Forcing Mechanisms Research Articles

CAGE21-3 Cruise Report: Ocean bottom seismics and acoustic surveys on the West-Svalbard margin – a study of local seismicity and its effect on methane seepage

Cruise CAGE21-3 with UiT’s research vessel R/V Helmer Hanssen was carried out to address the following objectives within the SEAMSTRESS project (Tectonic stress effects on Arctic methane seepage):1- Recovery of 10 Ocean Bottom Seismometers (OBS) deployed last year north of the Knipovich Ridge-Molloy Transform Fault corner (see report CAGE20-5).2- Conduction of an active source OBS experiment for the study of shear waves propagation in relation to local stress field and focused fluid flow.3- Acquisition of seismic lines for completing a side survey for the FRAME-IODP proposal.In 2020, 10 ocean-bottom seismometers (OBS) were deployed at the northern termination of the Knipovich Ridge to monitor seismic activity. The aim is to detect regional and local earthquakes. If localearthquakes are captured we will investigate the stress field in the region and how faults and fluids react to such stress (Figure 1). From the 10 instruments 8 are from Awi and 2 from UiT. IODP proposal 985 Eastern Fram Strait Palaeo-archive (FRAME) proposed to drill five sites on the western Svalbard margin in order to study chronostratigraphy, palaeoceanography, the palaeoclimatic evolution, and the forcing mechanisms responsible for main climatic transitions and the dynamics of the Svalbard Barents Sea Ice Sheet complex. This cruise acquired additional site survey seismic data for proposed primary and alternate drill sites. The cruise may be known as: CAGE21_3

Late Pleistocene to early Holocene ENSO-related hydroclimatic changes revealed by palynological records from the coastal Xixi Basin, eastern China

We report a palaeontological record for the last 37.0 cal kyr BP in the Xixi Basin, eastern China, to better understand the correlation between hydroclimatic change and vegetation history during the late Pleistocene to early Holocene. Principal component analysis based on the 25 main palynological taxa indicated that dominant taxa fall into three groups: conifers that preferred dry conditions on the left of leading principal component (PC) 1, ferns and wetland communities associated with a humid climate responding to the positive loading of PC1, and Quercus-dominated communities principally corresponding to PC2. Based on this analysis, hydroclimatic conditions were relatively warm/dry during the latter Marine Isotope Stage (MIS) 3 with a high abundance of conifers. This was followed by a generally cold/wet MIS 2 with expansion of ferns and herbs, and a warm/dry MIS 1 with more evergreen species. The sporadic occurrence of evergreen species and drier conditions since about 19.0 cal kyr BP coincides with the beginning of the last deglaciation. An increase in sedimentary accumulation rates and changes in vegetation components at the onset of the Holocene are likely related to maximum transgression. Wet conditions coincide with several cold intervals, including Heinrich events and the Last Glacial Maximum. The overall warm/dry and cold/wet climatic patterns agree well with the palaeoclimatic records elsewhere in the middle Yangtze region, showing similar hydroclimatic forcing mechanisms related to the migration of the Meiyu Front modulated by the El Niño-Southern Oscillation (ENSO) activities. Cold/wet conditions probably correspond to enhanced ENSO and reduced insolation while warm/dry conditions correspond to dampened ENSO and increased insolation. The extremely humid conditions that occur during cold events may result from the combined effects of the Atlantic Meridional Overturning Circulation and ENSO. These results expand our understanding of the monsoonal hydroclimate in eastern China during the late Quaternary period.

Slope and Shelf Flow Anomalies Off Oregon Influenced by the El Niño Remote Oceanic Mechanism in 2014–2016

