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Because colored dissolved materials (CDMs) trap incoming sunlight closer to the surface, they have the potential to affect sea surface temperatures. We compare two models, one with and one without CDMs, and show that their presence leads to an increase in the amplitude of the seasonal cycle over coastal and northern subpolar regions, which may exceed 2 degrees C. The size and sign of the change are controlled by the interplay between enhanced shortwave heating of the surface, shading and cooling of the subsurface, and the extent to which these are connected by vertical mixing. The changes in the seasonal cycle largely explain changes in the range of temperature extremes, an aspect of climate with important implications for ecosystem cycling. The modeled changes associated with CDMs have an intriguing resemblance to the observed trend in the annual cycle seen in recent decades, suggesting that more attention be paid to the role of "ocean yellowing" in global change.

Plain Language Summary Colored detrital materials produced by decay of organic matter turn the oceans yellow and trap light closer to the surface. This results in more heating of the surface but less heating of the ocean interior during the summer months. As a result, colder water is mixed up to the surface in winter months, and the annual cycle of sea surface temperature increases. The pattern of this change resembles the observed trend in the annual cycle of SST, which is particularly important in coastal areas where it may result in driving marine heatwaves that drive changes in ecosystem structure. Our work suggests that more attention to ocean optics and mixed layer dynamics will be essential to properly understanding such changes in extreme temperatures.