Upward Flux Research Articles

Turbulent Vertical Velocities in Labrador Sea Convection

AbstractTurbulent vertical velocity measurements are scarce in regions prone to convection such as the Labrador Sea, which hinders our understanding of deep convection dynamics. Vertical velocity, , is retrieved from wintertime glider deployments in the convective region. From , downward convective plumes of dense waters are identified. These plumes only cover a small fraction of the convective area. Throughout the convective area, the standard deviation of agrees with scaling relations for the atmospheric surface and boundary layers. It initially depends on surface buoyancy loss in winter, and later, on wind stress after mid‐March. Both periods are characterized by positive turbulent vertical buoyancy flux. During convective periods in winter, the positive buoyancy flux is mostly forced by surface heat loss. After mid‐March, when buoyancy loss to the atmosphere is reduced, the positive buoyancy flux results from a restratifying upward freshwater flux, potentially of lateral origins and without much atmospheric influence.

Estimating Phase Transition Rates in Shallow Cumulus Clouds from Mass Flux. Part II: Vertically dependent formulation

Abstract This work continued the investigation of the relationship between phase transition rates and mass flux in trade wind cumulus clouds. The latter were simulated by an LES model initialized with soundings from the RICO field project. In Part I (Kogan, 2022) we demonstrated that a very high correlation exists between integral phase transition rates and upward mass flux. In this study we focused on the vertically dependent variables and showed that a similar high correlation exists between the condensation rate 𝒞 (z) and the upward mass flux ℳ (z). Based on condensation theory, we showed that under quasi-steady approximation condensation rates can be calculated by a linear function of ℳ with the slope coefficient dependent only on temperature and pressure. The model data showed that the error of such approximation is less than a few tenths of a percent. The parameterization of the evaporation process is more complex, mostly because of the slow evaporation of raindrops as they fall through the cloud’s unsaturated areas. Nevertheless, it was possible to define the fraction of evaporation to condensation rate as a function of vertical coordinate z and cloud thickness H. This function can be quite accurately approximated by the 3rd order polynomials of z and H. It is suggested that proposed formulation of evaporation together with the quasi-steady formulation of condensation can serve as a parameterization of water phase transition rates in shallow cumulus clouds.

Interdecadal Variation in the Impact of Arctic Sea Ice on El Niño–Southern Oscillation: The Role of Atmospheric Mean Flow

Abstract This study reveals the remarkable interdecadal changes in the influence of boreal winter Arctic sea ice concentration (SIC) anomalies (ASICAs) in the Greenland–Barents Seas on the subsequent El Niño–Southern Oscillation (ENSO) development. Winter ASICA is strongly associated with the subsequent winter ENSO before the late 1980s and after the late 2000s, but their connection is weak during the 1990s and the 2000s. The interdecadal variations in the influence of ASICA on ENSO are associated with changes in the spatial structure of the ASICA-induced North Pacific atmospheric anomalies. During high-correlation periods, winter SIC increases in the Greenland–Barents Seas lead to tropospheric cooling via the suppression of upward surface heat fluxes, which further trigger an atmospheric teleconnection from the Arctic to the North Pacific. The accompanying North Pacific Oscillation–like atmospheric anomalies result in sea surface temperature (SST) warming in the subtropical North Pacific, which extends southward into the tropical Pacific via wind–evaporation–SST feedback and leads to surface westerly anomalies over the tropical western Pacific in the following summer. The tropical western Pacific westerly wind anomalies impact the subsequent ENSO development via triggering positive Bjerknes air–sea interaction. During low-correlation periods, atmospheric anomalies over the North Pacific generated by the winter ASICA are located more northward and cannot induce marked subtropical North Pacific SST anomalies and thus have a weak impact on the following ENSO development. Numerical experiments suggest that the interdecadal variation in the spatial structure of the North Pacific atmospheric anomalies induced by the winter ASICA is partly attributed to change in the atmospheric mean flow. Significance Statement Arctic sea ice is an important component of the global climate system. Studies have shown that Arctic sea ice has decreased significantly in recent decades, which is considered to be one of the most important responses of Earth’s climate system to global climate change. A number of studies have shown that Arctic sea ice anomalies have significant impacts on weather and climate over mid–high latitudes through tropospheric and stratospheric processes. Recent studies have suggested that the effects of Arctic sea ice anomalies could extend to the tropics via air–sea interactions. El Niño–Southern Oscillation (ENSO) is the strongest air–sea coupled system in the tropics and can have a significant impact on climate over the globe. ENSO is also considered to be one of the most important sources of subseasonal–seasonal climate predictability over many parts of the globe. It is therefore important to study the factors for the ENSO occurrence. A recent study showed that Arctic sea ice anomalies during boreal winter in the Greenland–Barents Seas have a significant impact on the following winter ENSO. In this study, we further reveal that the influence of winter Arctic sea ice anomalies in the Greenland–Barents Seas on the subsequent ENSO is unstable and has undergone significant interdecadal variation. We then investigate the mechanisms underlying the interdecadal variation via observational analysis and numerical simulations. The results of this study not only have implications for improving the prediction of ENSO but also could improve our understanding of the physical link between high-latitude climate systems and tropical air–sea coupling systems.

