Although air temperatures rise early in summer, the ocean warms more slowly and typically reaches peak temperatures later in the season, often between August and September.
Why the sea heats slowly
Water has a high heat capacity and mixes vertically, so it absorbs and stores heat over weeks and months before surface temperatures reach their maximum. As a result the warmest sea temperatures usually lag peak air temperatures and occur near the end of summer.
When temperatures peak
In many temperate coastal regions sea surface temperatures climb through June and July and commonly peak in August or September after prolonged solar heating and reduced mixing from seasonal wind changes.
Benefits for recreation
Later summer warmth makes water more comfortable for swimming and recreation, extending the season for beach activities and coastal tourism without the chill of earlier summer months.
Ecological impacts
Warmer surface waters alter marine ecosystems by shifting plankton communities and the timing of blooms, a process that can disrupt food webs and the seasonal cycles of fish and invertebrates.
Algae growth and dissolved oxygen
Higher temperatures favour faster algal growth in many coastal waters, increasing the likelihood of large blooms and in some cases harmful algal events. As decomposition of bloom material accelerates it can deplete dissolved oxygen, stressing or killing marine life.
Practical notes
Bathing waters are often warmest late in summer, but swimmers and coastal managers should be aware that unusually warm seasons can coincide with algal blooms, low oxygen zones or marine heatwave conditions that affect water quality and ecosystem health.
Takeaway
Because the ocean stores heat and releases it slowly, sea temperatures usually peak at the end of summer. This benefits recreation but also increases ecological risks such as altered plankton dynamics, algal blooms and reduced dissolved oxygen, so monitoring and cautious use of coastal waters are important.