Northern Hemisphere Summer Research Articles

Future changes in the extratropical storm tracks and cyclone intensity, wind speed, and structure

Abstract. Future changes in extratropical cyclones and the associated storm tracks are uncertain. Using the new CMIP6 models, we investigate changes to seasonal mean storm tracks and composite wind speeds at different levels of the troposphere for the winter and summer seasons in both the Northern Hemisphere (NH) and Southern Hemisphere (SH). Changes are assessed across four different climate scenarios. The seasonal mean storm tracks are predicted to shift polewards in the SH and also in the North Pacific, with an extension into Europe for the North Atlantic storm track. Overall, the number of cyclones will decrease by ∼5 % by the end of the 21st century, although the number of extreme cyclones will increase by 4 % in NH winter. Cyclone wind speeds are projected to strengthen throughout the troposphere in the winter seasons and also summer in the SH, with a weakening projected in NH summer, although there are minimal changes in the maximum wind speed in the lower troposphere. Changes in wind speeds are concentrated in the warm sector of cyclones, and the area of extreme winds may be up to 40 % larger by the end of the century. The largest changes are seen for the SSP5-85 scenario, although a large amount of change can be mitigated by restricting warming to that seen in the SSP1-26 and 2-45 scenarios. Extreme cyclones show larger increases in wind speed and peak vorticity than the average-strength cyclones, with the extreme cyclones showing a larger increase in wind speed in the warm sector.

Arctic glaciers and ice caps through the Holocene:a circumpolar synthesis of lake-based reconstructions

Abstract. The recent retreat of nearly all glaciers and ice caps (GICs) located in Arctic regions is one of the most clear and visible signs of ongoing climate change. This paper synthesizes published records of Holocene GIC fluctuations from lake archives, placing their recent retreat into a longer-term context. Our compilation includes 66 lake-based GIC records (plus one non-lake-based record from the Russian Arctic) from seven Arctic regions: Alaska, Baffin Island in northeastern Canada, Greenland, Iceland, the Scandinavian peninsula, Svalbard, and the Russian high Arctic. For each region and for the full Arctic, we summarize evidence for when GICs were smaller than today or absent altogether, indicating warmer-than-present summers, and evidence for when GICs regrew in lake catchments, indicating summer cooling. Consistent with orbitally driven high boreal summer insolation in the early Holocene, the full Arctic compilation suggests that the majority (50 % or more) of studied GICs were smaller than present or absent by ∼10 ka. We find the highest percentage (>90 %) of Arctic GICs smaller than present or absent in the middle Holocene at ∼ 7–6 ka, probably reflecting more spatially ubiquitous and consistent summer warmth during this period than in the early Holocene. Following this interval of widespread warmth, our compilation shows that GICs across the Arctic began to regrow and summers began to cool by ∼6 ka. Together, the Arctic records also suggest two periods of enhanced GIC growth in the middle to late Holocene from ∼ 4.5–3 and after ∼2 ka. The regional records show variability in the timing of GIC regrowth within and between regions, suggesting that the Arctic did not cool synchronously despite the smooth and hemispherically symmetric decline in Northern Hemisphere summer insolation. In agreement with other studies, this implies a combined response to glacier-specific characteristics such as topography and to other climatic forcings and feedback mechanisms, perhaps driving periods of increased regional cooling. Today, the direction of orbital forcing continues to favor GIC expansion; however, the rapid retreat of nearly all Arctic GICs underscores the current dominance of anthropogenic forcing on GIC mass balance. Our review finds that in the first half of the Holocene, most of the Arctic's small GICs became significantly reduced or melted away completely in response to summer temperatures that, on average, were only moderately warmer than today. In comparison, future projections of temperature change in the Arctic far exceed estimated early Holocene values in most locations, portending the eventual loss of most of the Arctic's small GICs.

