Wavier jet streams driven by zonally asymmetric surface thermal forcing

Abstract

Recent studies have argued that global warming is responsible for a wavier jet stream, thereby driving midlatitude extreme flooding and drought. Polar amplification-the relative enhancement of high-latitude temperatures under global warming-is argued to be the principal climate state driving midlatitude extremes. Namely, the decreased meridional temperature gradient suppresses the mean zonal winds, leading to wavier midlatitude jets. However, although observations are consistent with such a linkage, a detailed dynamical mechanism is still debated. Here, we argue that the Northern Hemisphere land-sea thermal forcing contrast that underlies zonally asymmetric forcing drives a response in the planetary geostrophic motion, which provides balanced mean fields for synoptic eddies in midlatitudes and thus for wavier jet streams. We show that when the barotropic zonal mean wind U is smaller than a threshold, proportional to the β-plane effect and dry static stability, the flow field exhibits a dramatic transition from a response confined near the surface to one reaching the upper atmosphere. As global warming enhances polar amplification, the midlatitude jet stream intensity is suppressed. The confluence of these effects leads to wavier jet streams.

Keywords: Arctic amplification; planetary geostrophic motion; wavier jet stream; zonally-asymmetric thermal forcing.

Fig. 1.

The response of planetary geostrophic motion to thermal forcing when $U>4H^2{\beta }_{L}S$, with $G(x)=\text{cos}(2x),\hspace{0.17em}F(x)=\text{sin}(2x)/2$, k_s = 2, H = 0.5, ${\beta }_L=1.0$, S = 1.0, and U = 1.5. (A–E) The pressure field, ${\varphi }_L$ (A); the vertical wind, w_L (B); the meridional wind, v_L (C); and the total pressure field $P_L=-Uy+{\varphi }_L$ at (D) the surface (z = 0) and (E) the top of atmosphere (z = 1). Note that the flow is confined to a surface region less than approximately H.

Fig. 2.

The response of planetary geostrophic motion to surface thermal forcing when $U<4H^2{\beta }_{L}S$. Here U = 0.5 and the other parameters are the same as in Fig. 1. (A–E) The pressure field, ${\varphi }_L$ (A); vertical wind, w_L (B); meridional wind, v_L (C); and the pressure field $P_L=-Uy+{\varphi }_L$ at (D) the surface (z = 0) and (E) the top of atmosphere (z = 1).

Fig. 3.

The response of planetary geostrophic motion to thermal forcing confined near the surface when $U>4H^2{\beta }_{L}S$ and U = 1.5, with the other parameters the same as in Fig. 1, but note the difference in the vertical scale. (A–E) The pressure field, ${\varphi }_L$ (A); vertical wind, w_L (B); meridional wind, v_L (C); and the total pressure field $P_L=-Uy+{\varphi }_L$ at (D) the surface (z = 0) and (E) the tropopause (z = 1).

Fig. 4.

The response of planetary geostrophic motion to surface thermal forcing when $U<4H^2{\beta }_{L}S$. Here U = 0.5 and the other parameters are the same as in Fig. 1. (A–E) The pressure field, ${\varphi }_L$ (A); vertical wind, w_L (B); meridional wind, v_L (C); and the total pressure field $P_L=-Uy+{\varphi }_L$ at the (D) surface (z = 0) and (E) top of the atmosphere (z = 1).