of mangrove associates. Mangroves in the island have been adversely affected due to the numerous anthropogenic activities, including land reclamation, tourism, coastal aquaculture and agriculture and other industrial activities etc. Proper conservations of mangroves are urgently required to the island to avoid further decline of mangrove ecosystem. It is imperative to evaluate policies, legal instruments and development strategies to effectively protect this valuable ecosystem.

Comparison of OMI-DOAS total ozone column with ground-based measurements in Argentina

<p>Total ozone column (TOC) measurements through the Ozone Monitoring Instrument (OMI/NASA EOSAura) are compared with ground-based observations made using Dobson and SAOZ instruments for the period 2004–2019 and 2008–02/2020, respectively. The OMI data were inverted using the Differential Optical Absorption Spectroscopy algorithm (overpass OMI-DOAS). The four ground-based sites used for the analysis are located in subpolar and subtropical latitudes spanning from 34°S to 54°S in the Southern Hemisphere, in the Argentine cities of Buenos Aires (34.58°S, 58.36°W; 25 m a.s.l.), Comodoro Rivadavia (45.86°S, 67.50°W; 46 m a.s.l.), Río Gallegos (51.60°S, 69.30°W; 72 m a.s.l.) and Ushuaia (54.80°S, 68.30°W; 14 m a.s.l.). The linear regression analyzes showed correlation values greater than 0.90 for all sites. The OMI measurements revealed an overestimation of less than 4 % with respect to the Dobson instruments, while the comparison with the SAOZ instrument presented a very low underestimation of less than 1 %.</p>

El Niño‐Southern Oscillation Impacts on Global Wave Climate and Potential Coastal Hazards

AbstractWave direction and wave power are the main climate‐related drivers of large‐scale coastal morphological hazards on open sandy coastlines. El Niño‐Southern Oscillation (ENSO) is the main climate driver affecting global wave climates on interannual timescales. Hence, a quantification of wave climate variability with ENSO would aid the characterization of coastal risk as a first‐pass assessment tool. However, no such data are available on the global scale at present. To address this, this work (1) quantifies ENSO impacts on global mean wave direction and wave power, and (2) classifies the global coast according to the ENSO wave climate hazard. The correlation coefficient (R) of the Multivariate ENSO Index between wave power and mean wave direction was calculated. Then, employing a multivariate technique, the resulting R values were used to classify the global coastlines. The results show that ENSO has distinct latitudinal impacts around the world. El Niño increases the wave power of swells generated in the extra‐tropics which propagate toward the tropics, a phenomenon which is particularly notable in the Pacific Basin, while the most significant rotations in wave direction with ENSO occur at tropical and subtropical latitudes. Based on this new information, the global coastline was classified into five distinct types, depending on the directional wave climate response to ENSO.

Dickinsonia discovered in India and late Ediacaran biogeography

The discovery of Dickinsonia in India allows assessment of biogeographic provinces and plate tectonic reconstructions for the late Ediacaran. The fossils were found in the roof of Auditorium Cave at Bhimbetka Rock Shelters, a UNESCO World Heritage Site for Paleolithic and Mesolithic cave art, near Bhopal, Madhya Pradesh. The fossils are identical with Dickinsonia tenuis from the Ediacara Member of the Rawnsley Quartzite in South Australia, and like them also show deformation due to lateral impingement, arcuate pieces missing, and alignment. They are within the late Ediacaran, Maihar Sandstone of the Bhander Group, in red sandstones formed in coastal plain paleoenvironments, including eolian, tsunamite, and intertidal bedforms. This new occurrence confirms assembly of Gondwanaland by 550 Ma, but not reconstructions adjusted for true polar wander. Cloudina and other small shelly marine fossils were low latitude, but vendobionts such as Dickinsonia were at temperate to subtropical latitudes.

North Atlantic Western Boundary Currents Are Intense Dissolved Organic Carbon Streams

In the North Atlantic, there are two main western boundary currents related to the Atlantic Meridional Overturning Circulation (AMOC): the Gulf Stream flowing northward and the Deep Western Boundary Current (DWBC) flowing southward. Here we analyze data from the OVIDE section (GO-SHIP A25 Portugal-Greenland 40–60°N) that crosses the DWBC and the northward extension of the Gulf Stream, the North Atlantic Current. We show that North Atlantic western boundary currents play a key role in the transport of dissolved organic matter, specifically dissolved organic carbon (DOC). Revisited transports and budgets of DOC with new available data identify the eastern Subpolar North Atlantic (eSPNA) as an important source of locally produced organic matter for the North Atlantic and a key region in the supply of bioavailable DOC to the deep ocean. The East Greenland Current, and its upstream source the East Reykjanes Ridge Current on the eastern flank of the mid-Atlantic ridge, are export pathways of bioavailable DOC toward subtropical latitudes. The fast overturning and subsequent remineralization of DOC produced in the autotrophic eSPNA explains up to 38% of the total oxygen consumption in the deep North Atlantic between the OVIDE section and 24°N. Carbon budgets that do not take into account this organic remineralization process overestimates the natural uptake of carbon dioxide (CO2) from the atmosphere by one third. The inclusion of DOC transports in regional carbon budgets reconciles the estimates of CO2 uptake in the North Atlantic between model and observations.

