Vertical Temperature Structure Research Articles

An objective algorithm for reconstructing the three-dimensional ocean temperature field based on Argo profiles and SST data

While global oceanic surface information with large-scale, real-time, high-resolution data is collected by satellite remote sensing instrumentation, three-dimensional (3D) observations are usually obtained from in situ measurements, but with minimal coverage and spatial resolution. To meet the needs of 3D ocean investigations, we have developed a new algorithm to reconstruct the 3D ocean temperature field based on the Array for Real-time Geostrophic Oceanography (Argo) profiles and sea surface temperature (SST) data. The Argo temperature profiles are first optimally fitted to generate a series of temperature functions of depth, with the vertical temperature structure represented continuously. By calculating the derivatives of the fitted functions, the calculation of the vertical temperature gradient of the Argo profiles at an arbitrary depth is accomplished. A gridded 3D temperature gradient field is then found by applying inverse distance weighting interpolation in the horizontal direction. Combined with the processed SST, the 3D temperature field reconstruction is realized below the surface using the gridded temperature gradient. Finally, to confirm the effectiveness of the algorithm, an experiment in the Pacific Ocean south of Japan is conducted, for which a 3D temperature field is generated. Compared with other similar gridded products, the reconstructed 3D temperature field derived by the proposed algorithm achieves satisfactory accuracy, with correlation coefficients of 0.99 obtained, including a higher spatial resolution (0.25° × 0.25°), resulting in the capture of smaller-scale characteristics. Finally, both the accuracy and the superiority of the algorithm are validated.

Reanalysis comparisons of upper tropospheric/lower stratospheric jets and multiple tropopauses.

The representation of upper tropospheric/lower stratospheric (UTLS) jet and tropopause characteristics is compared in five modern high-resolution reanalyses for 1980 through 2014. Climatologies of upper tropospheric jet, subvortex jet (the lowermost part of the stratospheric vortex), and multiple tropopause frequency distributions in MERRA (Modern Era Retrospective Analysis for Research and Applications), ERA-I (the ECMWF interim reanalysis), JRA-55 (the Japanese 55-year Reanalysis), and CFSR (the Climate Forecast System Reanalysis) are compared with those in MERRA-2. Differences between alternate products from individual reanalysis systems are assessed; in particular, a comparison of CFSR data on model and pressure levels highlights the importance of vertical grid spacing. Most of the differences in distributions of UTLS jets and multiple tropopauses are consistent with the differences in assimilation model grids and resolution: For example, ERA-I (with coarsest native horizontal resolution) typically shows a significant low bias in upper tropospheric jets with respect to MERRA-2, and JRA-55 a more modest one, while CFSR (with finest native horizontal resolution) shows a high bias with respect to MERRA-2 in both upper tropospheric jets and multiple tropopauses. Vertical temperature structure and grid spacing are especially important for multiple tropopause characterization. Substantial differences between MERRA and MERRA-2 are seen in mid- to high-latitude southern hemisphere winter upper tropospheric jets and multiple tropopauses, and in the upper tropospheric jets associated with tropical circulations during the solstice seasons; some of the largest differences from the other reanalyses are seen in the same times and places. Very good qualitative agreement among the reanalyses is seen between the large scale climatological features in UTLS jet and multiple tropopause distributions. Quantitative differences may, however, have important consequences for transport and variability studies. Our results highlight the importance of considering reanalyses differences in UTLS studies, especially in relation to resolution and model grids; this is particularly critical when using high-resolution reanalyses as an observational reference for evaluating global chemistry climate models.