AbstractOutputs of the regional ocean circulation model are analyzed to demonstrate the measurable impact of the El Niño remote oceanic forcing mechanism along the US West Coast during the major heat wave period of 2014–2016. The 2‐km horizontal resolution model, based on the Regional Ocean Modeling System (ROMS), was run for the period of 2009–2018. Though the model does not assimilate observations, it performs well by comparison with time series data explaining observed variability on temporal scales from several days to seasonal and interannual. The El Niño‐related oceanic anomalies provided by a global state estimate are introduced in the regional model at the southern boundary at 24N. These propagate alongshore with coastally trapped waves (CTWs) and influence the variability off Oregon (41°–46°N). In particular, CTWs are evident in the subsurface along‐slope current, vs, and in the depth of the 26.5 kg m−3 isopycnal surface over the slope, z26.5. In summer 2014 and 2015, vs anomalies are positive (northward) and z26.5 anomalies are negative (deeper) along the US West Coast. In addition to the CTW patterns, z26.5 anomalies also exhibit slow‐moving features associated with undercurrent widening, separation, and subsurface eddy variability. Over the Oregon shelf, El Niño conditions contributed to the sharp weakening of the southward alongshore current throughout the water column in July 2014 and 2015, despite the near‐average southward, upwelling‐favorable winds.

Threshold behavior of local gradient Richardson number in strongly stratified nonequilibrium turbulence

A nonequilibrium, strongly stratified turbulent flow is examined with a numerically simulated data set. We find self-organization of the flow around a critical state with a local gradient Richardson number (Ri) of about 1/4. The turbulence spontaneously organizes into layered, anisotropic flow structures within which the local Ri probability density function peaks around 1/4 for regions with the most significant dissipation rates and eddy viscosities. The results suggest that self-organization of the flow around Ri = 1/4 could be a property of layered anisotropic stratified turbulence and may not require the specific external forcing mechanisms previously reported.

Seasonal land-ice-flow variability in the Antarctic Peninsula

Abstract. Recent satellite-remote sensing studies have documented the multi-decadal acceleration of the Antarctic Ice Sheet in response to rapid rates of ice-sheet retreat and thinning. Unlike the Greenland Ice Sheet, where historical, high-temporal-resolution satellite and in situ observations have revealed distinct changes in land-ice flow within intra-annual timescales, observations of similar seasonal signals are limited in Antarctica. Here, we use high-spatial- and high-temporal-resolution Copernicus Sentinel-1A/B synthetic aperture radar observations acquired between 2014 and 2020 to provide the first evidence for seasonal flow variability of the land ice feeding George VI Ice Shelf (GVIIS), Antarctic Peninsula. Our observations reveal a distinct austral summertime (December–February) speed-up of ∼0.06±0.005 m d−1 (∼ 22±1.8 m yr−1) at, and immediately inland of, the grounding line of the glaciers nourishing the ice shelf, which constitutes a mean acceleration of ∼15 % relative to baseline (time-series-averaged) rates of flow. These findings are corroborated by independent, optically derived velocity observations obtained from Landsat 8 imagery. Both surface and oceanic forcing mechanisms are outlined as potential controls on this seasonality. Ultimately, our findings imply that similar surface and/or ocean forcing mechanisms may be driving seasonal accelerations at the grounding lines of other vulnerable outlet glaciers around Antarctica. Assessing the degree of seasonal ice-flow variability at such locations is important for quantifying accurately Antarctica's future contribution to global sea-level rise.

Influence of the El Niño-Southern Oscillation on SST Fronts Along the West Coasts of North and South America.

Along the west coasts of North, Central, and South America, sea surface temperature (SST) fronts are important for circulation dynamics and promoting biological activity. Prevailing equatorward winds during summer results in offshore Ekman transport and upwelling along the coast, where fronts often form between cold, upwelled water and warmer offshore waters. The interannual variability in winds, coastal upwelling, sea level anomalies, and SST in these regions have been linked to the El Niño-Southern Oscillation (ENSO), however SST fronts have received less attention. Here, we investigate the interannual variability of SST fronts off North, Central, and South America using satellite SST data spanning 1982-2018. Anomalies of fronts within 0-300km offshore indicate interannual variability that coincides with ENSO events in most regions. Frontal activity generally decreases during El Niño events and increases during La Niña events. The decrease in fronts off Peru and Chile during El Niño coincides with the seasonal peak in frontal activity, while off the United States the decrease occurs when frontal activity is at a seasonal minimum. We also utilized satellite measurements of wind stress and sea level anomaly to investigate how ENSO oceanic and atmospheric forcing mechanisms affect frontal activity. Decreases in frontal activity during El Niño events are largely due to oceanic forcing (i.e., coastal Kelvin waves) off Central and South America and to both oceanic forcing and atmospheric teleconnections off the United States. This study furthers our understanding of the influence of ENSO on coastal upwelling regions in the eastern Pacific Ocean.