An updated checklist of ctenophores (Ctenophora: Nuda and Tentaculata) of Mexican seas

Ctenophores are one of the most conspicuous and frequent groups of the gelatinous zooplankton community, but their regional diversity in tropical and subtropical latitudes remains largely unknown. We provide an overview and update of the current knowledge of the diversity in Mexican seas, including ocean and coastal-neritic environments of the Gulf of Mexico, the Mexican Caribbean Sea, and the Mexican Pacific Ocean. Ctenophore records were reviewed based on the available scientific and gray literature, the Naturalista network (www.naturalista.mx), and the ctenophore species collected in the Gulf of California by the Monterey Bay Aquarium Research Institute. A total of 33 taxa (Class Nuda and Tentaculata) were found to occur in Mexican seas, of which 12 of the 33 taxa (36.4 % of the total) were recorded in the Gulf of Mexico, 7 (21.2 %) in the Mexican Caribbean Sea, 25 (75.8 %) in the Gulf of California, 11 (33.3 %) in the Eastern Tropical Pacific, and only 1 (3.0 %) are known in the Northeastern Pacific. Up to nine taxa included in our account represent first records for Mexico (i.e., Bathocyroe fosteri, Kiyohimea usagi, Lampocteis cruentiventer, Leucothea sp., Aulacoctena sp., Haeckelia beehleri, Charistephane fugiens, Bathyctena chuni, and Hormiphora californensis). Due to the lack of data on benthic ctenophores and the sparse studies on oceanic and deep-living species, it is expected that the list will grow as new surveys are performed in the deep sea. The lack of long-term studies on Mexican ctenophores have limited our capacity to draw valid conclusions on their abundance, total diversity, endemicity, and trophic ecology in Mexico.

Composite Vertical-Motion Patterns near North American Polar–Subtropical Jet Superposition Events

AbstractA polar–subtropical jet superposition is preceded by the development of a polar cyclonic potential vorticity (PV) anomaly at high latitudes and a tropical anticyclonic PV anomaly at subtropical latitudes. A confluent large-scale flow pattern can lead to the juxtaposition of these respective PV anomalies at middle latitudes, resulting in the addition of the nondivergent circulations induced by each PV anomaly and an increase in upper-tropospheric wind speeds at the location of jet superposition. Once these PV anomalies become juxtaposed, vertical motion within the near-jet environment facilitates the advection and diabatic redistribution of tropopause-level PV, and the subsequent formation of the steep, single-step tropopause structure that characterizes a jet superposition. Given the importance of vertical motion during the formation of jet superpositions, this study adopts a quasigeostrophic (QG) diagnostic approach to quantify the production of vertical motion during three types of jet superposition events: polar dominant, eastern subtropical dominant, and western subtropical dominant. The diagnosis reveals that the geostrophic wind induced by polar cyclonic QGPV anomalies is predominantly responsible for QG vertical motion in the vicinity of jet superpositions. The QG vertical motion diagnosed from the along-isotherm component of the Q vector, which represents the vertical motion associated with synoptic-scale waves, is dominant within the near-jet environment. The QG vertical motion diagnosed from the across-isotherm component of the Q vector, which represents the vertical motion associated with frontal circulations in the vicinity of the jet, is subordinate within the near-jet environment, but is relatively more important during eastern subtropical dominant events compared to polar dominant and western subtropical dominant events.

Wuhan MST radar: technical features and validation of wind observations

Abstract. The Wuhan mesosphere–stratosphere–troposphere (MST) radar is a 53.8 MHz monostatic Doppler radar, located in Chongyang, Hubei Province, China, and has the capability to observe the dynamics of the mesosphere–stratosphere–troposphere region in the subtropical latitudes. The radar system has an antenna array of 576 Yagi antennas, and the maximum peak power is 172 kW. The Wuhan MST radar is efficient and cost-effective and employs more simplified and more flexible architecture. It includes 24 big transmitter–receiver (TR) modules, and the row or column data port of each big TR module connects 24 small TR modules via the corresponding row or column feeding network. Each antenna is driven by a small TR module with peak output power of 300 W. The arrangement of the antenna field, the functions of the timing signals, the structure of the TR modules, and the clutter suppression procedure are described in detail in this paper. We compared the MST radar observation results with other instruments and related models in the whole MST region for validation. Firstly, we made a comparison of the horizontal winds in the troposphere and low stratosphere observed by the Wuhan MST radar with the radiosonde on 22 May 2016, as well as with the ERA-Interim data sets (2016 and 2017) in the long term. Then, we made a comparison of the observed horizontal winds in the mesosphere with the meteor radar and the Horizontal Wind Model 14 (HWM-14) model in the same way. In general, good agreements can be obtained, and this indicates that the Wuhan MST is an effective tool to measure the three-dimensional wind fields of the MST region.