Observed multi-decadal increase in the surface ocean’s thermal inertia

The ocean’s surface layer has a crucial role in Earth’s climate, absorbing excess atmospheric heat, thereby regulating global temperatures. Here, using global daily sea surface temperature (SST) data, we document a notable increase in the persistence of SST anomalies across the global ocean since 1982. This trend is also evident in frequency space, showing a decreased variance in SSTs on timescales shorter than a month, but a slight increase on longer timescales. A simple stochastic model attributes this prolonged memory to three key factors––a deepening of the surface mixed layer, a weakening of oceanic forcing and reduced damping rates. The first two factors decrease the variance on shorter timescales, while the third increases it on longer timescales. Our findings have great relevance to the observed increase in the duration of marine heatwaves and the associated heightened thermal threats to marine organisms. Our study also suggests that the ocean’s ability to sequester heat is weakening. Analysis of 42 years of daily sea surface temperature data shows increasing persistence of anomalies. These changes, which are attributed to deepening of the mixed layer, reduced oceanic forcing and reduced damping associated with stronger stratification, have implications for marine heatwave duration.