Importance of Pacific oceanic front for the North Atlantic blocking

Recent studies have examined the local impact of the Gulf Stream front on North Atlantic blocking. In this study we used semi-idealized atmospheric-only experiments to identify the additional crucial role of the Kuroshio Extension front in setting the climatological North Atlantic blocking activity.

Blocking refers to a warm-core anticyclone remaining quasi-stationary throughout the extratropical troposphere for minimum of 4-5 days. It frequently triggers extreme weather events, such as prolonged cold spells, heat waves, droughts and floods. An accurate prediction of blocking is crucial for mitigating damages associated with extreme weather events. However, general circulation models (GCMs) still struggle to simulate the climatology of blocking frequency, particularly over the North Atlantic. Climatologically, North Atlantic blocking activity is strongest in winter with two distinct peaks: the primary peak is located downstream of the North Atlantic storm track or eddy-driven jet and is known as Euro-Atlantic blocking; the secondary peak occurs poleward of the North Atlantic storm track and is known as Greenland-blocking. 
Recent studies have examined the local impact of the Gulf Stream front on North Atlantic blocking. However, atmospheric circulation over the North Atlantic is also influenced by remote forcing from the North Pacific, where the physical mechanism linking North Pacific and North Atlantic blocking remains poorly understood. In this study we used semi-idealised atmospheric-only experiments to identify the crucial role of the Kuroshio Extension front in the climatological North Atlantic blocking activity

Importance of the remote Pacific midlatitude oceanic fronts

Our experiments examine the separate and combined impacts of the Gulf Stream and Kuroshio Extension fronts, as well as the impact of tropical sea surface temperature (SST) on winter Atlantic blocking activity. In the absence of the Northern hemisphere midlatitude oceanic fronts and tropical SST-asymmetry, the primary blocking peak shifts from Euro Atlantic into Greenland (See Figure). The unrealistic shift in the primary blocking peak into Greenland is largely improved when including the midlatitude North Atlantic and Pacific midlatitude oceanic fronts. Our semi-idealized atmospheric-only experiments suggested that the Pacific oceanic front accounts for 41.2% and 24.1% of the impact of oceanic fronts on Euro-Atlantic and Greenland blocking frequency, respectively.

Figure: Impacts of the midlatitude oceanic fronts and the tropical SST on the winter (DJF) climatological blocking frequency across the Euro-Atlantic region: Spatial distribution of the blocking frequency in (a) the FULL run (with realistic SST in the Northern Hemisphere and tropics; shading) and the ZUNF run (with zonally uniform tropical SST and without oceanic fronts; contour interval: 4%), (b)–(f) response in semi-idealised experiments (i.e. difference between the experiments and the ZUNF run; shading) and the climatology in the ZUNF run (contours): (b) tropical SST and two oceanic fronts (FULL run), (c) two oceanic fronts (EXT_ALL run), (d) tropical SST (TROP_ALL run), (e) Atlantic oceanic front (EXT_ATL run), and (f) Pacific oceanic front (EXT_PAC run). Stippling in (b)–(f) indicates differences exceeding the 95% confidence interval based on the two-tailed Students’ t-test.

Dynamics of the response of North Atlantic blocking to midlatitude oceanic fronts

Midlatitude oceanic fronts influence the blocking frequency by affecting (1) the dynamics of blocking events, (2) the storm-track activity and (3) the overall mean atmospheric circulation.

Impact through the dynamics of the blocking events: Realistic Euro-Atlantic blocking events are maintained by an eastward moving trough-ridge system over the North Atlantic region that is attributed partly to synoptic-scale or high-frequency eddy-forcing, as well as a persisting European ridge downstream of the storm-track that is attributed mainly to low-frequency eddy-forcing. The midlatitude oceanic fronts play a crucial role in maintaining this wave train and the low and high frequency eddy-forcing of the Euro-Atlantic blocking events. Our simulation results suggest that the synoptic-scale eddy-forcing of Euro-Atlantic blocking is mainly contributed by Atlantic oceanic fronts, whereas low-frequency eddy-forcing of Euro-Atlantic blocking is due to the joint effect of the Atlantic and Pacific oceanic fronts. On the other hand, realistic Greenland Blocking events are initiated by low frequency eddy-forcing and are strengthened by high-frequency forcing characterized by north-eastward migration of a synoptic-scale wave train from North America to Greenland. Our simulation results suggest that the North Atlantic midlatitude oceanic fronts play a dominant role for the synoptic-scale forcing of Greenland blocking, whereas the low-frequency forcing of Greenland blocking is contributed by both Atlantic and Pacific SST-fronts.

Impact through the storm track activity: Intensification of extratropical cyclones or storm-track activity is crucial for the development of blocking. As shown in previous studies, Euro-Atlantic blocking frequency is closely related to the Atlantic storm track activity. An enhanced eastward extension of the Atlantic storm track represents more passage of synoptic-scale wave trains towards Eurasia, such that more extratropical cyclones could interact with a planetary-scale ridge over Europe and such interaction favours the formation of blocking. The enhanced low atmospheric temperature gradients (or baroclinicity) maintained by the midlatitude oceanic fronts play a crucial role in maintaining and anchoring the storm-track. Our simulation results show that the Atlantic storm-track activity is enhanced by the Atlantic midlatitude oceanic front and is highly amplified by the Pacific midlatitude oceanic front. Therefore, the two midlatitude oceanic fronts are crucial for the Atlantic storm-track activity and North Atlantic blocking frequency.

Impact through the mean circulation state: The Atlantic midlatitude oceanic front enhances the synoptic-wave forcing which in turn strengthens the eddy-driven jet locally. Because of the geostrophic balance, the intensification of the Atlantic eddy-driven jet corresponds to cyclonic response and a reduction in Greenland blocking frequency. Therefore, the Atlantic midlatitude oceanic front acts to reduce Greenland blocking frequency. Our numerical experiments show that the Pacific midlatitude oceanic front also has a significant remote impact on the Atlantic eddy-driven jet via enhancing synoptic-wave activity from the North Pacific into the North Atlantic, which further reduces Greenland blocking frequency.  In addition to strengthening of the eddy-driven jet, the Atlantic and Pacific midlatitude oceanic fronts jointly enhance the planetary-scale ridge over the eastern Atlantic, which is favorable for higher blocking frequency over the Euro-Atlantic.

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