02 August 2018

Drivers of Low Cloud Responses to Poleward Jet Shifts in Observations & Models

Jet shifts have little impact on total cloud cover because of attendant changes in the environment.

Science

Observed poleward shifts of the midlatitude jet stream do not cause substantial changes in total cloud cover or top-of-atmosphere radiation on interannual timescales. Whereas upper-level clouds shift poleward with the jet, low clouds increase dramatically across much of the North Pacific. This increase is primarily due to enhanced cold air advection. Global climate models largely fail to capture the observed responses of clouds & radiation to interannual jet shifts because they systematically underestimate how sensitive low clouds are to their environmental controls.

Impact

This work indicates that observed jet shifts do not cause reductions in total cloud cover, primarily because the environment in the region vacated by the jet becomes highly favorable for low clouds. Models systematically underestimate the observed increase in low clouds with cold advection, preventing them from capturing the observed insensitivity of cloud-radiative effects to jet shifts. The results suggest that jet shifts – in and of themselves – are not capable of causing substantial cloud feedbacks, in contrast to common expectations.

Summary

The long-standing expectation that poleward shifts of the midlatitude jet under global warming will lead to poleward shifts of clouds and a positive radiative feedback on the climate system has been shown to be misguided by several recent studies. On interannual timescales, free tropospheric clouds are observed to shift along with the jet, but low clouds increase across a broad expanse of the North Pacific Ocean basin, resulting in negligible changes in total cloud fraction and top-of-atmosphere radiation. Here it is shown that this low cloud response is consistent across eight independent satellite-derived cloud products. It is demonstrated that the spatial pattern and magnitude of the low cloud coverage response is primarily driven by anomalous surface temperature advection. In the Eastern North Pacific, anomalous cold advection by anomalous northerly surface winds enhances sensible and latent heat fluxes from the ocean into the boundary layer, resulting in large increases in low cloud coverage. Local increases in low-level stability make a smaller contribution to this low cloud increase. Despite closely capturing the observed response of large-scale meteorology to jet shifts, global climate models largely fail to capture the observed response of clouds and radiation to interannual jet shifts because they systematically underestimate how sensitive low clouds are to surface temperature advection, and to a lesser extent, low-level stability. More realistic model simulations of cloud-radiation-jet interactions require that parameterizations more accurately capture the sensitivity of low clouds to surface temperature advection.

Contact
Mark Zelinka
Lawrence Livermore National Laboratory (LLNL)
Publications
Zelinka, M, K Grise, S Klein, C Zhou, A DeAngelis, and M Christensen.  2018.  "Drivers of Low Cloud Responses to Poleward Jet Shifts in Observations & Models."  Journal of Climate, doi:10.1175/jcli-d-18-0114.1.