Mixed-layer Budget Research Articles

Trend and interannual variability of summer marine heatwaves in the tropical Indian ocean: Patterns, mixed layer heat budget, and seasonal prediction

Marine heatwaves (MHWs) are extreme sea surface temperature (SST) events in all ocean basins, with far-reaching impacts on marine ecosystems and socio-economy. The leading patterns, trend, and interannual variability of summer MHWs in the tropical Indian Ocean (TIO) are investigated in this study. The first empirical orthogonal function (EOF) mode of frequency of MHWs exhibits a monopole pattern over the entire basin. This mode is highly associated with the concurrent Indian Ocean Basin warming, indicating a remarkable trend over the past four decades. The linear trend in MHW properties largely relates to increased summer Indian Ocean mean SST. The second EOF mode exhibits a zonal dipole with the MHW numbers increasing in the west and decreasing in the east. On the interannual timescale, the first two EOF modes are remotely affected by antecedent and concurrent El Niño events, respectively. The ocean mixed layer budget is utilized for examining the formation of different summer MHW patterns. During the preceding spring, the surface heat flux is important for the development of MHWs, while the ocean advections play a secondary role in the South Indian Ocean for the MHW monopole. Once the SST anomaly rises in summer, the ocean advections play a dominant role in maintaining the SST. Last, we assess the prediction skill of summer TIO MHWs by performing a bilinear seasonal statistical prediction model. Our results suggest the frequency of summer MHWs in the TIO could be predicted one season in advance. This study has great implications for understanding and predicting ocean extreme events in the TIO.

Carbon Outgassing in the Antarctic Circumpolar Current Is Supported by Ekman Transport From the Sea Ice Zone in an Observation‐Based Seasonal Mixed‐Layer Budget

AbstractDespite its importance for the global cycling of carbon, there are still large gaps in our understanding of the processes driving annual and seasonal carbon fluxes in the high‐latitude Southern Ocean. This is due in part to a historical paucity of observations in this remote, turbulent, and seasonally ice‐covered region. Here, we use autonomous biogeochemical float data spanning 6 full seasonal cycles and with circumpolar coverage of the Southern Ocean, complemented by atmospheric reanalysis, to construct a monthly climatology of the mixed layer budget of dissolved inorganic carbon (DIC). We investigate the processes that determine the annual mean and seasonal cycle of DIC fluxes in two different zones of the Southern Ocean—the Sea Ice Zone (SIZ) and Antarctic Southern Zone (ASZ). We find that, annually, mixing with carbon‐rich waters at the base of the mixed layer supplies DIC which is, in the ASZ, either used for net biological production or outgassed to the atmosphere. In contrast, in the SIZ, where carbon outgassing and the biological pump are weaker, the surplus of DIC is instead advected northward to the ASZ. In other words, carbon outgassing in the southern Antarctic Circumpolar Current (ACC), which has been attributed to remineralized carbon from deep water upwelled in the ACC, is also due to the wind‐driven transport of DIC from the SIZ. These results stem from the first observation‐based carbon budget of the circumpolar Southern Ocean and thus provide a useful benchmark to evaluate climate models, which have significant biases in this region.

Estimations of vertical diffusivity and applications on a mixed layer budget analysis of the Bay of Bengal using Argo data

Estimations of vertical diffusivity and applications on a mixed layer budget analysis of the Bay of Bengal using Argo data

Surface ocean carbon budget in the 2017 south Georgia diatom bloom: Observations and validation of profiling biogeochemical argo floats

Estimates of the partial pressure of CO2 (pCO2) derived from biogeochemical Argo floats have the potential to improve our knowledge of the highly variable and partially observed Southern Ocean carbon sink through sampling at improved temporal and spatial resolution. Here we use the data from six biogeochemical Argo floats to characterise near-surface dissolved inorganic carbon (DIC) concentrations and fluxes at the site of an intense (Chl-a >3 mg m−3) mesoscale diatom bloom situated northwest of South Georgia. Concurrently, we provide independent analysis and validation of the methodology used by the Southern Ocean Carbon and Climate Observational and Modelling (SOCCOM) project for deriving surface pCO2 from float-based pH and oxygen measurements. We compare the float observations with co-located ship data from bottle samples over a month-long period. When compared to data sampled within 24 h and 25 km of each float profile, we find good agreement with a mean offset of −0.005 ± 0.018 (1σ) between float pH and bottle-derived pH. This translates to comparable pCO2 estimates between ship measurements and floats with a mean difference of 2.6 ± 12.8 (1σ) μatm, providing support for the use of biogeochemical Argo float data to supplement shipboard pCO2 measurements in the Southern Ocean. Based on float-derived pCO2 we calculate a sizeable local flux of CO2 of 24 ± 7 mmol C m−2 d−1 (over a 27-day period) from the atmosphere into the surface mixed layer, driven by a large air-sea pCO2 gradient and strong but variable winds. Despite the considerable air-sea flux, the local mixed layer carbon budget appears to be dominated by entrainment and detrainment of carbon-rich waters into and out of the mixed layer. However, given the large uncertainties associated with these fluxes and the significant challenges associated with closing the mixed layer budget, further research is required to refine float-based mixed layer DIC fluxes.

