Horizontal Flux Research Articles

The Relative Importance of Factors Influencing Tropical Cyclone Rapid Intensity Changes

AbstractPredicting rapid intensity changes in tropical cyclones (TCs) is a major challenge due to the influence of multiple competing processes within the vortex and in the TC environment. We present an empirical framework that quantifies the relative importance of the various factors that influence critical transitions in TC intensities. Our analysis of model simulations of recent TCs over the Bay of Bengal identifies the following variables within the vortex as the biggest influence on TC rapid intensity changes: the amplitudes of wave number 1 of 700‐ to 850‐mb horizontal moisture flux convergence and precipitation in the rainband region and the amplitude of wave number 0 of precipitation within the radius of maximum winds. Likewise, the most important environmental variables identified are the angle between the driest air and the shear vector and the magnitude of vertical wind shear. These findings provide guidance on guidance for future observational efforts and data assimilation into TC forecasting models.

Heavy rainfall in Mediterranean cyclones, Part II: Water budget, precipitation efficiency and remote water sources

In this study, we use convection-permitting high resolution (3 km) simulations to quantify and analyse the water budget, precipitation efficiency and water sources of 100 intense Mediterranean cyclones. To this end, we calculate the water content, advection and microphysical processes of water vapour and rain water by implementing new diagnostics to the Weather Research and Forecasting (WRF) model. The 100 intense cyclones have been randomly selected from a 500 intense cyclones dataset, identified and tracked in an 11-year time period in part I of this study. Results are presented in a composite approach showing that most rainfall takes place to the north-east side of the cyclones, close to their centre. Rainfall location is concomitant to the area of horizontal moisture flux convergence and is quasi-equal to the amount of water vapour loss due to microphysical processes. Similar results were found regardless if cyclones produce high or low rainfall amounts. Vertical profiles of the water budget terms revealed deeper clouds for the cyclones producing high rainfall, consistent with higher values of vertical advection of both water vapour and rain water. Finally, cyclones were analysed with respect to their precipitation efficiency, i.e. the ratio between the rainwater produced in an atmospheric column and the consequent rainfall, and showed that cyclones tend to be more efficient when their rainfall production takes place over land. Therefore, there is a complex relation between water vapour advection, precipitation efficiency and rainfall which is discussed through the comparison of two tropical-like cyclones with two cyclones that produced low rainfall amounts. Finally, our analysis is complemented by applying a Lagrangian approach to all 100 cyclones in order to quantify the water vapour source regions that contribute to the cyclones’ rainfall due to local surface evaporation. Results showed that these regions are located over both the Atlantic and the Mediterranean, however we show that cyclones producing high rainfall are related with higher water transport from both the subtropical Atlantic and the Mediterranean Sea. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.

Observations of solar small-scale magnetic flux-sheet emergence

Aims. Two types of flux emergence were recently discovered in numerical simulations: magnetic loops and magnetic sheet emergence. While magnetic loop emergence has been documented well in recent years using high-resolution full Stokes data from ground-based telescopes as well as satellites, magnetic sheet emergence is still an understudied process. We report here on the first clear observational evidence of a magnetic sheet emergence and characterise its development. Methods. Full Stokes spectra from the Hinode spectropolarimeter were inverted with the Stokes Inversion based on Response functions (SIR) code to obtain solar atmospheric parameters such as temperature, line-of-sight velocities, and full magnetic field vector information. Results. We analyse a magnetic flux emergence event observed in the quiet-Sun internetwork. After a large-scale appearance of linear polarisation, a magnetic sheet with horizontal magnetic flux density of up to 194 Mx cm−2 hovers in the low photosphere spanning a region of 2–3 arcsec. The magnetic field azimuth obtained through Stokes inversions clearly shows an organised structure of transversal magnetic flux density emerging. The granule below the magnetic flux sheet tears the structure apart leaving the emerged flux to form several magnetic loops at the edges of the granule. Conclusions. A large amount of flux with strong horizontal magnetic fields surfaces through the interplay of buried magnetic flux and convective motions. The magnetic flux emerges within 10 minutes and we find a longitudinal magnetic flux at the foot points of the order of ∼1018 Mx. This is one to two orders of magnitude larger than what has been reported for small-scale magnetic loops. The convective flows feed the newly emerged flux into the pre-existing magnetic population on a granular scale.

