Assessing variability in the size and strength of the North Atlantic subpolar gyre

Recent studies on the size and strength of the North Atlantic subpolar gyre (SPG) offer contrasting assessments of the gyre's temporal variability: studies that use empirical orthogonal function (EOF) analyses of satellite sea-surface height (SSH) report a rapid decline in SPG size and strength since 1992 (similar to 20% per decade), while concurrent in situ observations report either no trend or a slight decline. Here we investigate this discrepancy by analyzing the size and strength of the SPG with satellite SSH from 1993 to 2015 with two separate methods: indirectly via EOF analysis and more directly through measurements of the gyre center and boundary. We define the boundary of the gyre as the largest closed contour of SSH, the center as the minimum SSH, and the strength as the difference between the SSH at the boundary and the center. We identify a linear decline over the study period in the SPG strength (5.1% per decade), but find no statistically significant trend in the SPG area. The trend in the gyre strength is weaker than the EOF-based trend and is most likely below the level of detection of the in situ measurements. We conclude that the variability previously identified as a sharp decline in SPG circulation can be more appropriately attributed to basin-wide sea level rise during the satellite altimetry period. In addition, we find that the properties of the eastern SPG do not covary with the SPG size, suggesting that SPG dynamics do not control the strength of the intergyre throughput.