Analyses of ship-based measurements of sea level pressure reveal a systematic weakening of the horizontal pressure gradient across the Pacific in the last fifty years. This reduction is also present in the NCAR/NCEP and ECMWF reanalysis sea level pressure products. The magnitude is estimated to be between 2% to 13%. This weakening is consistent with simulations from general circulation models when sea-surface temperatures are uniformly raised. It is also consistent with reductions of the large-scale subsidence over the eastern Pacific in the models. Since the reduction of vertical overturning circulation in the models can be explained through fundamental thermodynamic constraints on the atmospheric circulation, we postulate that the weakening of the sea-level pressure gradient is an intrinsic characteristic of the tropical atmosphere in a warmer climate, and the observed trend in the sea-level pressure provides an indirect evidence of the reduction of atmospheric vertical overturning circulation in the tropical Pacific. It is also pointed out that the weakening of the vertical overturning circulation does not mean the weakening of the hydrological cycle.