“The Blob”: A Warm Anomaly’s Effect on Interannual Variability of Eastern Subtropical Mode Water

Abstract

A high sea surface temperature (SST) anomaly occurred off the western coast of North America in the winters of 2013-2014 and 2014-2015, which scientists and journalist named the “Blob”. The region affected by this anomaly overlaps one of three areas in the North Pacific where water passes through the permanent pycnocline, and into a relatively homogenous, subsurface body of water, termed mode water. We examine the Blob’s effects on North Pacific Eastern Subtropical Mode Water (ESTMW) formation region, 15-45ºN by 125-150ºW. Multiyear averages, climatologies, of the effective subduction rate, subduction volume, and SST are determined from global, empirical datasets. In addition, we compared the climatologies to the years affected by the Blob, determined through yearly datasets and objectively mapped Argo float products. In 2014 and 2015, a SST increase of 2ºC was most prominent across the southern portion of the region. Instead of decreasing mixed-layer depth uniformly across the region, the affected years were marked by expansion of shallow 15-45 meter mixed-layers on the southern edge of the ESTMW formation site and an increase in subduction volume by 1 Sverdrup. Therefore, warm anomalies like “the Blob” could lead to temporary negative feedback on climate change by increasing subduction at mode water formation sites and increasing long-term heat storage as warmer water subducts below the surface.