Determining the Origin of Tidal Oscillations in the Ionospheric Transition Region With EISCAT Radar and Global Simulation Data

AbstractAt high‐latitudes, diurnal and semidiurnal variations of temperature and neutral wind velocity can originate both in the lower atmosphere (UV or infrared absorption) and in the thermosphere‐ionosphere (ion convection, EUV absorption). Determining the relative impact of different forcing mechanisms gives insight to the vertical coupling in the ionosphere. We analyze measurements from the incoherent scatter radar (ISR) facility operated by the EISCAT Scientific Association. They are complemented by meteor radar data and compared to global circulation models. The amplitudes and phases of tidal oscillations are determined by an adaptive spectral filter (ASF). Measurements indicate the existence of strong semidiurnal oscillations in a two‐band structure at altitudes ≲110 and ≳130 km, respectively. Analysis of several model runs with different input settings suggest the upper band to be forced in situ while the lower band corresponds to upward‐propagating tides from the lower atmosphere. This indicates the existence of an unexpectedly strong, in situ forcing mechanism for semidiurnal oscillations in the high‐latitude thermosphere. It is shown that the actual transition of tides in the altitude region between 90 and 150 km is more complex than described so far.

Numerical Investigation of Tropical Indian Ocean Barrier Layer Variability

AbstractThe barrier layer (BL) is part of the surface ocean isolated from the local air‐sea exchange processes despite it's great potential to store the heat beneath the mixed layer. The tropical oceans are known for the presence of BL. The interannual variability of the BL of the tropical Indian Ocean is examined in this study with a concomitant analysis of observations and outputs from an ocean general circulation model. The model faithfully reproduces the tropical variability of the dynamics and thermodynamics of the Indian Ocean when compared with observations and other studies. The BL thins (thickens) in the eastern (central‐to‐western) tropical Indian Ocean during the positive phase of the Indian Ocean Dipole Mode (IOD). The individual roles of five major forcing mechanisms on BL‐IOD coupling are examined. The thinning in the east is caused by upward heaving thermocline due to upwelling Kelvin waves during the positive phase of the IOD, causing the isothermal layer to shoal steeper than the mixed layer. The surface freshwater fluxes substantiate the thinning of BL in the east during this time, while the net heat fluxes thin the BL in the entire tropical Indian Ocean. The basin‐wide salinity (temperature) dynamics have widespread and complementary impacts, a BL thickening (thinning) in the equatorial and eastern tropical Indian Ocean during positive IOD years, with the former overwhelming the latter. The thickening of BL in the central‐to‐western tropical Indian Ocean during positive IOD is due to a combined action of salinity and freshwater forcing. The thermocline heaving, temperature and salinity dynamics and buoyancy forcing could explain the net variability of BL in the tropical Indian Ocean almost entirely in the model. The ocean‐only response shows that a BL weakens the shoaling (deepening) of the ocean mixed layer during positive (negative) IOD years in the eastern tropical Indian Ocean. As barrier layers store heat beneath the mixed layer, their role in Indian Ocean weather and climate needs to be examined with coupled model experiments.

An overview of Atlantic forcing of the North Sea with focus on oceanography and biogeochemistry

This paper provides an overview of the current knowledge of Atlantic forcing of the North Sea's hydro- and biogeochemical dynamics. The North Atlantic Oscillation (NAO) causes the largest short-term variations via common large scale atmospheric variations and the wind-driven exchange of water masses. On longer time scales the zonal shifts in the position of the subpolar front (SPF) in the eastern North Atlantic, also triggered by the NAO, determine the oceanic settings in the northern and central North Sea. The hydrodynamics of the southern North Sea are influenced more by local forcings. The influence of Atlantic forcing extends to the biogeochemistry of the northern North Sea, but its influence on primary production is still unclear. Climate change is expected to reinforce some of the forcing mechanisms but may reduce others, and further work is advised to firm up the projections available to date.

