Sensitivity of cyclogenesis to sea surface temperature in the northwesternAtlantic

During the Intensive Observation Period 15 (13-15 February 1997) of the FAS TEX Experiment, a major cyclone crossed the Atlantic Ocean from the Newfoun dland 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 degreesC) to warm water (15 degreesC). To elucidate the role of sea surface temperature (SST) and air-sea fluxes i n the dynamics of oceanic cyclones, three nonhydrostatic mesoscale simulati ons were performed. The first one is a control experiment with a realistic SST field describing in detail the oceanic front associated with the NAC sy stem. The two following simulations are sensitivity experiments where the S ST front is removed: the first one uses a uniformly cold SST equal to 3 deg reesC and the second one uses a uniformly warm SST equal to 15 degreesC. The frontogenetic function and the vertical velocity sources in the lower-a tmospheric layers of the three simulations were diagnosed. In the control simulation, the surface heat fluxes were found to be negativ e in the perturbation warm sector and positive in the region behind the col d front. As reported by numerous authors, this pattern of surface heating a nd cooling did not intensify the cyclone, except in the occlusion when the phasing with the SST front occurs. This configuration enhances the horizont al 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 affecte d 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 differ ent surface flux fields. There are several consequences: (i) the uniform SS Ts tend to decrease the cold front intensity of the wave, (ii) a weaker buo yancy flux source of vertical velocity is found above a uniform cold SST ac ross the occlusion in comparison with the control case, and (iii) surprisin gly, 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 f orcings. The thermal wind forcing and the thermal wind imbalance forcing were the mo st sensitive to the SST compared to the turbulent momentum forcing. This re sult means that (i) the feed back of the ageostrophic circulation induced b y the surface is greater on the kinematic forcings than on the turbulent fo rcings 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 forcin g than in the control case. This result is explained by the convective acti vity that increases the low-level convergence and vorticity spinup. This po int means that latent heat release and baroclinicity are in tight interacti on. In the first 12 h and at the scale of the simulation domain, the three cycl ones evolve similarly, but at a small scale their internal structures diver ge strongly and rapidly. The scale at which the surface turbulent fluxes ac t 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 a ttributed to the strong positive surface heat fluxes because they precondit ioned the MABL by moistening and heating the low levels during the incipien t stage of the cyclone development.