Northward Transport Research Articles

Estimation the effect of accumulated long-range transported pollutants during a PM2.5 event in Taiwan

Fine particulate matter (PM2.5) pollution has attracted much attention in recent years due to its adverse impact on human health and the environment. This study examined a major PM2.5 event observed in Taiwan on March 02–03, 2022, using the Weather Research and Forecasting - Community Multiscale Air Quality (WRF-CMAQ) models. This period also coincided with an outage caused by the power failure of three power plants in Southern Taiwan on March 03, 2022, which created a unique situation to distinguish the impacts of long-range transported (LRT) pollutants and the operation of power plants on the air quality in Taiwan during the high-air-pollution event.Four numerical experiments, Baseline (real case), Replace (Baseline with power plants), No_LRT (Baseline without LRT), and No_0303_LRT (Baseline without LRT on March 03, 2022), were conducted with different initial emission scenarios. The simulation results revealed that the PM2.5 concentrations emitted from three power plants only counted a relatively small proportion in total, while the LRT pollutants were the key factor contributing to the high-concentration PM2.5 during the event. Besides, we figured out that the high PM2.5 concentrations on March 03, 2022 were dominated by the contribution of accumulated LRT. In addition, the results also suggested that the existence of a lee-side vortex, which is considered a potential key issue of the mesoscale system contributes to extreme air pollutant events in Taiwan, causing encouraging conditions for the upward diffusion and northward transport of the PM2.5 concentrations over the southern part of Taiwan, as a result, the distribution of pollution was modulated.

Large-scale background and role of quasi-biweekly moisture transport in the extreme Yangtze River rainfall in summer 2020

A severe flooding hit southern China along the Yangtze River in summer 2020. The floods were induced by extreme rains, and the associated dynamic and thermodynamic conditions are investigated using daily gridded rainfall data of China and NCEP-NCAR reanalysis. It is found that the June–July rainfall over the Yangtze River Basin (YRB) experienced pronounced subseasonal variation in 2020, dominated by a quasi-biweekly oscillation (QBWO) mode. The southwestward-moving anomalous QBWO circulation was essentially the fluctuation of cold air mass related to the tropospheric polar vortex or trough-ridge activities over the mid-high latitude Eurasian in boreal summer. The southwestward-transport of cold air mass from mid-high latitudes and the northeastward-transport of warm and moist air by the strong anomalous anticyclone over the western North Pacific provided important large-scale circulation support for the extreme rainfall in the YRB. The analysis of streamfunction of water vapor flux demonstrates that a large amount of water vapor eastward zonal transport from the Bay of Bengal and Indo-China and northward transport from the South China Sea provided the background moisture supply for the rainfall. The quasi-biweekly anomalies of potential and divergent component of vertically integrated water vapor flux played an important role in maintaining the subseasonal variability of rainfall in June–July of 2020. The diagnosis of moisture tendency budget shows that the enhanced moisture closely related to the quasi-biweekly fluctuated rainfall was primarily attributed to the moisture convergence. Further analysis of time-scale decomposition in the moisture convergence indicates that the convergence of background mean specific humidity by the QBWO flow and convergence of QBWO specific humidity by the mean flow played dominant roles in contributing to the positive moisture tendency. In combination with adiabatic ascent over the YRB induced by the warm temperature advection, the boundary layer moisture convergence strengthened the upward transport of water vapor to moisten the middle troposphere, favoring the persistence of rainfall during June–July. The vertical moisture transport associated with boundary layer convergence was of critical importance in causing low-level tropospheric moistening. By comparison, the horizontal moisture advection played a secondary important role in the quasi-biweekly oscillation of rainfall in June–July 2020.