Assessment of thermal stratification, stability and characteristics of deep water zone of the southern Caspian Sea

Variations of seawater properties and stratification of water column in the southern deep water zone (470 m depth) of the Caspian Sea were evaluated based on field measured data. Variations of physical characteristics (temperature, salinity, density and sound speed) of seawater mainly were limited to the upper 100 m layers. Below this layer, seasonal changes of seawater parameters were minor. Vertical structure of temperature in the southern Caspian Sea waters was characterized by a significant seasonal thermocline approximately between 20 and 50 m depths in summer with a vertical gradient of around 16 °C. Destruction of the thermocline occurred with the general cooling of the sea surface water and deepening of the mixed layer during late of autumn and winter. Frequency of stability in water column often across the upper layers (surface mixed and thermocline) was more than deeper levels. Variations of salinity were around 12.4 PSU in the time of measurements. Structures of thermocline and pycnocline were characterized by a significant homogeneity especially in offshore stations during the year. According to high agreement between density and temperature in the Caspian Seawater, a strong seasonal pycnocline was observed in the position of the thermocline. Furthermore, structure of sound speed variations followed vertical variations of seawater temperature. Variations of chemical characteristics (dissolved oxygen and pH) of seawater mainly were continued from sea surface to near the sea bed in deep water zone.

Thermodynamic constraint on the depth of the global tropospheric circulation

The troposphere is the region of the atmosphere characterized by low static stability, vigorous diabatic mixing, and widespread condensational heating in clouds. Previous research has argued that in the tropics, the upper bound on tropospheric mixing and clouds is constrained by the rapid decrease with height of the saturation water vapor pressure and hence radiative cooling by water vapor in clear-sky regions. Here the authors contend that the same basic physics play a key role in constraining the vertical structure of tropospheric mixing, tropopause temperature, and cloud-top temperature throughout the globe. It is argued that radiative cooling by water vapor plays an important role in governing the depth and amplitude of large-scale dynamics at extratropical latitudes.

A consistent structure of phytoplankton communities across the warm–cold regions of the water mass on a meridional transect in the East/Japan Sea

Three cruises were undertaken along a meridional transect in the East/Japan Sea (EJS) in spring (May 2007), summer (July 2009), and fall (October 2012) to determine the geographic variations in phytoplankton biomass and community composition. This study revealed a gradient of surface temperature and a fluctuation of hydrographic conditions along the transect. Although a subpolar front (SPF) formed between the warm- and cold-water masses (37–40°N), no significant differences in phytoplankton biomass and community composition were detected between the southern and northern parts of the EJS. These results disprove our initial hypothesis that different water masses may contain differently structured phytoplankton communities. In the present study, isothermal layers (≤ 12°C) fluctuated over a depth of 50m in both warm- and cold-water masses, depending on the SPF. In contrast, the nitracline (i.e. 2.5μM nitrate isopleth) depth was recorded within a limited range of 20–40m in spring, 30–50m in summer, and 40–60m in fall. The chlorophyll a concentrations at the subsurface chlorophyll maxima (SCM) were significantly higher in spring and summer (356 ± 233 and 270 ± 182ngL–1, respectively) than in fall (117 ± 89ngL–1). The relative contributions of individual phytoplankton groups to the depth-integrated chlorophyll a concentration conformed to the composition of the phytoplankton community in the SCM layer, showing a dominance of diatoms (58 ± 19% in spring, 48 ± 11% in summer, and 30 ± 20% in fall). Canonical correspondence analysis revealed that the geographic structures of phytoplankton communities were strongly associated with the vertical structures of water temperature and nutrient concentration in the water column rather than with horizontal gradients of hydrographic conditions. Finally, our findings suggest that water column stability and light–nutrient availability in the euphotic zone play a key role in determining geographical consistency of the biomass and the community structure of phytoplankton in the EJS.

