Local Vertical Mixing Research Articles

Short-terms variability of water properties and phytoplankton blooms along the central eastern Florida shelf edge

The central eastern Florida shelf edge is a highly dynamic area that supports an important yet vulnerable deep-sea coral reefs ecosystem, the Oculina Coral Habitat Area of Particular Concern. Rapid and large short-terms (days to weeks) changes of bottom temperature (up to 9 °C) and pCO2 (up to 180 μatm) were observed at the shelfbreak during a two-month (May 13-July 10, 2017) deployment of a lander package. An analysis suggests that these changes are the combined results of tides, the Gulf Stream meandering, and submesoscale eddies and filaments. The processes responsible for sub-tidal variability may include 1) the Gulf Stream frontal movements, 2) upwelling/downwelling of slope waters in association with the Gulf Stream variability, and 3) submesoscale processes and associated vertical movements. Satellite images also frequently show a narrow plume of elevated chlorophyll concentration that stretches from the coast northward up to >200 km along the Gulf Stream front during late spring and early summer. Our analysis indicates that these phytoplankton blooms in the plume are likely supported by the nutrient supply from the nutrients-rich slope waters to the shelf edge and subsequent local vertical mixing. Carbon export associated with these blooms can be an important food source to the Oculina corals. Upwelling of slope waters, on the other hand, will lead to increased CO2 and reduced pH and aragonite saturation state along the shelf edge. Therefore, these dynamic processes may have strong impacts on the health and sustainability of the Oculina coral ecosystem.

Analysis of Radon Near-Surface Measurements, Using Co-Located Ozone Data, Radio-Sounding Vertical Profiles, Sensible Heat Flux and Back-Trajectory Calculation

Simultaneous and co-located observations of near-surface Radon-222, ozone and meteorological parameters in a central Italy observation site operated by the University of L’Aquila (Italy), are used to study the physical drivers of the radon abundance during night-time hours. The knowledge of the potential temperature vertical gradient in the surface layer of nocturnal thermal inversion is made possible using co-located radio-sounding vertical profiles of pressure and temperature, thus making possible to indirectly infer the local surface flux of atmospheric radon (16 ± 6 mBq m−2 s−1). The dynamical removal due to turbulent convective motions is found to be the dominant controlling process, determining large differences in the near-surface radon abundance between stable and unstable conditions of the nocturnal Planetary Boundary Layer (PBL). Usual unstable PBL conditions during daytime hours induce an effective dynamical vertical dilution of surface radon, which rapidly reaches a quasi-steady-state abundance during mid-day and afternoon hours, with very low concentration values (5.1 ± 2.0 Bq m−3). Using back-trajectory reanalyses, estimates of local radon fluxes and vertical mixing efficiencies inside the PBL along the air mass latitudinal-longitudinal path and finally the irreversible radon loss due to radioactive decay, we have explored the fraction of daytime radon attributable to long-range advection in the continental near-mountain measurement site of L’Aquila (44 ± 18%).

Effects of Horizontal Transport and Vertical Mixing on Nocturnal Ozone Pollution in the Pearl River Delta

Based on the meteorological, ozone (O3), and vertical observation data of 2020, this study sought to evaluate the daily variation in O3, particularly the characteristics of nocturnal ozone pollution. We also discuss the effect of local and mesoscale horizontal transport and vertical mixing on the formation of nocturnal O3 pollution. Distinct seasonal characteristics of the daily O3 variation in Shenzhen were identified. In particular, significant nocturnal peaks were found to regularly occur in the winter and spring (November–December and January–April). The monthly average of daily variation had a clear bimodal distribution. During the period, O3 pollution frequently occurred at night, with the maximum hourly O3 concentration reaching 203.5 μg/m3. Nocturnal O3 pollution was closely associated with horizontal transport and vertical mixing. During the study period, the O3 maximum values were recorded on 68 nights, primarily between 23:00 and 03:00, with occasional observation of two peaks. The impact of horizontal transport and vertical mixing on the nocturnal secondary O3 maximum values was elaborated in two case studies, where vertical mixing was mainly associated with low-level jets, with strong wind shear enhancing turbulent mixing and transporting O3 from the upper layers to the surface.

