Atmospheric Response to SST anomalies. Part II: Background-state dependence, teleconnections and local effects in summer

Abstract

In this paper and its companion, Part I, we explore the response of the atmosphere to sea-surface temperature anomalies in different geographical locations and seasons. In Part 1we focussed on northern-hemisphere winter (DJF) whereas in this paper, Part 2, we focus on summer (JJA) and inter-seasonal comparisons. We use two different configurations of the same idealised atmospheric model, constructed using two different configurations of continents and topography. These configurations give rise to slightly different background wind fields and variability within the same season, and therefore give a measure of how robust a response is to small changes in the background-state. We characterise the types of responses that are found to SST anomalies in the midlatitudes and tropics in JJA, and compare these with the corresponding responses in DJF. We find that the responses to midlatitude SST anomalies in JJA are generally on a much smaller spatial scale than those in DJF. Responses in the tropical Pacific are much less dependent on season, although teleconnections between the tropical Pacific and the North Atlantic are not found in JJA as robustly as they are in DJF. Given insight from our model results, however, we do find some summer periods in reanalysis data where there is a strong association between the tropical Pacific and the summer North-Atlantic Oscillation. We discuss the reasons for these effects and the implications for Northern Hemisphere seasonal prediction in summer.