The Seasonal Cycle of the Eastern Boundary Currents of the North Atlantic Subtropical Gyre

AbstractFor the first time, four dedicated hydrographic cruises—one in each season—took place in 2015 around the Canary Islands to determine the seasonality of the flows at the eastern boundary of the North Atlantic Subtropical Gyre. The Canary Current (CC) is the eastern boundary current of the North Atlantic Subtropical Gyre and links the Azores Current with the North Equatorial Current. The 2015 estimations show that the CC has a seasonal behavior in its path and strength, flowing on its easternmost position in winter (3.4 ± 0.3 Sv), through the Canary Islands in spring (2.1 ± 0.7 Sv) and summer (2.0 ± 0.6 Sv) and on its westernmost position in fall (3.2 ± 0.4 Sv). At the Lanzarote Passage (LP), the dominant flow is southward except in fall, where a northward transport is observed at the surface (1.1 ± 0.3 Sv) and intermediate (1.3 ± 0.2 Sv) layers. Combining all the available transport estimations, a historical composite observational seasonal cycle is constructed which fits the 2015 seasonal cycle. Hence, a solid seasonal cycle is constructed supported by all the available observations in the area. The LP seasonal cycle and its seasonal amplitude match the seasonal cycle and amplitude of the Atlantic Meridional Overturning Circulation (AMOC) measured by the RAPID‐MOCHA data array. These results indicate that the seasonal cycle of the AMOC of the RAPID‐MOCHA data array is driven by the dynamics of the eastern boundary current system.

Fingerprint of Climate Change on Southern Ocean Carbon Storage

AbstractThe Southern Ocean plays a critical role in the uptake, transport, and storage of carbon by the global oceans. It is the ocean's largest sink of CO2, yet it is also among the regions with the lowest storage of anthropogenic carbon. This behavior results from a unique combination of high winds driving the upwelling of deep waters and the subduction and northward transport of surface carbon. Here we isolate the direct effect of increasing anthropogenic CO2 in the atmosphere from the indirect effect of climate variability and climate change on the reorganization of carbon in the Southern Ocean interior using a combination of modeling and observations. We show that the effect of climate variability and climate change on the storage of carbon in the Southern Ocean is nearly as large as the effect of anthropogenic CO2 during the period 1998–2018 compared with the climatology around the year 1995. We identify a distinct climate fingerprint in dissolved inorganic carbon (DIC), with elevated DIC concentration in the ocean at 300–600 m that reinforces the anthropogenic CO2 signal, and reduced DIC concentration in the ocean around 2,000 m that offsets the anthropogenic CO2 signal. The fingerprint is strongest at lower latitudes (30°–55°S). This fingerprint could serve to monitor the highly uncertain evolution of carbon within this critical ocean basin, and better identify its drivers.

Long-lasting impact of winter North Atlantic Oscillation on Barents-Kara sea ice anomaly in recent decades

In this paper, the long-lasting impact of the winter North Atlantic Oscillation (NAO) on Arctic sea ice is investigated using reanalysis data, with focus on the Barents–Kara (BK) sea where the air-sea-ice response is closely associated with the interdecadal shift in the northern action center of the NAO. A significant negative correlation between the winter NAO and the late autumn BK sea ice has been dominant since the early 2000s, which is in sharp contrast to the extremely weak correlation before the late 20th century. When the northern center of the NAO retreats westward, a prevailing low-level southerly wind anomaly creates significant positive air temperature anomalies over the BK sea, and the induced ocean current strengthens the northward transport of warm sea water, resulting in a positive BK upper-layer ocean temperature anomaly and a negative sea ice anomaly until early spring. Entering summer, the preexisting less-than-normal sea ice causes the amount of solar shortwave radiation absorbed by the upper-layer sea water to significantly increase and thereby continues to warm up the upper layer of sea water and reduce the sea ice. The warmed sea water enables the negative sea ice anomaly to last until late autumn owing to its relatively large specific heat capacity. Thus, the NAO in the previous winter exerts a long-lasting impact on the BK sea ice.