Multidecadal variation of northern hemisphere summer monsoon forced by the SST inter-hemispheric dipole

The sea surface temperature inter-hemispheric dipole (SSTID) is an important variability mode of global SST anomalies, characterized by an anti-phase variation of SST between the two hemispheres. In this study, the decadal variation of the northern hemisphere summer monsoon (NHSM) is found to be strongly regulated by the SSTID, with positive (negative) phases of the SSTID corresponding to the strengthening (weakening) of NHSM. Both observation and SST-forced atmospheric model simulations suggest that the SSTID related thermal forcing modulates the NHSM by causing planetary-scale atmospheric circulation adjustments. Positive SSTID events lead to coherent increase (decrease) of surface air temperature over the entire northern (southern) hemisphere, increasing the inter-hemispheric thermal contrast (ITC). As sea level pressure changes are just opposite to air temperature, the increase of ITC enhances the inter-hemispheric pressure gradient (southern hemisphere minus northern hemisphere), leading to the strengthening of summer monsoonal circulation and the increase of monsoon rainfall in the northern hemisphere.

A high-resolution stalagmite record from Luoshui Cave, Central China over the past 23.5 kyr

A continuous high-resolution record from a precisely-dated stalagmite spanning the last precessional cycle provides new insight into the dynamics of climate change from orbital to centennial scales. A 600-mm length stalagmite (LS46) collected from the Luoshui Cave, Hubei Province, Central China, was well-dated (a total of 24 230Th dates) and high-resolved sampled for oxygen and carbon stable isotopic measurements (2047 subsamples in total). The stalagmite LS46 δ18O is consistent with other published stalagmite δ18O time series in monsoonal China, and closely tracks changes in the Northern Hemisphere summer insolation (NHSI) on July 21st throughout the past 23.5 kyr. Besides, the stalagmite LS46 δ18O shows significant correlations with the reconstructed Dali lake-level history and tree-ring δ18O records in the marginal monsoon regions. On this basis, we interpret that the stalagmite LS46 δ18O reflects the large-scale monsoon circulation with a superimposed local climate signal that might be related to effective humidity. Furthermore, the stalagmite LS46 δ18O exhibited four long-lasting extremely weak monsoon events since 7 kyr B.P.. These four weak monsoon events corresponded well with the tree-ring record in the northwestern fringe of the East Asian summer monsoon (EASM) region. Notably, the good correspondence between the stalagmite record and the tree-ring record also provides a good chronological support for our assessment of the climate and cultural evolution during the middle-late Holocene transition, the medieval warm period (MWP) and Little Ice Age (LIA). During the early-middle Holocene, the detrended δ18O profile revealed 10 multi-centennial monsoon cycles with a duration of nearly 500 yr and the amplitude of ∼0.8‰. The wavelet analysis further discloses that the EASM has nearly 500 yr and 200 yr periodic oscillations in the early-middle Holocene, research indicated that the solar activity and the North Atlantic Oscillation (NAO)/North Atlantic Deep Water (NADW) jointly drive the dominant nearly 500 yr cycle of the Asian monsoon.

Early Holocene palaeoceanographic and glaciological changes in southeast Greenland

Sediment core ER11-16 from Køge Bugt in Southeast Greenland is used to assess early Holocene palaeoceanographic changes and sediment rafting from icebergs calved from the large outlet glaciers in the area. Diatom analysis reconstructs variability in surface water temperature, salinity and sea-ice concentrations, and benthic foraminiferal assemblages is used to reconstruct subsurface ocean conditions. We report Holocene Thermal Maximum in Southeast Greenland during the early Holocene (at least since onset of the record 9100 cal yr BP) until around 4500 cal yr BP, which contrasts with a delay until the mid-Holocene of the Holocene Thermal Maximum in South and Southwest Greenland. The early Holocene warming in Southeast Greenland was caused by a combination of high solar insolation and a weakened subpolar gyre, both of which served to warm the Irminger Current waters subducting onto the shelf. At the same time, the surface temperature was relatively high and sea-ice cover in the polar surface waters of the East Greenland Current was relatively low. High levels of iceberg rafting occurred in Køge Bugt during the early Holocene, synchronously with these warm oceanic temperatures. This is attributed to an increase in iceberg production from the extensive, but retreating, Greenland Ice Sheet. The warm surface conditions were interrupted by a marked and short-lived increase in sea ice around 8200 years ago, providing the first evidence of this global cold episode in Southeast Greenland. After 4500 cal. yr BP, sea-ice cover increased with an expansion of the East Greenland Current, suppressing the inflow of warmer subsurface Irminger Current water to the Southeast Greenland shelf. We relate this oceanic shift to the decreased Northern Hemisphere summer solar insolation. Multi-centennial variability is observed in the grain size spectrum of iceberg rafted debris; a finding we interpret in the context of palaeoceanographic changes.

