An Investigation of the Effect of Ocean Mesoscale Variability on the Dynamics of the North Pacific Jet Stream and Storm Track

This talk presents the results of an investigation of the effects of ocean mesoscale SST variability on the midlatitude atmospheric flow in the North Pacific. The investigation is based on numerical experiments that were carried out with the NCAR CESM, using a configuration in which the atmospheric global circulation model was thermodynamically coupled to a slab ocean model. Two ensembles of model simulations were generated at a quarter-degree horizontal resolution. While the resolution was sufficient to resolve a significant part of the mesoscale SST variability, that variability was retained in only one of the two ensembles. The effect of mesoscale SST variability on the atmosphere is explored by comparing the different terms of the atmospheric eddy kinetic energy equation for the two ensembles. It is found that mesoscale SST variability enhances the upper tropospheric transfer of kinetic energy from the core of the polar jet to the atmospheric eddies in the Kuroshio Extension region. The enhanced transfer of kinetic energy weakens the core of the jet stream and leads to a reduction of the vertical wind shear (baroclinic instability) of the time-mean flow. While the static stability of the atmosphere is also weaker in the presence of ocean mesoscale variability, which compensates for some of the weakening of the baroclinic instability, the production of eddy kinetic energy by baroclinic energy conversion is reduced. The reduced generation of eddy kinetic energy in the Kuroshio Extension leads to a downstream reduction of the kinetic energy that can be transferred from the atmospheric eddies to the jetstream. Because the jetstream is weakened both in the Kuroshio Extension and downstream, mesoscale SST variability has an effect on the large scale flow that is similar to that of a drag force.