Seasonal variations in Circumpolar Deep Water intrusions into the Ross Sea continental shelf

Intrusions of the warm and nutrient-rich Circumpolar Deep Water (CDW) across the Ross Sea shelf break play an important role in providing heat for ice shelf basal melting and setting the physical environment for biochemical processes. Several mechanisms driving CDW intrusions into the Ross Sea were proposed such as mesoscale eddies, tidal rectification, and interactions between the Antarctic Slope Current (ASC) and topographic features. The seasonal variations in the poleward transport of CDW are investigated using ERA-Interim wind data and a Ross Sea circulation model based on the Regional Ocean Modeling System (ROMS) between September 1999 and September 2014. The analyses focus on the currents along the shelf break and deep troughs on the Ross Sea shelf and discuss the wind-driven Ekman pumping in both shelf and adjacent open ocean regions. The results reveal that the poleward intrusions generally move up onto the continental shelf along the eastern flanks of deep troughs. Seasonal variations of the ocean surface stress torque exerted by wind and sea ice in the off-shelf area are correlated with CDW intrusions. The maxima of CDW intrusions usually occur in austral summer. There is a significant temporal correlation on the seasonal time scale between the on-shelf intrusions in deep troughs in the western Ross Sea shelf and poleward Sverdrup transports in the adjacent off-shelf open ocean driven by the Ekman pumping. The analysis of ocean surface stress fields also indicates that the vorticity fluxes through the Ekman pumping are in favor of southward and northward transports in the eastern and western parts of the Ross Sea, respectively. The relationships between currents, CDW intrusions, and ocean surface stress fields imply the importance of air–sea interactions and potential climate change to the environment in the Ross Sea.

Sediment dispersal and basin evolution during contrasting tectonic regimes along the western Gondwanan margin in the central Andes

A >15-20 km-thick succession in southern Bolivia forms the most-complete stratigraphic record in western Gondwana. Upper Neoproterozoic–Carboniferous clastic rocks record ∼300 Myr of marine, nonmarine, and glacially influenced sedimentation in diverse basin systems generated by variable tectonic regimes along the western edge of Gondwana during active and passive-margin conditions. New provenance results help resolve key uncertainties regarding source regions and sediment dispersal patterns. The findings are integrated with spatial variations in stratigraphic thicknesses to evaluate regional patterns of basin subsidence, magmatism, and deformation during long-term evolution of the western Gondwanan margin in the central Andes.Detrital zircon U–Pb geochronological data for 17 sandstone samples reveal sedimentary input from Precambrian cratonic basement provinces and pre-Andean basement, magmatic arc, and fold-thrust belt source regions. The basement age signatures indicate derivation from the flanking Brasiliano (900-560 Ma) and Pampean (650-500 Ma) provinces to the south and east, and the distal Rio de la Plata craton (2400-2000 Ma) along the eastern South American margin ∼1000–1500 km to the southeast. Although the greater Amazonian craton was not a major contributor, subordinate Amazonian signatures from the Sunsás (1300-950 Ma) province to the east and northeast selectively fed the northern basin regions of the central Andes. Despite the lack of Paleozoic igneous rocks in Bolivia, detrital zircons of Ordovician age attest to the pre-Andean influence of the subduction-related Famatinian magmatic arc. Limited Devonian-Carboniferous igneous material was contributed locally from western pre-Andean highlands or regionally by axial northward transport from selected igneous sources in Argentina and Chile. Episodic recycling of Neoproterozoic–Paleozoic basin fill, including a sharp reappearance of Famatinian-age detritus, can be linked to periods of Paleozoic crustal shortening and foreland sedimentation ascribed to Famatinian, Ocloyic, Chañic, or Gondwanide phases of deformation.The spatial distribution of sediment sources along with temporal shifts in sediment routing highlight several stages in the paleogeographic evolution of the western Gondwanan margin preserved in the central Andes. Initial regional subsidence spanned a multiphase Neoproterozoic to early Paleozoic history of Rodinia breakup, Brasiliano-Pampean orogenesis, and post-orogenic back-arc extension prior to final late Paleozoic amalgamation of Gondwana. The early Paleozoic onset of subduction and Famatinian arc magmatism led to high-magnitude subsidence (>10–15 km) likely driven by Ordovician slab rollback in an extensional back-arc basin. Thereafter, intermittent Paleozoic contraction in a poorly understood pre-Andean system (best expressed in the Eastern Cordillera of Bolivia and neighboring segments of northern Argentina and southern Peru) generated transient topographic loads that produced superimposed flexural foreland and successor basin systems.