Variability of indian monsoon and its forcing mechanisms since late quaternary

The Indian monsoon is an important part of the global monsoon system, allowing important transfers of moisture at a large geographical scale and deeply affecting human populations and economic prosperity of regions. The tropical summer monsoon in the Northern Hemisphere is generally considered to be driven by low latitude solar radiation. Therefore, the summer monsoon strength is near zero-phase to the maximum of Northern Hemisphere Summer Insolation (NHSI). However, records from the Arabian Sea and some other parts of the Indian Ocean (e.g., Andaman Sea) show that a ∼8 kyr phase difference exists between the Indian summer monsoon (ISM) strength and the northern Hemisphere Summer Insolation maxima, which is obviously different from the records of stalagmites in the East Asia and other marine sediments (e.g., Bay of Bengal). This leads to the “sea-land precession phase paradox” in indian summer monsoon research. This paper systematically summarizes the Indian monsoon variability on orbital scale indicated by various records from the Indian monsoon regions (including oceans and continents) since the late Quaternary. The orbital forcing of Indian monsoon, the potential phase difference between indian summer monsoon and northern Hemisphere Summer Insolation and its possible forcing mechanism(s) are further discussed. The observed phase lag between indian summer monsoon and northern Hemisphere Summer Insolation may be controlled by the Atlantic Meridional Overturning Circulation (AMOC), latent heat transfer between the southern Indian Ocean and the Asian continent, or caused by the lack of tightly coupling between the Arabian Sea summer monsoon proxies and the monsoon intensity. In addition, it is still unclear whether previous monsoon proxies can provide a strong constraint on the intensity of summer monsoon. Environmental magnetism has been widely used in high-resolution dating and the analysis of paleoclimate variabilities in marine and terrestrial sediments, due to its high sensitivity on the rainfall and temperature. Therefore, in order to solve these issues, it is necessary to combine magnetic parameters with geochemical and paleontological parameters for more systematic work in the future.

Mixed convective thermal transport in a lid-driven square enclosure with square obstacle

The prime motive of this disquisition is to scrutinize simultaneous aspects of external forcing mechanism and internal volumetric forces on non-Newtonian liquid filled in square enclosure. Inertially driven upper lid is assumed by providing constant magnitude of slip velocity whereas thermal equilibrium is disturbed by assuming uniform temperature at lower boundary and by keeping rest of walls as cold. To enhance thermal diffusion transport with in the flow domain cold as well as adiabatic temperature situation is provided. In view of velocity constraints all the extremities at no-slip except the upper wall which is moving with ULid. Formulation is attained in dimensional form initially and afterwards variables are used to convert constructed differential system into dimensionless representation. A numerical solution of leading formulation is sought through Galerkin finite element discretization. Momentum and temperature equations are interpolated by quadratic polynomials whereas pressure distribution is approximated by linear interpolating function. Domain discretized version is evaluated in view of triangular and rectangular elements. Newton’s scheme is employed to resolve the non-linearly discretized system and a matrix factorization based non-linear solver renowned as PARADISO is used. Validation of results is ascertained by forming agreement with existing studies. In addition, grid independence test is also performed to show credibility of performed computations. Stream lines and isothermal contours patterns are portrayed to evaluate variation in flow distributions. Kinetic energy and local heat flux for uniform and non-uniform heating situations are also divulged in graphical and tabular formats. Increase in Reynold number produces decrease in kinetic energy of fluid. Enhancement in Grashof number causes enrichment of thermal buoyancy forces due to which Nusselt number uplifts. Clock wise rotations increase against upsurge in magnitude of Reynold number which is evidenced form stream lines. Squeezing of secondary vortex against Prandtl number arises due to dominance of viscous forces.