Seasonal variability of hydrographic structure in Sharm Obhur and water exchange with the Red Sea

Sharm Obhur is a narrow coastal inlet about 10 km long. The maximum depth at the entrance is about 35 m, which decreases gradually towards the head. Nine field trips were conducted for hydrographic survey in the Sharm during April 2015–January 2016 covering pre-summer transition, summer, pre-winter transition and winter seasons. In each trip, eight stations along the central axis of the Sharm were occupied for the measurement of temperature and salinity. In addition, an Acoustic Doppler Current Profiler (ADCP) mooring was deployed near the entrance (at station 2) during 18 February–26April 2015. The vertical structures of temperature and salinity show two distinct layers—a relatively low saline surface layer and a high saline bottom layer. The thermohaline properties increase from the entrance towards the head in all the seasons except for a slight decrease in temperature during December. Near the head, the observed maximum temperature and salinity are 33.22 °C (August) and 40.36 psu (April), respectively, while the observed minimum temperature and salinity are 25.05 °C and 38.97 psu, respectively, during January. The water exchange between the Sharm and the Red Sea shows two-layer structure, with a surface inflow and a deep outflow which is typical of basins where evaporation exceeds precipitation. The pressure gradient generated by the increasing density towards the head pushes the relatively low saline surface water from the Red Sea to the Sharm with a gradient in surface salinity influenced by the evapouration and heat exchange. Near the head, it sinks and returns as a deep water flow. The estimated flushing time of the Sharm varies between 7 and 12 days with an average of 9.5 days.

Sensitivity of AGCM‐simulated regional JJAS precipitation to different convective parameterization schemes

ABSTRACTThis study examines the effects of convective parameterization schemes (CPSs) on the simulated June–September (JJAS) precipitation climatology, variability and predictability over southwestern Arabian Peninsula and northeast Africa during 1981–2014 within an atmospheric global climate model framework. The three CPSs used are: the simplified Arakawa–Schubert (SAS) scheme; the SAS scheme with Tokioka modification (STOK) and the Emanuel (EMAN) scheme coupled with a probability distribution function‐based cloud parameterization scheme. SAS and STOK overestimate JJAS total precipitation over the selected domain compared to observations, while EMAN underestimates. EMAN provides the most realistic distribution of precipitation and spatial distribution of convective precipitation despite the tendency for underestimating the total precipitation. The principal patterns of precipitation variability (as measured by the leading empirical orthogonal functions) in EMAN and STOK are clearly related to the observed pattern, while the maximum variability in SAS occurs in a completely different location. The El Niño Southern Oscillation‐related JJAS precipitation teleconnection simulated by the SAS scheme is very weak as compared to observations and other two CPSs. The signal‐to‐noise ratio estimated by SAS and STOK is very low as compared to EMAN. Correlation analysis shows that EMAN performs better than the other two CPSs. Low (high) outgoing longwave radiation (OLR) values are indicative of enhanced (suppressed) convection and hence more (less) cloud coverage. The root mean square error estimated for OLR in EMAN is lower than that of the other two CPSs. The vertical structure of specific humidity and temperature shows less error in EMAN than that of the other two CPSs, which could be a reason for better predictability over the region of interest.

Importance of the Vertical Resolution in Simulating SST Diurnal and Intraseasonal Variability in an Oceanic General Circulation Model

Abstract In this paper, the influence of high vertical resolution near the surface in an oceanic general circulation model in simulating the observed sea surface temperature (SST) variability is investigated. In situ observations of vertical temperature profiles are first used to quantify temperature variability with depth near the ocean surface. The analysis shows that there is a sharp vertical temperature gradient within the top 10 m of the ocean. Both diurnal and intraseasonal variabilities of the ocean temperatures are largest near the surface and decrease with the ocean depth. Numerical experiments with an oceanic general circulation model are next carried out with 1- and 10-m vertical resolutions for the upper ocean to study the dependence of the simulated SST and vertical temperature structure on the vertical resolution. It is found that the simulated diurnal and intraseasonal variabilities, as well as the associated vertical temperature gradient near the surface, are strongly influenced by the oceanic vertical resolution, with the 1-m vertical resolution producing a stronger vertical temperature gradient and temporal variability than the 10-m vertical resolution. These results suggest that a realistic representation of SST variability with a high vertical resolution in the upper ocean is required for a coupled atmosphere–ocean model to correctly simulate the observed tropical intraseasonal oscillations, including the Madden–Julian oscillation and the boreal summer monsoon intraseasonal oscillation, which are strongly linked with the underlying SST.

