Laura Feudale

On the Need of Intermediate Complexity General Circulation Models: A “SPEEDY” Example

processes that allows realistic and fast climate simula -tions that often involve large ensembles for the purpose of reducing uncertainty and estimation of the forced and internal variability of the system. The forced signal is typically estimated by an ensemble mean of many simulations, but ensembles of state-of-the-art models are often too small to reduce the remaining internal variability. The ensemble size needed to estimate the mean accurately depends on the signal-to-noise ratio for the variable and region under consideration. For example, the ensemble size to estimate midlatitude 500-hPa height accurately is about 20, which is larger than most ensembles used in seasonal hindcast data-sets or climate projections performed by individual centers. Intermediate complexity models can also be used efficiently to investigate the sensitivity of simu-lated climate to changes in parameters in the physical parameterizations. Another application is related to climate change. For example, Forest et al. (2002) and Sokolov at al. (2009) use the MIT Integrated Global System Model (MIT IGSM) to investigate topics such as climate sensitivity, aerosol forcing, ocean heat uptake rate, and probabilistic projections of climate change. There are many intermediate complexity system models of intermediate complexity (EMICs). A number of them are participating in the IPCC Fifth Assessment Report and can be found at http://climate .uvic.ca/EMICAR5 (one of which is based on a previous version of the model introduced here). This website also provides information about experiments that are performed with these models that range from en-sembles of 1,000-year-long historical simulations to the assessment of different CO

Seasonal Ensemble Predictions of West African Monsoon Precipitation in the ECMWF System 3 with a Focus on the AMMA Special Observing Period in 2006

Abstract The West Africa monsoon precipitation of the ECMWF operational Seasonal Forecast System (SYS3) is evaluated at a lead time of 2–4 months in a 49-yr hindcast dataset, with special attention paid to the African Monsoon Multidisciplinary Analysis (AMMA) special observation period during 2006. In both the climatology and the year 2006 the SYS3 reproduces the progression of the West Africa monsoon but with a number of differences, most notably a southerly shift of the precipitation in the main monsoon months of July and August and the lack of preonset rainfall suppression and sudden onset jump. The model skill at predicting summer monsoon rainfall anomalies has increased in recent years indicating improvements in the ocean analysis since the 1990s. Examination of other model fields shows a widespread warm sea surface temperature (SST) bias exceeding 1.5 K in the Gulf of Guinea throughout the monsoon months in addition to a cold bias in the North Atlantic, which would both tend to enhance rainfall over...

A common mode of variability of African and Indian monsoon rainfall at decadal timescale

There are many factors and mechanisms capable of influencing and perturbing rainfall in both African and Indian monsoon regions. Using observed data and ensembles of Atmospheric General Circulation Model simulations, evidence is presented that an association between the two systems exists on decadal timescales and the mechanism responsible for this common mode is suggested. Decadal variability of rainfall in the two monsoon systems results from a large scale forcing induced by an interplay of different ocean basins. The emerging pattern is characterized by warmer (cooler) equatorial and cooler (warmer) extratropical regions, more visible in the northern hemisphere. This large scale forcing pattern leads to an upper-level pressure gradient between the equator and the monsoon regions which modifies also the Tropical Easterly Jet, thus providing a potential link between the African and Indian monsoon. The response is baroclinic, therefore at low levels, the pressure gradient reverses and leads to increased (reduced) pressure over the Saharan and Indian region, both being favourable for a weakening (strengthening) of the respective monsoons. Therefore, the predictability of the monsoon trends depends mainly on how well the sea surface temperature modes, which modulate the monsoons variability, can be predicted.