Stability of polycyclic aromatic hydrocarbon clusters in protoplanetary discs

Context. The infrared signature of polycyclic aromatic hydrocarbons (PAHs) is present in many protostellar discs, and these species are thought to play an important role in the heating of the gas in the photosphere. Aims. We consider PAH cluster formation as one possible cause for non-detections of PAH features in protoplanetary discs. We test the necessary conditions for cluster formation and cluster dissociation by stellar optical and far-UV photons in protoplanetary discs using a Herbig Ae/Be and a T Tauri star disc model. Methods. We perform Monte Carlo and statistical calculations to determine dissociation rates for coronene, circumcoronene, and circumcoronene clusters with sizes of between 2 and 200 cluster members. By applying general disc models to our Herbig Ae/Be and T Tauri star model, we estimate the formation rate of PAH dimers and compare these with the dissociation rates. Results. We show that the formation of PAH dimers can take place in the inner 100 AU of protoplanetary discs in sub-photospheric layers. Dimer formation takes seconds to years, allowing them to grow beyond dimer size in a short time. We further demonstrate that PAH clusters increase their stability while they grow when they are located beyond a critical distance that depends on stellar properties and PAH species. The comparison with the local vertical mixing timescale allows a determination of the minimum cluster size necessary for the survival of PAH clusters. Conclusions. Considering the PAH cluster formation sites, cluster survival in the photosphere of the inner disc of Herbig stars is unlikely because of the high UV radiation. For the T Tauri stars, survival of coronene, circumcoronene, and circumcircumcoronene clusters is possible, and cluster formation should be considered as one possible explanation for low PAH detection rates in T Tauri star discs.

On the Consequences of PBL Scheme Diffusion on UTLS Wave and Turbulence Representation in High-Resolution NWP Models

AbstractMesoscale numerical weather prediction (NWP) models are routinely exercised at kilometer-scale horizontal grid spacings (Δx). Such fine grids will usually allow at least partial resolution of small-scale gravity waves and turbulence in the upper troposphere and lower stratosphere (UTLS). However, planetary boundary layer (PBL) parameterization schemes used with these NWP model simulations typically apply explicit subgrid-scale vertical diffusion throughout the entire vertical extent of the domain, an effect that cannot be ignored. By way of an example case of observed widespread turbulence over the U.S. Great Plains, we demonstrate that the PBL scheme’s mixing in NWP model simulations of Δx = 1 km can have significant effects on the onset and characteristics of the modeled UTLS gravity waves. Qualitatively, PBL scheme diffusion is found to affect not only background conditions responsible for UTLS wave activity, but also to control the local vertical mixing that triggers or hinders the onset and propagation of these waves. Comparisons are made to a reference large-eddy simulation with Δx = 250 m to statistically quantify these effects. A significant and systematic overestimation of resolved vertical velocities, wave-scale fluxes, and kinetic energy is uncovered in the 1-km simulations, both in clear-air and in-cloud conditions. These findings are especially relevant for upper-level gravity wave and turbulence simulations using high-resolution kilometer-scale NWP models.

Characteristics of vertical mixing in a sea-cage farm and its environmental influences in a strong tide system: A case study in the Nanji Archipelago, East China Sea