Surface cross-equatorial pathways of seawater from the Bay of Bengal

Cross-equatorial pathways of heat and salt in the Eastern Indian Ocean are analyzed using Lagrangian trajectories based on the ECCO2 model simulation and drifter-derived currents combined with Argo data. The results show that most of the particles released in the Bay of Bengal move across the equator near the eastern boundary of the upper Indian Ocean (< 50m) during the southwest monsoon season and join the South Equatorial Current of the wind-driven circulation. Meridional salt and heat transports associated with the cross-equatorial currents feature significant seasonal variations, with stronger southward transports during the southwest monsoon than northward transports during the northeast monsoon, which are correlated significantly with the surface Ekman transports. The estimated annual mean salt and heat transports, based on the model (observational) data, are -0.06×109 (-0.11×109) kg·s-1 and -0.20 (-0.38) PW (1PW = 1015 W), respectively, southward across the equator.

Climate-driven shifts in pelagic fish distributions in a rapidly changing Pacific Arctic

Baseline surveys of offshore pelagic fishes in the eastern Chukchi Sea in 2012 and 2013 found that age-0 Arctic cod (Boreogadus saida) dominated the pelagic fish community in summer, with relatively few adults present in the region. Since this time, drastic changes in the ocean-atmosphere-ice feedback loop have led to continued warming, further reducing ice cover, and increased northward transport has led to an increase in Pacific-origin waters on the Chukchi shelf in summer. To examine potential bottom-up effects of these environmental changes on pelagic fishes in this rapidly changing environment, we extended a time series of large-scale acoustic-trawl surveys with additional surveys in 2017 and 2019. Age-0 Arctic cod were the most abundant pelagic fish in all four survey years, comprising 68–93% of fish abundance. However, age-0 walleye pollock (Gadus chalcogrammus), which were scarce (<0.1% of fishes) and confined to the southern Chukchi in 2012 and 2013, were present in high abundance (>21% of fish abundance) throughout the Chukchi shelf in 2017 and 2019. Age-0 Arctic cod were substantially more abundant in 2017 than in other years, possibly due to increased survivorship of larvae under warm conditions. Unlike in 2017, Arctic cod and pollock were spatially separated in 2019 due to enhanced transport, with Arctic cod primarily present in the northeastern portion of the survey area, which was characterized by cool surface and bottom temperatures. The substantial increase in abundance of age-0 pollock in recent years suggests that environmental conditions now allow this species to extend its northern range into the southern and central Chukchi Sea, at least on a seasonal basis. The changes in abundance and species composition of pelagic fishes in the 2012–2019 time series are tightly coupled to recent changes in sea ice, temperature, and the increasing transport of Bering Sea waters through Bering Strait into the Chukchi Sea. Given that the environment is expected to experience further warming and increased transport, these northward shifts in species distribution are likely to persist in the future.

Roles of Surface Forcing in the Southern Ocean Temperature and Salinity Changes under Increasing CO2: Perspectives from Model Perturbation Experiments and a Theoretical Framework

