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Environmental DNA (eDNA) is increasingly used to noninvasively monitor aquatic animals in freshwater and coastal areas. However, the use of eDNA in the open ocean (hereafter referred to OceanDNA) is still limited because of the sparse distribution of eDNA in the open ocean. Small pelagic fish have a large biomass and are widely distributed in the open ocean. We tested the performance of two OceanDNA analysis methods—species-specific qPCR (quantitative polymerase chain reaction) and MiFish metabarcoding using universal primers—to determine the distribution of small pelagic fish in the open ocean. We focused on six small pelagic fish species ( Sardinops melanostictus , Engraulis japonicus , Scomber japonicus , Scomber australasicus , Trachurus japonicus , and Cololabis saira ) and selected the Kuroshio Extension area as a testbed, because distribution of the selected species is known to be influenced by the strong frontal structure. The results from OceanDNA methods were compared to those of net sampling to test for consistency. Then, we compared the detection performance in each target fish between the using of qPCR and MiFish methods. A positive correlation was evident between the qPCR and MiFish detection results. In the ranking of the species detection rates and spatial distribution estimations, comparable similarity was observed between results derived from the qPCR and MiFish methods. In contrast, the detection rate using the qPCR method was always higher than that of the MiFish method. Amplification bias on non-target DNA and low sample DNA quantity seemed to partially result in a lower detection rate for the MiFish method; the reason is still unclear. Considering the ability of MiFish to detect large numbers of species and the quantitative nature of qPCR, the combined usage of the two methods to monitor quantitative distribution of small pelagic fish species with information of fish community structures was recommended.

Arctic cyclones, as significant weather systems at high latitudes, are often accompanied by strong winds or heavy precipitation, leading to prolonged and more destructive disasters. In this study, we identified summer (JJAS) Arctic cyclones in the ERA5 reanalysis dataset from 1979 to 2022 using a machine-learning cyclone identification and tracking algorithm. We selected 40 sets of the strongest cyclones on the northern edge of Eurasia (NEE), one of the most identifiable Arctic Frontal Zones, over the past 44 years to assess the role of tropopause polar vortices (TPVs) in intense summer cyclones by using the Weather Research and Forecasting model (WRF). Our sensitivity experiments show that removing the horizontal temperature gradient of the lower stratosphere significantly reduces cyclone intensity, particularly for TPVs-matched cyclones, increasing by 7.4 hPa at their maximum intensity moment (2.8 hPa for TPVs-unmatched cyclones). TPVs-matched cyclones typically exhibit a lower tropopause and an “upper warm-lower cold” thermodynamic structure. High potential vorticity (PV) induced by the downward intrusion of TPVs due to the thermal structure closely links with the intensified development of these cyclones, leading to prolonged lifetimes. On the other hand, TPVs-unmatched cyclones display a distinct frontal structure in the lower troposphere but are not sensitive to changes in the upper-level horizontal temperature gradient. This may help to highlight the role of TPVs downward intrusion on TPVs-matched intense cyclones on NEE.

On 7 February 2020, precipitation within the comma-head region of an extratropical cyclone was sampled remotely and in situ by two research aircraft, providing a vertical cross section of microphysical observations and fine-scale radar measurements. The sampled region was stratified vertically by distinct temperature layers and horizontally into a stratiform region on the west side, and a region of elevated convection on the east side. In the stratiform region, precipitation formed near cloud top as side-plane, polycrystalline, and platelike particles. These habits occurred through cloud depth, implying that the cloud-top region was the primary source of particles. Almost no supercooled water was present. The ice water content within the stratiform region showed an overall increase with depth between the aircraft flight levels, while the total number concentration slightly decreased, consistent with growth by vapor deposition and aggregation. In the convective region, new particle habits were observed within each temperature-defined layer along with detectable amounts of supercooled water, implying that ice particle formation occurred in several layers. Total number concentration decreased from cloud top to the −8°C level, consistent with particle aggregation. At temperatures > −8°C, ice particle concentrations in some regions increased to >100 L −1 , suggesting secondary ice production occurred at lower altitudes. WSR-88D reflectivity composites during the sampling period showed a weak, loosely organized banded feature. The band, evident on earlier flight legs, was consistent with enhanced vertical motion associated with frontogenesis, and at least partial melting of ice particles near the surface. A conceptual model of precipitation growth processes within the comma head is presented.

