Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 4Bryden et al.Figure 4.5. Strength of the Atlantic MOC at 25ºN. derived from an ensemble average of threestate estimation models (solid curve), and the model spread (shaded), for the period 1962–2002(courtesy of the CLIVAR Global Synthesis and Observations Panel, GSOP). The estimates fromindividual hydrographic sections at 24ºN. (from Bryden et al., 2005), from the WOCE inverse modelestimates at 24ºN. (Ganachaud, 2003a; Lumpkin and Speer, 2007), and from the 2004–05 RAPID–MOC array at 26ºN. (Cunningham et al., 2007) are also indicated, with respective uncertainties.tion of the AMOC, from March 2004 to March2005, was 18.7 ± 5.6 Sv, with instantaneous(daily) values varying over a range of nearly10–30 Sv. The Florida Current, Ekman, andmid-ocean geostrophic transport were foundto contribute about equally to the variabilityin the upper ocean limb of the AMOC. Thecompensating southward flow in the deepocean (identical to the red curve in Figure 4.6but opposite in sign), also shows substantialchanges in the vertical structure of the deepflow, including several brief periods where thetransport of lower NADW across the entiresection (associated with source waters originatingin the Norwegian-Greenland Sea denseoverflows) is nearly, or totally, interrupted.These results show that the AMOC can, anddoes, vary substantially on relatively shorttime scales and that AMOC estimates derivedfrom one-time hydrographic sections arelikely to be seriously aliased by short-termvariability. Although the short-term variabilityof the AMOC is large, the standard error inthe 1-year RAPID estimate derived from theautocorrelation statistics of the time seriesis approximately 1.5 Sv (Cunningham et al.,2007). Thus, this technique should be capableof resolving year-to-tear changes in the annualmean AMOC strength of the order of 1–2 Sv.The 1-year (2004–05) estimate of the AMOCstrength of 18.7 ±1.5 Sv is consistent, withinerror estimates, with the corresponding valuesnear 26°N. determined from the WOCE inverseanalysis (16–18 ±2.5 Sv). It is also consistentwith the 2004 hydrographic section estimateof 14.8 ±6 Sv, which took place during the firstmonth of the RAPID time series (April 2004),during a period when the AMOC was weakerthan its year-long average value (Fig. 4.5).3.4 Time-Varying Ocean StateEstimationWith recent advances in computing capabilitiesand global observations from bothsatellites and autonomous in-situ platforms,132