Ocean impact on decadal Atlantic climate variability revealed by sea-level observations

Abstract

Ocean impact on decadal Atlantic climate variability revealed by sea-level observations. Nature, 521 (7553). 508-510. 10.1038/nature14491 Contact NOC NORA team at publications@noc.soton.ac.ukThe NERC and NOC trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. properties and offer timeseries of sufficient length (Ext. Data Fig. 1) to study decadal 46 ocean circulation variations. Investigating ocean circulation using tide gauges is not new: 47 the first attempt to estimate the Gulf Stream using tide gauges was made in 1938 14 . The 48 principle is based on geostrophic dynamics: on timescales longer than a few days, ocean 49 circulation is in geostrophic balance so, looking downstream, the sea level is seen to 50increase from left to right in the northern hemisphere. 51 52Estimates of the Gulf Stream using tide gauges have focused on the use of gauges on the 53American east coast with an offshore estimate of sea level from either an island gauge 15 54 or a reconstructed sea level 16 . A weakness of this method is that the offshore 55 measurement lies in the eddy-filled ocean where sea-level fluctuations at any one point 56 are influenced by the mesoscale 17 even on long timescales, increasing the difficulty of 57 making estimates of ocean circulation that is coherent on large spatial scales. This is the 58 case for sea level at Bermuda, whose decadal fluctuations can be reproduced by 59 considering a Rossby wave response to wind forcing 16 . To make estimates of ocean 60 circulation that capture the fluctuations in large-scale circulation and less eddy variability, 61 measurements close to or on the western boundary are necessary 18 . We account for this 62 by focusing on the gradient of sea level along the US east coast. The mean dynamic sea 63 level decreases to the north along the east coast of the US (Fig. 1a) (Fig. 1a), we can construct a single sea-level composite 82 representative of the subtropical (subpolar) circulation by averaging sea-level from 83 linearly detrended, deseasonalised tide gauges, with the inverse barometer effect removed, 84 south (north) of the Cape (Fig. 1b, c). The difference, south minus north (Fig. 1d), 85 represents our circulation index. This index projects onto observed surface velocities 86 during the satellite era in the intergyre region, with a positive index associated with more 87 northwards flow and a more northerly path of this circulation (Extended Data Fig. 4). captured by the accumulated sea-level index, observationally supporting the hypothesis 113 6 that circulation changes and not only air-sea fluxes were involved in these changes 28 . For 114 the purposes of statistical analyses, the timeseries have had a 7-year low-pass, Tukey 115 filter applied to them, which is referred to with the prefix '7-year' from here on. The 7-116 year sea-level index leads the 7-year rate of heat content change by 2 years with a 117 maximum correlation of ...

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