Oxygen Variability During ENSO in the Tropical South Eastern Pacific

The Oxygen Minimum Zone (OMZ) of the Tropical South Eastern Pacific (TSEP) is one of the most intensely deoxygenated water masses of the global ocean. It is strongly affected at interannual time scales by El Nino (EN) and La Nina (LN) due to its proximity to the equatorial Pacific. In this work, the physical and biogeochemical processes associated with the subsurface oxygen variability during EN and LN in the period 1958 – 2008 were studied using a regional coupled physical-biogeochemical model and in situ observations. The passage of intense remotely forced coastal trapped waves caused a strong deepening (shoaling) of the OMZ upper limit during EN (LN). A close correlation between the OMZ upper limit and thermocline depths was found close to the coast, highlighting the role of physical processes. The subsurface waters over the shelf and slope off central Peru had different origins depending on ENSO conditions. Offshore of the upwelling region (near 88°W), negative and positive oxygen subsurface anomalies were caused by Equatorial zonal circulation changes during LN and EN, respectively. The altered properties were then transported to the shelf and slope (above 200 m) by the Peru-Chile undercurrent. The source of nearshore oxygenated waters was located at 3°S – 4°S during neutral periods, further north (1°S – 1°N) during EN and further south (4°S – 5°S) during LN. The offshore deeper (< 200 – 300 m) OMZ was ventilated by waters originating from ~ 8°S during EN and LN. Enhanced mesoscale variability during EN also impacted OMZ ventilation through horizontal and vertical eddy fluxes. The vertical eddy flux decreased due to the reduced vertical gradient of oxygen in the surface layer, whereas horizontal eddy fluxes injected more oxygen into the OMZ through its meridional boundaries. In subsurface layers, remineralization of organic matter, the main biogeochemical sink of oxygen, was higher during EN than during LN due to oxygenation of the surface layer and in spite of a reduced primary production. Sensitivity experiments highlighted the larger impact of equatorial remote forcing with respect to local wind forcing during EN and LN.

Wind erosion changes induced by different grazing intensities in the desert steppe, Northern China

Desert steppes are fragile ecosystems, which generally suffer severe wind erosion hazards, especially when they experiencing external disturbances such as grazing. Therefore, the effects of grazing on the wind erosion process are very important for improving our understanding of the effects of the stocking rate on grassland degradation and enacting reasonable grazing strategies in grasslands. However, few studies have discussed the wind erosion changes induced by different grazing intensities thus far. Herein, a series of grazing experiments was carried out in a desert steppe in Urad Back Banner, Inner Mongolia. These experiments included 2 types of grasslands that are shrub dominated grassland (SDG) and grass dominated grassland (GDG). Each grassland type had 3 grazing rate treatments: no grazing as the control (CK), moderate grazing (MG), and heavy grazing (HZ). After the grazing experiments lasting for 4 years, sand traps were set in the grazing plots to detect the horizontal sand flux at different grazing intensities. In addition to the wind erosion observed, the vegetation and soil particles were also surveyed and sampled. Our results showed that the vegetation cover (VC) and aboveground biomass (AGB) in the MG and HG plots all had obvious differences compared to the CK plot, regardless of whether it was SDG or GDG. The soil particle analysis showed that the fractions of erodible particles (60–200 μm) in the MG and HG plots decreased significantly. In the GDG, the soil particles exhibited a coarsening trend from CK to HG plots for the sorting action of the wind. Suffering a grazing disturbance, the wind erosion in a desert steppe exhibited significant changes. For instance, in the SDG, the horizontal sand flux in the MG and HG plots were 1.78 and 2.06 times the sand flux in the CK plots, respectively, and, in the GDG, they were 17.86 and 56.53 times the flux in the CK plots, respectively. Except in the MG and HG plots of the GDG, the saltation height of sand was generally lower than 20 cm. In the SDG, the particle size of the entrained sand mainly ranged from 80 to 200 μm, and, in the GDG, it ranged from 80 to 300 μm. This finding also indicated the validity of Shao’s sand entrainment equation. We also found that particle size of saltating sand increased with their saltation height for the tough surface of the MG and HG plots in the GDG. Based on the different fractions of horizontal flux in different directions, we also discussed the threshold wind velocity for sand entrainment, and we confirmed that the threshold wind velocities for no grazing in SDG and GDG were approximately 16.5 m·s−1 and 15 m·s−1, respectively. Using our previous wind erosion model, we estimated the dust emission in the total desert steppe of China and found that different grazing scenarios would change the magnitude and distribution of dust emissions.