Heat and salinity transport between the permanent pycnocline and the mixed layer due to the obduction process evaluated from a gridded Argo dataset

It is necessary for air–sea interaction and climate research to evaluate heat and salinity transport between the mixed layer (ML) and the permanent pycnocline and its contribution to the ML properties. The authors have calculated the obduction and subduction rates with the Eulerian definition using a 1°-grid Argo dataset and incorporated the obduction rate into the ML budget analysis in order to assess the impact of the entrained permanent pycnocline water on the ML temperature and salinity. They have successfully separated the effect of obduction on the ML from the entrainment of the seasonal pycnocline, and specified the regions where the obduction impact is noticeable. The ML heating rate due to obduction was smaller than ± 0.1 K/yr on average over most of the domain, but it was larger than − 0.5 K/yr in places around the Circumpolar Current, in the northeastern tropical Pacific, the southwestern tropical Indian Ocean, and the northeastern North Atlantic. The salinizing rate due to obduction exceeded ± 0.02 psu/yr in these areas, and in the subarctic North Pacific and the eastern South Pacific. The waters entrained from the permanent pycnocline warm and salinize the ML in the high latitudes due to the dichothermal structure. The method applied in this study also enabled us to assess the residence time and route of obducted and subducted water parcels below the ML. Furthermore, the authors examined the changes of obduction and subduction related with the marine heatwave that occurred in the northeastern North Pacific around 2014.

Mixed Layer Budget Terms on Acoustic Propagation A Study based on the Butterfly Track Experiment in the South Eastern Arabian Sea

A butterfly type of repeat track cruise was carried out in the South Eastern Arabian Sea (off Minicoy) onboard INS Sagardhwani during July 2016 to Aug 2016. We have also made use of the data from OMNI buoy, AD09, which is about 6 km close to the centre station of butterfly track. Air sea flux, the horizontal current data from AD09 and the time series data collected from the butterfly experiment were analyzed to compute the mixed layer heat and salt budget. The short-term thermo-haline variability off Minicoy, relative contribution of heat/salt budget terms in MLD and its effects on acoustic propagation are addressed in this paper. In this study, we found that most dominating term in the mixed layer heat budget estimation is net surface heat flux followed by the advective terms. However the salinity in the mixed layer is dominated by the contribution of buoyancy mixing due to night time evaporative cooling. During the calm, sunny day, the so-called afternoon effect due to the diurnal heating restricts the sonar range. But during the windy day, the wind/wave mixing prevents the warming of the surface layer which in turn enhances the sonar range. Similarly, the night time cooling also enhance the acoustic propagation range. The presence of Arabian Sea High Salinity Watermass in the surface layer also enhances the acoustic propagation.

Influence of biological carbon export on ocean carbon uptake over the annual cycle across the North Pacific Ocean

AbstractWe evaluate the influences of biological carbon export, physical circulation, and temperature‐driven solubility changes on air‐sea CO2 flux across the North Pacific basin (35°N–50°N, 142°E–125°W) throughout the full annual cycle by constructing mixed layer budgets for dissolved inorganic carbon (DIC) and pCO2, determined on 15 container ship transects between Hong Kong and Long Beach, CA, from 2008 to 2012. Annual air‐sea CO2 flux is greatest in the western North Pacific and decreases eastward across the basin (2.7 ± 0.9 mol C m−2 yr−1 west of 170°E, as compared to 2.1 ± 0.3 mol C m−2 yr−1 east of 160°W). East of 160°W, DIC removal by annual net community production (NCP) more than fully offsets the DIC increase due to air‐sea CO2 flux. However, in the region west of 170°E influenced by deep winter mixing, annual NCP only offsets ~20% of the DIC increase due to air‐sea CO2 flux, requiring significant DIC removal by geostrophic advection. Temperature‐driven solubility changes have no net influence on pCO2 and account for <25% of annual CO2 uptake. The seasonal timing of NCP strongly affects its influence on air‐sea CO2 flux. Biological carbon export from the mixed layer has a stronger influence on pCO2 in summer when mixed layers are shallow, but changes in pCO2 have a stronger influence on air‐sea CO2 flux in winter when high wind speeds drive more vigorous gas exchange. Thus, it is necessary to determine the seasonal timing as well as the annual magnitude of NCP to determine its influence on ocean carbon uptake.

