Mid-Latitude Mesoscale Temperature Variability in Six Multiship XBT Surveys

Abstract

We discuss the spatial character of mesoscale temperature variability as observed in six midlatitude multiship expendable bathythermograph (XBT) surveys, two from the North Pacific and four from the North Atlantic. These surveys sample mid-ocean regions as well as regions adjacent to the western boundary currents of both subtropical gyres. Mesoscale characteristics exhibit substantial changes between surveys, between ship tracks within each survey and between different portions of individual ship tracks. These results suggest that a single XBT section across a region is unlikely to represent the range of variability within that region satisfactorily. Fluctuations below the mixed layer are strongly vertically correlated, but surface temperatures are not well correlated with those below the mixed layer. We concentrate on the horizontal variability of the temperature at 450 m, and present rms thermal variability values and zonal autocorrelation functions as conventional measures of the variability. We also discuss the utility of analysis perspectives that focus on the ?event-like? character of much of the variability and describe the properties of the different features in the data. Using climatology rather than a linear trend to remove the large scale can result in markedly different feature characteristics and survey statistics. In particular, warm and cold features (defined relative to climatology) often have quite different scales. Assuming that the mesoscale temperature variability is a Guassian process with 150 km decorrelation length is acceptable at the 95% level in several surveys using linearly detrended data, but is generally unacceptable when climatology is removed.