The Sensitivity of the Jet Stream Response to Climate Change to Radiative Assumptions

Abstract

Comprehensive climate models exhibit a large spread in the magnitude of projected poleward eddy‐driven jet shift in response to warming. The spread has been connected to the radiative response to warming. To understand how different radiative assumptions alone affect the jet shift in response to warming, we introduce a new clear‐sky longwave radiation hierarchy that spans idealized (gray versus four bands; without or with interactive water vapor) through comprehensive (correlated‐k) radiation in the same general circulation model. The new hierarchy is used in an aquaplanet configuration to explore the impact of radiation on the jet stream response to warming, independent of mean surface temperature and meridional surface temperature gradient responses. The gray radiation scheme produces a split jet and its eddy‐driven jet shifts equatorward as climate warms, whereas the storm track shifts equatorward then poleward. Including four longwave bands leads to a merged jet that shifts slightly poleward with warming, and the storm track shifts monotonically poleward. Including interactive water vapor makes the poleward jet shift comparable to the jet shift with comprehensive radiation and interactive water vapor. These jet and storm track differences are linked to the radiation response of the stratospheric temperature, the tropopause height, and the meridional gradient of the radiative forcing to warming. Dynamically, the equatorward jet shift with the gray scheme is consistent with reduced wave reflection on the poleward flank of the jet, whereas the poleward jet shift with the other schemes is consistent with increased eddy length scale that favors equatorward wave propagation.

Mechanisms of Future Predicted Changes in the Zonal Mean Mid-Latitude Circulation

“…Prescribed diabatic perturbations require knowledge of the equilibrium response to increased CO 2 , which is shaped by the circulation. The perturbations, including their meridional and vertical extent, have not been shown to occur in the absence of the circulation response, i.e., in radiative-convective equilibrium either without [60] or with the climatological circulation. Thus, it is possible the meridional extension of the prescribed diabatic heating into the subtropics is shaped by the circulation and encodes the circulation response.…”

“…Thus, it is possible the meridional extension of the prescribed diabatic heating into the subtropics is shaped by the circulation and encodes the circulation response. In order for tests involving prescribed diabatic perturbations to be useful the perturbations must be shown to occur in response to increased CO 2 in radiative-convective equilibrium either without [60] or with the climatological circulation.…”

“…Future progress in understanding the zonal mean midlatitude circulation response to increased CO 2 relies on (1) carefully designed climate model experiments that attempt to falsify the thermodynamic starting points and assumptions underlying the proposed mechanisms and (2) subsequently using the remaining mechanisms to create emergent constraints. Several approaches can be used to test mechanisms, for example, (1) imposing diabatic perturbations based on the response to increased CO 2 in radiative-convective equilibrium either in the absence [60] or presence of the climatological circulation, (2) quantifying the transient evolution of the circulation response to an abrupt CO 2 increase [9,11,17,36,42,67,68], (3) imposing latitudinally dependent CO 2 concentration [54], (4) quantifying the seasonality of the mechanisms, and (5) nudging parameters to their climatological value, e.g., turning off wind induced surface heat exchange [43]. Emergent constraints (see [19]) are needed in order to quantify and compare the modeled mechanisms to those in the real atmosphere.…”

Imagining Simpler Worlds to Understand the Complexity of Our Own

“…As explored by Tan et al (), the circulation would respond rather differently on each planet. On the first, with the gray atmosphere, the midlatitude jet streams and storm tracks would initially shift equatorward in response to enhanced greenhouse gas concentrations.…”

“…Tan et al () were not motivated by recent discoveries of exoplanets: Understanding the response of the storm tracks on these hypothetical worlds has direct relevance to climate change on Earth. We have little confidence in our climate model projections of the atmospheric circulation response to global warming, particularly for the midlatitude jet streams and storm tracks (e.g., Shepherd, ; Vallis et al, ).…”