This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF ...Re-Analysis) surface fluxes with a 1-h frequency in solar heat flux (6 h for the other forcing fields) over the 1959-2001 period. The model has 124 vertical levels with a vertical resolution of 1 m near the surface and 500 m at the bottom. This ocean forced experiment is used to assess the impact of the North Atlantic weather regimes on the surface ocean. Composites of sea surface temperature (SST) anomalies associated with each weather regime are computed and the mechanisms explaining these anomalies are investigated. Then, the SST anomalies related to each weather regime in the ocean-forced experiment are prescribed to the ARPEGE Atmosphere General Circulation Model. We show that the interaction with the surface ocean induces a positive feedback on the persistence of the Blocking regime, a negative feedback on the persistence of the NAO-regime and favours the transition from the Atlantic Ridge regime to the NAO-regime and from the Atlantic Low regime toward the Blocking regime.
During the Intensive Observation Period 15 (13-15 February 1997) of the FASTEX Experiment, a major cyclone crossed the Atlantic Ocean from the Newfoundland Basin to southern Iceland. Its surface low ...center deepened by 17 hPa in 7 h when the perturbation crossed the North Atlantic Current (NAC) from cold (3 degree C) to warm water (15 degree C). To elucidate the role of sea surface temperature (SST) and air-sea fluxes in the dynamics of oceanic cyclones, three nonhydrostatic mesoscale simulations were performed. The first one is a control experiment with a realistic SST field describing in detail the oceanic front associated with the NAC system. The two following simulations are sensitivity experiments where the SST front is removed: the first one uses a uniformly cold SST equal to 3 degree C and the second one uses a uniformly warm SST equal to 15 degree C. The frontogenetic function and the vertical velocity sources in the lower-atmospheric layers of the three simulations were diagnosed. In the control simulation, the surface heat fluxes were found to be negative in the perturbation warm sector and positive in the region behind the cold front. As reported by numerous authors, this pattern of surface heating and cooling did not intensify the cyclone, except in the occlusion when the phasing with the SST front occurs. This configuration enhances the horizontal gradient of surface buoyancy flux across the occlusion, which increases the buoyancy flux source of vertical velocity (w). When the SST front is removed, the surface heat fluxes are strongly affected in magnitude and in spatial variability. The marine atmospheric boundary layer (MABL) stability, the convective activity, the warm advection in the core of the wave, and the heating depth are strongly affected by the different surface flux fields. There are several consequences: (i) the uniform SSTs tend to decrease the cold front intensity of the wave, (ii) a weaker buoyancy flux source of vertical velocity is found above a uniform cold SST across the occlusion in comparison with the control case, and (iii) surprisingly, a weaker w buoyancy flux source is also found above a uniform warm SST because of a higher heating depth. Vertical velocity depends not only on the buoyancy flux forcing but also on the thermal wind, the turbulent momentum, and the thermal wind imbalance forcings. The thermal wind forcing and the thermal wind imbalance forcing were the most sensitive to the SST compared to the turbulent momentum forcing. This result means that (i) the feed back of the ageostrophic circulation induced by the surface is greater on the kinematic forcings than on the turbulent forcings and (ii) the turbulent momentum forcing does not play a crucial role in cyclogenesis. Above a uniform warm SST, the strongest intensity of the occlusion is due to the strongest w thermal wind forcing and w thermal wind imbalance forcing in the MABL, in spite of a weaker w buoyancy flux forcing than in the control case. This result is explained by the convective activity that increases the low-level convergence and vorticity spinup. This point means that latent heat release and baroclinicity are in tight interaction. In the first 12 h and at the scale of the simulation domain, the three cyclones evolve similarly, but at a small scale their internal structures diverge strongly and rapidly. The scale at which the surface turbulent fluxes act on the dynamics of marine cyclones is therefore important. Finally, the cyclone simulated in the warm SST case developed more rapidly than those simulated in the real and the cold SST cases. This behavior is attributed to the strong positive surface heat fluxes because they preconditioned the MABL by moistening and heating the low levels during the incipient stage of the cyclone development.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