Climate-inferred distribution estimates of mid-to-late Pliocene hominins

During the mid-to-late Pliocene (ca. 4–3 Ma), several hominin species were present in central Sahel, eastern and southern Africa. The potential for the discovery of hominin remains from this interval is limited by the availability of exposed Pliocene deposits and the ability to investigate them. As a result, most discoveries have been made in the Afar region of Ethiopia and in the Lake Turkana basin, thus unveiling only a portion of Pliocene hominins' probable geographical presence. In this study we provide a continental view of geographic areas potentially accessible to these hominins. To do so, we estimate the climatic envelope suitable for mid-to-late Pliocene hominin presence, using the earth system model IPSL-CM5A and the Maxent habitat suitability algorithm. Our analysis reveals high habitat suitability for these hominin species in semi-arid regions where annual thermal amplitude and mean annual precipitation are moderate, mostly corresponding to tropical xerophytic shrublands. Our habitat model estimates geographically continuous, suitable climatic conditions for hominins between central Sahel and northeastern Africa, but not between eastern and southern Africa. This discontinuity suggests that southern African and eastern African hominins were separated by an environmental barrier that could only be crossed during particularly favourable periods or by undertaking long-range dispersal over climatically hostile habitats. Under climate conditions of northern hemisphere summer at perihelion this climatic barrier is not present. In contrast, the Turkana basin, the Laetoli region, and a large part of southern Africa remain suitable for all precession angles, suggesting that these areas may have functioned as refugia. The constant presence of these stable areas combined with the periodic establishment of corridors for dispersion can potentially explain hominin diversity in eastern Africa. • Paleoclimatic reconstructions and HSM depict high suitability in semi-arid regions • Eastern and southern suitable areas are disconnected under mean Pliocene climate • Under specific precession configurations this climatic barrier disappears • Turkana basin and the Laetoli region could have work as climatic refugia • High Pliocene Hominin diversity could be linked with periodic presence of corridors.

Stepwise evolution of the Asian summer monsoon during the Holocene revealed by a stalagmite record from Heifeng Cave, Southwest China

The abrupt changes of the Asian summer monsoon (ASM) during the Holocene have long been of interest to environmental scientists. Here, RAMPFIT and Bayesian Change Point (BCP) analyses are applied to analyze the stalagmite δ18O record from Heifeng Cave in southern China, and the results show that it is characterized by a distinct stepwise pattern of variation which can be divided into six stages (Stages, S) and five transitional phases (Transitions, T). In the early Holocene, when Northern Hemisphere summer insolation (NHSI) was at its maximum, the ASM underwent step-like increases in strength, comprising two stages. With the decrease in NHSI during the middle to late-Holocene, the weakening of ASM intensity was characterized by further step-like changes comprising three stages. The transitional phases of the monsoon are broadly correlative with millennial-scale monsoon weakening events associated with Bond events in the North Atlantic region, suggesting that the stepwise evolution of the ASM during the Holocene was caused by the combined effects of changes in NHSI and Bond events. We propose that Bond events may have acted as triggers that caused the ASM to cross a critical threshold, prompting a shift from one regime to another.

Orbital‐ and Millennial‐Scale Variability in Northwest African Dust Emissions Over the Past 67,000 years

