1 Title: The evolution of the equatorial Pacific zonal SST gradient during the twentieth century 1 2 Authors: 3

Abstract: 23 Two popular reconstructions of observed sea surface temperature, HadISST1 and NOAA 24 ERSST (recently released version 3), are analyzed to clarify the evolution of the equatorial 25 Pacific zonal SST gradient over the period 1880-2005. A brief survey of recently published 26 analyses illustrates the widespread disagreement over the observed change in the SST gradient, 27 which has implications for its projected response to global warming. Differences exist between 28 the two reconstructions, but our straightforward analyses show that the observations allow us to 29 be confident of three things: (1) the western equatorial Pacific has been warming, (2) the eastern 30 equatorial Pacific has not been warming as much as the west, and (3) the equatorial Pacific zonal 31 SST gradient has not been weakening. Rather, the observations suggest that the mean SST in the 32 eastern equatorial Pacific has remained quite steady. Remaining uncertainties, however, prohibit 33 making a stronger statement such as that the SST gradient has been strengthening. 34 The observational results are compared with and discussed in the context of IPCC AR4 35 model simulations of the twentieth century climate and beyond. Five of the six models analyzed 36 indicate a strengthening of the zonal SST gradient over the 20th century, while the GFDL CM2.1 37 exhibits a substantial weakening of the zonal SST gradient caused by a large warming in the east. 38 Beyond the 20th century, there is no convergence of results among the six models regarding the 39 response of the zonal SST gradient to a continued increase of atmospheric CO2 concentration. 40