Dr. Laifang Li

Duke University, USA

Room 303, Keyan building

15:00 Aug 11, 2017 

The Atlantic multidecadal variability (AMV) has a significant impact on global and regional climate. However, the role of the ocean, if any, in driving the AMV remains a matter of debate. Based on historical sea surface temperature (SST) records and atmospheric reanalysis datasets, we provide evidence that atmospheric forcing alone is insufficient to explain the AMV SSTA. First, from an investigation of SST patterns, we show that the low-frequency decadal variability of the AMV SSTA is largely attributable to the long persistence of extratropical SSTA. The SSTA associated with NAO does not show the same persistence, suggesting that the AMV SSTA, especially its extratropical branch, is not solely a response to atmospheric forcing. Secondly, we evaluate the sufficiency of atmospheric forcing in driving AMV SSTA by constructing an idealized red-noise model which mimics the mixed layer heat balance in the North Atlantic. In the model, the oceanic contribution is neglected in order to isolate the effects of atmospheric forcing. Model parameters, including system decay time scale and atmospheric forcing, are derived from EN4.2.0 and 20th century reanalysis datasets. The model is run for many simulations using parameters for either the extratropical or tropical North Atlantic. These simulations reveal that the long persistence of extratropical SSTA is tied to a weak system decay, a result of both deep mixed layers and cold mean SSTs in the region. Importantly, the modeled extratropical SSTA underestimate the observed SSTA variability: none of the simulations produces an SSTA standard deviation higher than that observed. In contrast, the variability of observed tropical SSTA is within the 95% confidence interval quantified from the idealized model simulations, indicating that tropical SSTA can be explained by atmospheric forcing. Our study is consistent with previous studies that emphasize the importance of atmospheric circulation in creating the spatial coherence of AMV SSTA. However, in terms of extratropical SSTA, our work suggests that atmospheric forcing alone is insufficient to explain the AMV. 

Prolonged effect of the stratospheric pathway in linking Barents–Kara—Sea sea ice variability to the midlatitude circulation in a simplified model

Dr. Pengfei Zhang

Purdue University, USA

To better understand the dynamical mechanism that accounts for the observed lead-lag correlation between the early winter Barents–Kara Sea (BKS) sea ice variability and the later winter midlatitude circulation response, a series of experiments are conducted using a simplified atmospheric general circulation model with a prescribed idealized near-surface heating over the BKS. A prolonged effect is found in the idealized experiments following the near-surface heating and can be explicitly attributed to the stratospheric pathway and the long time scale in the stratosphere. The analysis of the Eliassen-Palm flux shows that, as a result of the imposed heating and linear constructive interference, anomalous upward propagating planetary-scale waves are excited and weaken the stratospheric polar vortex. This stratospheric response persists for approximately 1–2 months accompanied by downward migration to the troposphere and the surface. This downward migration largely amplifies and extends the low-level jet deceleration in the midlatitudes and cold air advection over central Asia. The idealized model experiments also suggest that the BKS region is the most effective in affecting the mid-latitude circulation than other regions over the Arctic.