AbstractReconstructions of aeolian dust flux to West African margin sediments can be used to explore changing atmospheric circulation and hydroclimate over North Africa on millennial to orbital timescales. Here, we extend West African margin dust flux records back to 37 ka in a transect of sites from 19° to 27°N, and back to 67 ka at Ocean Drilling Program (ODP) Hole 658C, in order to explore the interplay of orbital and high‐latitude forcings on North African climate and make quantitative estimates of dust flux during the core of the Last Glacial Maximum (LGM). The ODP 658C record shows a Green Sahara interval from 60 to 50 ka during a time of high Northern Hemisphere summer insolation, with dust fluxes similar to levels during the early Holocene African Humid Period, and an abrupt peak in flux during Heinrich event 5a (H5a). Dust fluxes increase from 50 to 35 ka while the high‐latitude Northern Hemisphere cools, with peaks in dust flux associated with North Atlantic cool events. From 35 ka through the LGM dust deposition decreases in all cores, and little response is observed to low‐latitude insolation changes. Dust fluxes at sites from 21° to 27°N were near late Holocene levels during the LGM time slice, suggesting a more muted LGM response than observed from mid‐latitude dust sources. Records along the northwest African margin suggest important differences in wind responses during different stadials, with maximum dust flux anomalies centered south of 20°N during H1 and north of 20°N during the Younger Dryas.

Potential CO2 forcing and Asian summer monsoon precipitation trends during the last 2,000 years

Abstract Holocene records for the Asian Summer Monsoon (ASM) indicate that, apart for the last 2,000 years (“2 kyr shift”), solar insolation was the dominant factor controlling the monsoon climate. The aim of this review is to provide a synopsis of climate characteristics over the last 2 kyr, clarifying mechanisms for the diverse trend with Northern Hemisphere Summer Insolation (NHSI) records. Here, we initially review proxy-based climate reconstructions for the last 2 kyr, and then compare them with records from the last five interglacial periods. Finally, we examine potential physical mechanisms responsible for the “2 kyr shift.” Findings from this review indicate that the “2 kyr shift” is a representative pattern of Holocene climate change within the core area of the ASM, and the “2 kyr shift” could be mainly controlled by changes in atmospheric CO2 concentration. In addition, suggestions to address a more humid condition dominating the Asian monsoonal margin zones are offered.

Field‐Aligned Current During an Interval of BY‐Dominated Interplanetary‐Field; Modeled‐to‐Observed Comparisons

AbstractWe model an interval of remarkable interplanetary magnetic field (IMF), for which we have a comprehensive set of observational data. This interval is associated with the arrival of an interplanetary coronal mass ejection. The solar wind densities at the time are particularly high and the IMF is primarily northward over many hours. This results in strong auroral emissions within the polar cap in a cusp spot, which we associate with lobe reconnection at the high‐latitude magnetopause. We also observe areas of upwards field‐aligned current (FAC) within the summer Northern Hemisphere polar cap that exhibit large current magnitudes. The model can reproduce the spatial distribution of the FACs well, even under changing conditions in the incoming IMF. Discrepancies exist between the modeled and observed current magnitudes. Notably, the winter Southern Hemisphere exhibits much lower current magnitudes overall. We also model a sharp transition of the location of magnetopause reconnection at the beginning of the interval, before the IMF remained northward for many hours. The reconnection location changed rapidly from a subsolar location at the low‐latitude magnetopause under southward IMF conditions, to a high‐latitude lobe reconnection location when the field is northward. This occurs during a fast rotation of the IMF at the shock front of a magnetic cloud.

Collapse of the Liangzhu and other Neolithic cultures in the lower Yangtze region in response to climate change.

The Liangzhu culture in the Yangtze River Delta (YRD) was among the world’s most advanced Neolithic cultures. Archeological evidence suggests that the Liangzhu ancient city was abandoned, and the culture collapsed at ~4300 years ago. Here, we present speleothem records from southeastern China in conjunction with other paleoclimatic and archeological data to show that the Liangzhu culture collapsed within a short and anomalously wet period between 4345 ± 32 and 4324 ± 30 years ago, supporting the hypothesis that the city was abandoned after large-scale flooding and inundation. We further show that the demise of Neolithic cultures in the YRD occurred within an extended period of aridity that started at ~4000 ± 45 years ago. We suggest that the major hydroclimatic changes between 4300 and 3000 years ago may have resulted from an increasing frequency of the El Niño–Southern Oscillation in the context of weakened Northern Hemisphere summer insolation.

Mechanical forcing of the North American monsoon by orography.

