Professor Myles Allen CBE FRS

Abstract:

Decadal fluctuations in the climate of the North Atlantic-European region may be influenced by interactions between the atmosphere and the Atlantic Ocean, possibly as part of a coupled ocean-atmosphere mode of variability. For such a mode to exist, a consistent atmospheric response to fluctuations in North Atlantic sea surface temperatures (SST) is required. Furthermore, this response must provide feedbacks to the ocean. Whether a consistent response exists, and whether it yields the required feedbacks, are issues that remain controversial. Here, these issues are addressed using a novel approach to analyze an ensemble of six integrations of the Hadley Centre atmospheric general circulation model HadAM1, all forced with observed global SSTs and sea-ice extents for the period 1949-93. Characterizing the forced atmospheric response is complicated by the presence of internal variability. A generalization of principal component analysis is used to estimate the common forced response given the knowledge of internal variability provided by the ensemble. In the North Atlantic region a remote atmospheric response to El Nino-Southern Oscillation and a further response related to a tripole pattern in North Atlantic SST are identified. The latter, which is most consistent in spring, involves atmospheric circulation changes over the entire region, including a dipole pattern in sea level pressure often associated with the North Atlantic oscillation. Only over the tropical/subtropical Atlantic, however, does it account for a substantial fraction of the total variance. How the atmospheric response could feed back to affect the ocean, and in particular the SST tripole, is investigated. Several potential feedbacks are identified but it has to be concluded that, because of their marginal consistency between ensemble members, a coupled mode that relied on these feedbacks would be susceptible to disruption by internal atmospheric variability. Notwithstanding this conclusion, the authors' results suggest that predictions of SST evolution could be exploited to predict some aspects of atmospheric variability over the North Atlantic, including fluctuations in spring of the subtropical trade winds and the higher latitude westerlies.

Abstract:

Recent validation studies have confirmed that the first along-track scanning radiometer (ATSR) can retrieve sea surface temperature (SST) to an accuracy of 0.3K even in the presence of heavy atmospheric aerosol. However, using the standard (pre-launch) retrieval, this accuracy is achieved only when data from all three thermal channels (3.7, 11 and 12 μm) are available; in the absence of 3.7 μm data, retrieved SSTs are subject to significant cold bias. As 3.7 μm data are useful only for nighttime observations, and ATSR's 3.7 μm channel failed in May 1992, only 11 and 12 μm data informed SST derivation for most of the 1991 - 1996 mission. We demonstrate the potential for improvement in this retrieval, based on comparison of observed brigthness temperatures with precise SSTs derived using 3.7 μm data. A reduction in global-mean cold bias from >0.6K to <0.1K is achieved, with standard deviation approximately halved. We also examine the treatment of optical pathlength variation around the ATSR swath.