Profiling measurement of internal tide in the Bali Strait by reciprocal sound transmission

A reciprocal sound transmission experiment was carried out from 10 to 12 June 2015 along one-crossed-strait line in the Bali Strait with strong tidal current to measure the vertical section structures of range-averaged current and temperature at a 3 minutes interval. The five-layer structures of those parameters in the vertical sections were reconstructed by the regularized of travel time data for 2 rays. The hourly mean current showed a generation of nonlinear internal tide with amplitudes of (1.0—1.5)m/s and period of 6 hours, superimposed on semi-diurnal internal tide with amplitudes, decreasing from the upper to lower layer. The hourly mean temperature was characterized by variation with amplitude of (1.0-1.5) and period of 6 and 8 hours. Current variation revealed an out of phase relation between the upper and lower layer while temperature varied in phase for all five layers. The 2-day average current formed a stratified structure, varying from -0.6 to -0.1m/s and from 23.8 to 28, respectively. The five-layer current and temperature were significantly over the inversion errors. It is suggested that thermal stratification in the Bali strait was caused by intrusion of dense cold water from the Indian Ocean due to coastal upwelling.

Impact of Different Cumulus Parameterization Schemes in SAUDI-KAU AGCM

In global climate models (GCMs), the convection is parameterized, since the typical scale of this process is smaller than the model resolution. This study examines the impact of two different cumulus parameterization schemes on the simulated climate using single-column model (SCM) as well as in an atmospheric global climate model (AGCM). The two schemes used are: the Simplified Arakawa–Schubert (SAS) scheme; and the Emanuel scheme coupled with a probability distribution function-based cloud parameterization scheme (EMAN). The humidity, temperature, cloud fraction, and precipitation simulations are improved in EMAN as compared to that of SAS in SCM. Climatological simulations (1981-2014) conducted using an AGCM at a moderate resolution (T106L44: 1.125° × 1.125°) indicated that the use of the EMAN improved the results. The precipitation over the tropical belt also showed improvements in terms of the distributions, biases, and association with observation. These improvements are attributable to a better vertical structure of temperature, especially in the tropics, due to the more realistic estimation of the temperature and moisture fields by the EMAN. The error estimated in outgoing long-wave radiation for EMAN is lower than that of the SAS. The vertical structure of specific humidity and temperature shows less error in EMAN as compared to SAS. Results using the SCM and AGCM reveal the benefits of using the EMAN in comparison to the SAS which includes better simulation of the relative humidity, temperature, and precipitation fields.

Modes of vertical thermodynamic and wind variability over the Maritime Continent

Abstract. The Maritime Continent (MC) is an exceedingly complex region from the perspective of both its meteorology and its aerosol characteristics. Convection in the MC is ubiquitous and assumes a wide variety of forms under the influence of an evolving large-scale dynamic and thermodynamic context. Understanding the interaction between convective systems and their environment, both individually and in the aggregate, requires knowledge of the dominant patterns of spatial and temporal variability. Ongoing cloud model ensemble studies require realistic perturbations to the atmospheric thermodynamic state to devise system sensitivities. Apart from modeling studies, evanescent signals in the tropical system are obscured by the underlying broad-scale meteorological variability, which if constrained could illuminate fine-scale physical processes. To this end, radiosonde observations from 2008 to 2016 are examined from three upper-air sounding sites within the MC for the purpose of exploring the dominant vertical temperature, humidity, and wind structures in the region. Principal component analysis is applied to the vertical atmospheric column to transform patterns present in radiosonde data into canonical thermodynamic and wind profiles for the MC. Both rotated and non-rotated principal components are considered, and the emerging structure functions reflect the fundamental vertical modes of short-term tropical variability. The results indicate that while there is tremendous spatial and temporal variability across the MC, the primary modes of vertical thermodynamic and wind variability in the region can be represented in a lower-dimensional subspace. In addition, the vertical structures are very similar among different sites around the region, though different structures may manifest more strongly at one location than another. The results indicate that, while different meteorology may be found in various parts of the MC at any given time, the processes themselves are remarkably consistent. The ability to represent this variability using a limited number of structure functions facilitates analysis of covariability between atmospheric structure and convective systems and also enables future systematic model-based ensemble analysis of cloud development, convection, and precipitation over the MC.

