The nature of observed variations in temperature-salinity (T-S) relationship between El Ni¿o and non-El Ni¿o years in the pycnocline of the eastern equatorial Pacific Ocean (NINO3 region, 5¿S--5¿N, 150¿W--90¿W) is investigated using an ocean general circulation model. The origin of the subject water mass is identified using the adjoint of a simulated passive tracer. The higher salinity during El Ni¿o is attributed to larger convergence of saltier water from the Southern Hemisphere and smaller convergence of fresher water from the Northern Hemisphere.