Ocean temperature extremes driven by an enhanced seasonal cycle- A role for ocean yellowing?

One of the major ways in which climate change affects ecosystems is through extremes in temperature. One possible driver for such extremes is an increase in the amplitude of the annual cycle. In this paper we analyze the pattern of changes in the annual cycle in the observational sea surface temperature record, finding increases in the annual cycle over many parts of the of the globe comparable to or exceeding mean warming trends. We then examine a previously overlooked control on the annual cycle, namely the distribution of colored dissolved materials (CDM) which result in a yellowing of the water column. CDM represents one of the major absorbers of sunlight in the in the ocean, but is generally not explicitly resolved in climate models. We examine a pair of simulations with an Earth System Model whose representation of ocean optics distinguishes absorption associated with chlorophyll from that associated with CDM. Adding CDM to the model produces a pattern of change in both temperature and extremes and annual cycle which with many similarities to the observed trends outside of the Arctic Ocean, but which is quite different from the pattern associated with global warming. Analysis of the energy budget of the surface layer highlights the importance of properly modeling mixed layer dynamics in order to simulate changes in marine heat waves. This in turn may require much higher vertical resolution and better representation of the dynamics of absorbers, particularly in coastal regions.