Field investigation of soil moisture profile in the seedbeds of clayey paddy fields composed of large clods (aggregates)

ABSTRACT Unstable germination and emergence of upland crops in clayey paddy fields is a pressing issue. As tillage leads to the formation of large clods measuring a few centimeters, the soil moisture content in the seedbed markedly decreases, potentially inhibiting germination of seeds. Although the effect of the clod size on the soil moisture profile was confirmed experimentally, the validity of this effect has not been tested in the field. In this study, we aimed to clarify the effect of clod size on the moisture profile and water balance in the seedbeds of clayey paddy field. We developed an apparatus for measuring soil moisture content in the soil layer with clods. This apparatus used a laser displacement sensor and large transparent plate horizontally fixed above the soil surface. Temporal changes in the soil moisture profile (0–10 cm depth) and evaporation rate of clayey paddy fields with different clod sizes were monitored during the sowing period of soybeans. We found that the larger the mean clod diameter (MCD), the greater the decrease in soil moisture content near the soil surface. In addition, soil moisture loss at the sowing depth (5 cm) was positively correlated with MCD. In terms of the water balance, the estimated upward water flux at a depth of 5 cm was less than 1 mm d−1 across all the plots and periods. By contrast, the estimated ratio of the apparent to molecular diffusion coefficient of water vapor in the dry soil layer of clayey paddy field with an MCD of 2.5 and 1.8 cm were 5.1 and 2.0, respectively. These results suggest that more significant soil moisture loss causing reduction of germination and emergence rate can occur in a clayey paddy field with large clods, likely owing to the increased enhancement of water vapor movement. To increase the germination and emergence rate, therefore, a numerical model that can evaluate the moisture profile of soil layer with clods should be developed and the sowing depth should be optimized based on the MCD.

Observations of Significant Ion Energy Outflows Associated With Cusp Ion Outflows and the Role of Poynting Flux as an Energy Source

AbstractThe cusp ion outflow on Earth has been extensively studied for decades. However, the energy flux associated with the ion outflow, which is of equivalent importance to the number flux or mass flux, has been rarely studied. Here, we present the first systematic study on the cusp ion energy flux and the energy budget along the cusp magnetic flux tube using quasi‐conjunction observations from the Polar and Fast satellites. Significant ion energy fluxes (several 10s mW/ mapped to 100 km altitude) away from the Earth are frequently observed in the mid‐altitude cusp (3–6 Re). Observations at low altitudes (2 Re) show that the upward ion energy flux is associated with ion outflows originating from the ionosphere. In addition, we show that the ion outflows experience intense energization well above the ionosphere. The only possible energy source for this energization is the earthward Poynting flux in mid‐altitude. The electrons are accelerated downward and are another energy sink (not source). The altitude profile of the energy fluxes suggests a transition region between 2 and 4 Re where both ion heating and electron acceleration primarily occur and where significant Poynting flux is dissipated. Analysis of the E/B ratio shows that the Poynting flux is carried by both the Alfven waves and quasi‐static structures. The above results place important constraints on possible local ion energization mechanisms.

