| Summary: | This thesis investigates climate model errors in the simulation of the El Niño-Southern Oscillation (ENSO) and its teleconnections and their relevance to the ‘signal-to-noise paradox’ in climate predictions. ENSO is highly predictable on the seasonal timescale and it is a key driver of interannual climate variability across the globe, but its role in the signal-to-noise paradox is unclear. It is found that the simulated late winter Aleutian Low response to ENSO has an amplitude which is around half that observed in multiple seasonal forecasting systems. The response is found to be weak throughout the depth of troposphere during both El Niño and La Niña. In the case of El Niño, tropical precursors of the teleconnection are found to be accurate, and so the underestimated amplitude likely arises in the extratropics. The impact of increased ocean resolution on the simulation of ENSO and its late winter teleconnection to the North Pacific is tested using coupled free- running simulations of varying ocean resolution (from 1◦ to 0.25◦ ). It is found that increased resolution largely eliminates errors in the zonal extent of El Niño and La Niña SST anomalies, as well as reducing SST mean state biases in the tropical Pacific. SST asymmetry in the eastern Pacific also improves with increased resolution due to an increased probability of extreme El Niño events, but it remains underestimated. The position of the El Niño teleconnection is too far west in low resolution models, but in-line with the improvement in SST anomalies, this error is largely eliminated with increased ocean resolution. Weaker improvement is found for the La Niña teleconnection, which is attributed to tropical Pacific precipitation biases which persist even with increased resolution. The problem of underestimated teleconnection amplitude is not fully resolved with increased ocean resolution. The ratio of predictable components metric, often used to diagnose signal-to-noise errors, is expressed in terms of three key parameters: forecast correlation skill, ...
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