A numerical study of tropical sea-air interactions using a cloud resolving model coupled with an ocean mixed-layer model

Tao et al. (1999) simulated thermodynamic quasi-equilibrium states ("climate" regimes) over a tropical ocean domain by using a cloud-resolving model (CRM; the Goddard Cumulus Ensemble model, GCE) in which large-scale atmospheric conditions and a constant sea surface temperature (SST) were prescribed. They found that the imposed large-scale horizontal winds and surface fluxes played a crucial role in determining tropical climate states. Accordingly, a warm/humid climate was associated with maintained strong vertical wind shears or strong surface winds when a well-organized convective system prevailed. On the other hand, a cold/dry climate was produced due to weak wind shears and weak surface winds, when horizontal winds were mixed by convective processes. As a continued study to Tao et al. (1999), Shie et al. (2000) further presented a detailed budget analysis for both temperature and moist as well as several specific characteristics that were found in diverse modeled climates. In a study examining multiscale coupled air-sea variations, Sui et al. (1997) applied TOGA COARE surface flux observations as external forcings that drove an ocean mixed-layer (OML) model. They suggested that to couple ocean mixing processes with atmospheric radiative-convective processes was necessary in order to better understand the relevant features of air-sea interactions.

In this paper, we are interested in a study of coupled sea-air phenomena that involve various tropical atmospheric climates and their interactions with ocean boundary layer. We developed a 2-D air-sea coupled model by implementing the 2-D OML model (Sui et al. 1997) to the 2-D GCE model (Tao et al., 1999), and performed several numerical simulations with various prescribed large-scale conditions. The focus of our study is then twofold. On one hand, we will present impacts on SST, mixed-layer depth, and fresh water flux by variously prescribed atmospheric large-scale components, such as atmospheric large-scale horizontal wind shear, surface wind, and radiation. On the other hand, a discussion on feedback of SST to the atmospheric radiative-convective system by affecting its cloud structures (well-organized stratiform clouds or erect convective clouds), climate regimes, and the involved specific characteristics, will also be presented.