We investigate thermohaline effects on the upper layer circulation of the North Pacific by carrying out numerical experiments. The Pacific circulation north of 30 ¿S is driven by wind forcing, thermohaline forcing at the sea surface, and Newtonian body forcing at the southern end of the model domain. The three driving agents for the circulation are incorporated step by step into the model to evaluate the role of each agent. The thermohaline forcing at the sea surface enhances the subsurface southward flow, which is the upper part of the layered deep Pacific meridional circulation. This flow suffers distortion but does not disappear, even when wind forcing is imposed. It follows the eastern and the southern perimeter of the subtropical gyre, becoming the southward western boundary undercurrent at low latitudes. The cross-gyre southward western boundary undercurrent to the tropics, which is indicated in the observed distribution of tracers, is associated with the thermohaline circulation. Thus it is found that at the upper thermocline depths in the subtropical and tropical regions the flows in the poleward and westward region are wind driven, and those in the equatorward and eastward region are thermohaline. The inflow into the tropical South Pacific and Atlantic of the Antarctic Intermediate Water may be accounted for by this mechanism. ¿ 1998 American Geophysical Union