A two-dimensional fine-scale atmospheric model was set across an idealized 80-km-wide east–west sea gulf at 60oN (‘Gulf of Finland’). During a moderate south-easterly basic flow in overcast early summer conditions, a robust low-level jet (LLJ) is formed, with diverging and relatively strong afternoon surface easterlies blowing along the cool gulf. The LLJ is caused primarily by inertial oscillation in space–time due to the large difference in roughness and stability over the land and the sea, but the surface easterlies are also enhanced by an anti-heat island circulation triggered by the slant basic flow across the cool sea gulf. The dynamics and details of the anti-heat island circulation are discussed. With warmer waters the primary LLJ relaxes and the anti-heat island circulation ceases while the enhanced vertical mixing still maintains strong surface winds over the warm sea, but with reduced cross-isobar angles. Sunny conditions enhance convection over land and may induce sea breezes. These may reinforce the LLJ and the associated surface easterlies over the sea.