Abstract A rapid warming and freshening of the Southern Ocean have been observed over the past several decades and are attributed to anthropogenic climate change. In this study, ocean model perturbation experiments are conducted to separate roles of individual surface forcing in the Southern Ocean temperature and salinity changes. Model-based findings are compared with results from a theoretical framework including three idealized processes defined on the θ–S diagram. Under the future scenario of CO2 doubling, the heat flux forcing dominates the large-scale warming, deepening of isopycnals, and spiciness changes along isopycnals, which can be captured by an idealized pure warming process to represent the subduction of surface heat uptake. The poleward-intensifying westerly winds account for 24% of the enhanced warming between 35° and 50°S and would have comparable contribution as the heat flux forcing after removing the global ocean warming effect. In contrast, the widespread freshening in the Southern Ocean driven by increased surface freshwater input is largely compensated by the wind-driven saltening. The response to freshwater forcing could not be approximated as a similar pure freshening process as the induced cooling and freshening have comparable effects on density. The wind-driven changes are primarily through the local heave of isopycnals, thus resembling an idealized pure heave process, but contain considerable spiciness signals especially in the midlatitude Southern Ocean, resulting from anomalous northward transport and subduction of heat and salt that are largely density-compensating. These distinct signatures of individual surface forcing help us to better understand observed and projected changes in the Southern Ocean. Significance Statement Considerable changes including a rapid warming and freshening have been observed in the Southern Ocean as it absorbs most of the extra heat from the anthropogenic climate change, receives increased surface freshwater input, and experiences a poleward shift and intensification of the westerly winds. The purpose of this study is to distinguish different contributions from surface heat flux, freshwater flux, and wind forcing to the Southern Ocean temperature and salinity changes, based on ocean model experiments and three idealized processes from a theoretical framework. Our study reveals distinct signatures of individual surface forcing that help us to understand linkages between changes seen at the surface and in the interior Southern Ocean.

Moored Observations of the Currents and Transports of the Maluku Sea

Abstract The mean circulation and volume budgets in the upper 1200 m of the Maluku Sea are studied using multiyear current meter measurements of four moorings in the Maluku Channel and of one synchronous mooring in the Lifamatola Passage. The measurements show that the mean current in the depth range of 60–450 m is northward toward the Pacific Ocean with a mean transport of 2.07–2.60 Sv (1 Sv ≡ 106 m3 s−1). In the depth range of 450–1200 m, a mean western boundary current (WBC) flows southward through the western Maluku Sea and connects with the southward flow in the Lifamatola Passage. The mean currents in the central-eastern Maluku Channel are found to flow northward at this depth range, suggesting an anticlockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83–2.25 Sv, which is balanced by three transports: 1) 0.62–0.93 Sv southward transport into the Seram–Banda Seas through the Lifamatola Passage, 2) 0.97–1.01 Sv returning to the western Pacific Ocean through the central-eastern Maluku Channel, and 3) a residual transport surplus, suggested to upwell to the upper layer joining the northward transport into the Pacific Ocean. The dynamics of the intermediate gyre circulation are explained by the potential vorticity (PV) integral constraint of a semienclosed basin. Significance Statement The Indonesian Throughflow plays an important role in the global ocean circulation and climate variations. Existing studies of the Indonesian Throughflow have focused on the upper thermocline currents. Here we identify, using mooring observations, an intermediate western boundary current with the core at 800–1000-m depth in the Maluku Sea, transporting intermediate waters from the Pacific into the Seram–Banda Seas through the Lifamatola Passage. Potential vorticity balance suggests an anticlockwise gyre circulation in the intermediate Maluku Sea, which is evidenced by the mooring and model data. Transport estimates suggest northward countercurrent in the upper Maluku Sea toward the Pacific, supplied by the Lifamatola Passage transport and upwelling from the intermediate layer in the Maluku Sea. Our results suggest the importance of the intermediate Indonesian Throughflow in global ocean circulation and overturn. More extensive investigations of the Indo-Pacific intermediate ocean circulation should be conducted to improve our understanding of global ocean overturn and heat and CO2 storages.

Pacific cod or tikhookeanskaya treska (Gadus macrocephalus) in the Chukchi Sea during recent warm years: Distribution by life stage and age-0 diet and condition