Floating active carpets drive transport and aggregation in aquatic ecosystems

Communities of swimming microorganisms often thrive near liquid–air interfaces. We study how such ‘active carpets’ shape their aquatic environment by driving biogenic transport in the water column beneath them. The hydrodynamic stirring that active carpets generate leads to diffusive upward fluxes of nutrients from deeper water layers, and downward fluxes of oxygen and carbon. Combining analytical theory and simulations, we examine the biogenic transport by studying fundamental metrics, including the single and pair diffusivity, the first passage time for particle pair encounters and the rate of particle aggregation. Our findings reveal that the hydrodynamic fluctuations driven by active carpets have a region of influence that reaches orders of magnitude further in distance than the size of the organisms. These non-equilibrium fluctuations lead to a strongly enhanced diffusion of particles, which is anisotropic and space dependent. Fluctuations also facilitate encounters of particle pairs, which we quantify by analysing their velocity pair correlation functions as a function of distance between the particles. We found that the size of the particles plays a crucial role in their encounter rates, with larger particles situated near the active carpet being more favourable for aggregation. Overall, this research broadens our comprehension of aquatic systems out of equilibrium and how biologically driven fluctuations contribute to the transport of fundamental elements in biogeochemical cycles.

Parameterization of Wave‐Induced Stress in Large‐Eddy Simulations of the Marine Atmospheric Boundary Layer

AbstractLarge‐eddy simulations (LES) of the Marine Atmospheric Boundary Layer (MABL) commonly utilize roughness length to parameterize wave effects on the sea surface. However, this method might not adequately capture the intricacies of wind‐wave interactions, especially in swell‐dominated conditions. In this study, we integrated a wave‐induced stress parameterization method into the open‐source LES code, PArallel Large‐eddy simulation Model (PALM), and assessed its performance under low wind conditions with varying wave ages and directions. Our method treats windsea effects as surface friction while employing an exponentially decaying profile to represent swell‐induced stress. We examined the model's sensitivity to wind and wave parameters and calibrated the wave damping rate values across various scenarios, leveraging data from wave‐phase‐resolved simulations and field measurements. The enhanced LES model with the proposed wave parameterization effectively reproduces wave‐influenced wind characteristics such as upward momentum flux, low‐level wind maximum, and negative wind gradients, showing good agreement with previous numerical and observational data. Moreover, our model accounts for wind‐wave misalignment scenarios by calculating each directional and spectral wave stress component individually and integrating them over the wave spectrum. We found that the swell‐induced stress is the dominant force in the wave boundary layer (WBL) with low wind speed, with its magnitude significantly varying with the wave direction relative to the wind. This variation influences the dynamic equilibrium within the WBL, modifying both wind shear and wind veer, and these effects persist up to the top of the boundary layer flow.

Effect of climate warming on subsurface temperature in basins with topography-driven groundwater flow

Climate warming has caused increased air temperature as well as increased subsurface temperature. Many previous studies on subsurface warming simplified heat advection by neglecting the horizontal component of regional groundwater flow or even neglected heat advection accompanying groundwater flow. In this study, the simultaneous control of heat advection and conduction on subsurface warming is numerically investigated in a 2D hypothetical basin cross section. By calculating the increment of subsurface temperature, we find that heat advection could accelerate subsurface warming. In a given basin, subsurface warming in the recharge area with downward groundwater flow is more significant than that in the discharge area with upward groundwater flow. By using a 1D model with vertical groundwater flow only for comparison, we find that in any part of a basin, even if the horizontal flux is very small compared with the vertical flux, neglecting the horizontal flux would underestimate the propagation depth of climate warming. This implies that when the propagation depth of climate warming is a priori known variable, existing 1D models would overestimate downward groundwater flux or underestimate upward groundwater flux. Moreover, we find pumping would cause deeper propagation depths of climate warming by accelerating groundwater circulation, whereas basin-scale heterogeneity and anisotropy of hydraulic conductivity would cause shallower propagation depths of climate warming because of the relative dominance of horizontal flow. By demonstrating the importance of 2D groundwater flow on subsurface warming, the results provide new insight into understanding the regulation of temperature among the atmosphere, the hydrosphere and the lithosphere.