Vertical mixing is an important aspect of the sea-cage hydrodynamic since it is directly related to the reduction of the potential environmental risks of sea-cage farms. Previous studies have focused on the enhancement of vertical mixing due to the water blockage of the sea cage through numerical simulations and laboratory experiments. However, these experimental settings inevitably deviated from the field hydrodynamic environments where vertical mixing was contributed by not only the water blockage of the sea cage but also some natural dynamic processes. This paper investigated the vertical mixing in a sea-cage farm located in a strong tide system through an in-situ method. The results indicate that the mixing could be divided into three regimes, namely, the bottom-boundary-layer regime (BBL regime), the regime generated by the baroclinic process during the currents reversal (CR regime), and the regime mainly located under the cage bottom (CB regime). The first two regimes were generated by natural dynamic processes, while the latter one was caused by the water blockage of the sea cage as illustrated in previous studies. Highlight of this paper was the identification of the interaction between the BBL and CB regimes, i.e., the interaction between mixing driven by natural dynamic processes and the artificial sea cage. The mixing in the bottom layer was significantly enhanced by this interaction and can maintain strong for a longer duration than that generated by natural dynamic processes alone. The nutrients concentration and the size distribution of the suspended particulate matter (SPM) were chosen to illustrate the environmental influences of the vertical mixing in the sea-cage farm. The nutrient release from the sea bottom was enhanced by the local vertical mixing, and the concentrations of NO2−, PO43−, and SiO32− increased when the mixing was strong. The principal variation in the NO3− concentration was independent of vertical mixing but was controlled by the alternation of the water masses. The size variation of the SPM was negatively correlated with the intensity of the vertical mixing. The results in this paper inspire us to create a suitable vertical-mixing environment in future sea-cage farms by taking advantage of the interaction between the mixing generated by the artificial sea cage and natural dynamic processes, thereby reducing the potential environmental risk and guaranteeing the sustainable development of the sea-cage farming.

Evidences of strong sources of DFe and DMn in Ryder Bay, Western Antarctic Peninsula.

The spatial distribution, biogeochemical cycling and external sources of dissolved iron and dissolved manganese (DFe and DMn) were investigated in Ryder Bay, a small coastal embayment of the West Antarctic Peninsula, during Austral summer (2013 and 2014). Dissolved concentrations were measured throughout the water column at 11 stations within Ryder Bay. The concentration ranges of DFe and DMn were large, between 0.58 and 32.7 nM, and between 0.18 and 26.2 nM, respectively, exhibiting strong gradients from the surface to the bottom. Surface concentrations of DFe and DMn were higher than concentrations reported for the Southern Ocean and coastal Antarctic waters, and extremely high concentrations were detected in deep water. Glacial meltwater and shallow sediments are likely to be the main sources of DFe and DMn in the euphotic zone, while lateral advection associated with local sediment resuspension and vertical mixing are significant sources for intermediate and deep waters. During summer, vertical mixing of intermediate and deep waters and sediment resuspension occurring from Marguerite Trough to Ryder Bay are thought to be amplified by a series of overflows at the sills, enhancing the input of Fe and Mn from bottom sediment and increasing their concentrations up to the euphotic layer.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

An Edge-Referenced Surface Fresh Layer in the Beaufort Sea Seasonal Ice Zone

AbstractTo understand the factors causing the interannual variations in the summer retreat of the Beaufort Sea ice edge, Seasonal Ice Zone Reconnaissance Surveys (SIZRS) aboard U.S. Coast Guard Arctic Domain Awareness flights were made monthly from June to October in 2012, 2013, and 2014. The seasonal ice zone (SIZ) is where sea ice melts and reforms annually and encompasses the nominally narrower marginal ice zone (MIZ) where a mix of open-ocean and ice pack processes prevail. Thus, SIZRS provides a regional context for the smaller-scale MIZ processes. Observations with aircraft expendable conductivity–temperature–depth probes reveal a salinity pattern associated with large-scale gyre circulation and the seasonal formation of a shallow (~20 m) fresh layer moving with the ice edge position. Repeat occupations of the SIZRS lines from 72° to 76°N on 140° and 150°W allow a comparison of observed hydrography to atmospheric indices. Using this relationship, the basinwide salinity signals are separated from the fresh layer associated with the ice edge. While this layer extends northward under the ice edge as the melt season progresses, low salinities and warm temperatures appear south of the edge. Within this fresh layer, average salinity is correlated with distance from the ice edge. The salinity observations suggest that the upper-ocean freshening over the summer is dominated by local sea ice melt and vertical mixing. A Price–Weller–Pinkel model analysis reveals that observed changes in heat content and density structure are also consistent with a 1D mixing process.