The dynamics of marine stepwise mass extinction

Mass extinction is characterized by the loss of 50 - 90 + percent of genetically and ecologically diverse species within 1 - 3.5 Myr intervals. Three conflicting theories exist: (1) Graded Mass Extinction; (2) Stepwise Mass Extinction; and (3) Catastrophic Mass Extinction. These can only be adequately tested with high resolution (cm-scale) stratigraphic data spanning the entire mass extinction interval and adjacent strata. Such data are presently available only for the Eocene-Oligocene (E-O), Cretaceous-Tertiary (K-T) and Cenomanian-Turonian (C-T) extinctions. In general, prevalent uniformitarian stratigraphic philosophy and use of the modern Earth/Life system as a model for the Phanerozoic has hindered the search for, and expectations of, high-resolution stratigraphic data critical) to mass extinction research. The modern Earth/Life "model" predicts highly variable, environmentally and biologically resilient systems and predominantly autocyclic stratigraphic response to large-scale forcing mechanisms.

Modelling morphodynamic responses of a natural embayed beach to Typhoon Lekima encountering different tide types

Recent developments in process-based coastal area models such as XBeach provide new opportunities to predict coastal responses to primary forcing mechanisms such as storm hydrodynamic by using 2DH grids. However, due to the lack of measured data, there are few application scenarios of the models. Therefore, more measurement and research are needed. In this paper, the Typhoon Lekima that hitting Zhejiang Province during neap tide period was selected to simulate morphodynamic responses of the Huangcheng Beach by assuming encountering with different tide types. Cross shore measurements with eight cross-shore profiles (named S1 ~ S8 from north to south) of the Huangcheng Beach pre- and post- the Typhoon Lekima respectively were presented. Then a 2DH storm surge and wave coupled model was established with Delft3D Flow/Wave. The model was well calibrated with measured water levels and wave data and provided hydrodynamic boundary conditions of different typhoon and tide types coupling situations for a refined model. The refined model was built using XBeach and simulated the morphological responses of the Huangcheng beach with well verifications. On basis of the numerical results, bed level changes at the eight profiles were analyzed, and the character of erosion and deposition under different tide conditions were illustrated. The net sand volume changes were got smaller under the spring tide condition rather than middle tide and neap tide conditions. Further study of the distribution of wave induced current at different stages of collision, inundation and ebb shown that the stronger current under spring tide condition would increase the sediment transport rate and reduce the deposition volume at profiles S3 and S6 ~ S8, and the total amount of sediment involved in transportation had increased for the whole beach, leading to the reduction of net erosion volume at profiles S1, S2, S4 and S5.

When Eastern India Oscillated Between Desert Versus Savannah‐Dominated Vegetation

AbstractDuring the last glacial period, the tropical hydrological cycle exhibited large variability across orbital and millennial timescales. However, the response of the Indian summer monsoon (ISM), its related impact on terrestrial ecosystems, and associated forcing mechanisms remain controversial. Here we present a marine record of pollen‐inferred vegetation changes suggesting that eastern India shifted from woody‐savanna mosaics during Marine Isotopic Stage 3 to grasslands during the Last Glacial Maximum resulting from large‐scale drying. Our data shows that ISM maximum is in phase with obliquity and precession maxima suggesting a dominant role of the Indian Ocean interhemispheric temperature gradient on glacial ISM variability. Persistent and abrupt dryland expansions of varying magnitude suggest rapid‐scale onset of aridity during Heinrich Stadial events and during the Toba eruption. We propose that the amplitude of ISM drought events are initiated by high latitude and volcanic forcings, although modulated by precession.

Passive Control of Supersonic Shear Layer in Rectangular Multistream Jet

Supersonic multistream engines with single-expansion ramp nozzles use splitter plates to separate the core from bypass streams. The resulting shear-layer instability imposes unsteady loading on proximal surfaces and generates acoustic tones. The present work develops a passive control strategy to alleviate these effects. The thick splitter plate subcomponent of the full configuration is isolated, and Mach 1.6 and sonic boundary layers are allowed to develop on either side to form the shear layer. Instability mechanisms and optimal forcing-response characteristics are identified from linear analysis of time-averaged large-eddy simulations (LESs). These results are then used to guide splitter plate trailing-edge modifications with different wave numbers and amplitudes to interfere with the internal forcing mechanism. LESs of the altered configurations at small wave numbers reveal a diamond-grid-like pattern of cells and dislocations, similar to those observed in low-speed active control studies. Modal decomposition displays an increase in rank behavior, with the dominant mode in good agreement with linear predictions. At large wave numbers and amplitudes, where nonlinear effects are stronger, LESs show that control introduces streamwise vorticity into the wake, reduces coherent structure size (streamwise correlation length) and inhibits the tonal content. These effects are confirmed in companion experiments.

