Island cloud trails represent only a small fraction of ocean boundary layer clouds. However, they can be an important key to understanding how marine boundary layer clouds respond to perturbation. During the La Ni¿a period of 1999--2001 the island cloud of Nauru demonstrated daytime persistence over 100 kilometers. Our results show that boundary layer clouds over the eastern margin of the warm pool are affected by regional subsidence and lack of CCN (Cloud Condensation Nuclei) during this active warm pool convection phase of the ENSO (El Nino Southern Oscillation). The increased persistence of boundary layer clouds during the day, as reflected by the island cloud trails, combined with the dissipation of boundary layer clouds at night contribute to cooling the ocean surface. During La Nina, boundary layer cloudiness around Nauru decreased by a factor of 1.9 and SSTs decreased by about 3¿C.