A band of intense rainfall extends more than 1,000 km along Mexico's west coast during Northern Hemisphere summer, constituting the core of the North American monsoon1,2. As in other tropical monsoons, this rainfall maximum is commonly thought to be thermally forced by emission of heat from land and elevated terrain into the overlying atmosphere3-5, but a clear understanding of the fundamental mechanism governing this monsoon is lacking. Here we show that the core North American monsoon is generated when Mexico's Sierra Madre mountains deflect the extratropical jet stream towards the Equator, mechanically forcing eastward, upslope flow that lifts warm and moist air to produce convective rainfall. These findings are based on analyses of dynamic and thermodynamic structures in observations, global climate model integrations and adiabatic stationary wave solutions. Land surface heat fluxes do precondition the atmosphere for convection, particularly in summer afternoons, but these heat fluxes alone are insufficient for producing the observed rainfall maximum. Our results indicate that the core North American monsoon should be understood as convectively enhanced orographic rainfall in a mechanically forced stationary wave, not as a classic, thermally forced tropical monsoon. This has implications for the response of the North American monsoon to past and future global climate change, making trends in jet stream interactions with orography of central importance.

Enhanced aridity in the source region of the Yangtze River since 5.8 ka revealed by the sediments of Saiyong Co

The Yangtze River is a major component of the ‘Asian Water Tower’ and it provides freshwater resources for hundreds of millions of people in East Asia. Hence, climatic and environmental changes in the source region of the Yangtze River (SRYR) are of major significance for understanding changes in the status of its freshwater resources. However, due to the contradictory results shown by the available paleoclimatic records from the SRYR and neighboring regions, it remains unclear how the regional climate and hydrology have evolved since the middle Holocene. Here, we present records of hydrological variations based on multiple proxies (grain size, scanned micro-scale X-ray fluorescence (XRF) element composition, and loss on ignition (LOI)) of the sediments of alpine lake Saiyong Co, with the aim of reconstructing hydrological changes in the SRYR since 5.8 ka. The first principal component (PC1) of the XRF scanned element record is dominated by Ti, Fe and K, representing terrigenous inputs from the catchment. The LOI profile shows an inverse pattern of variation to that of PC1, suggesting that the sedimentary organic matter is primarily derived from aquatic plants. Since 5.8 ka the overall decrease in sample scores on PC1 and the increasing LOI, together with the grain-size based lake-level record, indicate an overall drying trend in SRYR. Furthermore, several major droughts are recorded, at ∼4.0 ka, ∼2.8 ka and ∼1.8 ka. The hydrological variations at Saiyong Co are supported by multiple records from the SRYR, suggesting that millennial-scale environmental variations were regionally synchronous and were influenced by Northern Hemisphere summer insolation and movement of the intertropical convergence zone.

Geochemical characteristics of Holocene loess-paleosol sequences in central Chinese Loess Plateau and their implications for East Asian monsoon evolution

The geochemical composition and magnetic susceptibility (MS) of three sections (Weinan, Jingchuan and Jingbian) in the central Chinese Loess Plateau, located along a north-south climate gradient, were analyzed at high resolution in order to reconstruct changes in weathering intensity and hence Asian monsoon evolution during the Holocene. The results show distinct differences in element composition between the loess and paleosol layer, as well as spatial differences which reflect changes in mean annual temperature and annual precipitation along the climatic gradient. The measured major element compositions, combined with the results of previous research, indicate that the provenance of the Holocene loess-paleosol sequences in the central Chinese Loess Plateau was constant over time and space. The degree of chemical weathering in the study area is relatively weak and at an intermediate weathering stage. Based on a magnetic susceptibility age model, the East Asian summer monsoon changes recorded by the loess-paleosol sequences show a distinct two-stage pattern of evolution, with a gradual increase during 13.1–8.5 ka and a gradual decrease thereafter. Comparison of the results with climate records from elsewhere indicates that the East Asian monsoon evolution was mainly controlled by changes in Northern Hemisphere summer insolation and ice sheets.