The East Asian summer climate displays a marked change after the late 1990s. This is principally due to a weakening of the Pacific–Japan (PJ) teleconnection pattern that was a dominant driver of precipitation variability over East Asia. Nevertheless, western Japan has frequently experienced heavy rainfall events over the past several years. Atmospheric reanalysis and observational datasets are used to investigate summer precipitation variability over East Asia in a view of interdecadal changes from 1979 to 2020. East Asian summer precipitation has increased the most in Japan, especially over and around the Southwest Islands in southern Japan, where cumulus convection has been more dominant for the characteristic of precipitation variability including mei-yu–baiu rainfall than frontal structure since the mid-2000s. Atmospheric analysis in vertical structures and convective instability indicates that moist instability is neutralized by cumulus convection over the southern East China Sea and off the Pacific coast of Japan, where recent warming in sea surface temperature (SST) along the Kuroshio exceeds the SST threshold for convection. This decadal shift in precipitation variability has a close relationship with the second precipitation mode over East Asia, which has taken the place of the PJ pattern as a leading driver of precipitation variability over western Japan in the past decade. The enhanced cumulus convection is enabled to act as a forcing mechanism for the Rossby wave train from East Asia toward North America along the westerly jet or for the eastward extension of the Silk Road pattern, which possibly favors a heatwave in the Pacific Northwest areas of North America.

Drake Passage is a key region for transport between the surface and interior ocean, but a mechanistic understanding of this exchange remains immature. Here, we present wintertime, submesoscale‐resolving hydrographic transects spanning the southern boundary of the Antarctic Circumpolar Current and the Polar Front (PF). Despite the strong surface wind and buoyancy forcing, a freshwater lens suppresses surface‐interior exchange south of the PF; ventilation is instead localized to the PF. Multiple lines of the analysis suggest submesoscale processes contribute to ventilation at the PF, including small‐scale, O(10 km), frontal structure in water mass properties below the mixed layer and modulation of a surface eddy diffusivity at sub‐50 km scales. These results show that ventilation is sensitive to both submesoscale properties near fronts and non‐local processes, for example, sea‐ice melt, that set stratification and mixed layer properties. This highlights the need for adaptive observing strategies to constrain Southern Ocean heat and carbon budgets. Plain Language Summary Drake Passage is a region of the Southern Ocean between the southern tip of South America and the Antarctic Peninsula. Due to its relative accessibility as compared to the rest of the polar ocean, it is the most frequently occupied region of the Southern Ocean. Most occupations by ships in Drake Passage acquire measurements at 20–100 km spacing or “mesoscale” resolution. Here, we present data collected by piloted robotic underwater vehicles that sampled across the southern section of Drake Passage with submesoscale, or 1–10 km, resolution in wintertime. These novel observations indicate that while the southernmost region of Drake Passage is strongly stratified in density, the Polar Front (PF), one of the major dynamical features of the Southern Ocean, is more weakly stratified. The reduced stratification at the PF presents a pathway for the localized exchange of water between the surface and interior ocean. In addition, this study finds that the PF is eddy‐suppressing, meaning that the mean flow of the PF can transport oceanic properties away before they can be stirred. These findings have implications for the estimation of carbon fluxes between the atmosphere and the Southern Ocean, a vital part of the climate system. Key Points High‐resolution hydrographic sections across the Drake Passage provide insight into spatial variability in surface‐interior exchange Wintertime observations suggest ventilation is spatially localized to the Polar Front and influenced by submesoscale processes A mixing length estimate shows modulation at submesoscales and mixing suppression in the upper layers of the Polar Front

Despite the long existence of theoretical studies, few statistical studies of precipitation characteristics on the northern Pacific storm track have been reported due to lack of observation. Using data from GPM DPR and ERA-Interim, we examined the precipitation features of extratropical cyclones in the northern Pacific stormtrack region. Extratropical cyclones were classified into four categories including developing, mature, dissipating, and short-term based on their life stages. Our results show that extratropical cyclones of all categories had a “comma” rainband and precipitation mostly occurred to the east of the cyclonic center. The extratropical cyclones promote precipitation to the east of their centers, but suppress precipitation to the west. Precipitation to the east of the extratropical cyclones had larger and more condensed droplets, a stronger intensity, and a higher rain top than the local seasonal average, while the opposite characteristics were seen to the west. Our results suggest that the different types of vertical air motion and moisture content in these two regions induced by the frontal structure of extratropical cyclones play important roles in the different impact of extratropical cyclones. Furthermore, the different life stages of extratropical cyclones had different degrees of impact on precipitation: the highest impact in the developing stage, followed by the mature stage, and the weakest impact in the dissipating stage.