Vertical mixing and internal wave energy fluxes in a sill fjord

We consider the distribution and level of local vertical mixing inside of the Drøbak Sill in the Oslofjord, Norway. The work is motivated by observations of long periods (∼years) of hypoxic or even anoxic conditions in the innermost basin, episodes attributed to weak vertical mixing. In line with earlier work on the subject we assume that the local vertical mixing level inside of the sill is predominantly determined by the loss of energy of propagating, tidally-induced internal waves whose source is the sill region. To investigate possible differences in vertical mixing we estimate the eddy diffusivity in the various basins based on model simulations and observations using three methods whereby the eddy diffusion coefficient is estimated. The model we use is an ultra high-resolution version of the three-dimensional, hydrostatic ocean model ROMS forced solely by barotropic tide well outside of the sill. To evaluate the sensitivity of the model results we perform sensitivity experiments in which the mesh size and various parameters and parameterizations are varied. We find indeed that the internal waves lose most of their energy before they reach the innermost basin, and hence set the scene for long periods of no deep water renewal. The sensitivity experiments reveal that it is important that the model's mesh size is small enough to resolve the dominant wavelengths of the topography. Moreover, we find that the strength of the turbulence production and hence the mixing depends on the initially chosen stratification. The method we use is generic and may be applied to any sill fjord.

Vertical mixing and hysteresis in the competition of buoyant and non-buoyant plankton prey species in a shallow lake

Some plankton species like cyanobacteria have an advantage in the competition for light in comparison to other species because of their buoyancy. This advantage can be diminished by vertical mixing in the surrounding water column. We introduce a non-spatial model, based on ordinary differential equations, which accounts for this effect. The main aim is to show that vertical mixing influences the outcome of competition between different species. Hysteresis is possible for a certain range of parameters. Introducing a grazing predator, the system exhibits different dynamics depending on the strength of mixing. In a diffusively coupled horizontal spatial model, local vertical mixing can also have a global effect on the biological system, for instance, destabilization of a locally stable solution, or the generation of new spatiotemporal behavior.

Dynamic and thermal responses of the Kuroshio to typhoon Megi (2004)

AbstractRelatively weak hurricane‐induced cooling (<1°C) is often observed in the Loop Current, while observed sea surface temperature (SST) cooling is much stronger in the Kuroshio. This was previously attributed to strong advection effects by the Loop Current. Based on a budget analysis, this study quantitatively diagnoses the dynamic and thermal responses of the Kuroshio to typhoon Megi and four additional tropical cyclones (TCs), which produce up to ~3°C SST cooling within the Kuroshio itself. The results show that the surface thermal response of the Kuroshio is a wind‐induced temperature cooling primarily due to vertical mixing (up to 96%). The dynamic response of the Kuroshio depends not only on TC conditions (intensity and wind direction) but also on prestorm ocean conditions (e.g., ocean mixed layer depth, baroclinicity, and current velocity). Differently from the open ocean where the ocean gains kinetic energy mainly from wind stress work, the Kuroshio can obtain greater kinetic energy from its baroclinic pressure work, especially in the upper ocean. The strong SST cooling observed in the Kuroshio region is therefore likely associated with the kinetic energy extracted from the Kuroshio baroclinic potential energy, which greatly enhances local vertical mixing.