Seasonal variations in the vertical structure of temperature and salinity fields in the shallow Baltic Sea off the Kaliningrad Region coast

Seasonal variations in the vertical structure of temperature and salinity fields in the shallow Baltic Sea off the Kaliningrad Region coast

Bias Correction for Retrieval of Atmospheric Parameters from the Microwave Humidity and Temperature Sounder Onboard the Fengyun-3C Satellite

The microwave humidity and temperature sounder (MWHTS) on the Fengyun (FY)-3C satellite measures the outgoing radiance from the Earth’s surface and atmospheric constituents. MWHTS, which makes measurements in the isolated oxygen absorption line near 118 GHz and the vicinity of the strong water vapor absorption line around 183 GHz, can provide fine vertical distribution structures of both atmospheric humidity and temperature. However, in order to obtain the accurate soundings of humidity and temperature by physical retrieval methods, the bias between the observed and simulated radiance calculated by the radiative transfer model from the background or first guess profiles must be corrected. In this study, two bias correction methods are developed through the correlation analysis between MWHTS measurements and air mass identified by the first guess profiles of the physical inversion; one is the linear regression correction (LRC), and the other is the neural network correction (NNC), representing the linear and nonlinear relationships between MWHTS measurements and air mass, respectively. The correction methods have been applied to MWHTS observed brightness temperatures over the geographic area (180° W–180° E, 60° S–60° N). The corrected results are evaluated by the probability density function of the differences between corrected observations and simulated values and the root mean square errors (RMSE) with respect to simulated observations. The numerical results show that the NNC method has better performance, especially in MWHTS Channels 1 and 7–9, whose peak weight function heights are close to the surface. In order to assess the effects of bias correction methods proposed in this study on the retrieval accuracy, a one-dimensional variational system was built and applied to the MWHTS brightness temperatures to estimate the atmospheric temperature and humidity profiles. The retrieval results also show that NNC has better performance. An indication of the stability and robustness of the NNC method is given, which suggests that the NNC method has promising application perspectives in the physical retrieval.

Application of 3-D ensemble variational data assimilation to a Baltic Sea reanalysis 1989–2013

A 3-D ensemble variational (3DEnVar) data assimilation method has been implemented and tested for oceanographic data assimilation of sea surface temperature (SST), sea surface salinity (SSS), sea ice concentration (SIC), and salinity and temperature profiles. To damp spurious long-range correlations in the ensemble statistics, horizontal and vertical localisation was implemented using empirical orthogonal functions. The results show that the 3DEnVar method is indeed possible to use in oceanographic data assimilation. So far, only a seasonally dependent ensemble has been used, based on historical model simulations. Near-surface experiments showed that the ensemble statistics gave inhomogeneous and anisotropic horizontal structure functions, and assimilation of real SST and SIC fields gave smooth, realistic increment fields. The implementation was multivariate, and results showed that the cross-correlations between variables work in an intuitive way, for example, decreasing SST where SIC was increased and vice versa. The profile data assimilation also gave good results. The results from a 25-year reanalysis showed that the vertical salinity and temperature structure were significantly improved, compared to both dependent and independent data.