Many fish species have moved poleward with ocean warming, and species distribution shifts can occur because of adult fish movement, or juveniles can recruit to new areas. In the Bering Sea, recent studies document a dramatic northward shift in the distribution of Gadus macrocephalus (Pacific cod in English and tikhookeanskaya treska in Russian) during a period of ocean warming, but it is unknown whether the current northward distribution shift continues into the Chukchi Sea. Here, we use catch data from multiple gear types to present larval, age-0, and older Pacific cod distributions from before (2010 and 2012) and during (2017, 2018, and 2019) recent Chukchi Sea warming events. We also report on the habitat, diet, and condition of age-0 Pacific cod, which were present in the eastern Chukchi Sea in recent warm years (2017 and 2019), but were absent in a cold year (2012). We hypothesize that age-0 recruitment to the eastern Chukchi Sea is associated with recent warm temperatures and increased northward transport through the Bering Strait in the spring. Age-0 fish were present in both benthic and pelagic habitats and diets reflected prey resources at these capture locations. Age-1 Pacific cod were observed in the western Chukchi Sea in 2018 and 2019, indicating possible overwinter survival of age-0 fish, although there was little evidence that they survive and/or remain in the Chukchi Sea to age-2. Observed low lipid accumulation in age-0 Pacific cod from the Chukchi Sea suggests juvenile overwinter mortality may be relatively high compared to more boreal regions (e.g. Gulf of Alaska). Adult Pacific cod were also observed in the Chukchi Sea during 2018 and 2019. Although densities in the western Chukchi Sea were very low compared to the Bering Sea, the adults are the first known (to us) records from the Chukchi Sea. The increased presence of multiple age-classes of Pacific cod in the Chukchi Sea suggests poleward shifts in both nursery areas and adult summer habitat beyond the Bering Sea, but the quantity and quality (e.g. summer productivity and overwintering potential) of these habitats will require continued surveys.

The influence of an atmospheric river on a heavy precipitation event over the western Alps

On 2–3 October 2020, a heavy precipitation event severely affected northern Italy and in particular the western Alps, with rainfall amount exceeding 600 mm over 24 h. This event was associated with an upper-level trough over the western Mediterranean basin, a large-scale configuration typical of heavy precipitation phenomena on the southern side of the Alps, since it induces a northward transport of large amounts of moisture impinging on the orography. The present study shows that a relevant amount of moisture moved towards the Mediterranean basin in the form of an atmospheric river (AR), a long and narrow filament-shaped structure crossing the whole Atlantic Ocean, characterized in the present case by a maximum Integrated Vapour Transport exceeding 1000 kg m−1 s−1. Therefore, in addition to the local contribution from the Mediterranean Sea, a relevant amount of moisture moved from the Tropics towards the Mediterranean, feeding the precipitation systems.The presence of an AR represented a distinguishing aspect of the event, superimposed on the well-known dynamic-thermodynamic mechanisms of heavy precipitation over the Alps. High-resolution numerical simulations and diagnostic tools have been exploited to investigate in detail how the transport of water vapour associated with the AR has influenced the dynamics and favoured the severity of the heavy precipitation processes.The results disclose the role of the AR and add further details to the theoretical framework of heavy precipitation mechanisms in the Alpine area, improving our understanding of the complex interaction between large-scale flows and mesoscale dynamics during extreme precipitation episodes. Due to the relatively fast evolution of the synoptic disturbance, the typical mesoscale mechanisms would have led only to an ordinary intense rainfall event. The contribution of the AR turned the event into a devastating flood.

Compound Heat Wave, Drought, and Dust Events in California

Abstract California is one of the nation’s top agriculture producers and is vulnerable to extreme events such as droughts and heat waves. Concurrent extreme events may further stress water and energy resources, exerting greater adverse socioeconomic, environmental, and health impacts than individual events. Here we examine the features of compound drought, heat wave, and dust events in California during spring and summer. From 2003 to 2020, 16 compound events are found in warm seasons, with a mean duration of ∼4 days. Compound events are characterized by enhanced surface temperature up to 4.5°C over northern and western California, reduced soil moisture and vegetation density, and an increase in dust optical depth (DOD) by 0.05–0.1 over central and southern California. The enhanced DOD is largely associated with severe vegetation dieback that favors dust emissions and southeasterly wind anomalies that support northward transport of dust from source regions in southern California. Surface fine dust and PM2.5 concentrations also increase by more than 0.5 and 5 μg m−3, respectively, during compound events associated with both enhanced dust emissions and a relatively stable atmosphere that traps pollutants. The development of the compound events is related to an anomalous high over the west coast in the lower to middle troposphere, which is a pattern favoring sinking motion and dry conditions in California. The anomalous high is embedded in a wave train that develops up to 7 days before the events. In comparison with heat wave extremes alone, compound events show significantly higher DOD and lower vegetation density associated with droughts.