Effect of the Trans‐Volcanic Axis on meridional propagation of summer precipitation in western Mexico

AbstractThe Trans‐Volcanic Axis (TVA)—comprising ridges and terrain elevations above 1,900 m—strongly affects meridional propagation of summer precipitation in western Mexico. This barrier modifies atmospheric moisture content and changes the annual precipitation cycle configuration, which distinguishes it from the precipitation cycle on both sides of the TVA. Using upper‐air observations and rainfall records, this study demonstrates that the TVA is the region where the North American monsoon (NAM) and the mid‐summer drought (MSD) influence intersections regarding precipitation. This region affects both northward extent of double precipitation maxima with the MSD and the southward extent of the NAM single precipitation peak. This study proposes that the forcing mechanism that controls the annual precipitation cycle in the TVA region relies on latent heat loss via ground cooling by reducing air temperature below dewpoint temperature in the 700–500 hPa layer, consistent with local terrain elevations. The orographic effect increases specific humidity in the atmospheric column—sufficiently large to enhance local precipitation accumulation—resulting in a mid‐summer drought intensity decline but not enough to yield a single peak in precipitation. Consequently, the TVA becomes an uncategorized region that does not display the NAM nor the characteristic features of the MSD.

Pollen-based climate changes since the middle MIS 3 in Jilantai Salt Lake in the marginal region of the Asian summer monsoon domain, Inner Mongolia, China

• The pollen record since mid-MIS 3 was provided for a current-hyperarid region of China. • The climate was subhumid during the mid-late MIS 3 and hyperarid during the Holocene . • Insolation and ice volume were the forcing mechanisms for climate changes in the Jilantai Basin. The climate conditions of the marginal region of the Asian summer monsoon (ASM) domain during Marine isotope stage (MIS) 3 (60–27 cal kyr BP) are poorly understood and require further study. Here, we collected fossil pollen samples from a 24.0-m-long drill core from Jilantai Salt Lake (Inner Mongolia, China) to reconstruct the regional climate changes since mid-MIS 3. During 47.0–43.0 cal kyr BP, the frequencies of the occurrence of aquatics and Pediastrum indicate relatively humid conditions. At approximately 43.0 cal kyr BP, the climate became dry, as revealed by the increase in Ephedra and the reduced aquatics and Pediastrum concentrations. The climate during 43.0–27.1 cal kyr BP was drier than that in the previous interval. Aquatics, ferns, and algae were barely detectable, indicating harsh climate conditions during 27.1–12.2 cal kyr BP. During the Holocene, aquatics and algae vanished, and the observed pollen assemblages were dominated by herb taxa; however, a substantial loss of taxa numbers occurred, suggesting that the climate was hyperarid. In general, the climate was relatively humid during the mid-late MIS 3, arid during MIS 2 (27.0–11.7 cal kyr BP), and hyperarid during the Holocene. The evaporation capacity connected with the Northern Hemisphere summer insolation and the Northern Hemisphere ice volume may have played a vital role in vegetation and climate changes in the Jilantai Basin since mid-MIS 3.

The seafloor heat flux driven by bottom water temperature variation in the Yellow and Bohai Seas

The Yellow and Bohai Seas (YBS) are shallow marginal seas located at the temperate latitudes of the western North Pacific. The seasonal bottom water temperature variation in the YBS can be greater than 20 °C and therefore generates significant seafloor heat flux, which significantly changes both the sediment temperature profile and heat content of the overlying water column. In this study, we investigated the seafloor heat flux driven by bottom water temperature variation in the YBS using a high-resolution numerical ocean model and a one-dimensional sediment temperature model. The results showed that seafloor heat flux in the area shallower than 50 m was on the order of 10 W/m2. The seafloor heat flux was larger than the lateral heat flux driven by circulation and ranked second only to surface net heat flux accounting for the most important forcing mechanism of ocean heat content variation in the coastal YBS. The accumulated seafloor heat flux during the sediment cooling/warming season could warm/cool the overlying water column by about 1 °C in the coastal YBS where the depth is shallower than 30 m. The depth of sediment affected by seafloor heat flux (seasonal sediment temperature variation >0.1 °C) could be as deep as 10 m. The complex spatial–temporal distribution of seafloor heat flux generated a complex distribution of sediment temperature and sound speed, which may have an important effect on acoustic wave propagation in the YBS.

