Bora event variability and the role of air‐sea feedback

Abstract

[1] A two-way interacting high resolution numerical simulation of the Adriatic Sea using the Navy Coastal Ocean Model (NCOM) and Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS 1 ) was conducted to improve forecast momentum and heat flux fields, and to evaluate surface flux field differences for two consecutive bora events during February 2003. (COAMPS 1 is a registered trademark of the Naval Research Laboratory.) The strength, mean positions and extensions of the bora jets, and the atmospheric conditions driving them varied considerably between the two events. Bora 1 had 62% stronger heat flux and 51% larger momentum flux than bora 2. The latter displayed much greater diurnal variability characterized by inertial oscillations and the early morning strengthening of a west Adriatic barrier jet, beneath which a stronger west Adriatic ocean current developed. Elsewhere, surface ocean current differences between the two events were directly related to differences in wind stress curl generated by the position and strength of the individual bora jets. The mean heat flux bias was reduced by 72%, and heat flux RMSE reduced by 30% on average at four instrumented over-water sites in the two-way coupled simulation relative to the uncoupled control. Largest reductions in wind stress were found in the bora jets, while the biggest reductions in heat flux were found along the north and west coasts of the Adriatic. In bora 2, SST gradients impacted the wind stress curl along the north and west coasts, and in bora 1 wind stress curl was sensitive to the Istrian front position and strength. The two-way coupled simulation produced diminished surface current speeds of $12% over the northern Adriatic during both bora compared with a one-way coupled simulation.Citation: Pullen, J., J. D. Doyle, T. Haack, C. Dorman, R. P. Signell, and C. M. Lee (2007), Bora event variability and the role of air-sea feedback,

Modeling the ocean and atmosphere during an extreme bora event in northern Adriatic using one-way and two-way atmosphere–ocean coupling

“…Our numerical experiments are consistent with earlier findings (Zore-Armanda and Gačić, 1987;Paklar et al, 2001;Kuzmić et al, 2007;Pullen et al, 2007;Malačič et al, 2012) in the sense that a prolonged bora generates a double cyclonic-anticyclonic gyre system in the northern Adriatic. A double gyre is related to the wind stress curl in the northern Adriatic, arising from pronounced Trieste and Senj jets to the north and south of Istria and moderate to low wind speeds in between.…”

“…This means that the initial conditions for ALADIN2w do not contain oceanographic information from the previous POM2w run; only OSTIA analyses are used for SST in the ALADIN assimilation cycle. The full two-way ocean-atmosphere assimilation system would consist of a fully coupled ocean-atmosphere model for integration during the data assimilation cycle of ALADIN as well: such a strategy was successfully implemented in (Pullen et al, 2007) and its implementation is left for future work.…”

On the onset of bora and the formation of rotors and jumps near a mountain gap

“…Smith, 1987;Klemp and Durran, 1987;Grubišić, 1989;Jurčec and Glasnović, 1991;Bajić, 1991). Recent investigations conducted since the Mesoscale Alpine Programme (MAP) have focused more specifically on three-dimensional and small-scale aspects of bora winds: the role of mountain gaps in the formation of shear lines and potential-vorticity banners (Grubišić, 2004); the influence of gravity-wave breaking on the formation of surface wakes and jets (Jiang and Doyle, 2005); the role of boundary-layer separation in the development of gap winds (Gohm and Mayr, 2005); the dynamics and predictability of bora turbulence and gusts (Belušić and Klaić, 2004;Belušić and Klaić, 2006); the sensitivity of bora winds to the sea-surface temperature (Cesini et al, 2004;Kraljević and Grisogono, 2006); and the impact of the bora flow on ocean currents and air-sea interactions Pullen et al, 2006;Pullen et al, 2007;Dorman et al, 2006;Kuzmić et al, 2006).…”

Summer breakout of trapped bottom dense water from the northern Adriatic

“…[14] Quality and relatively high resolution of the ECMWF boundary conditions allows us to choose a relatively small (in comparison with Pullen et al [2006Pullen et al [ , 2007) outer domain. Sensitivity simulations, as well as other highresolution simulations of the bora flow using COAMPS coupled with the ECMWF model [Belušić et al, 2007], confirm the validity of the chosen setup.…”

“…[5] The wintertime dynamics (bora and NADDW generation) has been extensively studied using the data and numerical modeling throughout the last two decades by numerous investigators [e.g., Zore-Armanda and Gačić, 1987;Bergamasco et al, 1999;Beg Paklar et al, 2001, 2005Loglisci et al, 2004;Pullen et al, 2006Pullen et al, , 2007Dorman et al, 2007], especially after the comprehensive experiments carried out in 2002/2003 [Jeffries and Lee, 2007;Kuzmić et al, 2006]. On the contrary, existing studies of the atmosphere-ocean interactions in the northern Adriatic in summertime have been mostly carried out in the frame of the various biogeochemical studies (e.g., mucilage aggregation [Supić et al, 2002;Cozzi et al, 2004;Grilli et al, 2005;Russo et al, 2005]).…”