A one-dimensional model is used to analyze, at the local scale, the response of the equatorial Atlantic Ocean under different meteorological conditions. The study was performed at the location of ...three moored buoys of the Pilot Research Moored Array in the Tropical Atlantic located at 10° W, 0° N; 10° W, 6° S; and 10° W, 10° S. During the EGEE-3 (Etude de la circulation océanique et de sa variabilité dans le Golfe de Guinee) campaign of May-June 2006, each buoy was visited for maintenance during 2 days. On board the ship, high-resolution atmospheric parameters were collected, as were profiles of temperature, salinity, and current. These data are used here to initialize, force, and validate a one-dimensional model in order to study the diurnal oceanic mixed-layer variability. It is shown that the diurnal variability of the sea surface temperatures is mainly driven by the solar heat flux. The diurnal response of the near-surface temperatures to daytime heating and nighttime cooling has an amplitude of a few tenths of degree. The computed diurnal heat budget experiences a net warming tendency of 31 and 27 W m⁻² at 0° N and 10° S, respectively, and a cooling tendency of 122 W m⁻² at 6° S. Both observed and simulated mixed-layer depths experience a jump between the nighttime convection phase and the well-stabilized diurnal water column. Its amplitude changes dramatically depending on the meteorological conditions occurring at the stations and reaches its maximum amplitude (~50 m) at 10° S. At 6° and 10° S, the presence of barrier layers is observed, a feature that is clearer at 10° S. Simulated turbulent kinetic energy (TKE) dissipation rates, compared to independent microstructure measurements, show that the model tracks their diurnal evolution reasonably well. It is also shown that the shear and buoyancy productions and the vertical diffusion of TKE all contribute to the supply of TKE, but the buoyancy production is the main source of TKE during the period of the simulation.
Abstract Current feedback (CFB) and thermal feedback (TFB) have been shown to strongly influence both atmospheric and oceanic dynamics at the oceanic mesoscale (10–250 km). At smaller scales, oceanic ...submesoscale currents (SMCs; 0.1–10 km) have a major influence on the ocean’s energy budget, variability, and ecosystems. However, submesoscale air–sea interactions are not well understood because of observational and modeling limitations related to their scales. Here, we use a realistic submesoscale-permitting coupled oceanic and atmospheric model to quantify the spatiotemporal variability of TFB and CFB coupling in the northwest tropical Atlantic Ocean. While CFB still acts as a submesoscale eddy killer by inducing an energy sink from the SMCs to the atmosphere, it appears to be more efficient at the submesoscale by approximately 30% than at the mesoscale. Submesoscale CFB affects the surface stress, however, the finite time scale of SMCs for adjusting the atmospheric boundary layer results in a diminished low-level wind response, weakening partial ocean reenergization by about 70%. Unlike at the mesoscale, submesoscale CFB induces stress/wind convergence/divergence, influencing the atmospheric boundary layer through vertical motions. The linear relationship between the surface stress derivative or wind derivative fields and sea surface temperature gradients, widespread at the mesoscale, decreases by approximately 35% ± 7% or 77% ± 10%, respectively, at the submesoscale. In addition, submesoscale TFB induces turbulent heat fluxes comparable to those at the mesoscale. Seasonal variability in meso- and submesoscale CFB and TFB coupling is mostly related to background wind speed. Also, disentangling submesoscale CFB and TFB is challenging because they can reinforce or counteract each other.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The aim of the Programme Océan Multidisciplinaire Méso Echelle (POMME) was to study the formation and subduction of 11°–13°C waters in the northeast Atlantic (21°–15°W and 38°–45°N). An extensive ...oceanic and atmospheric data set was collected over 1 year during the period September 2000–October 2001. Owing to the importance of energy and water exchanges between the top layers of the ocean and the atmosphere in the subduction process, a surface heat, freshwater, and momentum budget has been computed combining the use of satellite products, in situ data, and atmospheric model outputs. This data set has been compared and validated with observations collected from a moored buoy and an instrumented mast onboard a research vessel. Each component of the net heat, freshwater, and momentum flux has been individually evaluated, and turbulent fluxes were computed with a state‐of‐the‐art bulk flux algorithm deduced from turbulence measurements made during the experiment. We have adopted a 5 km grid spacing to take into account the oceanic mesoscale variability. The annual domain‐averaged heat flux is positive (+33 W m−2), indicating a heating of the ocean, whereas model estimates (European Centre for Medium‐Range Weather Forecasts (ECMWF) and the French operational weather forecast model, ARPEGE) indicate a negative (cooling) budget (−9 W m−2 and −25 W m−2, respectively). Sensitivity tests of the parameterization used and the sea surface temperature used place the accuracy of the budget to about 10 W m−2. The freshwater budget is negative, implying a freshening of the ocean, as in the ECMWF model. Our assessment proves that sea surface temperature patterns condition the mesoscale patterns of the heat budget, a feature that is not reproduced by models.