Linking moisture and near-surface wind with winter temperature to reveal the Holocene climate evolution in arid Xinjiang region of China

An increasing number of palaeo-climatic records have been reported to identify the Holocene climate history in the arid Xinjiang region of northwest China. However, few studies have fully considered the internal linkages within the regional climate system, which may limit our understanding of the forcing mechanisms of Holocene climate change in this region. Here, we systematically consider three major issues of the moisture/precipitation, temperature and near-surface wind relevant to the Holocene climate history of Xinjiang. First, despite there still has debated for the Holocene moisture evolution in this region, more climatic reconstructions from lake sediments, loess, sand-dunes and peats support a long-term regional wetting trend. Second, temperature records from ice cores, peats and stalagmites demonstrate a long-term winter warming trend during the Holocene in middle- to high-latitudes of Asia. Third, recent studies of aeolian sedimentary sequences reveal that the near-surface winds in winter gradually weakened during the Holocene, whereas the winter mid-latitude Westerlies strengthened in the Tienshan Mountains. Based on this evidence, in the arid Xinjiang region we propose an early to middle Holocene relatively cold and dry interval, with strong near-surface winds; and a warmer, wetter interval with weaker near-surface winds in the middle to late Holocene during winter. Additionally, we develop a conceptual model to explain the pattern of Holocene climate changes in this region. From the early to the late Holocene, the increasing atmospheric CO2 content and winter insolation, and the shrinking of high-latitude continental ice-sheets, resulted in increasing winter temperatures in middle to high latitudes in the Northern Hemisphere. Subsequently, the increased winter temperature strengthened the winter mid-latitude Westerlies and weakened the Siberian high-pressure system, which caused an increase in winter precipitation and a decrease in near-surface wind strength. This scenario is strongly supported by evidence from geological records, climate simulation results, and modern reanalysis data. Our hypothesis highlights the important contribution of winter temperature in driving the Holocene climatic evolution of the arid Xinjiang region, and it implies that the socio-economic development and water resources security of this region will face serious challenges presented by the increasing winter temperature in the future.

Characteristics, Evolution, and Formation of Cold Air Outbreaks in the Great Plains of the United States

Abstract Wintertime cold air outbreaks (CAOs) in the Great Plains of the United States have significant socioeconomic, environmental, and infrastructural impacts; the events of December 1983 and February 2021 are key examples of this. Previous studies have investigated CAOs in other parts of North America, particularly the eastern United States, but the development of CAOs in the Great Plains and their potential subseasonal-to-seasonal (S2S) predictability have yet to be assessed. This study first identifies 37 large-scale CAOs in the Great Plains between 1950 and 2021, before examining their characteristics, evolution, and driving mechanisms. These events occur under two dominant weather regimes at event onset: one set associated with anomalous ridging over Alaska and the other set associated with anomalous pan-Arctic ridging. Alaskan ridge CAOs evolve quickly (i.e., on synoptic time scales) and involve stratospheric wave reflection. Conversely, Arctic high CAOs are preceded by weak stratospheric polar vortex conditions several weeks prior to the event. Both categories of CAOs feature anomalous upward wave activity flux from Siberia, with downward wave activity flux over Canada seen only in the Alaskan ridge CAOs. The rapid development of the Alaskan ridge CAOs, also linked with a North Pacific wave train and anomalous wave activity flux from the central Pacific, suggests that these events could be forced by tropical modes of variability. These findings present evidence that different forcing mechanisms, with contrasting time scales, may produce distinct sources of predictability for these CAOs on the S2S time scale.