An investigation into Venusian atmospheric chemistry based on an open-access photochemistry-transport model at 0–112 km

Atmospheric chemistry plays a crucial role in the evolution of climate habitability on Venus. It has been widely explored by chemistry-transport models, but some characteristics are still poorly interpreted. This study is devoted to developing an open-access chemistry-transport model spanning both the middle and lower atmospheres of Venus. It provides a scheme for the structure of the chemistry, especially for the sulfur and oxygen, and investigates the influence of the cloud diffusivity and the SO2 dissolution that are adopted in the clouds. The developed model is based on the VULCAN framework and was updated with the state-of-the-art Venusian atmospheric chemistry. It includes vertical eddy diffusion retrieved recently with the Venus Express observations, and it resolves radiative transfer containing gas absorption and scattering, Mie scattering of the cloud droplets, and absorption of the unknown UV absorber. The obtained abundance profiles of SO, SO2, CO, COS, O, O2, O3, HCl, and NO are in overall agreement with the observations. The results show that the increase in cloud diffusivity has slight effects on the chemical structure. The SO2 mainly dissolves in 50–90 km and evaporates below the clouds. The rapid dissolution-release cycle is responsible for the large upward flux of SO2 at 58 km. At around 70 km, SO has a significant peak that is larger than that of previous studies by an order of magnitude, and S and SO2 also show slight increases. They are attributed to the buffering effects of liquid SO2 in the clouds. O2 is significantly eliminated by SO in this layer. We emphasize the superior regulation of the sulfur cycle on O2 at 70 km and its potential contributions to the long-standing problem of the overestimated O2 abundance.

Nutrient Replenishment by Turbulent Mixing in Suspended Macroalgal Farms

AbstractThis study uses large eddy simulations to investigate nutrient transport and uptake in suspended macroalgal farms. Various farm configurations and oceanic forcing conditions are examined, with the farm base located near the nutricline depth. We introduce the Damkohler number Da to quantify the balance between nutrient consumption by macroalgae uptake and supply by farm‐enhanced nutrient transport. Most cases exhibit low Da, indicating that farm‐generated turbulence drives sufficient upward nutrient fluxes, supporting macroalgae growth. High Da and starvation may occur in fully grown farm blocks, a configuration that generates the weakest turbulence, particularly when combined with densely planted macroalgae or weak flow conditions. Flow stagnation within the farm due to macroalgae drag may constrain the uptake efficiency and further increase the starvation risk. Mitigation strategies involve timely harvesting, avoiding dense macroalgae canopies, and selecting farm locations with robust ocean currents and waves. This study provides insights for sustainable macroalgal farm planning.

Effect of typhoon on suspended sediment concentration, bed erosion and sediment transport in the Yangtze Estuary

Annually 5–6 typhoons strike the Yangtze Estuary (YE) as extreme events. However, their high energy and importance for sediment transportation and geomorphic changes are still not fully understood. In this study, high-resolution observations of wind, wave, flow velocity, and suspended sediment concentration (SSC) at two in-situ stations were carried out during the 2022 Hinnamnor typhoon. Additionally, we simulated the change in SSC, estuarine bed erosion/deposition, and flow and sediment transport with and without a typhoon in the YE using MIKE3 numerical model. The findings revealed that the Hinnamnor typhoon-induced waves increased the SSC of the turbidity maximum zone (TMZ) by a factor of 5.6 times (maximum is 2.8 kg/m3). The TMZ area also extended by 2.68 times (maximum is 7880km2, 70.4% of YE) in the YE. Moreover, the typhoon caused a dramatic change in sediment transport and bed erosion/deposition in the YE. First, in the delta front area where the mean water depth is >5 m, the typhoon significantly increased the southward flux of residual flow and sediment, causing sediment transport into Hangzhou Bay to abruptly increase 26.3 times (increase of 52 million tons, accounting for 1/3 of the present annual flux of the Yangtze River (150 million tons)) during a single spring-neap period. The net erosional area and volume extended to 6770km2 (60.4% of YE) and 91.18 × 106 m3. Second, in the delta shoals (where the mean water depth is <5 m, including east Chongming Shoal, Hengsha Shoal, Jiuduansha Shoal, and east-south Nanhui Shoal), residual flow and sediment flux decreased northward from the typhoon and resulted in the erosion of the shoal. Third, in channels with trumpet-shaped mouths (North Branch (NB), North Channel (NC) and South Passage (SP), except for North Passage (NP)), the upward flux of residual flow and sediment increased due to the typhoon, resulting in bed deposition in these channels (NB, NC and SP). This study highlights the important influence of typhoons on flow and sediment transport and bed erosion in estuarine areas.

