Initial Conditions and Quantifying Model Climates: Does it Matter Where We Come From?


There does not exist an agreed-upon procedure to quantify model climates from their output. Here, we explore differences in characterisations of regional variable climatologies arising from the application of three climate quantification approaches to the output of an initial condition ensemble climate system model experiment. We find that that ensemble member trajectories, distinguished in set-up only by the round-off order differences in initial atmospheric temperature, can—over certain regional domains and for particular initial system states—produce significantly (\(p\ll 0.01\)) different variable probability distributions. In addition, using different quantification approaches to capture what might be presumed to be the same “climatic state”—which itself may be influenced by the initial climate system state—can yield significantly different distributions. We conclude that a multivariate distribution, sampled over both time and multiple ensemble members, together with measures of autocorrelation, may serve as a useful quantification approach for model climates.


Many definitions have been proposed for climate in the literature (e.g., Lorenz, 1995; Werndl, 2015; Lovejoy, 2013; IPCC, 2013; WMO, 2010). For many of the central concepts in climate science, there do not appear to be any widely accepted definitions (Todorov, 1986; Daron, 2012; Werndl, 2015). One might argue that definitions used are broadly similar, that there appears to be consensus on the intuitive idea of climate (Leith, 1985) and that the descriptions used are generally “good enough” for the specific contexts in which they are applied. However, Lorenz (1995) notes that “certain questions regarding climate may be answered either affirmatively or negatively, according to the precise [definition of climate used]”. Furthermore, Lorenz (1995) suggests that in different contexts—across which the nature of the available data varies—the definition that would lead to the most meaningful characterisation of a given climatic state, may differ (see also Schneider et al., 1974). In particular, definitions of climate which are applicable in observational studies, are not necessarily the most useful in theoretical or modelling studies (Lorenz, 1995; Schneider et al., 1974; Leith, 1978).

Of particular interest in this work are various climate quantifications—definitions which, when applied, provide a quantitative characterisation of a “model climate” (also