Eliciting features of 2D greenwater overtopping of a fixed box using modified dam break models

Greenwater overtopping of a 2D fixed rectangular box due to focused wave groups is investigated using Computational Fluid Dynamics (CFD). Simulations span a wide parameter space, such that a range of overtopping types previously observed in experiments - including dam-break, plunging plus dam-break and hammer fist events - are observed. For each greenwater event the horizontal momentum flux and overtopping volume are computed, as these represent integrated quantities linked to damage potential on offshore facilities. These values are compared to simplified predictions based on the classical dam-break model with the dam height based on a run-up calculation on the box. It is found that these predictions deviate from measurements made in the CFD simulations because the classical model omits physics evident during overtopping events. Modifications to the classical dam-break model are presented to account for (i) the finite duration of an overtopping event; (ii) the finite quantity of water in the crest of a wave which exceeds the freeboard; (iii) the local geometry at the edge of the box; and (iv) the horizontal momentum in the crest of a wave which exceeds the freeboard. Insight gained from the first of these corrections leads to a new parameter- the relative overtopping duration T', defined as the ratio of the duration for which fluid exceeds the freeboard to the natural time scale for runout of a dam with height equal to maximum run-up above freeboard. It is found that this new parameter dramatically reduces scatter in the integrated quantities. The results suggest that this new parameter, which combines freeboard exceedance and overtopping duration, may be an efficient indicator for screening metocean conditions to identify design sea states.

Interpreting Rainfall Anomalies Using Rainfall's Nonnegative Nature

AbstractPrevious studies emphasized the role of transient disturbances in linking interannual variation of average rainfall (IVAR; average period &gt; 1 month) to external climate forcings. Researchers conventionally use transient eddy‐driven horizontal fluxes of certain variables to explain the IVAR, which is not universally applicable to disturbance‐associated IVAR research because these variables' horizontal distributions can drastically affect their fluxes. Here we demonstrate that the strengthened transient disturbance activities can directly impact IVAR positively in almost all extratropical terrestrial regions in each season. This impact occurs because disturbance strengthened daily downward motions cannot generate negative rainfall to compensate for the increased rainfall generated by disturbance enhanced daily upward motions. This is much more universally applicable to disturbance‐associated IVAR research because it derives transient eddies' direct and independent impacts on IVAR. Because of this impact, the horizontal heterogeneity of global warming rate can modify the long‐term trend of extratropical terrestrial rainfall through disturbance activity trend.

Noble gas composition and 3H/3He groundwater ages in the Gardermoen Aquifer, Norway: Improved understanding of flow dynamics as a tool for water management