The Eurasian Arctic Ocean along the MOSAiC drift in 2019–2020: An interdisciplinary perspective on physical properties and processes

The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC, 2019–2020), a year-long drift with the Arctic sea ice, has provided the scientific community with an unprecedented, multidisciplinary dataset from the Eurasian Arctic Ocean, covering high atmosphere to deep ocean across all seasons. However, the heterogeneity of data and the superposition of spatial and temporal variability, intrinsic to a drift campaign, complicate the interpretation of observations. In this study, we have compiled a quality-controlled physical hydrographic dataset with best spatio-temporal coverage and derived core parameters, including the mixed layer depth, heat fluxes over key layers, and friction velocity. We provide a comprehensive and accessible overview of the ocean conditions encountered along the MOSAiC drift, discuss their interdisciplinary implications, and compare common ocean climatologies to these new data. Our results indicate that, for the most part, ocean variability was dominated by regional rather than seasonal signals, carrying potentially strong implications for ocean biogeochemistry, ecology, sea ice, and even atmospheric conditions. Near-surface ocean properties were strongly influenced by the relative position of sampling, within or outside the river-water influenced Transpolar Drift, and seasonal warming and meltwater input. Ventilation down to the Atlantic Water layer in the Nansen Basin allowed for a stronger connectivity between subsurface heat and the sea ice and surface ocean via elevated upward heat fluxes. The Yermak Plateau and Fram Strait regions were characterized by heterogeneous water mass distributions, energetic ocean currents, and stronger lateral gradients in surface water properties in frontal regions. Together with the presented results and core parameters, we offer context for interdisciplinary research, fostering an improved understanding of the complex, coupled Arctic System.

Vertical Energy Fluxes Driven by the Interaction between Wave Groups and Langmuir Turbulence

Abstract Data from an air–sea interaction tower are used to close the turbulent kinetic energy (TKE) budget in the wave-affected surface layer of the upper ocean. Under energetic wind forcing with active wave breaking, the dominant balance is between the dissipation rate of TKE and the downward convergence in vertical energy flux. The downward energy flux is driven by pressure work, and the TKE transport is upward, opposite to the downgradient assumption in most turbulence closure models. The sign and the relative magnitude of these energy fluxes are hypothesized to be driven by an interaction between the vertical velocity of Langmuir circulation (LC) and the kinetic energy and pressure of wave groups, which is the result of small-scale wave–current interaction. Consistent with previous modeling studies, the data suggest that the horizontal velocity anomaly associated with LC refracts wave energy away from downwelling regions and into upwelling regions, resulting in negative covariance between the vertical velocity of LC and the pressure anomaly associated with the wave groups. The asymmetry between downward pressure work and upward TKE flux is explained by the Bernoulli response of the sea surface, which results in groups of waves having a larger pressure anomaly than the corresponding kinetic energy anomaly, consistent with group-bound long-wave theory.

Impact of the Winter Regional Hadley Circulation over Western Pacific on the Frequency of Following Summer Tropical Cyclone Landfalling in China