Dynamical contribution to sea surface salinity variations in the eastern Gulf of Guinea based on numerical modelling

In this study, we analyse the seasonal variability of the sea surface salinity (SSS) for two coastal regions of the Gulf of Guinea from 1995 to 2006 using a high resolution model (1/12°) embedded in a Tropical Atlantic (1/4°) model. Compared with observations and climatologies, our model demonstrates a good capability to reproduce the seasonal and spatial variations of the SSS and mixed layer depth. Sensitivity experiments are carried out to assess the respective impacts of precipitations and river discharge on the spatial structure and seasonal variations of the SSS in the eastern part of the Gulf of Guinea. In the Bight of Biafra, both precipitations and river runoffs are necessary to observe permanent low SSS values but the river discharge has the strongest impact on the seasonal variations of the SSS. South of the equator, the Congo river discharge alone is sufficient to explain most of the SSS structure and its seasonal variability. However, mixed layer budgets for salinity reveal the necessity to take into account the horizontal and vertical dynamics to explain the seasonal evolution of the salinity in the mixed layer. Indeed evaporation, precipitations and runoffs represent a relatively small contribution to the budgets locally at intraseasonal to seasonal time scales. Horizontal advection always contribute to spread the low salinity coastal waters offshore and thus decrease the salinity in the eastern Gulf of Guinea. For the Bight of Biafra and the Congo plume region, the strong seasonal increase of the SSS observed from May/June to August/September, when the trade winds intensify, results from a decreasing offshore spread of freshwater associated with an intensification of the salt input from the subsurface. In the Congo plume region, the subsurface salt comes mainly from advection due to a strong upwelling but for the Bight of Biafra, entrainment and vertical mixing also play a role. The seasonal evolution of horizontal advection in the Bight of Biafra is mainly driven by eddy correlations between salinity and velocities, but it is not the case in the Congo plume.

Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean

Abstract. Sea-to-air and diapycnal fluxes of nitrous oxide (N2O) into the mixed layer were determined during three cruises to the upwelling region off Mauritania. Sea-to-air fluxes as well as diapycnal fluxes were elevated close to the shelf break, but elevated sea-to-air fluxes reached further offshore as a result of the offshore transport of upwelled water masses. To calculate a mixed layer budget for N2O we compared the regionally averaged sea-to-air and diapycnal fluxes and estimated the potential contribution of other processes, such as vertical advection and biological N2O production in the mixed layer. Using common parameterizations for the gas transfer velocity, the comparison of the average sea-to-air and diapycnal N2O fluxes indicated that the mean sea-to-air flux is about three to four times larger than the diapycnal flux. Neither vertical and horizontal advection nor biological production were found sufficient to close the mixed layer budget. Instead, the sea-to-air flux, calculated using a parameterization that takes into account the attenuating effect of surfactants on gas exchange, is in the same range as the diapycnal flux. From our observations we conclude that common parameterizations for the gas transfer velocity likely overestimate the air-sea gas exchange within highly productive upwelling zones.

Southern Ocean Gas Exchange Experiment: Setting the stage

[1] The Southern Ocean Gas Exchange Experiment (SO GasEx) is the third in a series of U.S.‐led open ocean process studies aimed at improving the quantification of gas transfer velocities and air‐sea CO2 fluxes. Two deliberate 3He/SF6 tracer releases into relatively stable water masses selected for large DpCO2 took place in the southwest Atlantic sector of the Southern Ocean in austral fall of 2008. The tracer patches were sampled in a Lagrangian manner, using observations from discrete CTD/Rosette casts, continuous surface ocean and atmospheric monitoring, and autonomous drifting instruments to study the evolution of chemical and biological properties over the course of the experiment. CO2 and DMS fluxes were directly measured in the marine air boundary layer with micrometeorological techniques, and physical, chemical, and biological processes controlling air‐sea fluxes were quantified with measurements in the upper ocean and marine air. Average wind speeds of 9 m s−1 to a maximum of 16 m s−1 were encountered during the tracer patch observations, providing additional data to constrain wind speed/gas exchange parameterizations. In this paper, we set the stage for the experiment by detailing the hydrographic observations during the site surveys and tracer patch occupations that form the underpinning of observations presented in the SO GasEx special section. Particular consideration is given to the mixed layer depth as this is a critical variable for estimates of fluxes and biogeochemical transformations based on mixed layer budgets.

