Quantifying the Role of Ocean Dynamics in Mixed-Layer Temperature Variability

Understanding the role of the ocean in climate variability requires first understanding the role of ocean dynamics in ocean mixed-layer and thus sea-surface temperature variability. However, key aspects of the spatially and temporally varying contributions of ocean dynamics to such variability remain unclear. Here, the authors quantify the contributions of ocean-dynamical processes to mixed-layer temperature variability on monthly to decadal timescales across the globe. To do so, they use two complementary but distinct methods: 1) a method in which ocean heat transport is estimated directly from a state-of-the-art ocean state estimate; and 2) a method in which it is estimated indirectly from observations and the energy budget of the ocean mixed layer. The results extend previous studies by providing quantitative estimates of the role of ocean dynamics in mixed-layer temperature variability throughout the globe, across a range of timescales, in a large range of available measurements, and using two different methods. Consistent with previous studies, both methods indicate that the ocean dynamical contribution to mixed-layer temperature variance is largest over the western boundary currents, their extensions, and regions of equatorial upwelling. In contrast to many previous studies, the results suggest that ocean dynamics reduce the variance of Northern Hemisphere mixed-layer temperatures on timescales longer than a few years. Hence, in the global-mean, the fractional contribution of ocean dynamics to mixed-layer temperature variability decreases at increasingly low-frequencies. Differences in the magnitude of the ocean dynamical contribution based on the two methods highlight the critical need for improved ocean observations.