Clear skies, subsequent to Hurricane Ivan's passage across the Gulf of Mexico in September 2004, provided a unique opportunity to investigate upper ocean responses to a major hurricane. Oceanic cyclonic circulation was rapidly intensified by the hurricane's wind field (59--62 m s-1), maximizing upwelling and surface cooling (3--7¿C) in two large areas along Ivan's track. Upward isothermal displacements of 50--65 m, computed from wind stress and sea surface height changes, caused rapid ventilation of thermoclines and nutriclines, leading to phytoplankton blooms with peak concentrations 3--4 days later. Wind speed changes along Ivan's track demonstrated that the cool waters (20--26¿C) provided immediate negative feedback to the hurricane's intensity. Although our study focused on a relatively small ocean area, it revealed that mesoscale cyclones, in addition to warm anticyclones, may play an important role in producing along-track hurricane intensity changes.