Paleolimnological evidence of the consequences of recent increased dissolved organic carbon (DOC) in lakes of the northeastern USA

As a result of reductions in sulfate deposition and changing climate, dissolved organic carbon (DOC) concentrations have increased in many lakes situated in forests of northeastern North America and northern Europe since the 1990s. Although this increase is well documented, the associated ecological implications remain unclear. In particular, DOC strongly influences the vertical temperature structure of lakes, with increasing DOC often leading to a shallower epilimnion. We investigated the effect of increased DOC concentrations on lake thermal structure using fossil diatom records from six remote Maine lakes. Sedimentary diatom profiles from three pairs of small (<0.5 km2) lakes were compared, with each pair containing one lake with a documented significant increase in DOC and the other experiencing no change in DOC since the early 1990s. Lake thermal structure was inferred from changes in the relative abundance of Discostella stelligera and Aulacoseira species, two diatom taxa that are associated with changes in thermal stratification. The three lakes without increasing DOC showed no change in diatom-inferred mixing depth over the past few decades. Of the lakes with documented increases in DOC, two showed the highest turnover in diatom community structure over time. Profiles from both of these lakes also indicated directional change in diatom-inferred mixing depth over the past 20 years, but the direction of change differed. This study demonstrates that recent increases in DOC have the potential to alter the physical and biological structure of lakes, but that these responses may differ across lakes.

On the secondary eyewall formation of Hurricane Edouard (2014).

A first observationally-based estimation of departures from gradient wind balance during secondary eyewall formation is presented. The study is based on the Atlantic Hurricane Edouard (2014). This storm was observed during the National Aeronautics and Space Administration's (NASA) Hurricane and Severe Storm Sentinel (HS3) experiment, a field campaign conducted in collaboration with the National Oceanic and Atmospheric Administration (NOAA). A total of 135 dropsondes are analyzed in two separate time periods: one named the secondary eyewall formation period and the other one referred to as the decaying-double eyewalled storm period. During the secondary eyewall formation period, a time when the storm was observed to have only one eyewall, the diagnosed agradient force has a secondary maxima that coincides with the radial location of the secondary eyewall observed in the second period of study. The maximum spin up tendency of the radial influx of absolute vertical vorticity is within the boundary layer in the region of the eyewall of the storm and the spin up tendency structure elongates radially outward into the secondary region of supergradient wind, where the secondary wind maxima is observed in the second period of study. An analysis of the boundary-layer averaged vertical structure of equivalent potential temperature reveals a conditionally unstable environment in the secondary eyewall formation region. These findings support the hypothesis that deep convective activity in this region contributed to spin up of the boundary layer tangential winds and the formation of a secondary eyewall that is observed during the decaying-double eyewalled storm period.

Field observations on water temperature and stratification in a seasonally ice-covered shallow thermokarst lake

In order to better understand the thermodynamic characteristics of seasonally ice-covered shallow lake, the freezing and melting processes, water/ice temperature variation, as well as meteorological conditions were measured in a typical thermokarst lake in central Qinghai-Tibet Plateau from October 2010 until July 2013. The temporal variations of lake temperature, development of thermocline, and the effects of lake on lake water thermal structure were investigated. Results indicated that a significant surface sublimation/ablation took place over the entire season. The daily and seasonal vertical structures of the lake temperature are significantly dominated by the change of air temperature, surface radiative fluxes, wind speed, thermodynamics, and heat fluxes from the lake bottom sediment. During the cycle of ice free-freezing onset-ice growing-melting onset-melting breakoff-ice free, the lake temperature takes up an annual cycle of stratification-overturning-inversion stratification-inversion-normal stratification-overturning-stratification. The thermal stratification consists only of epilimnion (mixing layer) and thermocline. Both layers can be replaced by a strong mixing process caused by strong gusts. Thus, compared with large lakes, the seasonal ice-covered shallow lakes have quite different thermodynamic characteristics.