This study presents a novel data set of noble gas compositions and 3H/3He measurements in groundwater samples from the Gardermoen Aquifer in Norway. The motivation was to test the applicability of noble gases as tracers in constraining the conceptual model and improve the understanding of deeper parts of the aquifer. Flow models have been used as tools for water resource management at the Oslo international airport, which is located within the aquifer recharge area, and bordering towards protected nature reserves. Current models are well calibrated for upper parts of the aquifer, and in line with new noble gas data. However, in some areas large inverse pressure gradients are observed. Interpretation of noble gas data proved useful in understanding these pressure deviations as a result of water retention in low permeability layers of clay and silt. Noble gas composition, isotopic ratios and tritium water ages define different water “types”. Mixing along flow path or as a consequence of pumping were found to be indicators of geological heterogeneity, and were used to evaluate degree of flow separation as well as vertical versus horizontal flux. 3H/3He water ages varied between 0 and 50 years, increasing with depth below the groundwater table and along flow paths. These data indicate lower vertical permeability and longer residence times in deeper parts of the aquifer than suggested by previous models. This study is of high relevance in the continued water resource management scheme at Gardermoen, for improving existing models and in sound, long-term monitoring of groundwater, ensuring sustained influx towards sensitive biotopes.

Temperature variability caused by internal tides in the coastal waters of east coast of Peninsular Malaysia

The effects of tidal currents (i.e., barotropic and internal tides) are important in the biogeochemistry of a coastal shelf sea. The high-frequency of currents and near-bottom temperatures collected in three consecutive southwest monsoon seasons (May, June, July and August of 2013 until 2015) is presented to reveal the role of the tidal currents to the temperature variability in the coastal shelf sea of the east coast of Peninsular Malaysia (ECPM), south of the South China Sea (SCS). The results of a spectral density and harmonic analysis demonstrate that the near-bottom temperature variability and the tidal currents are influenced by diurnal (O1 and K1) and semidiurnal (M2) tidal currents. The spectral density of residual currents (detided data) at 5, 10 and 16 m depth also shows significant peaks at the diurnal tidal frequency (K1) and small peaks at the semidiurnal tidal frequency (M2) indicating the existence of internal tides. The result of the horizontal kinetic energy (HKE) shows a strong intermittent energy of internal tides in the ECPM with the strongest energy is found at 16 m depth during a sporadic cooling event in June and July. A high horizontal cross-shore heat flux (16 m) also indicates strong intrusions of cooler water into the ECPM in June and July. During the short duration of cold pulse water observed in June and July, a cross-wavelet analysis also reveals the strong relationship between the near-bottom temperatures and the internal tidal currents at the diurnal tidal frequency. The intrusion of this cooler water is probably related to the monsoon-induced upwelling in June. It is loosely interpreted that the interaction between the strong barotropic tides and the steep slope in the central basin of the SCS under the stratified condition in southwest monsoon has generated these internal tides. The dissipation of internal tides from the slope area probably has driven the cold-upwelled water into the ECPM coastal shelf sea when the upwelling intensity is the highest in June and July.

Ocean Circulation Causes Strong Variability in the Mid‐Atlantic Bight Nitrogen Budget

AbstractUnderstanding of nitrogen cycling on continental shelves, a critical component of global nutrient cycling, is hampered by limited observations compared to the strong variability on a wide range of time and space scales. Numerical models have the potential to partially alleviate this issue by filling spatiotemporal data gaps and hence resolving annual area‐integrated nutrient fluxes. In this study, a three‐dimensional biogeochemical‐circulation model was implemented to simulate the Mid‐Atlantic Bight (MAB) nitrogen budget. Model results demonstrate that, on average, MAB net community production (NCP) was positive (+0.27 Tg N/year), indicating net autotrophy. Interannual variability in NCP was strong, with annual values ranging between 0.19 and 0.41 Tg N/year. Along‐shelf and across‐shelf horizontal transport fluxes were the other dominant terms in the nitrogen budget and were primarily responsible for this NCP variability. The along‐shelf current transported nitrogen from the north (0.65 Tg N/year) into the MAB, supplementing the nitrogen entering from terrestrial inputs (0.27 Tg N/year). However, NCP was highest in the year when total water volume transport and inorganic nitrogen input was strongest across the continental slope in the southern MAB, rather than when terrestrial inputs were greatest. Interannual variability of NCP appears to be linked to changes in the positions of the Gulf Stream and Slope Water Gyre. Overall, the strong spatiotemporal variability of the nitrogen fluxes highlights the importance of observations throughout all seasons and multiple years in order to accurately resolve the current status and future changes of the MAB nitrogen budget.

