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Hervé Giordani and Guy Caniaux

Abstract

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°C) to warm water (15°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°C and the second one uses a uniformly warm SST equal to 15°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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Hervé Giordani and Serge Planton

Abstract

In the conventional quasigeostrophic (QG) form of the ω equation developed by Hoskins et al., the unique forcing of vertical velocity is the geostrophic deformation. As the QG or even the semigeostrophic (SG) hypotheses are not adapted to study the frontal dynamics in the atmospheric boundary layer, this paper proposes a generalized expression of the Hoskins et al. form of the vertical velocity. Two thermal and three dynamical sources of the vertical velocity are identified. These forcings allow for identification of each of the physical processes acting simultaneously on the ageostrophic circulation in the boundary layer. This new form of the ω equation is used to explain wind increase in the atmospheric boundary layer over the warm waters of the sea surface temperature (SST) front observed during a fair anticyclonic day of the SEMAPHORE experiment (1993) and simulated with a nonhydrostatic mesoscale atmospheric model. Since the SST gradients are weak (of the order of 1.5°C 100 km−1), the surface turbulent heat forcing is not a dominant factor and all the five forcings of vertical velocity have rather the same intensity.

In order to answer the question of how and over what thickness does the oceanic thermal front disturb significantly the atmospheric flow in the marine atmospheric boundary layer in such conditions, the degree of coupling between the turbulent heat forcing and the net forcing directly linked to the atmospheric flow is examined. Their strong anticorrelations (r < −0.9) below 200 m indicate that the ageostrophic circulation and the turbulent heat fluxes are in interregulation in this atmospheric layer, which can be assimilated to an internal boundary layer for the flow. This interregulation works in such a fashion to minimize the atmosphere thermal wind imbalance through an adaptation of the atmospheric flow, but also, to some extent, of the surface turbulent heat fluxes themselves.

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Hervé Giordani, Guy Caniaux, and Louis Prieur

Abstract

A simplified oceanic model is developed to easily perform cheap and realistic mesoscale simulations on an annual scale. This simplified three-dimensional oceanic model is obtained by degenerating the primitive equations system by prescribing continuously analysis-derived geostrophic currents U g into the momentum equation in substitution of the horizontal pressure gradient. Simplification is provided by a time sequence of U g called guide, which is used as a low-resolution and low-frequency interpolator. This model is thus necessarily coupled to systems providing geostrophic currents—that is, ocean circulation models, analyzed/reanalyzed fields, or climatologies. In this model, the mass and currents fields are constrained to adjust to the geostrophic guide at all scales. The vertical velocity is deduced from the vorticity equation, which ensures the coherence between the vertical motion and the geostrophic structures evolution. Horizontal and vertical advection are the coupling processes that can be activated independently from each other and offer the possibility to (i) continuously derive a three-dimensional model when all processes are activated, (ii) understand how some retroaction loops are generated, and (iii) study development of structures as a function of the geostrophic environment. The model was tested during a 50-day lasting simulation over the Program Océan Multidisciplinaire Méso Echelle (POMME) experiment (northeast Atlantic Ocean, September 2000–October 2001). Optimal analyzed geostrophic currents were derived weekly during POMME from a quasigeostrophic model assimilating altimeter data. Comparison with independent in situ and satellite data indicates that this simulation is very realistic and does not drift, thanks to the prescribed geostrophic guide.

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Virginie Guemas, David Salas-Mélia, Masa Kageyama, Hervé Giordani, and Aurore Voldoire

Abstract

This study investigates the nonlinear processes by which the ocean diurnal variations can affect the intraseasonal sea surface temperature (SST) variability in the Atlantic Ocean. The Centre National de Recherches Météorologiques one-dimensional ocean model (CNRMOM1D) is forced with the 40-yr ECMWF Re-Analysis (ERA-40) surface fluxes with a 1-h frequency in solar heat flux in a first simulation and with a daily forcing frequency in a second simulation. This model has a vertical resolution of 1 m near the surface. The comparison between both experiments shows that the daily mean surface temperature is modified by about 0.3°–0.5°C if the ocean diurnal variations are represented, and this correction can persist for 15–40 days in the midlatitudes and more than 60 days in the tropics. The so-called rectification mechanism, by which the ocean diurnal warming enhances the intraseasonal SST variability by 20%–40%, is found to be robust in the tropics. In contrast, in the midlatitudes, diurnal variations in wind stress and nonsolar heat flux are shown to affect the daily mean SST. For example, an intense wind stress or nonsolar heat flux toward the atmosphere during the first half of the day followed by weak fluxes during the second half result in a shallow mixed layer. The following day, the preconditioning results in heat being trapped near the surface and the daily mean surface temperature being higher than if these diurnal variations in surface forcings were not resolved.

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Frédéric Marin, Guy Caniaux, Hervé Giordani, Bernard Bourlès, Yves Gouriou, and Erica Key

Abstract

A comparison of June 2005 and June 2006 sea surface temperatures in the eastern equatorial Atlantic exhibits large variability in the properties of the equatorial cold tongue, with far colder temperatures in 2005 than in 2006. This difference is found to result mainly from a time shift in the development of the cold tongue between the two years. Easterlies were observed to be stronger in the western tropical Atlantic in April–May 2005 than in April–May 2006, and these winds favorably preconditioned oceanic subsurface conditions in the eastern Atlantic. However, it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid-May 2005 over a broad region extending from 20°W to the African coast and from 6°S to the equator. This particular event underscores the ability of local intraseasonal wind stress variability in the Gulf of Guinea to initiate the cold tongue season and thus to dramatically impact the SST in the eastern equatorial Atlantic. Such intraseasonal wind intensifications are of potential importance for year-to-year variability in the onset of the African monsoon.

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Alice Pietri, Xavier Capet, Francesco d’Ovidio, Marina Levy, Julien Le Sommer, Jean-Marc Molines, and Hervé Giordani

Abstract

The quasi-geostrophic and the generalized omega equations are the most widely used methods to reconstruct vertical velocity (w) from in-situ data. As observational networks with much higher spatial and temporal resolutions are being designed, the question rises of identifying the approximations and scales at which an accurate estimation of w through the omega equation can be achieved and what are the critical scales and observables needed. In this paper we test different adiabatic omega reconstructions of w over several regions representative of main oceanic regimes of the global ocean in a fully eddy-resolving numerical simulation with a 1=60o horizontal resolution. We find that the best reconstructions are observed in conditions characterized by energetic turbulence and/or weak stratification where near-surface frontal processes are felt deep into the ocean interior. The quasi-geostrophic omega equation gives satisfactory results for scales larger than ~ 10 km horizontally while the improvements using a generalized formulation are substantial only in conditions where frontal turbulent processes are important (providing improvements with satisfactory reconstruction skill down to ~ 5 km in scale). The main sources of uncertainties that could be identified are related to processes responsible for ocean thermal wind imbalance (TWI), which is particularly difficult to account for (especially in observation-based studies) and to the deep flow which is generally improperly accounted for in omega reconstructions through the bottom boundary condition. Nevertheless, the reconstruction of mesoscale vertical velocities may be sufficient to estimate vertical fluxes of oceanic properties in many cases of practical interest.

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