Marine heatwave impacts are seasonally amplified by ocean biogeochemical gradients

Marine heatwaves are becoming more frequent and intense under climate change, yet their effects on ocean productivity remain poorly understood. Previous studies have focused on where marine heatwaves occur, but the importance of when they occur has received less attention, particularly in polar regions where winter darkness limits conventional satellite observations. Here we show, using year-round satellite lidar observations that overcome this polar data gap, that the ecological impacts of marine heatwaves are strongly amplified by season. Across the Southern Hemisphere, marine heatwaves intensify the contrast between reduced net primary production at low and mid latitudes and enhanced net primary production at high latitudes, with the strongest responses occurring during austral summer. Positive net primary production anomalies reach 0.1–0.2 × 10¹¹ mg C at high southern latitudes, while lower latitudes experience marked declines. This seasonal amplification arises from contrasting nutrient limitations across latitudes and enhanced summer light availability. Our findings identify event timing as a critical determinant of marine ecosystem responses to climate extremes. The ecological impacts of marine heatwaves in the Southern Ocean are subject to a pronounced seasonal amplification which aligns with biogechemical ocean gradients, according to analysis of lidar data combined with model simulations