Abstract The poleward migration of tropical cyclone (TC) activity in recent years has been linked to the expansion of the Hadley circulation (HC). Here, we investigate the impact of the winter regional HC over the western Pacific (WPHC) on the frequency of following summer landfalling TC (LTC) in China. Results show that interannual variation of the LTC frequency has a very close connection with the northern WPHC edge (WPHCE). After removing the El Niño–Southern Oscillation signal, there still exists a significant correlation between them. When the winter WPHCE shifts poleward, the associated lower-level southwesterly (easterly) wind anomalies over the subtropical western Pacific (tropical central-eastern Pacific) induce sea surface temperature (SST) warming (cooling) anomalies therein via suppressing (enhancing) upward surface heat flux. In turn, the SST warming (cooling) excites an anomalous cyclonic (anticyclonic) circulation to its west via a Rossby wave response, thus maintaining the southwesterly (easterly) wind anomalies. In addition, the negative rainfall anomalies over the tropical central-eastern Pacific induced by negative SST anomalies can stimulate an anomalous intensive Walker circulation with anomalous upward motion around the tropical western Pacific. Through this positive air–sea interaction, the winter WPHCE signal would be preserved in the ocean and maintained to the succeeding summer, then favoring LTC genesis landward by decreasing the vertical wind shear and increasing the low-level vorticity and midlevel humidity. Meanwhile, anomalous midtropospheric easterly winds over the subtropics are favorable for steering more LTCs toward China’s coast. This study suggests that the winter WPHCE variation is a potential predictor for the prediction of the following summer LTC activity over China. Significance Statement Tropical cyclone (TC) is one of the most catastrophic high-impact weather events, which may cause great casualties and severe property losses over the coastal areas, particularly when it makes landfall. Previous research studies have related the poleward migration trend of TC locations to the Hadley circulation (HC) expansion. Compared to the long-term trend, the magnitude of the year-to-year change of the HC edge (HCE) is even larger, leading to a stronger impact on the TC activity. A recent study has suggested that the northern HCE over the western Pacific (WPHCE) in boreal winter exhibits a notable interannual variability. In this study, we reveal that the wintertime WPHCE has a very close connection with the landfalling TC (LTC) frequency over China in the following summer. After removing the El Niño–Southern Oscillation (ENSO) signal, there still exists a significant positive correlation between them. Observational evidence and numerical model experiments consistently confirm that this time-lagged association is attributable to the air–sea interaction processes in the tropical Pacific. Thus, the results of this study could provide an additional predictor besides ENSO to improve understanding of the LTC activity in China.

Micro-scale experimental investigations of CO2-WAG injection and Ostwald ripening analysis in carbonate rocks with different pore structures

Carbonate reservoirs are rich in hydrocarbon resources. However, carbonate reservoirs usually have strong pore structure heterogeneity, and thus it is necessary to understand the effect of heterogeneity on pore-scale fluid distributions during CO2 enhanced oil recover (CO2-EOR) process. We thus conducted water-alternating-gas (CO2-WAG) injection in the three carbonate samples with strong to weak heterogeneity. The 15 sets of images showed that the fractured-vuggy carbonate with the strongest heterogeneity had the highest additional oil recovery. CO2-WAG injection significantly changed the residual oil distribution, and further oil production can be achieved by increasing the number of CO2-WAG cycles. For CO2 geological storage, with the increase of carbonate heterogeneity, CO2 capillary-trapping capacity decreased, CO2 dissolution trapping potentiality decreased, but the stability of CO2 capillary trapping enhanced. These key parameters related to CO2 storage were described through the saturation of trapped gas, the types of trapped gas, and the surface area-volume relationship of CO2 clusters. For example, the higher amount of network gas is detrimental to the stability of CO2 capillary trapping because the network gas is easily displaced when the hydrodynamic force is enhanced. The specific surface area of CO2 cluster determines dissolution dynamics, and a higher specific surface area is beneficial for CO2 dissolution. Another interesting phenomenon - Ostwald ripening, is related to long-term CO2 transport. The CO2 concentration gradient in aquifer generates a vertically upward diffusive flux, causing CO2 remobilization. By calculating the height of the transition zone and the time to reach gravity-capillary pressure equilibrium, we found that as carbonate heterogeneity increased, the height of transition zone (the area where CO2 is trapped by capillary pressure) increased, while equilibrium time shortened. By comparing the results of previous gas-water two-phase flow experiments, we believe that the relationship between CO2 storage characteristics and carbonate heterogeneity summarized in this paper are universally applicable.