Turbulent mixing in the upper ocean of the northwestern Weddell Sea, Antarctica

Turbulent mixing in the upper ocean (30–200 m) of the northwestern Weddell Sea is investigated based on profiles of temperature, salinity and microstructure data obtained during February 2014. Vertical thermohaline structures are distinct due to geographic features and sea ice distribution, resulting in that turbulent dissipation rates (e) and turbulent diffusivity (K) are vertically and spatially non-uniform. On the shelf north of Antarctic Peninsula and Philip Ridge, with a relatively homogeneous vertical structure of temperature and salinity through the entire water column in the upper 200 m, both e and K show significantly enhanced values in the order of O(10–7)–O(10–6) W/kg and O(10–3)–O(10–2) m2/s respectively, about two or three orders of magnitude higher than those in the open ocean. Mixing intensities tend to be mild due to strong stratification in the Powell Basin and South Orkney Plateau, where e decreases with depth from O(10–8) to O(10–9) W/kg, while K changes vertically in an inverse direction relative to e from O(10–6) to O(10–5) m2/s. In the marginal ice zone, K is vertically stable with the order of 10–4 m2/s although both intense dissipation and strong stratification occur at depth of 50–100 m below a cold freshened mixed layer. Though previous studies indentify wind work and tides as the primary energy sources for turbulent mixing in coastal regions, our results indicate weak relationship between K and wind stress or tidal kinetic energy. Instead, intensified mixing occurs with large bottom roughness, demonstrating that only when internal waves generated by wind and tide impinge on steep topography can the energy dissipate to support mixing. In addition, geostrophic current flowing out of the Weddell Sea through the gap west of Philip Passage is another energy source contributing to the local intense mixing.

Preliminary results of assessing the mixing of wave transport flux residualin the upper ocean with ROMS

The effects of the mixing of wave transport flux residual (Bvl) on the upper ocean is studied through carrying out the control run (CR) and a series of sensitive runs (SR) with ROMS model. In this study, the important role of Bvl is revealed by comparing the ocean temperature, statistical analysis of errors and evaluating the mixed layer depth. It is shown that the overestimated SST is improved effectively when the wave-induced mixing is incorporated to the vertical mixing scheme. As can be seen from the vertical structure of temperature 28°C isotherm changes from 20 min CR to 35 m in SR3, which is more close to the observation. Statistic analysis shows that the root-mean-square errors of the temperature in 10 m are reduced and the correlation between model results and observation data are increased after considering the effect of Bvl. The numerical results of the ocean temperature show improvement in summer and in tropical zones in winter, especially in the strong current regions in summer. In August the mixed layer depth (MLD) which is defined as the depth that the temperature has changed 0.5°C from the reference depth of 10 m is further analyzed. The simulation results have a close relationship with undetermined coefficient of Bvl, sensitivity studies show that a coefficient about 0.1 is reasonable value in the model.

Vertical temperature dynamics in the Northern Baltic Sea based on 3D modelling and data from shallow-water Argo floats

3D hydrodynamic models often produce errors in the depth of the mixed layer and the vertical density structure. We used the 3D hydrodynamic model NEMO to investigate the effect of vertical turbulence parameterisations on seasonal temperature dynamics in the Bothnian Sea, Baltic Sea for the years 2012 and 2013. We used vertical profiles from new shallow-water Argo floats, operational in the area since 2012, to validate our model. We found that NEMO was able to reproduce the general features of the seasonal temperature variations in the study area, when meteorological forcing was accurate. The k-ε and k-ω schemes were selected for a more detailed analysis. Both schemes showed clear differences, but neither proved superior. While sea surface temperature was better simulated with the k-ω scheme, thermocline depth was clearly better with the k-ε scheme. We investigated the effect of wave-breaking on the mixing of the surface layer. The Craig and Banner parameterisation clearly improved the representation of thermocline depth. However, further tuning of the mixing parameterisations for the Baltic Sea is needed to better simulate the vertical temperature structure. We found the autonomous Baltic Sea Argo floats valuable for model validation and performance evaluation.