Eddy Response to Changes in Jet Characteristics

The current scientific understanding is that two types of jets exist in the atmosphere, a subtropical jet (STJ) and an eddy-driven jet (EDJ). Two different mechanisms are responsible for the existence of these jets. The STJ is primarily driven by advection of planetary angular momentum by the thermally direct Hadley cell circulation, and eddies usually act to weaken this jet. On the other hand, the EDJ is driven by eddy momentum flux convergence (EMFC), which accelerates the zonal mean westerly wind. We investigate the sensitivity of eddies to changes in the jets' amplitude and position in a dry primitive equation general circulation model. A modified Newtonian relaxation scheme that has a very short relaxation time for the mean state, and a long relaxation time for eddies is used. This scheme allows obtaining any zonal symmetric temperature distribution, and is used to systematically modify the jets' amplitude and location. It is found that eddies are more sensitive to changes in the amplitude of the EDJ than in the amplitude of the STJ. Furthermore, when the EDJ is shifted poleward, eddies tend to increase. In addition, it is demonstrated that when reconstructing the mean temperature profile above the northern Pacific for different months, a midwinter minimum in the eddy kinetic energy is obtained, and we link the minimum to the poleward shift of the jet in transition seasons relative to winter.