Professor Myles Allen CBE FRS

Abstract:

In recent years, altimetric measurements of sea surface height (SSH) and infrared radiometric measurements of sea surface temperature (SST) have provided a wealth of information about ocean circulation and atmosphere/ocean interactions. We use SST data from the Along-Track Scanning Radiometer (ATSR) to examine the relationship between SST and SSH anomalies (from temporal means). It is found that zero-lag spatial cross correlations between SST and SSH anomalies are surprisingly strong (∼0.7) in specific geographical locations associated with mesoscale variability. The correlations are seasonal, being strongest in winter months and weakest in summer months. A scale analysis of the correlations suggests that they are mainly due to features with wavelengths ∼600 km, implying that large meanders in ocean fronts and/or mesoscale eddies could be responsible.

Abstract:

Singular spectrum analysis (SSA) provides a robust method of separating an arbitrary signal from "white" (independent, identically distributed) noise. In the presence of "coloured" noise, or any autocorrelated process, high-variance components of the noise can confuse the singular value decomposition, thereby obscuring genuine signals which are, in principle, detectable. A generalization of SSA is presented which yields both an optimal filter to discriminate against an arbitrary coloured noise and an objective method of quantifying uncertainty in signal reconstruction. The algorithm is applied to a simple synthetic signal-separation problem and used to resolve a degeneracy in the SSA of interannual and interdecadal variability of the Earth's global mean temperature. © 1997 Elsevier Science B.V.

Abstract:

The weather at middle latitudes is largely unpredictable more than a week or so in advance, whereas fluctuations in the ocean may be predictable over much longer timescales. If decadal fluctuations in North Atlantic sea surface temperature could be predicted, it might be possible to exploit their influence on the atmosphere to forecast decadal fluctuations in climate. Here we report analyses of shipboard observations that indicate significant decadal predictability of North Atlantic sea surface temperature, arising from the advective propagation of sea-surface-temperature anomalies and the existence of a regular period of 12-14 years in the propagating signals. The same timescale can be identified in a dipole-like pattern of North Atlantic sea-level pressure variability. We propose a mechanism which may connect these oceanic and atmospheric fluctuations, possibly as part of a coupled ocean-atmosphere mode of variability. Our results are encouraging for the prospects of forecasting natural fluctuations in the climate of the North Atlantic region several years in advance.