Structural decomposition of decadal climate prediction errors: A Bayesian approach

Decadal climate predictions use initialized coupled model simulations that are typically affected by a drift toward a biased climatology determined by systematic model errors. Model drifts thus reflect a fundamental source of uncertainty in decadal climate predictions. However, their analysis has so far relied on ad-hoc assessments of empirical and subjective character. Here, we define the climate model drift as a dynamical process rather than a descriptive diagnostic. A unified statistical Bayesian framework is proposed where a state-space model is used to decompose systematic decadal climate prediction errors into an initial drift, seasonally varying climatological biases and additional effects of co-varying climate processes. An application to tropical and south Atlantic sea-surface temperatures illustrates how the method allows to evaluate and elucidate dynamic interdependencies between drift, biases, hindcast residuals and background climate. Our approach thus offers a methodology for objective, quantitative and explanatory error estimation in climate predictions

Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere

Rainfall in the tropics is largely focused in a narrow zonal band near the Equator, known as the intertropical convergence zone. On average, substantially more rain falls just north of the Equator. This hemispheric asymmetry in tropical rainfall has been attributed to hemispheric asymmetries in ocean temperature induced by tropical landmasses. However, the ocean meridional overturning circulation also redistributes energy, by carrying heat northwards across the Equator. Here, we use satellite observations of the Earth's energy budget, atmospheric reanalyses and global climate model simulations to study tropical rainfall using a global energetic framework. We show that the meridional overturning circulation contributes significantly to the hemispheric asymmetry in tropical rainfall by transporting heat from the Southern Hemisphere to the Northern Hemisphere, and thereby pushing the tropical rain band north. This northward shift in tropical precipitation is seen in global climate model simulations when ocean heat transport is included, regardless of whether continents are present or not. If the strength of the meridional overturning circulation is reduced in the future as a result of global warming, as has been suggested, precipitation patterns in the tropics could change, with potential societal consequences.close9