Physical oceanography

Developing a detailed circulation model of Palmyra Atoll based on physical oceanographic data from remote sensing and field measurements.

Through our work measuring and modeling characteristics of the waters around Palmyra Atoll (e.g., water temperature, currents, and waves) we aim to better understand how the physical environment influences the communities that live on and feed near the seafloor. Our work includes two primary tasks: compiling and synthesizing existing oceanographic data from remote sensing and field measurements; and development of a detailed circulation model that we will couple with the benthic model to explore the physical drivers of coral reef resilience.

Based on synthesis of existing field (e.g., temperature loggers and current meters in the water) and remotely sensed data (e.g., satellite and model-based currents, sea surface temperature, winds, and waves from NOAA and NASA), we have developed a working hypothesis of the dominant mechanisms for water circulation around Palmyra Atoll: the main circulation patterns on Palmyra’s reefs are primarily caused when waves push water over the reef. While winds create waves at the scale of an ocean, at the scale of an atoll like Palmyra, wind has only a minor effect on wave creation. In our work, therefore, we are separating out the direct effect of the wind on the surface from the large ocean swells coming from far away. The way this plays out at Palmyra is that winds tend to push flow to the west, while tidal flows serve to exchange flow approximately twice per day. We believe that the larger offshore currents from the North Equatorial Counter Current (NECC) have a secondary effect on the reef, but are important in some locations.

Development of our detailed circulation model is based on a coupled ROMS/SWAN program (click here for more information about these models). We have populated the model with data we collected from 64 instruments installed around Palmyra Atoll in September 2012. These instruments measure wave velocity, currents (from the surface to the seafloor), water temperature, and other physical properties of the sites other RTI researchers are working on. We also installed a weather station on Palmyra in September 2013, which automatically uploads data to the internet, including standard weather variables (e.g., wind, relative humidity, air and water temperature) and amount of sunlight (solar radiation), which is important for model heat calibration.

We are closely collaborating with the benthic modeling team to ensure that RTI’s two modelling efforts are compatible. A key task is identifying the relevant scales in both time and space for model coupling. When these models are working in concert, we will have a better understanding of how physical factors, like temperature and waves, cause changes in the communities living on the seafloor and, therefore, how they impact reef resilience.

The project is led by Dr. Rob Dunbar and Dr. Stephen Monismith from Stanford University, whose team includes Justin Rogers, David Koweek, and Isabella Arzeno.