A stratospheric connection to Atlantic climate variability

Thomas Reichler, Junsu Kim, Elisa Manzini, Jürgen Kröger
2012 Nature Geoscience  
The stratosphere is connected to tropospheric weather and climate. In particular, extreme stratospheric circulation events are known to exert a dynamical feedback on the troposphere 1 . However, it is unclear whether the state of the stratosphere also affects the ocean and its circulation. A co-variability of decadal stratospheric flow variations and conditions in the North Atlantic Ocean has been suggested, but such findings are based on short simulations with only one climate model 2 . Here
more » ... assess ocean reanalysis data and find that, over the previous 30 years, the stratosphere and the Atlantic thermohaline circulation experienced low-frequency variations that were similar to each other. Using climate models, we demonstrate that this similarity is consistent with the hypothesis that variations in the sequence of stratospheric circulation anomalies, combined with the persistence of individual anomalies, significantly affect the North Atlantic Ocean. Our analyses identify a previously unknown source for decadal climate variability and suggest that simulations of deep layers of the atmosphere and the ocean are needed for realistic predictions of climate. The ocean has a large thermal inertia and is dominated by variability on timescales of years to decades. Traditionally, atmospheric influences on the ocean are understood from the stochastic climate model paradigm, in which the troposphere is thought to provide a white-noise forcing that is integrated by the ocean to yield a low-frequency response 3 . In this study we propose another relevant influence, which is related to the stratosphere. The stratosphere is characterized by persistent flow dynamics 4 and considerable multi-decadal energy 5-7 . Variations in the strength of the wintertime northern hemispheric stratospheric vortex, so called 'polar vortex events', are known to last for many weeks, as does their impact on the troposphere 8 . An example is stratospheric sudden warmings (SSWs), prolonged time periods with an unusually weak and warm polar vortex. SSWs occur on average every second year, but observations over the past 30 years reveal an intriguing quasi-decadal rhythm in the year-to-year occurrence of such events: during the 1990s, the Arctic winter stratosphere was characterized by an almost complete absence of SSWs, but during the 1980s and also during the 2000s the stratosphere experienced a record number of such events (Fig. 1a) . A connection between the stratosphere and the ocean can be established by the North Atlantic Oscillation (NAO), a largescale pattern of near-surface circulation anomalies over the North Atlantic. Polar vortex events modulate the NAO polarity, with a strong vortex leading to a positive and a weak vortex to a negative NAO (ref. 8). NAO variations in turn are linked to circulation variability in the North Atlantic. The NAO induces anomalous
doi:10.1038/ngeo1586 fatcat:q5lx46a2fjhfdppk7fg2s5qasm