Tropopause folding events (TFs) are characterized by the rapid and deep descent of the tropopause and are considered to play a significant role in mass exchange between the stratosphere and troposphere. In the present study, TFs occurring in the Antarctic coastal region were examined using the ERA5 dataset. First, the climatological distribution of TF frequency in the extratropics of the Southern Hemisphere was examined. Similar to results from previous studies, TFs were found to often occur along the coast of Antarctica, which is located more than 1000 km south of the maximum of the eddy kinetic energy of synoptic-scale disturbances. This result suggests that the climatological pattern of frequency of TFs in the southern high latitudes cannot be explained only by the geographical distribution of storm tracks. Next, a composite analysis of TFs at Syowa Station was performed. When the negative anomaly of the tropopause height was greatest, strong Q-vector divergence and downwelling were observed in the vicinity of the TF locations. The distribution of Q vectors is related to a local westerly jet and strengthening of the frontal structure associated with meridionally contracted synoptic-scale disturbances. The roles of the topography of the Antarctic Plateau and the radiative cooling on the surface of the continent during the contraction of the disturbances are also discussed based on ray-tracing theory.

The frontal structure of the Southern Ocean is investigated using the Wavelet/Higher Order Statistics Enhancement (WHOSE) frontal detection method, introduced in Chapman’s work. This methodology is applied to 21 yr of daily gridded absolute dynamic topography (ADT) data to obtain daily maps of the locations of the fronts. By forming frontal occurrence frequency maps and then approximating these occurrence maps by a superposition of simple functions, the time-mean locations of the fronts, as well as a measure of their capacity to meander, are obtained and related to the frontal locations found by previous studies. The spatial and temporal variability of the frontal structure is then considered. The number of fronts is found to be highly variable throughout the Southern Ocean, increasing (splitting) downstream of large bathymetric features and decreasing (merging) in regions where the fronts are tightly controlled by the underlying topography. These splitting/merging events are related to changes in the underlying frontal structure whereby regions of high frontal occurrence cross or spread over streamfunction contours. In contrast to the number of fronts, frontal meandering remains relatively constant throughout the Southern Ocean. Little to no migration of the fronts over the 1993–2014 time period is found, and there is only weak sensitivity of frontal positions to atmospheric forcing related to the southern annular mode or the El Niño–Southern Oscillation. Finally, the implications of these results for the study of cross-stream tracer transport are discussed.

Marine boundary layer (MBL) cloud morphology associated with two summertime cold fronts over the eastern North Atlantic (ENA) is investigated using high-resolution simulations from the Weather Research and Forecasting (WRF) Model and observations from the Atmospheric Radiation Measurement (ARM) ENA Climate Research Facility. Lagrangian trajectories are used to study the evolution of post-cold-frontal MBL clouds from solid stratocumulus to broken cumulus. Clouds within specified domains in the vicinity of transitions are classified according to their degree of decoupling, and cloud-base and cloud-top breakup processes are evaluated. The Lagrangian derivative of the surface latent heat flux is found to be strongly correlated with that of the cloud fraction at cloud base in the simulations. Cloud-top entrainment instability (CTEI) is shown to operate only in the decoupled MBL. A new indicator of inversion strength at cloud top that employs the vertical gradients of equivalent potential temperature and saturation equivalent potential temperature, which can be computed directly from soundings, is proposed as an alternative to CTEI. Overall, results suggest that the deepening–warming hypothesis suggested by Bretherton and Wyant explains many of the characteristics of the summertime postfrontal MBL evolution of cloud structure over the ENA, thereby widening the phase space over which the hypothesis may be applied. A subset of the deepening–warming hypothesis involving warming initially dominating over moistening is proposed. It is postulated that changes in climate change–induced modifications in cold-frontal structure over the ENA may be accompanied by coincident changes in the location and timing of MBL cloud transitions in the post-cold-frontal environment.

For the first time using computerized ionospheric tomography (CIT) and leveraging ultra-dense slant total electron content (STEC) measurements derived from two ground-based Global Navigation Satellite System (GNSS) receiver networks in Japan, we have reconstructed the 3-D field-aligned structure of nighttime medium-scale traveling ionospheric disturbances (MSTIDs) with high spatiotemporal resolution. The CIT algorithm focuses on electron density perturbation components, allowing for the imaging of disturbances with small amplitudes and scales. Slant TECs used for CIT are setup to consist of two components: the background derived from IRI-2016 model and TEC perturbations obtained by subtracting a 30-min running average from observations. The resolution is set to 0.25º in latitude and longitude, 10 km in altitude, 30 s in time. Simulations were conducted to assess the performance of the CIT algorithm, revealing that this technique has good fidelity by accurately reconstructing more than 80% of the electron density perturbations. The focus is on the nighttime event of July 4, 2022, when data were accessible. The reconstruction results show that the MSTIDs initially form at lower altitudes and subsequently develop to exhibit large amplitudes and scales that extend to higher altitudes, characterized by a well-defined frontal structure with electrodynamic signatures. These results are consistent with theories and snippets of observational evidence regarding electromagnetic-influenced MSTIDs, hence affirming the effectiveness of the developed CIT technique in probing of the variations in the 3-D structure of ionospheric electron density. This is expected to contribute to a compressive understanding of the underlying mechanisms of ionospheric inhomogeneities. Graphical Abstract