Impacts of waves, sea spray, and the upper ocean on extratropical storms

A coupled atmosphere-wave-sea spray-ocean model system is used to study North Atlantic extratropical hurricanes to evaluate the combined impacts of wave drag, sea spray, and upper ocean sea surface temperature (SST) on storm development. Our focus is on storm intensity and development. We consider the role of air-sea fluxes and boundary layer/atmosphere implications. The composite model system consists of a mesoscale atmospheric model, an operational wave model, a recent parameterization for heat and momentum fluxes due to sea spray, and an advanced ocean circulation model. The atmospheric model is the Canadian MC2 model, the wave model is the operational NCEP model WaveWatch3, the sea spray parameterizations follows Andreas and DeCosmo (2002), and the ocean model is POM (Princeton Ocean Model).

Case studies are extratropical hurricanes Earl (1998), Danielle (1998), Gustav (2002), Juan (2003) and two intense winter bombs from January 2000 and 2002. Results show that, when wind speeds exceed 25 ms-1, sea spray can significantly increase the sea surface fluxes, especially the latent heat flux over the high-wind core of the hurricane region. Wave-induced drag is important during the rapid-development phase of the storm and decreases as the storm waves reach maturity. Thus, sea spray tends to intensify storms particularly during the storm peak when winds are maximal, whereas wave-related momentum drag takes energy from the storm, primarily during the rapid development phase of the storm. The impact of coupling to POM is the generation of sea surface temperature (SST) cooling through entrainment mixing at the bottom of the mixed layer with the passage of the storm. SST cooling reduces the sea surface heat fluxes compared to uncoupled simulations, which use a time-invariant SST. Reduced heat fluxes lead to reduced storm intensity. These also tend to become notable during the storm peak, when winds are maximal.

In high-wind midlatitude extratropical hurricanes, sea spray, by itself, can increase the storm intensity by as much as ~ 15%. By comparison, wave-related momentum drag can reduce storm intensity by as much as ~ 5-10%, and SST cooling by a further ~ 5-10%, when the mixed layer is thin in summer and autumn storms. The overall manner in which these processes combine depends on the developmental properties of the storm. For example, if the storm occurs in winter, when the mixed layer is quite deep, SST cooling will not occur. If the high-wind center of the storm tends to be geographically relatively far from central SLP, then the sea spray impacts will not be as strong as when these two centers are geographical close together. This is because of the dominant role of wind speed in the sea spray formulation.