Observing System Simulation Experiments for the Atlantic Meridional Overturning Circulation

PIs: George Halliwell (NOAA/AOML), Carlisle Thacker (AOML/CIMAS), and Haoping Yang (AOML/CIMAS)

Collaborators: Robert Atlas (NOAA/AOML)

The long-term objective of this project is to design optimal observing system strategies to monitor changes in the Atlantic Meridional Overturning Circulation (AMOC), particularly changes that are potentially related to rapid climate change. The strategies include developing the capability of performing Observing System Simulation Experiments (OSSEs) and Observing System Experiments (OSEs) at NOAA/AOML and then using these systems to perform observing system design studies. A prototype OSSE system has been developed and tested at AOML using the HYCOM ocean model and the Sequential Evolutive Extended Kalman (SEEK) as the initial data assimilation method.

The initial OSSE experiment will use a medium resolution (0.72°) global HYCOM simulation as the nature run and a lower-resolution (1.44°) extended Atlantic Ocean model nested within the global model as the data-assimilative OSSE model. The domains of the two models are illustrated in sea surface height (SSH) maps (Figure 1). The Atlantic domain was extended southward to include the Antarctic Circumpolar Current, westward to include Drake Passage, and eastward to include the Algulhas retroflection region within the domain. This will enable the impact of observing systems deployed in the Southern Ocean on changes in the AMOC to be evaluated using OSSEs.

The initial two experiments that are being performed are designed to test the impact of extending ARGO profiles into the deep ocean, and to test the impact of zonal monitoring sections such as the RAPID/MOCHA section at 26.5°N. The global model was run from 1948 through 2010 forced by the NCEP atmospheric reanalysis and will also be used for scientific studies of the global overturning circulation and other ocean climate processes.

Figure 1. Sea Surface Height (SSH) maps for 29 February 2000 from the 0.72° global HYCOM simulation (left) and from the nested 1.44° extended Atlantic domain nested within the global HYCOM run (right).