The Relationship between the Atmospheric Heat Source over Tibetan Plateau and the Westerly-Monsoon Evolution in August and Its Physical Mechanism

In this study, the relationship between the East Asian subtropical westerly jet (EASWJ) and the East Asian summer monsoon (EASM) (westerly monsoon) and the correlation with the atmospheric heat source (AHS) on the Tibetan plateau (TP), especially the possible connection of the sudden enhancement of the correlation in August were analyzed. The results show that there is a significant correlation between the EASWJ and the EASM from June to October in terms of both intra-annual variability and interannual fluctuations, and the correlation between the AHS over TP and the EASWJ and the EASM during the same period is significantly enhanced in August. The synthetic analysis indicated that when the AHS was strong, a positive anomaly of a horizontal temperature gradient appeared over TP, which was conducive to the southward shift of the high-altitude temperature gradient center, resulting in the southward position of the axis of the 200 hPa westerly jet, and an upward and downward inclined westerly anomaly zone appeared from the south slope of TP to the main body and its north slope. Meanwhile, the East Asia–Pacific (EAP) teleconnection pattern with a negative phase appeared at 500 hPa, and TP to western Japan was located in the negative value area of the wave train. The AHS was conducive to the enhancement of the EAP negative phase, which was not conducive to the further northward transportation of water vapor by the EASM. On the contrary, when the AHS on TP was weak, the position of the westerly jet was northward and the EAP positive phase enhanced, contributing to the further northward transport of water vapor from the EASM.

North Atlantic Current and European Slope Current Circulation in the Rockall Trough Observed Using Moorings and Gliders

AbstractThe Rockall Trough (RT) accommodates the warmest and saltiest branch of the North Atlantic Current, which delivers water from the Gulf Stream into the marine environment around western Europe. In addition, the European Slope Current (ESC) carries warm water northward along the eastern boundary of the RT, and exchange between the ESC and the continental shelf is a dominant factor in determining the oceanographic conditions around the British Isles. However, the picture of the mean circulation and variability in the RT is still emerging, with a continuous observational campaign still in its relative infancy. The ESC, in particular, is poorly constrained by ship‐based, mooring, and satellite observations. In this paper, we examine the RT circulation and volume transport using a temporally extended and spatially expanded observing network. Six years of continuous mooring occupation reveal that a large‐amplitude, basin‐scale freshening event, previously detected south of Iceland around 2015, impacted the RT around 2017. Geostrophic transport was greatly reduced during this period, driven by a concurrent subsurface temperature increase at the western boundary. The circulation regained strength during the latter part of the record. We gathered 110 glider transects over 22 months which capture the ESC velocity field in unprecedented detail. The data are sufficient to characterize both the mean state and the emergent seasonal variability of the ESC, and reveal the year‐round presence of a southward countercurrent at depth. Variability in the strength and structure of this previously unstudied feature modulates net northward transport in the eastern boundary current system.

Transport of Antarctic Bottom Water Entering the Brazil Basin in a Planetary Geostrophic Inverse Model

AbstractThe abyssal circulation in the interior portions of the Brazil Basin and northern Argentine Basin were estimated using an overdetermined inverse method conserving neutral surface planetary potential vorticity and salinity. Antarctic Bottom Water (AABW) flows with a northward component through 30°S west of the Rio Grande Rise (RGR), as expected. However, in the area east of the RGR, the model shows southward transport in the upper part of the AABW across 30°S. This finding suggests that previous calculations of northward transport through Hunter Channel reaching the broader Brazil Basin may be overestimates. Reduced northward transport through the Hunter Channel has significant implications for the heat budget, implying less diapycnal mixing across the warmer AABW classes, compared to earlier studies.

