NSFGEO-NERC: Large-Scale Atmospheric Circulation Response to Oyashio Extension Frontal Variability

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Winter over the North Pacific features an endless parade of frontal storms, forming near Japan and building as they cross the ocean to reach North America. Model simulations can produce realistic North Pacific storms even if the sea surface temperatures (SSTs) are held fixed, indicating that the storms owe their existence to the strong upper-level jet stream and accompanying north-south atmopsheric temperature contrast rather than air-sea interactions. Nevertheless there are intriguing hints that storm behavior can be influenced by variations in the underlying SSTs, and some indications that slowly varying SSTs may change storm paths and behavior in ways that could prove useful for making long-range weather predictions. The SST variations of interest occur as a result of the meandering of the Kuroshio-Oyashio Extension (KOE), a narrow ocean current that extends into the Pacific from the coast of Japan. The KOE is a warm current and the sharp temperature contrast between the current and the cold water north of it produces relatively strong SST variations when the KOE shifts to the north or south.Attempts to identify and analyze the influence of KOE-related SST variations on storms and atmospheric circulation have had mixed results. But recent work by the Principal Investigators (PIs) and others suggests that the key to capturing the atmospheric response is to dramatically increase the resolution of atmospheric models used for this purpose. In particular the PIs have developed a variable resolution version of the Community Atmosphere Model version 6 (VR-CAM6) which takes advantage of the spectral element dynamical core to increase resolution over the North Pacific (a box from 20N to 60N and 130E to 110W). The lower resolution outside the North Pacific allows the model to be run cheaply enough to allow large ensembles of simulations, with 60 or more separate simulations for each imposed SST anomaly pattern, so that even weak atmospheric responses can be captured and analyzed.A further asset in identifying the atmospheric response to KOE-related SST variations is a simplified ocean model which allows the near-surface ocean to respond to changes in surface winds and other atmospheric forcing. The model is referred to as a "pencil" ocean model, or PenOM, because it represents the up and down transfer of heat in a column of the ocean below each grid box of the surface atmosphere. PenOM does not simulate the lateral movement of water in ocean currents, but work under this award adds the capability to represent changes in SST due to Ekman transport, meaning the sideways drift caused by the Coriolis force acting on water pushed along by the surface wind.The work is of societal as well as scientific interest due to its relevance to long-term weather prediction as noted above. Also, the PenOM configuration with Ekman transport will be made available to the worldwide research community through the Community Earth System Model as part of its suite of simpler model configurations. The work promotes international collaboration in atmospheric research through its collaboration with Dr. Arnaud Czaja at Imperial College London, through the arrangement noted below. Also, the project supports several outreach activities including the production of a short educational video featuring model simulations and research results. The project also provides support and training to a postdoctoral research associate.This project is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. The Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own country.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.