Sensitivity of mixed layer heat budgets to wind forcing: A case study for the equatorial Pacific cold tongue

Ocean General Circulation Models (OGCMs) are very useful tools to analyze and quantify heat budget terms governing the sea surface temperature variability. However, model results are strongly dependant on the momentum forcing used. In this study, five simulations of a climate-type OGCM, forced by different wind stress products from atmospheric reanalyses, satellites and in situ data are performed. They are analyzed in the eastern tropical Pacific from 1993 to 2000, in the Pacific cold tongue, a key region where all terms of the heat budget are important. The differences in thermodynamical and dynamical oceanic parameters relevant for mixed layer budgets are first documented in the five simulations, and validations against observations are presented at annual and intraseasonal scales. It is then shown that the different components of the oceanic heat budget are quantitatively greatly affected by the choice of the wind forcing, and the amplitude of the terms explaining sea surface temperature variations can double from one simulation to the other. In contrast, the relative contributions of the terms, i.e. mean advection, atmospheric forcing, mixing at the base of the mixed layer and contributions of eddies are the same in all simulations forced by different wind stress products. This is true for the mean 1993–2000 period, at seasonal timescales and during interannual events such as the abrupt cooling observed in the cold tongue in May 1998. This paper thus suggests that model users can be confident in the physical processes governing sea surface temperature variability whatever the wind stress used to force their model is, but they must be very cautious when estimating quantitatively heat budgets terms with one simulation only.

The tropical forest and fire emissions experiment: Emission, chemistry, and transport of biogenic volatile organic compounds in the lower atmosphere over Amazonia

Airborne and ground‐based mixing ratio and flux measurements using eddy covariance (EC) and for the first time the mixed layer gradient (MLG) and mixed layer variance (MLV) techniques are used to assess the impact of isoprene and monoterpene emissions on atmospheric chemistry in the Amazon basin. Average noon isoprene (7.8 ± 2.3 mg/m2/h) and monoterpene fluxes (1.2 ± 0.5 mg/m2/h) compared well between ground and airborne measurements and are higher than fluxes estimated in this region during other seasons. The biogenic emission model, Model of Emissions of Gases and Aerosols from Nature (MEGAN), estimates fluxes that are within the model and measurement uncertainty and can describe the large observed variations associated with land‐use change in the region north‐west of Manaus. Isoprene and monoterpenes accounted for ∼75% of the total OH reactivity in this region and are important volatile organic compounds (VOCs) for modeling atmospheric chemistry in Amazonia. The presence of fair weather clouds (cumulus humilis) had an important impact on the vertical distribution and chemistry of VOCs through the planetary boundary layer (PBL), the cloud layer, and the free troposphere (FT). Entrainment velocities between 10:00 and 11:30 local time (LT) are calculated to be on the order of 8–10 cm/s. The ratio of methyl‐vinyl‐ketone (MVK) and methacrolein (MAC) (unique oxidation products of isoprene chemistry) with respect to isoprene showed a pronounced increase in the cloud layer due to entrainment and an increased oxidative capacity in broken cloud decks. A decrease of the ratio in the lower free troposphere suggests cloud venting through activated clouds. OH modeled in the planetary boundary layer using a photochemical box model is much lower than OH calculated from a mixed layer budget approach. An ambient reactive sesquiterpene mixing ratio of 1% of isoprene would be sufficient to explain most of this discrepancy. Increased OH production due to increased photolysis in the cloud layer balances the low OH values modeled for the planetary boundary layer. The intensity of segregation (Is) of isoprene and OH, defined as a relative reduction of the reaction rate constant due to incomplete mixing, is found to be significant: up to 39 ± 7% in the ∼800‐m‐deep cloud layer. The effective reaction rate between isoprene and OH can therefore vary significantly in certain parts of the lower atmosphere.