Linking benthic hydrodynamics and cold-water coral occurrences: A high-resolution model study at three cold-water coral provinces in the NE Atlantic

Observations from numerous cold-water coral locations in the NE Atlantic show energetic near-bottom flow dynamics along the European continental margin at individual coral mounds and mound clusters. Dynamics are largely controlled by tide-topography interaction generating and enhancing periodic motions such as trapped waves, freely propagating internal tides and internal hydraulic jumps. In this study, linkages between key abiotic parameters and cold water coral occurrences are explored across entire cold-water coral mound provinces using an integrated modelling and observational approach. The 3-D ocean circulation model ROMS-AGRIF was applied to simulate near-bottom hydrodynamic conditions at three provinces in the NE Atlantic (Logachev mounds, Arc mounds and Belgica mounds) adopting a nested model setup with a central grid resolution of 250m. Simulations were carried out with a focus on accurate high-resolution topography and tidal forcing. The central model bathymetry was taken from high-resolution INSS (Irish National Seabed Survey) seafloor mapping data. The model was integrated over a full one-year reference period starting from the 1st January 2010. Interannual variability was not considered. Tidal forcing was obtained from a global solution of the Oregon State University (OSU) inverse tidal model. Modelled fields of benthic currents were validated against available independent in situ observations. Coral assemblage patterns (presence and absence locations) were obtained from benthic surveys of the EU FP7 CoralFISH programme and supplemented by data from additional field surveys. Modelled near-bottom currents, temperature and salinity were analysed for a 1-month subset (15th April to 15th May 2010) corresponding to the main CoralFISH survey period. The model results show intensified near-bottom currents in areas where living corals are observed by contrast with coral absence and random background locations. Instantaneous and time-mean current speeds at mound clusters in the Logachev province exceeded typical values in non-coral areas by up to a factor of three. Currents at cold-water coral locations in the Arc and Belgica mound provinces were less energetic, but still elevated compared to non-coral locations. An analysis of dynamical processes associated with oscillatory flow interacting with topography suggests that these motions are locally important food supply mechanisms to cold-water corals by promoting large amplitude local vertical mixing and organic matter fluxes. It is shown that their presence varies considerably between provinces based on the interplay of topographic slope, flow magnitude and ambient stratification.

The Great Whirl: Observations of its seasonal development and interannual variability

In situ measurements are used, together with sea surface height data, to study the development and variability of the Great Whirl (GW), a large quasi‐stationary anticyclone that appears off the coast of Somalia during the southwest monsoon season. We find that anticyclonic circulation indicative of the GW appears on average in April, almost two months before the onset of the southwest monsoon winds. This early initiation is coincident with the arrival of annual Rossby waves at the western boundary. With the onset of the monsoon winds in early June, the GW‐proper intensifies quickly, remaining at its peak throughout July, August, and September, and dissipating about one month after the winds have died. The GW is present for an average 166 ± 30 days per year and the position of its northern flank, close to 9°N, coincides with the latitude of zero wind stress curl. The intraseasonal variability of the GW is intense as a result of mutual advection with one to three flanking cyclones, which accompany the GW 70% of the time and tend to circulate clockwise around it. There is no consistent seasonal pattern for the development or dissipation of the GW, although movement to the southwest is common toward the end of the season. The GW of 1995 deepened from 200 m in June to over 1000 m in September, and strengthened from a swirl transport of 10 to 60 Sv. Cool waters in its core resulted from advection via the Somali Current and some local vertical mixing.

A High-Resolution Coral-Based Δ14C Record of Surface Water Processes in the Western Mediterranean Sea

The first high-resolution time series of pre- and post-bomb radiocarbon measurements is reported for surface waters in the western Mediterranean Sea. The Δ14C record was obtained from the aragonite skeleton of Cladocora caespitosa using a 50-yr-old corallite collected in the Ligurian Sea in 1998. Laser-ablation ICP measurements of trace elements (Li/Mg and Sr/Ca) show a strong seasonal variability, enabling the chronology of the Δ14C record to be determined at annual timescales. The mean Δ14C of pre-bomb surface water is -56 ± 3%, corresponding to a reservoir age of 262 ± 29 yr. The post-bomb maximum occurs in 1972 with a Δ14C value of 90%, significantly lower than the peak of 150% observed in the North Atlantic. The dilution of the peak-amplitude of Δ14C in western Mediterranean surface waters is attributed to mixing of North Atlantic Central Water inflow with relatively depleted underlying Intermediate Mediterranean and Levantine Intermediate waters. Intensification of this mixing is observed in 1963–1964, consistent with the change in atmospheric circulation from a positive to negative NAO phase (1960–1967). The post-peak Δ14C variability is relatively limited, reflecting mainly local vertical mixing forced by wind stress.