OBSERVATIONS OF HORIZONTAL AND VERTICAL SEDIMENT FLUXES ON A SANDBAR IN THE SUSPENDED AND SHEET FLOW LAYERS

The majority of prior sandbar migration studies have been conducted from the morphological standpoint, whereby, (i) bathymetric profiles are recorded over periods of time ranging from days to decades, at frequencies ranging from hourly to yearly (Ruessink et al., 2003), and (ii) hydrodynamic observations typically consist of far-field wave and environmental conditions. Subsequent modeling efforts have generally focused on tuning parameters in the sediment transport formulations (suspended load and bed load) to maximize model skill in predicting observed beach profiles over time (Fernández-Mora et al., 2015; Hoefel and Elgar, 2003). However, little emphasis at the operational level has been placed on tuning coastal morphology models to the true relative contributions of the physical processes (e.g. suspended load, bed load and/or sheet flow) that drive the changing bathymetry. This is due, in part, to the lack of detailed sediment transport observations (field and lab) under realistic wave forcing conditions and spatially variable bathymetry. Such a modeling approach leads to the improper quantification (magnitude and/or direction) of each modeled sediment transport component under skewed-asymmetric and/or breaking waves, often observed in the surf zone. The present study aims to better understand the physical mechanisms responsible for driving cross-shore sediment transport over a sandbar by quantifying (a) the vertical exchange of sediment at the near-bed interface (i.e. pick-up layer), and (b) intra-wave horizontal sediment fluxes in the suspended load and sheet layers.

Identify the impacts of waves and tides to coastal suspended sediment concentration based on high-frequency acoustic observations

High-frequency measurements of wind, waves, tides, suspended sediment concentration (SSC), and turbulence were made using an autonomous meteorological station and optical and acoustic instruments over 34 days during January 20–February 23, 2016, in Heini Bay, Yellow Sea. The responses of SSC on different dynamic processes are analyzed by decomposing the velocity and SSC fluctuations into wave and turbulent components. In the presence of SSC fronts, the horizontal advection by the reciprocating tidal flow would cause an asymmetry of SSC in a flood-ebb tidal cycle. The presence of swells induced intensified sediment resuspension and a flood-ebb symmetry of SSC. In the bottom boundary layer, the net horizontal sediment fluxes were significantly increased in the presence of strong waves. Significant amount of suspended sediments were taken away from the study area, implying the occurrence of coastal erosion.

Impact of the Horizontal Heat Flux in the Mixed Layer on an Extreme Heat Event in North China: A Case Study

Extreme heat over the North China Plain is typically induced by anomalous descending flows associated with anticyclonic circulation anomalies. However, an extreme heat event that happened in the North China Plain region on 12–13 July 2015, with maximum temperature higher than 40°C at some stations, was characterized by only a weak simultaneous appearance of an anomalous anticyclone and descending flow, suggesting that some other factor(s) may have induced this heat event. In this study, we used the forecast data produced by the Beijing Rapid Updated Cycling operational forecast system, which predicted the heat event well, to investigate the formation mechanism of this extreme heat event. We calculated the cumulative heat in the mixed-layer air column of North China to represent the change in surface air temperature. The cumulative heat was composed of sensible heat flux from the ground surface and the horizontal heat flux convergence. The results indicated that the horizontal heat flux in the mixed layer played a crucial role in the temporal and spatial distribution of high temperatures. The horizontal heat flux was found to be induced by distinct distributions of air temperatures and horizontal winds at low levels during the two days, implying a complexity of the low-level atmosphere in causing the extreme heat.

