Projected Narrowing and Reinforcement of the NH Jet in Winter—Effects of Arctic and Tropical Processes

Projected changes in the mid-latitude atmospheric circulation at the end of the 21^st century are investigated using coupled ocean-atmosphere simulations from the Community Earth System Model Large Ensemble (CESM-LENS). Averaging over all 40 ensemble members gives robust results of the forced response in terms of waviness of the flow and the evolution of both the zonal-mean eddy-driven jet, as well as the sector-by-sector averages over the 21^st century. In winter there is no significant change in the zonal-mean zonal index, but sinuosity is decreased. Only over the North American sector does sinuosity increase in winter and the zonal index decrease (equivalent to a negative NAM response).

In the zonal average, the jet becomes sharper (narrower path) and stronger in winter, thereby increasing its Rossby-wave elasticity and resisting perturbations, consistent with decreased sinuosity. This is a signal that first becomes apparent around 2070 in the LENS. The response over the Atlantic and secondarily over the Pacific sector is driving this zonal mean response.

The ensemble-mean, zonal-mean temperature response reveals three “hot-spots” of change: upper-tropospheric tropical warming (UTW), Arctic amplification (AA) and polar lower-stratospheric warming (PST). There is a large spread in circulation changes due to internal variability between the 40 ensemble members. Causes for this spread are associated with AA in the North American sector and the PST over the Atlantic sector. The UTW is too ubiquitous to allow firm conclusions. Results from CMIP5 ensemble-mean show a similar jet evolution over the 21^st century over the Atlantic. Only over the American continent does AA appear to be the dominating process for the flow evolution