A High-Resolution Coral-Based Δ14C Record of Surface Water Processes in the Western Mediterranean Sea

The first high-resolution time series of pre- and post-bomb radiocarbon measurements is reported for surface waters in the western Mediterranean Sea. The Δ14C record was obtained from the aragonite skeleton of Cladocora caespitosa using a 50-yr-old corallite collected in the Ligurian Sea in 1998. Laser-ablation ICP measurements of trace elements (Li/Mg and Sr/Ca) show a strong seasonal variability, enabling the chronology of the Δ14C record to be determined at annual timescales. The mean Δ14C of pre-bomb surface water is –56 ± 3‰, corresponding to a reservoir age of 262 ± 29 yr. The post-bomb maximum occurs in 1972 with a Δ14C value of 90‰, significantly lower than the peak of 150‰ observed in the North Atlantic. The dilution of the peak-amplitude of Δ14C in western Mediterranean surface waters is attributed to mixing of North Atlantic Central Water inflow with relatively depleted underlying Intermediate Mediterranean and Levantine Intermediate waters. Intensification of this mixing is observed in 1963–1964, consistent with the change in atmospheric circulation from a positive to negative NAO phase (1960–1967). The post-peak Δ14C variability is relatively limited, reflecting mainly local vertical mixing forced by wind stress.

Monsoon and eddy forcing of chlorophyll-a variation in the northeast South China Sea

The physical and biological environments of the northeast South China Sea (SCS) were investigated using 11 year satellite and reanalysis data, including ocean chlorophyll-a (Chl-a) concentrations, sea surface wind (SSW) values, sea surface temperatures (SSTs), sea surface height anomalies (SSHAs), etc. The findings reveal that ocean Chl-a concentrations west of the Luzon Strait have the most significant annual cycles in the SCS. The dominant forcing mechanisms are monsoon winds and mesoscale eddies studied by multiple regression analysis. In the offshore regions, strong winds directly caused the enhancements by local vertical mixing and entrainment. As in the near-shore regions, the alongshore winds indirectly caused the enhancements by inducing coastal upwelling. Although SST is highly correlated with Chl-a concentration, SST cooling is difficult to observe in the bloom region. It is considered a consequence of monsoon and eddy forcings. Other mechanisms, such as local Ekman pumping by the wind stress curl and the geostrophic potential vorticity, have little effect on Chl-a seasonal variations.

Physical atmospheric structure and tropospheric mixing information in vertical profiles of atmospheric CO2 mixing ratios

Vertical distribution of atmospheric CO2 mixing ratios, as well as CO2 vertical variance and gradient are related to the vertical stability at the time of measurement, to the transport of coherent upstream plumes studied through changes in the upstream surface influence or to the historic mixing processes and dispersive behavior. Three vertical profiles of CO2 mixing ratios measured from 900 to 4200 m above sea level (masl) in 2006 at La Muela, Spain (LMU, 41.60°N 1.10°W, 570 masl) are examined. Changes in CO2 mixing ratio are associated with changes of the atmospheric physical parameters on the day of the survey; and with the transport of coherent air masses. Its consistency is examined through changes of the historical horizontal dispersion and chaotic mixing dynamics of air masses during the four days prior to measurements. A climatology of Lagrangian backward simulations run once a week at four altitudes (600, 1200, 2500 and 4000 masl) at LMU for 2006 shows that dispersion in these altitudes is superdiffusive (exponent coefficient γ > 1/2) and mixing follows a chaotic dynamics (power law exponent λ > 0) at all altitudes and in all seasons. Furthermore, a Horizontal Mixing Discontinuity (HMD) at ∼2500 masl separates two layers with different constraints on vertical mixing. Above the HMD, more coherent air masses and laminar transport characterizes the dynamics of atmospheric horizontal mixing whereas below it, filamentation and chaotic mixing dominate horizontal mixing. In the lower part of the vertical profile, within the ABL, mixing takes place by convection. Chaotic mixing below the HMD induces the boundary layer entrainment. Results highlight that there are two main discontinuities in the air column which separates different atmospheric dynamics. The ABL is driven by local meteorological conditions of the site at the sampling time; the HMD is driven by the synoptic‐scale historical mixing conditions of air masses. The effect of horizontal transport in the free troposphere should be considered equally as important as the local vertical mixing processes in the atmospheric boundary layer when interpreting CO2 vertical gradients. The dispersive exponents can be used to identify the transport of coherent plumes from anthropogenic emissions with different carbon composition that the one‐single backtrajectories models do not detect.