Nonlinear stability of rotating two superposed magnetized fluids with the technique of the homotopy perturbation

In the present work, the Rayleigh-Taylor instability of two rotating superposed magnetized fluids within the presence of a vertical or a horizontal magnetic flux has been investigated. The nonlinear theory is applied, by solving the equation of motion and uses the acceptable nonlinear boundary conditions. However, the nonlinear characteristic equation within the elevation parameter is obtained. This equation features a transcendental integro-Duffing kind. The homotopy perturbation technique has been applied by exploitation the parameter growth technique that results in constructing the nonlinear frequency. Stability conditions are derived from the frequency equation. It's illustrated that the rotation parameter plays a helpful result. It's shown that the stability behavior within the extremely uniform rotating fluids equivalents to the system while not rotation. A periodic solution for the elevation function has been performed. Numerical calculations area unit created for linear analysis furthermore the nonlinear scope. Moreover, the elevation function has been premeditated versus the time parameter. The strategy adopted here is vital and powerful for solving nonlinear generator systems with a really high nonlinearity arising in nonlinear science and engineering.

Prolonged sampling time increases correlation between wind tunnel and integrated horizontal flux method

Agriculture is still the main source of NH3 emissions. Consequently, ammonia reduction and monitoring are included in the strict EU targets. Unfortunately, the main issue with the quantification of NH3 emissions is that some uncertainties are still related to NH3 volatilization measurements, mainly because of the absence of a standard measurement technique.This study compares two different methods for measuring NH3 fluxes: the Integrated Horizontal Flux (IHF) method used with glass tubes and the wind tunnels (WT) with acid traps, in order to provide further insights into quantification of NH3 emissions. Specifically, three field trials were carried out, using different fertilizers on bare soil: urea, raw manure and liquid separated digestate from Mediterranean buffalo cows (Bubalus bubalis L.).Results show that the same diurnal flux variations for both methods was observed, except for those in urea trials, mainly because of the dynamics of urea volatilization. Generally, WT underestimated ammonia emissions, especially during the first six hours of experimentation, since the fixed air speed inside the tunnel is lower than the outside wind speed at the same height. Cumulative ammonia volatilization measured with WT differs from those measured with IHF by 32% (urea), 43% (raw manure) and 46% (liquid separated digestate), respectively. Moreover, WT estimations seemed to be affected by a short exposition time of the acid traps, demonstrating a poor correlation of IHF and WT method within 2–3 h. Indeed, linear correlation between two methods was improved to R2 = 0.92, by increasing the minimum samplers exposition time up to 3 h.

Difference in wind erosion characteristics between loamy and sandy farmlands and the implications for soil dust emission potential

AbstractDust emission depends directly and indirectly on soil particle distribution, but little information is available that examines dust emission potential and its relationship with wind erosion for different soils. This study aimed to elucidate the difference in wind erosion characteristics between loamy farmland (LF) and sandy farmland (SF) and discuss the influence of this difference on dust emission potential. Field observations were conducted during 2012–2016 in Bashang district, North China. With increasing friction wind speed, the horizontal mass flux increased exponentially in LF but obeyed the power function in SF. The horizontal mass flux was higher in SF than in LF, but the gap narrowed from 766 times at friction wind speed of 0.3 m s−1 to 50 times at 0.6 m s−1. The dry aggregate size distribution of sediment in LF was finer than in SF. Fine particle content (diameter &lt; 0.05 mm) in windblown sediment was much higher in LF (31.39%) than in SF (5.92%), which was influenced by parent soils. With height increasing, the mass percentage of sediment decreased more rapidly in SF than in LF, with 86% and 66% of sediment clustering at height 0–20 cm, respectively. Because the horizontal mass flux and dust content in sediment both increased more quickly in LF than in SF with increasing friction wind speed, the dust emission potential was larger in LF than in SF under strong winds. The adverse effect of dust emission in loamy soils on atmospheric pollution should receive more attention in strong wind erosion events.