Record-breaking rainfall over the middle reaches of the Yangtze River in August 2021: Sub-seasonal perspective and its predictability

An extremely heavy rainfall cluster was observed over the middle reaches of the Yangtze River (MYR) in August 2021, breaking the historical record since 1981 and causing severe floods. The controlling circulation regime and possible predictability sources of the un-expected rainfall are still unclear, especially from a sub-seasonal perspective. Our results show that the successive heavy rainfall events in August 2021 had significant intraseasonal oscillations for 10–24 d and 30–60 d, and they were synergistically influenced by tropical and extra-tropical circulation regime. Above all, the East Asia–Pacific teleconnection pattern (EAP) turned into a negative phase in late July and maintained throughout August, providing favorable background conditions for the northward transport of tropical water vapor and the southward intrusion of cold air from mid-high latitudes. The 30–60-d oscillation of precipitation was dominated by the first mode of boreal summer intraseasonal oscillation (BSISO1, with 30–60-d period), which strengthened the tropical moisture transport, while the quasi-biweekly oscillation of the western Pacific subtropical high and the anomalous mid-latitude cyclone of EAP intensified the precipitation on the scale of 10–24 d. In addition, the second mode of BSISO (BSISO2, with 10–30-d period) may also intensify the 10–24-d precipitation in late August. The forecast leading time of the S2S models for the persistent heavy rainfall in August 2021 was basically 1–2 weeks, while the EAP and BSISO1 were revealed as the main predictable sources of the abnormal rainfall events. These results highlight the importance of superposition of different intraseasonal oscillations in forming the extreme rainfall event and demonstrate a potential chance to enhance the prediction skill of extreme event if the ensemble members in S2S models could be reasonably selected according to their performances on key predictability sources.

Anthropogenic Carbon Transport Variability in the Atlantic Ocean Over Three Decades

AbstractThe change in anthropogenic CO2 (Canth) in the Atlantic Ocean is linked to the Atlantic Meridional Overturning Circulation (AMOC), that redistributes Canth meridionally and in depth. We have employed direct biogeochemical measurements and hydrographic data from the last 30 years, adjusted using inverse models for each decade with both physical and biogeochemical constraints. We then have computed the meridional transports and the vertical transports between two sections at the interphases by advection and diffusion. We have focused on the repeated sections at three latitudes—30°S, 24, and 55°N, dividing the Atlantic into two boxes. We have divided the net transport into upper, deep and abyssal layers, with an upper and abyssal northward transport of Canth and a southward component in deep layers. The change in time in the net transports of Canth appears to be mainly due to modifications in the transport of upper layers. The lower layer of the AMOC, a combination of deep and abyssal waters, maintain more consistent transports in time. Vertical advection plays an important role in the North Atlantic, exporting Canth from upper to deep layers. In the South Atlantic, the newly formed Antarctic Bottom Water exports Canth from abyssal to deep layers. The strong gradient in Canth concentration at the interphase of upper and deep layers results in a strong vertical diffusion.

Impingement of Subsurface Anticyclonic Eddies on the Kuroshio Mainstream East of Taiwan

AbstractThis study shows that North Pacific subtropical mode water (STMW) characterized by low potential vorticity can approach and impinge on the Kuroshio mainstream east of Taiwan. The results of the analyses of observations and output from eddy‐resolving models show that the STMW appears with the frequency of 32.1% and 19.9% in the Kuroshio east of Taiwan. A case study with the output of the ocean general circulation model for the earth simulator during October 2015–March 2016 indicates that mode water moves southwestward toward the Kuroshio in accordance with a vertically lens‐shaped subsurface anticyclonic eddy (SAE). After the shoreward edge of the SAE impinges on the offshore edge of the Kuroshio, the northward velocity, especially subsurface velocity, of the Kuroshio increases significantly in the subsurface layer and the Kuroshio central position (KCP) extends eastward. While the SAE moves and dissipates, the offshore edge of the Kuroshio moves inshore, and then transport decreases. Both the northward velocity and the KCP changes result in the Kuroshio transport change during the impingement of SAE in the case. Statistic analysis further demonstrate that eddies those can trap moving STMW are dominated by SAEs. When SAEs encounter the Kuroshio, their subsurface northward velocity increases by 67.8% and the KCP moves eastward, and thus the northward transport of the Kuroshio increases by 43.4%.