Two-way coupling

Large image of ocean response to forcing

A snapshot from the two-way coupled Adriatic model (2 km resolution ocean/ 4-km resolution atmosphere) on September 29 2002 0600 UTC illustrates the Adriatic bora structure.

Bora Dynamics

Bora events are generated by synoptic pressure systems interacting with orography. During a bora event, cold northeasterly winds flow over the Dinaric Alps and result in large-amplitude gravity waves and wave breaking along the western slopes near the Adriatic shore. The strongest winds are found in the lee of mountain gaps, where wave breaking is minimized and bora “jets” are orographically accelerated. A gap in topography on the mainland, to the northeast of the Croatian Islands (close to this cross-section) accelerates flow across Kvarner Bay. There is also a wind speed maximum over the southern Gulf of Trieste in the far northern Adriatic. The highest terrain forces larger amplitude waves that overturn and break, thereby inducing a downstream “wake” of low-speed winds such as is found off of the Istrian Peninsula. The jets and wakes are organized in bands perpendicular to the axis of the Adriatic and possess a characteristic width less than 25 km (Grubisic, 2004). The alternating jets and wakes generate a wind stress curl pattern that drives a double gyre circulation in the ocean below - with a counter-clockwise cell dominating the northern coastal waters, and a clockwise circulation nestled against the Istrian Peninsula.

The rapidly moving air in the bora flow is typically 10°C colder than the sea, creating unstable atmospheric conditions. The near-surface air warms in the fast moving winds through air-sea interaction processes as it transits the basin in contact with the warmer sea temperatures. The atmospheric boundary layer (ABL) depth over the ocean is approximately 1 km. Above the bora flow, the unstable convective marine ABL is capped by a temperature inversion. In the ocean, large upward heat flux leads to a >20 m deep surface mixed layer as evidenced by the vertical temperature contours.

Air-Sea Coupling

A main goal of our work was to quantify and examine the impact of two-way coupled high-resolution coastal ocean temperatures on the overlying air properties. This necessitated verification that the two-way coupled SST generated during the course of the model simulation was realistic and reproduced well the observed SST spatial structure. We established the superior predictive skill of the two-way coupled SST via mean bias (MB) and root mean square error (RMSE) scores using satellite and in situ measurements of ocean temperature. Indeed, for the satellite data comparison MB was reduced by 45% while RMSE was reduced by 26% using two-way coupling. And for the in situ data, ocean temperature MB was reduced by 90% with RMSE reduced by 53% over one-way coupling. In absolute terms this represented a reduction in MB and RMSE of over 0.5°C relative to the one-way coupled simulation.

We found that mean wind speeds at four stations (three over-water and one coastal) in the northern Adriatic were systematically lower in the two-way coupled simulation relative to the one-way coupled simulation. The two-way coupled mean wind speeds were always closer to observed values and produced a 53% average reduction in MB for the four stations. The interaction of the air with comparatively cooler ocean temperatures on the west and north coasts reduced the local upward heat flux by approximately 20% in the two-way coupled simulation relative to the one-way coupled simulation. In those regions the two-way coupled SST had a relatively stabilizing effect by reducing mixing in the overlying atmosphere and slowing the winds and muting their variability. Although mean SST gradients were more diffuse than the instantaneous fields, this effect was apparent both during bora events and in the month-long mean and suggests the importance of air-sea interaction in shaping both episodic and long-term dynamics.

Pullen, J., J. D. Doyle, R. Hodur, A. Ogston, J. Book, H. Perkins, R. P. Signell, "Coupled ocean-atmosphere nested modeling of the Adriatic Sea during winter and spring 2001," Journal of Geohphysical Research, 108(C10), 3320, doi:10.1029/2003JC001780, 2003 (pdf)

Pullen, J., J. D. Doyle and R. P. Signell, “Two-way air-sea coupling: A study of the Adriatic,” Monthly Weather Review, 134(5), 1465–1483, 2006 (pdf)

Pullen, J., J. D. Doyle, C. Dorman, R. P. Signell, T. Haack, and C. M. Lee, "Bora event variability and the role of air-sea feedback," Journal of Geophysical Research, 112, C03S18, doi:10.1029/2006JC003726, 2007 (pdf)