Contributions of Atmospheric and Oceanic Feedbacks to Subtropical Northeastern Sea Surface Temperature Variability
Sea surface temperature (SST) variability in northeastern subtropical ocean basins is important for tropical-extratropical teleconnections, low-frequency modes of climate variability, and precipitation on continental coasts. A remarkable example of these SST variations was the “Blob” in the Pacific occurring in 2013 through 2015 off the coast of Baja California which led to severe drought conditions in the western USA. Despite the importance of SST variability in this region, uncertainty about the mechanisms controlling this variability remains. This study uniquely separates the roles of cloud feedbacks from oceanic processes to better understand SST variability in the northeastern Pacific and Atlantic.
This study investigates SST variability in the northeastern subtropical oceans using CESM1.2 in fully-coupled and slab-ocean configurations. The difference between them provides a measure of the impact of explicit ocean dynamics. The “cloud-locking” methodology is also used, whereby an external cloud dataset from a previous simulation is used in the radiative transfer; the effect is to decouple the cloud radiative effects from the circulation while maintaining the mean climate. Comparing cloud-locked to standard runs provides a measure of the cloud radiative feedback. Cloud-locked simulations were conducted with both the dynamic and slab ocean configurations, providing a complete experimental suite. The conclusion of these comparisons, through a detailed analysis of the upper ocean heat budget, shows that the impact of positive cloud radiative feedback on SST variability is determined by the role of ocean dynamics. Clouds enhance variability in both regions, but the enhancement is stronger in the Pacific than in the Atlantic. While in the Pacific the ocean dynamics act to always damp SST variations, this damping is much more muted in the Atlantic.
Climate variability in western North America and Europe is strongly impacted by SST variations in the northeastern ocean basins. Understanding the mechanisms of such SST variability is crucial for understanding the predictability of societally relevant climate variability, and ultimately for improving predictions at seasonal to decadal time scales. This study shows that SST variability differs in the Pacific and Atlantic because of the role that ocean dynamics play in responses to atmospheric forcing. This highlights the importance of having a credible simulation of the upper ocean as well as proper cloud radiative effects for capturing these large-scale variations.