High SST variability south of Martha's Vineyard: Observation and modeling study

High, small-scale SST variability (6 °C over 5–10 km) observed South of Martha's Vineyard during the low-wind component of the Coupled Boundary Layers Air–Sea Transfer (CBLAST-Low) oceanographic field program in August 2003 is investigated using the Navy Coastal Ocean Model (NCOM), with atmospheric forcing provided by the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®). 1 1 COAMPS® is a registered trademark of the Naval Research Laboratory. The ocean model includes tidal boundary forcing by the eight major tidal constituents, which is superimposed on the non-tidal lateral boundary conditions obtained from the 1/8° global NCOM real-time hindcast. The simulation is conducted with a high-resolution, 200-m grid, with bathymetry from the NGDC 3-arc-second Coastal Relief Model. The COAMPS fields, tidal forcing and NCOM results are evaluated with the CBLAST-Low observations and previous results. Both the simulation and observation analyses reveal that SST variability south of Martha's Vineyard is significant on August 18 and 25 and is strongly related to the cooling events on August 17 to 18 and August 24 to 25. The northeast winds passing through Muskeget Channel generate sharp horizontal SST gradients on August 18 by accelerating the westward transport of cold water from the cold, tidally-mixed Nantucket Shoals and by wind-induced upwelling and surface-cooling-induced vertical mixing. The mechanism of SST change on August 25 is differentiated from the change on August 18 by the northwest winds being unfavorable to the westward transport of cold water. The SST cooling on August 25 is mainly caused by local vertical mixing induced by heat lost.

Evidence of Vertical Mixing Caused by High Frequency Internal Waves along the Eastern Coast of Korea

Internal waves and internal tides occur frequently along the eastern coast of Korea. During the spring-tide period in April 2003, the East Korean Warm Current (EKWC) flowed near the Korean East Coast Farming Forecast System (KECFFS; a moored oceanographic measurement system), creating a strong thermocline at the intermediate layer. Weakened stratification and well-mixed water appeared frequently around the KECFFS, with duration of approximately 1 day. The results suggest the following scenario. Baroclinic motion related to the internal tide generated high frequency internal waves around the thermocline. The breaking of those waves then created turbulence around the thermocline. After well-mixed water appeared, a current component with perpendicular direction to the EKWC appeared within the inertial period. The change in stratification around the KECFFS locally broke the geostrophic balance as a transient state. This local vertical mixing formed an ageostrophic current within the inertial period.

Bay of Bengal summer monsoon 10–20 day variability in sea surface temperature using model and observations

Observed sea surface temperature (SST) in the Bay of Bengal exhibits intraseasonal variability during summer monsoon. An ocean general circulation model (OGCM) driven by satellite derived daily winds and heat fluxes during summer of 2000 is able to reproduce aspects of intraseasonal variability (10–20 days) in SST. The intra‐seasonal SST simulated using satellite derived shortwave flux compares more favorably with buoy data than that produced using the NCEP reanalysis flux. Analysis of the terms in the heat balance equation of MLD suggests that in the central Bay of Bengal local net heat flux and vertical diffusive mixing changes drive the SST ISOs. Thus, the SST ISOs in this region are governed primarily by one‐dimensional processes.