Strengthened impact of late autumn Arctic sea ice on Asian winter cold extremes after 1999/2000

Winter cold extremes (WCEs) frequently plague densely populated areas of Asia, leading to substantial economic losses and even fatalities. It has been found that the late autumn sea ice concentration (SIC) anomalies in the northern (SICN) and southern Arctic (SICS) are significantly positively and negatively correlated with the occurrence frequency of WCE in Asia, respectively (Wang and Su 2024). Our study demonstrates that the impacts of SICN and SICS have strengthened after 1999/2000. Specifically, before 1999/2000, the influences of SICN and SICS on the Asian WCE (AWCE) were relatively weak, possibly related to the weak intensity of SICS and the limited correlation between SICN and SICS. After 1999/2000, the interannual variability of SICS became larger and anti-correlated with that of SICN, resulting in a stronger teleconnection between the Arctic SIC and AWCE. It is revealed that after 1999/2000, the greater loss of SICS modified atmospheric stability through changes in surface heat fluxes and surface upward longwave radiation fluxes. This alteration weakened the magnitudes of westerly winds and increased the frequency of blocking events over the northern Eurasian continent, leading directly to a higher occurrence of cold extremes in Asia. These interdecadal differences in the influence of Arctic SIC on AWCE may be associated with long-term climate change.

Ocean Surface Warming and Cooling Responses and Feedback Processes Associated With Polar Lows Over the Nordic Seas

AbstractStrong surface winds induced by polar lows (PLs) may affect the upper ocean. However, understanding of the oceanic responses and feedback processes associated with PLs remains insufficient, especially for observations. Using a combined analysis of satellite‐based sea surface temperature (SST) and PL tracking data, we investigated the oceanic response to 380 PL passages over the Nordic Sea occurring between 1999 and 2018. Consequently, two types of oceanic responses—warming and cooling—occurred in 32% and 40% of the total occurrences, respectively. The average magnitude of SST response was approximately ±0.2 K. Significant differences in upward surface turbulent heat flux (THF) between warming and cooling response cases were found, causing a significant difference in the decay rate after maximum PL development. By analyzing changes in the state variables of the THF, we identified two different feedback processes depending on the oceanic warming/cooling response. During a warming (cooling) response, the atmosphere near the surface becomes more unstable (stable), and the turbulence of the marine atmospheric boundary layer increases (decreases), which strengthens (weakens) the ocean surface wind and decreases (increases) temperature and specific humidity. These changes contribute to increasing (decreasing) the upward THF that influences PL development. The differences between these two responses may be caused by the state of the upper ocean layer, including temperature inversion. The analysis of the in situ observations of the upper ocean supports the hypothesis that a warming response occurs when inversion is strong. This study emphasizes the importance of feedback through oceanic responses for understanding and predicting PL.

Changes in the tropical upper-tropospheric zonal momentum balance due to global warming

Abstract. We study the zonal momentum budget of the deep upper tropics in the context of present and future climates. In the zonal mean, as is known, a robust balance exists between the acceleration by the horizontal eddy momentum flux convergence and the deceleration by the mean meridional momentum advection. During summer, climatological stationary Rossby waves over the Asian monsoon longitudes converge westerly momentum into the tropics and are the primary contributors to the eddy term. During winter, anomalous westerly winds over the tropical east Pacific allow extratropical waves to propagate into the deep tropics, where they tend to break and decelerate the flow. When integrated over all longitudes, we find that eddies from these two regions sum constructively in summer and destructively in winter, always yielding a net positive momentum forcing that balances the mean flow term. The state-of-the-art CMIP6 suite qualitatively captures these features in the historical run and shows that the momentum fluxes change due to global warming. In summer, stationary eddy circulations in the Asian monsoon zone weaken in the upper troposphere (UT) but strengthen in the lower stratosphere (LS). Greater upward mass flux from the troposphere forces a stronger divergence and a more intense circulation in the LS following a Sverdrup vorticity balance. We observe this strengthening of summertime tropical and subtropical stationary waves in the LS over all longitudes and verify it in an idealized aquaplanet general circulation model experiment. In winter, we find that eddy westerlies over the east Pacific longitudes decrease in strength due to the expected weakening of the subtropical stationary waves with warming. This causes a significant decrease in the propagation of extratropical waves into this region, along with a drop in eddy potential vorticity fluxes associated with these waves. Thus, apart from the mean meridional flux, which weakens due to the projected weakening of the Hadley cells, our analysis of warming simulations clearly suggests significant and robust changes in the eddy momentum fluxes in the deep tropics. Potential implications of these changes in the context of the zonal mean flow and regional circulations are discussed.