Nutrient fluxes into an isolated coral reef atoll by tidally driven internal bores

AbstractScott Reef is a remote yet biodiverse coral reef atoll, rising from depths &gt; 1000 m on the edge of the continental shelf of northwestern Australia. An intensive 2‐week field study was conducted to assess how physical oceanographic processes influence reef communities in South Scott's deep (~ 50 m) and expansive (~ 300 km2) semi‐enclosed lagoon. The study covered a spring–neap tidal cycle, during which moored instruments measured temperature and velocities at a lagoon entrance and ship‐based vertical profilers measured water quality parameters. Tidally driven internal bores advected cooler and deeper offshore waters (up to 4.5°C, mean 2.4°C) into the lagoon, which were also richer in nutrients and chlorophyll a than lagoon waters. This offshore water originated from approximately 75 m depth. We used the strong observed relationship between in situ measurements of nitrate and temperature to estimate nitrate concentrations from high temporally and spatially resolved temperature measurements at the lagoon entrance, and then estimated the horizontal nitrate fluxes into the lagoon using the velocity measurements. The depth‐integrated, time‐averaged nitrate flux during spring tides (1.46 kg m−1 d−1) was over three times larger than during neap tides (0.46 kg m−1 d−1), despite a passing tropical storm that temporarily deepened the offshore thermocline and reduced the magnitude of the fluxes. Our observations indicate that colder, deeper, and nutrient‐rich water associated with internal tidal bores during spring tide is likely to be the primary mechanism through which allochtonous nutrients are delivered to benthic and pelagic communities within the lagoon of this isolated atoll.

Estimation of Horizontal Eddy Heat Flux in Upper Mixed-Layer in the South China Sea by Using Satellite Data

In this study, the horizontal eddy heat flux in the upper mixed-layer in the South China Sea (SCS) is derived from satellite-derived observational data of sea surface height anomalies and optimally interpolated sea surface temperature, as well as a reanalysis dataset of mixed-layer depth. The long-term heat flux shows a northward transport on the west side of the SCS, comparable with that in the Kuroshio extension with strong eddy activities. The eddy flux in the SCS has a prominent semi-annual cycle and becomes the strongest in winter and summer with the inflow flux in the south and the outflow in the northwest into the East China Sea through the Taiwan Strait. The semi-annual cycle is related to the strong semi-annul variabilities of the velocity and the temperature in areas southeast of Vietnam and in the northern SCS, respectively. In some areas of the SCS, the eddy heat flux can reach more than ~ 60% of the mean flow heat flux. The convergence of the eddy flux indicates that heat accumulates southeast of Vietnam, which may result in heat storage increases in the upper mixed-layer.

Mechanisms of northern North Atlantic biomass variability

Abstract. In the North Atlantic Ocean north of 40∘ N, intense biological productivity occurs to form the base of a highly productive marine food web. SeaWiFS satellite observations indicate trends of biomass in this region over 1998–2007. Significant biomass increases occur in the northwest subpolar gyre and there are simultaneous significant declines to the east of 30–35∘ W. These short-term changes, attributable to internal variability, offer an opportunity to explore the mechanisms of the coupled physical–biogeochemical system. We use a regional biogeochemical model that captures the observed changes for this exploration. Biomass increases in the northwest are due to a weakening of the subpolar gyre and associated shoaling of mixed layers that relieves light limitation. Biomass declines to the east of 30–35∘ W are due to reduced horizontal convergence of phosphate. This reduced convergence is attributable to declines in vertical phosphate supply in the regions of deepest winter mixing that lie to the west of 30–35∘ W. Over the full time frame of the model experiment, 1949–2009, variability of both horizontal and vertical phosphate supply drive variability in biomass on the northeastern flank of the subtropical gyre. In the northeast subpolar gyre horizontal fluxes drive biomass variability for both time frames. Though physically driven changes in nutrient supply or light availability are the ultimate drivers of biomass changes, clear mechanistic links between biomass and standard physical variables or climate indices remain largely elusive.

Microstructure Observations of Turbulent Heat Fluxes in a Warm-Core Canada Basin Eddy

AbstractAn intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6°C, significantly warmer than the surrounding −1°C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy ε was quite low . However, at the edges of the eddy core, ε was elevated to . Three different processes were associated with elevated ε. Double-diffusive steps were found at the eddy’s top edge and were associated with an upward heat flux of 5 W m−2. At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m−2 downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m−2. Integrating these fluxes over an idealized approximation of the eddy’s shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1–2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin.