The Synergistic Effect of The Preceding Winter Northern Hemisphere Annular Mode and Spring Tropical North Atlantic SST On Spring Extreme Cold Events in The Mid-high Latitudes of East Asia

In this paper, the synergistic effect of the preceding winter positive Northern Hemisphere annular mode (pNAM) and spring negative tropical North Atlantic (nTNA) sea surface temperature anomaly (SSTA) on spring extreme cold events in the mid-high latitudes of East Asia (MHEA) is investigated. The results show that the co-occurrence of the two factors is unfavorable for extreme cold events during spring in the MHEA via the snow cover and atmospheric bridges. Over the Atlantic, the spring nTNA SSTA can lead to an atmospheric response that is similar to the North Atlantic Oscillation (NAO), which enhances the persistence of the pNAM and in turn amplifies the negative spring Eurasian snow cover extent (EASCE) anomaly caused by the preceding winter pNAM. Meanwhile, the spring EASCE is closely related to the spring MHEA anomalous anticyclone. In addition to storing its signal in the spring EASCE, the spring nTNA SSTA can also lead to the spring MHEA anomalous anticyclone via the eastward Rossby wave train. The evidence shows that the Rossby wave energy can propagate eastward to the MHEA as a result of the enhanced negative spring EASCE anomaly and Rossby wave induced by the spring nTNA SSTA, and the two factors have an obvious synergistic effect on the spring MHEA anomalous anticyclone. This anomalous MHEA anticyclone becomes a barrier that can hinder the intrusion of cold air from the polar region and can increase the thickness of the atmospheric layer. The anomalous sinking motion of the spring MHEA anomalous anticyclone can also lead to an increase in net radiation received at the surface and increase the air temperature through the vertical motion of air. The southerly wind over the west side of the spring MHEA anomalous anticyclone leads to horizontal warm advection. All of the above processes favor an increase in air temperature and dampen extreme cold events, implying the synergistic effect of the preceding winter pNAM and spring nTNA SSTA on spring extreme cold events in the MHEA.

Contribution of variations in Northern Hemisphere annular mode to the near-surface wind speed changes over Eastern China for 1979-2017

Studies have shown that large-scale ocean-atmosphere circulations (LOACs) played the major role to the near-surface wind speed (NWS) changes over China; however, the mechanisms whereby LOACs influences NWS to have received little attention. In this study, the processes of the Northern Hemisphere annular mode (NAM) influencing the NWS changes are revealed over eastern China for 1979–2017. The results showed a slowdown in NWS, at a rate of − 0.09 ± 0.01 m s− 1 decade− 1; meanwhile, this decline could be partly driven by the weakening of the zonal wind component. When the NAM exhibits positive phases, the zonal-mean westerly weakens at the low-to-mid-latitudes (10°–40°N); meanwhile, in the troposphere descending flows prevail near 40°N and ascending flows prevail near 65°N, and in the lower troposphere there are northerly anomalies at the low-to-mid-latitudes and southerly anomalies at mid-to-high latitudes (40°–70°N). The anomalous meridional flows transport heat from lower latitudes to higher latitudes and weaken the north–south air temperature gradient. The decreased air temperature gradient over East Asia reduces the pressure-gradient near the surface in eastern China, thereby decreasing the NWS. Furthermore, the effects of NAM on NWS changes are more significant at interannual scale than decadal scale. 32.0 ± 15.8 % of the changes in the annual mean NWS are caused by the variations in NAM; meanwhile, the NAM contribution to the interannual changes in the zonal component of NWS reach 45.0 ± 12.9 %.

A Midlatitude Climatology and Interannual Variability of 200- and 500-hPa Cut-Off Lows

A climatology of midlatitude 200- and 500-hPa cut-off low systems in the Northern and Southern Hemispheres is constructed from the NCEP–NCAR reanalysis by detecting and tracking, under one consistent method, all of the systems that persisted for more than 36 h for the 58 years of 1960–2017. This method identifies a cut-off low as a cold-core geopotential height minimum that is isolated from the main westerlies and with a strong temperature gradient on its eastern flank. The obtained spatial and seasonal distributions show preferred regions of occurrence and that within these regions there is a level-dependent seasonality of cut-off lows. Whereas 200-hPa systems are more frequent in summer and autumn, 500-hPa systems are more evenly distributed throughout the seasons. Within each region and at each level, the annual number of cut-off lows has been increasing over time, trends that are consistent with documented signals of climate change such as a weakening and poleward shift of the subtropical jets and an increase in blocking frequency. These trends explain as much as 64% of the variance in the annual number of cut-off lows. The contribution of the annular modes and El Niño–Southern Oscillation to the interannual variability of the number of cut-off lows per season in each hemisphere is also investigated. Only the Northern Hemisphere annular mode has a statistically significant negative correlation throughout all seasons that explains 18%–45% of the variance in the yearly number of Northern Hemisphere 500-hPa cut-off lows.

The Long- and Short-Lived North Atlantic Oscillation Events in a Simplified Atmospheric Model

This study investigates the North Atlantic Oscillation (NAO) events with relatively long and short lifetimes based on an 8000-day perpetual-boreal-winter [December–February (DJF)] run result of the idealized Geophysical Fluid Dynamics Laboratory (GFDL) dynamical core atmospheric model. We identify the so-called long- and short-lived positive and negative NAO events from the 8000-day model output. The composite 300-hPa geopotential height anomalies show that the spatial patterns of the composite long-lived NAO events closely resemble the Northern Hemisphere annular mode (NAM) because the NAO dipole is accompanied with a statistically significant North Pacific meridional dipole (NPMD) at similar latitudes as that of the NAO dipole. The composite short-lived NAO events exhibit the locally confined canonical NAO. Twelve sets of modified initial-value experiments indicate that an absence (a presence) of the NPMD-type perturbations at the early stage of the long (short)-lived NAO events will decrease (increase) their intensities and naturally shorten (lengthen) their lifetimes. Thus, the preceding NPMD is an early factor that is conducive to the emergence of the long-lived NAO events in the model. We argue that through directly modulating the synoptic eddy forcing over the North Atlantic region, the preceding NPMD can gradually arouse the NAO-like circulation anomalies on the following days. That is the reason why the preceding NPMD can modulate the intensities and lifetimes of the NAO events.

Autumn Sea Ice Cover, Winter Northern Hemisphere Annular Mode, and Winter Precipitation in Eurasia

This paper examines the impacts of the previous autumn sea ice cover (SIC) on the winter Northern Hemisphere annular mode (NAM) and winter precipitation in Eurasia. The coherent variations among the Kara–Laptev autumn SIC, winter NAM, and Eurasian winter precipitation appear after the year 1982, which may prove useful for seasonal prediction of winter precipitation. From a physical point of view, the Kara–Laptev SIC and sea surface temperature (SST) anomalies develop in autumn and remain in winter. Given that winter NAM is characterized by an Arctic–midlatitude seesaw centered over the Barents Sea and Kara–Laptev Seas, it is closely linked to the Arctic forcing that corresponds to the Kara–Laptev sea ice increase (reduction) and the associated surface temperature cooling (warming). Moreover, based on both model simulations and observations, the diminishing Kara–Laptev sea ice does induce positive sea level pressure (SLP) anomalies over high-latitude Eurasia in winter, which is accompanied by a significant surface warming in northern Eurasia and cooling south of the Mediterranean. This surface air temperature (SAT) anomaly pattern facilitates increases of specific humidity in northern Eurasia with a major ridge extending southward along the East Asian coast. As a result, the anomalous Eurasian winter precipitation has a more zonal band structure.