Multi-decadal Variability of Gulf Stream and Slope Water

Abstract:

The oceanographic conditions in the Northwest Atlantic Ocean are determined by an interplay of the major circulation systems and water masses — the Gulf Stream (GS), Labrador Current (LC), and Slope Water (SW). We continued our study of the GS path variability to extend it to decadal variability of the SW coupled with the GS and LC multi-decadal change. We report some new results of the historic in situ data analysis aimed at a better understanding of the long-term climate trends in this region. Long-term variability of the seawater temperature within the upper 300m layer reveal decadal-scale warming in GS-SW region, which cannot be explained by changes in summer seasonal thermocline formation alone. In the last 15 to 20 years, beginning in 2005, the pentadal near-surface temperature in SW rose quickly in both summer and winter and, therefore, annually. The annual temperature in the SW increased by ~4.5°C. The summer temperature increased by ~3.8°C, while the winter temperature increased by almost 5°C. The ~55-year time-series of the upper 300m layer temperature also suggest quasi-periodicity with a 30 to 40-year period overlapped by lower frequency oscillations with a period longer than 55 years. Notably, however, the winter temperature began to decline in the years 2016 and 2017. To illustrate the long-term ocean climate shift in the SW, we computed 3D temperature anomalies in the region by subtracting the climate of 1965-1984 from that of 1995-2017. The differences demonstrate that most of the annual warming (up to 1.8°C) occurs along the Nova Scotia (NS) shelf-break and over the slope. Seasonal climate shifts, i.e., the temperature anomalies relative to ~40-year averages, indicate that within the upper 300m layer the warming is stronger in winter than in summer, although most of the sea surface warming occurs in summer. The strongest warming was detected in the western area of the NS shelf, near entrance to the Gulf of Maine. Although substantial warming within the SW occurred in the last 20 years and accelerated in the most recent years, the possibility of continuing of fast and excessive warming in the SW in the near future is not yet certain.