Evaluation of short-term temporal evolution of Pluto’s surface composition from 2014–2017 with APO/TripleSpec

In this work we present the results of a spectral observing campaign of Pluto to search for temporal changes in surface composition on 1- to 3-year timescales. Near-infrared spectra of Pluto were obtained from June 2014 to August 2017 with the TripleSpec cross-dispersed spectrograph at the Apache Point Observatory’s 3.5-meter Astrophysical Research Consortium (ARC) telescope. Observations were requested in order to obtain spectra of approximately the same sub-observer hemisphere ∼14 months apart, thus removing the effects of viewing geometry and rotation phase. Comparison of the CH4 (methane) band areas and band center shifts between each component of these “matched pairs” revealed a surface in transition. Band areas for the 1.66 and 1.72μm CH4 absorption features exhibited a >5-σ increase between 2014-06-17 and 2015-08-19, corresponding to a sub-observer hemisphere centered at ∼280°E, with the latter date only 1 month after the New Horizons flyby of Pluto. The majority of matched pairs were obtained of the anti-Charon hemisphere, home to the bright, volatile-rich Sputnik Planitia, and did not present statistically significant changes in CH4 band areas. CH4 band center shifts, which provide information on the mixing state of CH4 and N2 in solid solution, were calculated between components of each matched pair, with no significant band shifts detected. The favored explanation for these combined results is the sublimation of more-volatile N2 from the northern latitudes of Pluto in the lead-up to northern hemisphere summer solstice in 2029, leading to an increase in CH4 concentration.

Last interglacial hydroclimate in the Italian Prealps reconstructed from speleothem multi-proxy records (Bigonda Cave, NE Italy)

Past climate archives show the Last Interglacial (LIG) period as similar to slightly warmer than current temperatures. However, there is a lack of LIG proxy evidence regarding variations of the climate across large topographic features and how this manifests at different altitudes. Here, we analysed two flowstones from Bigonda Cave, northeast Italy, to reconstruct LIG climate conditions in a region where precipitation is strongly influenced by the presence of the Alps. Stable isotope ratios, trace element concentrations and speleothem petrography from 133 ka to 105 ka allowed the detection of various hydroclimate and environmental conditions. Composite speleothem δ18O records from northeast Italy were found to closely follow the Northern Hemisphere summer insolation curve, attesting to the influence of solar isolation on Earth's hydroclimate. Our reconstructions indicate a northward shift of the Intertropical Convergence Zone during the LIG maximum, permitting Atlantic-derived moisture to travel further east across North Africa than at present, before being directed north towards the Alps. The latter part of the LIG exhibited frequent, extreme precipitation events in the SE Prealps due to orographic lifting of moisture from both the Atlantic and Mediterranean seas. Socio-economic planning must prepare for extreme autumnal flooding events and serious summer droughts, particularly important in vulnerable mountainous regions.

Effect of Interplanetary Magnetic Field on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents During Different Seasons

AbstractWe present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF B in NH (B in SH) in most local seasons under both signs of IMF B. For B in NH (B in SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During B and B in NH (B and B in SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.18 0.09 in winter and 1.17 0.09 in autumn. During B and B in NH (B and B in SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.16 0.07 for FAC. We also find an explicit B effect on auroral currents in a given hemisphere: on average B in NH and B in SH causes larger currents than vice versa. The explicit B effect on divergence‐free current during IMF B is in very good agreement with the B effect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer.

Evaluation of Version 3 Total and Tropospheric Ozone Columns From Earth Polychromatic Imaging Camera on Deep Space Climate Observatory for Studying Regional Scale Ozone Variations

