Laminated sediments provide a rare opportunity to examine seasonal-scale ocean/atmosphere variation during abrupt climate transitions. Seasonal-scale changes through the transition from Heinrich Stadial 5 to Greenland Interstadial 12 are recorded in a sediment core (MD02-2515) from the Guaymas Basin, Gulf of California. The laminated sediments of the full interstadial have the highest opal concentrations (59.6 wt % biogenic silica) of the entire 55 kyr sequence and contain monospecific concentrations of the tropical oceanic diatom Azpeitia nodulifera that record flux from exceptional bloom events. Enhanced silica supply was likely initiated from the Southern Ocean during Heinrich Stadial 5, when Southern Hemisphere warming, via the bipolar seesaw mechanism, led to increased upwelling around Antarctica and an increase in silica supply to Subantarctic Mode Water formation. The resulting pulse of silica-rich waters was rapidly transferred north to the Equatorial Undercurrent and thence via the subsurface Mexican Coastal Current to the Gulf of California. Overall, the sequence shows a resurgence in El Niño strength and an intensification of the North American Monsoon associated with Northern Hemisphere warming on the transition to Greenland Interstadial 12. Penetration of tropical waters to the Gulf was aided by El Niño events. Diatom production in the summer stratified waters was in the subsurface, tapping nutrients from the nutricline. Marine varves up to 9 mm thick record as many as 8 within-year flux events. Repeated summer flux events of actively reproducing diatoms in subsurface blooms were driven by recurrent intense monsoonal Gulf surge storms. Around 60% of A. nodulifera cells examined were actively dividing and every stage of the cell division cycle is represented. Other diatom species record renewed vigour of winter – spring upwelling associated with strengthening northwesterly winds as the North Pacific high migrated northward. The monsoon intensification, the resurgence in El Niño strength and the strengthened winter-spring northwesterlies were likely all driven by the reduction to a minimum extent of the Laurentide Ice Sheet in Greenland Interstadial 12. The changes recorded in this transition to a warm interstadial may serve as an indicator of future changes in the region driven by global warming.