Planetary Climate Dynamics

Abstract:

A laboratory investigation is presented with the aim of studying systematically the occurrence and characteristics of low-frequency variability of flows resulting from the interaction of a baroclinic flow with periodic bottom topography. Low-frequency variability within the baroclinic wave regime occurred in two distinct forms in separate regions of parameter space. One corresponded to the transition region between the baroclinic travelling and stationary wave regimes. It involved primarily an interaction between the drifting baroclinic waves and stationary components of the topographically forced wave. The resulting flow had characteristics similar to amplitude vacillation and had a time-scale of 30–60 annulus revolutions (days), which also corresponded to the wave drift period. A new regime of low-frequency amplitude vacillation was discovered in the transition region with the axisymmetric flow regime. As the complexity of the flow increased the period of the vacillation cycles grew to ∼100–180 “days”. This slower vacillation seemed to involve a cyclic enabling and disabling of nonlinear interactions between the forced stationary wave and the growing and azimuthally drifting wave, which in turn was linked to a decrease in mean flow shear. Subsequent chains of wave-wave interactions characterised the complex but robust oscillation phenomenon. The resulting behaviour has several features in common with some recent models of intraseasonal oscillations in the mid-latitude troposphere and with sudden stratospheric warmings.

Abstract:

The long-term warming from an anthropogenic increase in atmospheric CO2 is often assumed to be proportional to the forcing associated with that increase. This paper examines this linear approximation using a zero-dimensional energy balance model with a temperature-dependent feedback, with parameter values drawn from physical arguments and general circulation models. For a positive feedback temperature dependence, warming increases Earth's sensitivity, while greater sensitivity makes Earth warm more. These effects can feed on each other, greatly amplifying warming. As a result, for reasonable values of feedback temperature dependence and preindustrial feedback, Earth can jump to a warmer state under only one or two CO2 doublings. The linear approximation breaks down in the long tail of high climate sensitivity commonly seen in observational studies. Understanding feedback temperature dependence is therefore essential for inferring the risk of high warming from modern observations. Studies that assume linearity likely underestimate the risk of high warming.