For many people this summer – especially those across Northland Auckland and Coromandel – showery days and bursts of heavy rain have become all too familiar.

This week, fresh downpours on already saturated ground have again triggered flood warnings and road closures across the upper North Island. These are individual weather events, but they are unfolding against unusually warm seas that load the atmosphere with extra moisture and energy.

Understanding ocean heat – and how it shapes rainfall, storms and marine heatwaves – is central to explaining what we experience on land.

For decades, scientists have recognised sea surface temperatures as a key influence on weather and climate. Warmer surfaces mean more evaporation, altered winds and shifting storm tracks.

But surface temperatures are only the skin of a deeper system. What ultimately governs how those sea surface temperatures persist and evolve is the ocean heat content stored through the upper layers of the ocean.

A clearer global picture of that deeper heat began to emerge in the early 2000s with the deployment of profiling floats measuring temperature and salinity down to 2,000 metres worldwide.

Those observations made it possible to extend ocean analyses back to 1958; before then, measurements were too sparse to provide a global view.

While sea surface temperatures remain vital for day-to-day weather, ocean heat content provides the foundation for understanding........

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