Estimating daytime CO2 fluxes over a mixed forest from tall tower mixing ratio measurements

Difficulties in estimating terrestrial ecosystem CO2 fluxes on regional scales have significantly limited our understanding of the global carbon cycle. This paper presents an effort to estimate daytime CO2 fluxes over a forested region on the scale of 50 km in northern Wisconsin, USA, using the tall‐tower‐based mixed layer (ML) budget method. Budget calculations were conducted for 2 years under fair‐weather conditions as a case study. With long‐term measurements of CO2 mixing ratio at a 447‐m‐tall tower, daytime regional CO2 fluxes were estimated on the seasonal scale, longer than in earlier studies. The flux derived from the budget method was intermediate among those derived from the eddy‐covariance (EC) method at three towers in the region and overall closest to that derived from EC measurements at 396 m of the tall tower. The dormant season average daytime‐integrated regional CO2 flux was about 0.35 ± 0.18 gC m−2. During the growing season, the monthly averaged daytime‐integrated regional CO2 flux varied from −1.58 ± 0.19 to −4.15 ± 0.32 gC m−2, suggesting that the region was a net sink of CO2 in the daytime. We also discussed the effects on theses estimates of neglecting horizontal advection, selecting for fair‐weather conditions, and using single‐location measurements. Daytime regional CO2 flux estimates from the ML budget method were comparable to those from three aggregation experiments. Differences in results from the different methods, however, suggest that more constraints are needed to estimate regional fluxes with more confidence. Despite uncertainties, our analyses indicate that it is feasible to estimate daytime regional CO2 fluxes on long timescales using tall tower measurements.

Air–sea gas exchange at extreme wind speeds measured by autonomous oceanographic floats

Measurements of the air–sea fluxes of N 2 and O 2 were made in winds of 15–57 m s − 1 beneath Hurricane Frances using two types of air-deployed neutrally buoyant and profiling underwater floats. Two “Lagrangian floats” measured O 2 and total gas tension (GT) in pre-storm and post-storm profiles and in the actively turbulent mixed layer during the storm. A single “EM-APEX float” profiled continuously from 30 to 200 m before, during and after the storm. All floats measured temperature and salinity. N 2 concentrations were computed from GT and O 2 after correcting for instrumental effects. Gas fluxes were computed by three methods. First, a one-dimensional mixed layer budget diagnosed the changes in mixed layer concentrations given the pre-storm profile and a time varying mixed layer depth. This model was calibrated using temperature and salinity data. The difference between the predicted mixed layer concentrations of O 2 and N 2 and those measured was attributed to air–sea gas fluxes F BO and F BN. Second, the covariance flux F CO( z) = 〈 wO 2′〉( z) was computed, where w is the vertical motion of the water-following Lagrangian floats, O 2′ is a high-pass filtered O 2 concentration and 〈〉( z) is an average over covariance pairs as a function of depth. The profile F CO( z) was extrapolated to the surface to yield the surface O 2 flux F CO(0). Third, a deficit of O 2 was found in the upper few meters of the ocean at the height of the storm. A flux F SO, moving O 2 out of the ocean, was calculated by dividing this deficit by the residence time of the water in this layer, inferred from the Lagrangian floats. The three methods gave generally consistent results. At the highest winds, gas transfer is dominated by bubbles created by surface wave breaking, injected into the ocean by large-scale turbulent eddies and dissolving near 10-m depth. This conclusion is supported by observations of fluxes into the ocean despite its supersaturation; by the molar flux ratio F BO/ F BN, which is closer to that of air rather than that appropriate for Schmidt number scaling; by O 2 increases at about 10-m depth along the water trajectories accompanied by a reduction in void fraction as measured by conductivity; and from the profile of F CO( z), which peaks near 10 m instead of at the surface. At the highest winds O 2 and N 2 are injected into the ocean by bubbles dissolving at depth. This, plus entrainment of gas-rich water from below, supersaturates the mixed layer causing gas to flux out of the near-surface ocean. A net influx of gas results from the balance of these two competing processes. At lower speeds, the total gas fluxes, F BO, F BN and F CO(0), are out of the ocean and downgradient.