A modeling study of physical processes occurring in an area of the northeast Atlantic (21.33°–15.33°W, 38.00°–45.00°N) that was extensively sampled during the Programme Océan Multidisciplinaire Méso ...Echelle (POMME) (October 2000–September 2001) is carried out. The model is a mesoscale version of the ocean general circulation model OPA developed at the Laboratoire d'Océanographie Dynamique et de Climatologie in Paris. It is used in a three‐dimensional limited area domain with a high‐resolution grid (approximately 5 km horizontal spacing, 69 vertical levels) and realistic boundary conditions (initial state, air‐sea fluxes, open boundary fluxes, and bottom topography). The objectives of the study are to properly simulate the upper ocean dynamics, particularly mesoscale activity and mixed layer evolution, during a key period (restratification) of the POMME experiment (POMME 1 and POMME 2, from February to May 2001) and to compare model results with oceanographic observations collected during the experiment in order to establish confidence in the model. Some results provided by the high‐resolution simulation, in particular features related to mixed layer depth and vertical velocities, are also presented. There is no pronounced north‐south mixed layer depth gradient, but strong filament‐shaped structures associated with stirring at the periphery of eddies are present. Mixed layer restratification is simulated. It is associated with submesoscale mixed layer depth structures and intense vertical velocity filaments in the upper ocean correlated with the relative vorticity gradient field.
The near-sea surface meteorological conditions associated with the Mediterranean heavy precipitation events constitute, on a short time scale, a strong forcing on the ocean mixed layer. This study ...addresses the question of the optimal time frequency of the atmospheric forcing to drive an ocean model in order to make it able to capture the fine scale ocean mixed layer response to severe meteorological conditions. The coupling time frequency should allow the ocean model to reproduce the formation of internal low-salty boundary layers due to sudden input of intense precipitation, as well as the cooling and deepening of the ocean mixed layer through large latent heat fluxes and stress under the intense low-level jet associated with these events. In this study, the one-dimensional ocean model is driven by 2.4-km atmospheric simulated fields on a case of Mediterranean heavy precipitation, varying the time resolution of the atmospheric forcing. The results show that using a finer temporal resolution than 1 h for the atmospheric forcing is not necessary, but a coarser temporal resolution (3 or 6 h) modifies the event course and intensity perceived by the ocean. Consequently, when using a too coarse temporal resolution forcing, typically 6 h, the ocean model fails to reproduce the ocean mixed layer fine scale response under the heavy rainfall pulses and the strong wind gusts.
► Oceanic coastal observations are used to study mixing processes. ► The atmospheric heating signal and its downward propagation can be isolated in those. ► Sensitivity studies are conducted using ...the K-profile parameterisation (KPP). ► The bottom boundary layer is responsible for the most important part of mixing. ► Here, the nonlocal effects in KPP have to be switched off for a better agreement.
In this article, the authors first present oceanic observations collected in a coastal area in May 2007. The evolution of temperature profiles exhibits a very clear atmospheric heating signal and is used to study mixing. Modelled atmospheric fluxes are evaluated using the oceanic measurements. The K-profile parameterisation (KPP) is chosen to identify the most important mixing processes and its parameters are tuned to minimise differences with respect to the observations.
It is found that:
• the tuned KPP is able to accurately represent the effect of mixing in this case;
• surface and bottom boundary layers, as well as interior shear instability mixing processes all play an important role in the observed evolution of the temperature profile, the bottom boundary being the source of the most intense mixing;
• the nonlocal effects in KPP (activated during nocturnal cooling periods) have to be switched off for a better agreement.
...the presence of the forcing Ddr means that the ageostrophic circulations in the MABL are not stationary and this result was confirmed by an analysis of the convective clouds evolution performed ...from the Meteosat and Advanced Very High Resolution Radiometer satellital images (Lambert et al. 1997). ...there is no pure forcing of the surface on the atmosphere and inversely, but rather a mutual influence or self-regulation process between the turbulent fluxes and the circulation that plays here in the first 200 m above the surface.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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