Discrete wavelength radiance measurements from the Deep Space Climate Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC) allows derivation of global synoptic maps of total and tropospheric ozone columns every hour during Northern Hemisphere (NH) Summer or 2 hours during Northern Hemisphere winter. In this study, we present version 3 retrieval of Earth Polychromatic Imaging Camera ozone that covers the period from June 2015 to the present with improved geolocation, calibration, and algorithmic updates. The accuracy of total and tropospheric ozone measurements from EPIC have been evaluated using correlative satellite and ground-based total and tropospheric ozone measurements at time scales from daily averages to monthly means. The comparisons show good agreement with increased differences at high latitudes. The agreement improves if we only accept retrievals derived from the EPIC 317 nm triplet and limit solar zenith and satellite looking angles to 70°. With such filtering in place, the comparisons of EPIC total column ozone retrievals with correlative satellite and ground-based data show mean differences within ±5-7 Dobson Units (or 1.5–2.5%). The biases with other satellite instruments tend to be mostly negative in the Southern Hemisphere while there are no clear latitudinal patterns in ground-based comparisons. Evaluation of the EPIC ozone time series at different ground-based stations with the correlative ground-based and satellite instruments and ozonesondes demonstrated good consistency in capturing ozone variations at daily, weekly and monthly scales with a persistently high correlation (r2 > 0.9) for total and tropospheric columns. We examined EPIC tropospheric ozone columns by comparing with ozonesondes at 12 stations and found that differences in tropospheric column ozone are within ±2.5 DU (or ∼±10%) after removing a constant 3 DU offset at all stations between EPIC and sondes. The analysis of the time series of zonally averaged EPIC tropospheric ozone revealed a statistically significant drop of ∼2–4 DU (∼5–10%) over the entire NH in spring and summer of 2020. This drop in tropospheric ozone is partially related to the unprecedented Arctic stratospheric ozone losses in winter-spring 2019/2020 and reductions in ozone precursor pollutants due to the COVID-19 pandemic.

Impacts of multi‐timescale circulations on meridional heat transport

AbstractRelative contributions to the zonal mean meridional heat transport by the climatological annual mean, climatological annual variation, synoptic, intra‐seasonal and lower‐frequency motions were examined based on the ERA‐Interim reanalysis data for the period of 1981–2015. The meridional heat transport analysed in this study only includes the component related to meridional wind and temperature. In the tropics, the climatological annual mean circulations dominate the long‐term mean meridional heat transport, while the interaction between the climatological annual mean temperature and the seasonal anomalous flow largely contributes to the seasonal variation of the meridional heat transport. In the middle latitudes, the climatological annual mean circulations and transient eddies (mostly synoptic and intra‐seasonal eddies) are of roughly equal importance in the poleward heat transport, leading to the maximum poleward heat transport around 50°N/S. The upper‐ and lower‐tropospheric heat transports by the climatological annual mean circulations appear opposite, with the magnitude of the lower‐tropospheric transport being greater. The preferred maximum zonal mean heat transport at 50°N by the climatological mean flow is attributed to the maximum zonal mean low‐level southerly in situ. The preferred peak latitude of the mid‐latitude poleward heat transport by synoptic eddies near 50°N arises from the combined effect of the strong synoptic‐scale meridional wind and temperature variabilities in situ and their in‐phase relationship. The heat transport by tropical cyclones (TCs) was estimated by applying a statistical relationship between TC intensity and the vertically integrated temperature averaged over the TC core region derived from high‐resolution Weather Research and Forecasting (WRF) model simulations. For northern hemisphere summer, TCs contribute about 35% of the total heat transport in the active TC regions, suggesting that TCs play a critical role in the regional meridional heat transport.

Characteristic signatures of Northern Hemisphere blocking events in a Lagrangian flow network representation of the atmospheric circulation.

In the past few decades, boreal summers have been characterized by an increasing number of extreme weather events in the Northern Hemisphere extratropics, including persistent heat waves, droughts and heavy rainfall events with significant social, economic, and environmental impacts. Many of these events have been associated with the presence of anomalous large-scale atmospheric circulation patterns, in particular, persistent blocking situations, i.e., nearly stationary spatial patterns of air pressure. To contribute to a better understanding of the emergence and dynamical properties of such situations, we construct complex networks representing the atmospheric circulation based on Lagrangian trajectory data of passive tracers advected within the atmospheric flow. For these Lagrangian flow networks, we study the spatial patterns of selected node properties prior to, during, and after different atmospheric blocking events in Northern Hemisphere summer. We highlight the specific network characteristics associated with the sequence of strong blocking episodes over Europe during summer 2010 as an illustrative example. Our results demonstrate the ability of the node degree, entropy, and harmonic closeness centrality based on outgoing links to trace important spatiotemporal characteristics of atmospheric blocking events. In particular, all three measures capture the effective separation of the stationary pressure cell forming the blocking high from the normal westerly flow and the deviation of the main atmospheric currents around it. Our results suggest the utility of further exploiting the Lagrangian flow network approach to atmospheric circulation in future targeted diagnostic and prognostic studies.