Coupling between CO2, water vapor, temperature, and radon and their fluxes in an idealized equilibrium boundary layer over land

We propose a new approach for relating concentration measurements in the atmospheric boundary layer to surface fluxes based on a simple equilibrium boundary layer model (an extension of Betts [2000]). This is a major shift from the traditional focus on the growth of the daytime dry boundary layer. We show equilibrium solutions for the diurnally averaged properties of the boundary layer that link the mixed layer equilibrium (on timescales longer than a day) of potential temperature, water vapor, CO2 (and other trace gases such as radon) with an interactive cloud layer and with the surface energy, water, and carbon balance. We examine these processes as a function of a set of external parameters: soil water content, which directly impacts respiration and photosynthesis, and hence transpiration; the surface net shortwave, directly linked to net radiation, which is also coupled to photosynthesis and transpiration; and the radiative cooling of the mixed layer (ML), which in the equilibrium model directly affects the surface sensible heat. We also show solutions where the net shortwave and radiative cooling are coupled to the cloud field. Our other variable model parameters are the properties of air entrained into the CBL, the midtropospheric values of vapor mixing ratio, CO2 and radon, and the lapse rate above cloud base. We considered two idealized ecosystems: forest (based on observations in Wisconsin) and grassland (using parameter estimates from the literature) to show how the vegetation model affects the ML equilibrium. We show how the mass transport out of the subcloud layer and the mass exchange with the free troposphere couples the mixed layer equilibrium of water vapor, CO2, and radon with the corresponding surface fluxes. We suggest that regional ML budgets may give useful constraints on regional carbon budgets, and that the coupling with the cloud field is a fundamental part of the ML equilibrium.

Physical oceanographic conditions during GasEx‐2001

GasEx‐2001 is a study of air‐sea gas exchange in a region of CO2 outgassing. The bulk of the experiment followed a drifting array of near‐surface instruments deployed during the second half of February 2001 just south of the equator in the central Pacific Ocean. Physical oceanographic conditions including local currents, the seasonal cycle, Kelvin waves, and tropical instability waves are described using shipboard data and a variety of other data sources to set the large‐scale oceanographic context for GasEx‐2001. Local physical oceanographic conditions during GasEx‐2001 are then analyzed using shipboard data and a simple one‐dimensional mixed layer model. The thermocline shoals about 13 m over the 15‐day experiment, implying an upwelling rate of 1 × 10−5 m s−1. Zonal velocity is surface‐intensified and westward, with vertical shear mostly through the thermocline. Meridional velocity is also strongly sheared with a maximum equatorward flow in the thermocline that is much reduced by 17‐m depth. The mixed layer model exhibits more near‐surface warming over the course of the experiment than is observed. Prescribing upwelling in the model closes the heat budget within error estimates. Entrainment at the base of the mixed layer plays a limited role in the mixed layer budgets of carbon and other water properties. Vertical shear of horizontal velocity within the mixed layer and slippage of the array through the surface water also have small (but uncertain) roles in these budgets.

Spring-summer imbalance of dissolved inorganic carbon in the mixed layer of the northwestern Sargasso Sea

The surface concentration of dissolved inorganic carbon (DIC) at the Bermuda Atlantic Time-series Study site (BATS) decreased gradually by ca. 30 μ mol kg − 1 from April to October in 1989. This decrease occurred almost in the absence of measurable nitrate in the mixed layer. Although ancillary data about the C system point to the possible importance of lateral transport, horizontal gradients in surface [DIC] and the mean flow in the area indicate that local effects should prevail in the seasonal drawdown of DIC. On the basis of an one-dimensional model, we hence estimate the mixed layer budget of DIC for this period, from surface [DIC] data, temperature profiles, and concomitant meteorological records. According to model uncertainties, the [DIC] drawdown should be mostly explained (71–93%) by a net community production (NCP) averaging 1.4–2.3 mgC m −3 d −1 , and to a lesser extent, by outgassing of CO 2 to the atmosphere. These losses are partially compensated ( 50%) is not respired in the mixed layer. DOI: 10.1034/j.1600-0889.1996.00011.x

Thermodynamics of Mixed Stratocumulus Layers: Saturation Point Budgets

Abstract Saturation point (SP) diagrams and a circulation model are used to analyze the thermodynamics of mixed stratocumulus layers, and develop conceptual tools for observational and parametric analyses. These models show the relation between SP structure and the thermodynamic fluxes for steady and unsteady mixed layers, couple convective and radiative fluxes at cloud top and within the layer, and suggest closure conditions on the circulation energetics using θv, isopleths. The steady-state model shows a mixing line SP structure below cloud top with correspondingly simple surface flux parameterizations. The time-dependent mixed layer budgets are formulated using an SP vector diagram to show the approach to equilibrium, the internal circulation, and the relationship between Eulerian and Lagrangian flux descriptions.