No abstract

Ageostrophic ocean processes such as frontogenesis, submesoscale mixed‐layer instabilities, shelf break fronts, and topographic interactions on the continental shelf produce surface‐divergent flows that affect buoyant material over time. This study examines the ocean processes leading to clustering, i.e., the increase of material density over time, on the ocean surface. The time series of divergence along a material trajectory, the Lagrangian divergence (LD), is the flow property driving clustering. To understand the impacts of various ocean processes on LD, numerical ocean model simulations at different resolutions are analyzed. Although the relevant processes differ, patterns in clustering evolution from the deep ocean and the continental shelf bear similarities. Smaller‐scale ocean features are associated with stronger surface divergence, and the surface material clustering is initially dominated by these features. Over time, the effect of these small‐scale features becomes bounded, as material traverses small‐scale regions of both positive and negative divergence. Lower‐frequency flow phenomena, however, continue the clustering. As a result, clustering evolves from initial small‐scale to larger‐scale patterns.

Recent technological advances over the past few decades have enabled the development of fully coupled atmosphere-ocean modeling prediction systems that are used today to support short-term (days to weeks) and medium-term (10-21 days) needs for both the operational and research communities. We overview the coupling framework, including model components and grid resolution considerations, as well as the coupling physics by examining heat fluxes between atmosphere and ocean, momentum transfer, and freshwater fluxes. These modeling systems can be run as fully coupled atmosphere-ocean and atmosphere-ocean-wave configurations. Examples of several modeling systems applied to complex coastal regions including Madeira Island, Adriatic Sea, Coastal California, Gulf of Mexico, Brazil, and the Maritime Continent are presented. In many of these studies, a variety of field campaigns have contributed to a better understanding of the underlying physics associated with the atmosphere-ocean feedbacks. Examples of improvements in predictive skill when run in coupled mode versus standalone are shown. Coupled model challenges such as model initialization, data assimilation, and earth system prediction are discussed.

In situ experimental data and numerical model results are presented for the Ligurian Sea in the northwestern Mediterranean. The Ligurian Sea Air-Sea Interaction Experiment (LASIE07) and LIGURE2007 experiments took place in June 2007. The LASIE07 and LIGURE2007 data are used to validate the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) 1 developed at the Naval Research Laboratory. This system includes an atmospheric sigma coordinate, nonhydrostatic model, coupled to a hydrostatic sigma-z-level ocean model (Navy Coastal Ocean Model), using the Earth System Modeling Framework (ESMF).A month-long simulation, which includes data assimilation in the atmosphere and full coupling, is compared against an uncoupled run where analysis SST is used for computation of the bulk fluxes. This reveals that COAMPS has reasonable skill in predicting the wind stress and surface heat fluxes at LASIE07 mooring locations in shallow and deep water. At the LASIE07 coastal site (but not at the deep site) the validation shows that the coupled model has a much smaller bias in latent heat flux, because of improvements in the SST field relative to the uncoupled model. This in turn leads to large differences in upper-ocean temperature between the coupled model and an uncoupled ocean model run.

Intraseasonal sea surface temperature anomalies generally cool during the convectively active phase of the intraseasonal oscillation in the Indian Ocean, but the behavior of intraseasonal ocean heat content anomalies is quite different. This is demonstrated using satellite observations and ocean reanalysis data. Ocean heat content anomalies increase during the convectively active phase of the intraseasonal oscillation and decrease during the convectively suppressed phase. Much of the intraseasonal variability of ocean heat content is westward propagating, moving in the opposite direction of the intraseasonal oscillation's convective envelope. While sea surface temperature anomalies are strongly regulated by variations in surface fluxes, their out of phase relationship with ocean heat content suggests that different processes are modulating the reservoir of warm water in the upper ocean. We hypothesize that oceanic equatorial waves are the primary forcing of intraseasonal ocean heat content anomalies during intraseasonal oscillation events. Plain Language SummaryThe intraseasonal oscillation organizes rainfall in the tropics and often initiates over the Indian Ocean. It is associated with strong surface winds and widespread cloudiness, which decreases incoming solar radiation, resulting in a cooling of the sea surface. We observe that while the sea surface cools, the reservoir of warm water in the upper Indian Ocean actually increases. This seeming contradiction between the behavior of the sea surface temperature and the ocean heat content is explained by oceanic equatorial wave dynamics. Much of the ocean heat content variability moves westward along lines of latitude that are just poleward of the equator, consistent with the location, timing, and direction of equatorial Rossby wave propagation. By increasing the ocean heat content, these waves are hypothesized to reduce the amount of sea surface cooling produced by the intraseasonal oscillation.