The Inter‐Model Uncertainty of Projected Precipitation Change in Northern China: The Modulating Role of North Atlantic Sea Surface Temperature

AbstractPrecipitation changes in northern China are projected to increase in the Coupled Model Inter‐comparison Project Phase 6 (CMIP6) multi‐model ensemble. However, these projections are accompanied by notable inter‐model uncertainty, and the sources of this uncertainty remain largely unexplored. By analyzing 30 CMIP6 models, this research explores the source of inter‐model uncertainty in projected precipitation change and reveals the fundamental mechanism driving uncertainty spread. Following the empirical orthogonal function of inter‐model projected precipitation change, the leading mode displays a seesaw spatial pattern between northwest and north China. This phenomenon predominantly stems from the inter‐model divergence of projected sea surface temperature (SST) warming in the North Atlantic. Further scrutinizing the ocean mixed layer heat budget, we discover that the combined effect of surface sensible heat flux, net surface shortwave radiation flux, and ocean heat transport convergence influences heat flux and SST change of North Atlantic. The multi‐model projections indicate that localized increases in solar radiation and heat convergence warm sea surface, raising SST and initiating convective motion. This convective motion subsequently transforms the 200 hPa teleconnection wave train, leading to an anti‐phase pattern over northern China. This wave pattern modulates total cloud cover percentage, influences surface upward latent heat flux, and adjusts the top of atmosphere outgoing longwave radiation, collectively resulting in the seesaw pattern. Our study underscores the pivotal role of inter‐model disparities in North Atlantic SST warming projection, which is a primary driver of precipitation uncertainty in northern China. These insights offer an essential foundation for refining and diminishing inter‐model uncertainty.

The underestimated role of manganese in modulating the nutrient structure in a eutrophic seasonally-stratified reservoir

Accumulation and subsequent release of nutrients have great potential to trigger algal blooms in lakes and reservoirs. We conducted high vertical resolution (2 m interval) monitoring at ∼monthly intervals over a year for hydrological parameters, Chl-a, ammonium (NH4+), nitrate (NO3−) and different species of phosphorus (P) and manganese (Mn) in a 40-meter-deep subtropical reservoir (Shanmei Reservoir) in Fujian, southern China. In this seasonally stratified reservoir featured with high nutrient loading, the consistent trend in the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) between the euphotic zone and the hypolimnion, coupled with its mirrored correlation with Chl-a concentration indicates that upward flux from the hypolimnion affects phytoplankton growth in the euphotic zone. The monthly variation of the depth-integrated multiple species of N and P indicates that during the stratification period in the hypoxic hypolimnion, approximately 80% of the DIP is removed, leading to a remarkable decoupling phenomenon between NH4+ and DIP. This process effectively increases the ratio of DIN to DIP in the hypolimnion, thereby significantly reducing the potential of algal blooms caused by the upward flux. A robust positive linear correlation between iron-manganese bound phosphorus (CBD-P) and particulate Mn was observed during stratification period implying that DIP was scavenged by sediment-released Mn throughout the water column. Vertical profiles during stratification showed that upward diffusion of Mn2+ facilitated the formation of Mn oxide zones near the oxycline. The most significant decrease in DIP inventory occurs when Mn oxide zones migrate either upwards from the bottom or downwards from the oxycline, indicating that the migration of Mn oxides on the vertical profile is a key factor in the decoupling of NH4+and DIP.Our findings underscore the importance of Mn cycling as an underappreciated DIP self-immobilization process in the water column of reservoirs characterized by high nutrient loading. Furthermore, we propose that denitrification and Mn cycling establish a consecutive feedback mechanism, preventing excessive nutrient accumulation in low oxygen bottom water. In the context of global changes, we anticipate a heightened prominence of this feedback mechanism.