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Even though winter land–sea thermal contrast (LSC) is expected to undergo a strong weakening in the future warmer climate, its effects have been poorly investigated. Here we run a set of idealized winter simulations featuring reduced LSC in the Northern Hemisphere (NH) extratropics, or in individual extratropical sectors of the NH (Atlantic and Pacific), using an intermediate-complexity atmospheric general circulation model. Reduced LSC is obtained by imposing a warming of surface land temperatures in East Asia and North America. For similar warming intensities over the two regions, the response of the model to East Asia forcing is significantly stronger and dominates the response to the sum of the two forcing patterns. We find that the LSC reduction causes a weakening and poleward shift of the midlatitude jet streams, and a strong interference with zonal wavenumbers 1 and 2. In particular, East Asian warming reduces the amplitude of waves 1 and 2, producing a strengthening of the stratospheric vortex, while a weaker vortex due to a moderate amplification of wave 1 is detected when warming North America. Eventually, stratospheric signals propagate downward in the troposphere affecting the midlatitude winter NH even remotely from the forcing. In this work we pinpoint some mechanisms by which weakened winter LSC influences the NH extratropical circulation: the results may become useful to interpret the response to long-term projections displaying reduced LSC along with other climate change forcing patterns. [ABSTRACT FROM AUTHOR]

A new regime index is constructed to capture the seasonal development of the stratospheric polar vortex in the northern winter, based on the standard deviation of Ertel's potential vorticity in the upper stratosphere in November–December. The narrow-jet flow regime is characterized by a polar vortex that is more confined to high latitudes in the early winter upper stratosphere. This upper-level vortex configuration is more susceptible to the disturbances of upward propagating planetary-scale Rossby waves; the stratospheric polar vortex thus weakens earlier and is vertically shallower. The wide-jet flow regime is characterized by a broader-than-average polar vortex that extends further into the subtropics in the early winter upper stratosphere. The polar night jet then gradually strengthens, moves poleward, and penetrates deep into the lowermost stratosphere, with a sharply defined polar vortex edge due to more frequent Rossby wave breaking. Composite difference analyses show that the wide- and narrow-jet regimes, defined in the uppermost stratosphere in November–December, lead to different circulation anomalies of the lower stratosphere and the troposphere in January–February, offering the potential for improved predictability of subseasonal to seasonal forecasts up to two months ahead. The lower-tropospheric responses in January–February are zonally asymmetric. The narrow-jet regime projects most strongly over the North Atlantic, with an equatorward-shifted and/or broader midlatitude westerly jet. The wide-jet-regime response is characterized by a weakened midlatitude westerly jet over the North Pacific. The two flow regimes also differ in their impacts on the storm track over western Europe and the east coast of North America, which may have implications for extreme weather events in those regions. [ABSTRACT FROM AUTHOR]

In the upper troposphere and lower stratosphere (UTLS), large-scale anticyclones associated with monsoons play major roles in tropospheric and stratospheric transport and mixing. To understand the forcing of the North American monsoon anticyclone (NAMA), this study examines the connection between precipitation over the tropics and subtropics of the North American longitude sector and the variability of the troposphere and lower stratosphere. Using ERA5 and outgoing longwave radiation (OLR) data from 1979 to 2019, we assess the relationship at the intraseasonal time scale using pentad-mean time series. We show that OLR anomalies are correlated with circulation anomalies northwest and northeast of the region of precipitation. Decreased OLR (increased precipitation) corresponds to increased geopotential heights and anticyclonic circulation anomalies in the 300–100-hPa layer and an opposite response in the lower-tropospheric 850–600-hPa layer. The results are consistent with the established theory of the Rossby wave response to latent heating. The increase in height, which is strongest near 150 hPa, indicates that increased precipitation is associated with a strengthened NAMA. UTLS temperatures also have significant correlations with OLR, with cold (warm) anomalies occurring above (below) the core of the anticyclonic anomaly consistent with large-scale balance. The vertical structure of geopotential and temperature anomalies is compared to simulations using an idealized general circulation model, which shows that such a vertical structure is a consistent response to diabatic heating. Correlations at the interannual time scale resemble those at the intraseasonal time scale, demonstrating that precipitation is related to the NAMA at both time scales. [ABSTRACT FROM AUTHOR]

Based on the statistical analyses using ERA data sets, this study reveals a significant linkage between the tropospheric Western Hemisphere (WH) circulation pattern and the stratospheric EOF1 mode (North America [NA] pattern) of the wintertime daily height field at 50 hPa, which exhibits the displacement of the polar vortex to NA. The results of composite analysis indicate that the WH pattern leads the NA pattern at about 5 days. This time‐lagged linkage between the two patterns is further confirmed by the results of time‐lagged singular value decomposition (SVD) analyses. Through the linear interference with climatological waves, the WH pattern contributes to the enhanced (suppressed) wave‐2 (wave‐1) activities propagating into the stratosphere. The characteristics of planetary waves, with opposite sign of anomalous wave‐1 and wave‐2 activities, lead to the formation of the stratospheric NA pattern. We suggest the need for caution in interpreting the stratospheric influences on cold air outbreaks over NA. Key Points: There is an obvious time‐lagged linkage between the WH circulation pattern and the EOF1 mode of NH 50‐hPa daily height anomaliesThe WH pattern can enhance (suppress) the planetary wave‐2 (wave‐1) fluxes into the stratosphere through the linear interferenceThe anomalous wave‐1 and wave‐2 fluxes into the stratosphere with opposite sign are both important for the polar vortex morphology [ABSTRACT FROM AUTHOR]

Understanding and predicting midlatitude cold spells is of scientific and public interest, given often associated severe impacts. However, large-scale atmospheric dynamics related to these events are not fully understood. The winter of 2017/18 was characterized by several cold spells affecting large parts of North America and Eurasia. Here, the role of stratosphere–troposphere coupling for the occurrence of cold spells in this winter is investigated using different wave propagation diagnostics. While the European cold spell in late February 2018 was influenced by a major sudden stratospheric warming (SSW) associated with wave absorption, the cold spells over North America at the end of December 2017 and early February 2018 were related to downward reflected waves over the North Pacific. Previously proposed wave reflection indices, however, either miss these reflection events or are not able to distinguish them from the major SSW related to wave absorption. To overcome this, a novel simple index based on eddy heat flux is proposed here, capturing regional wave reflection over the North Pacific. Reflection events detected with this index are shown to be followed by North Pacific blocking and negative temperature anomalies over North America. An improved understanding of the contribution of wave reflection for cold spells is crucial to better predict such events in the future. [ABSTRACT FROM AUTHOR]

This study examines the preconditioning of events in which the Arctic stratospheric polar vortex shifts toward Eurasia (EUR events), North America (NA events), and the Atlantic (ATL events) using composite analysis. An increase in blocking days over northern Europe and a decrease in blocking days over the Bering Strait favor the movement of the vortex toward Eurasia, while the opposite changes in blocking days over those regions favor the movement of the vortex toward North America. An increase in blocking days over the eastern North Atlantic and a decrease in blocking days over the Bering Strait are conducive to movement of the stratospheric polar vortex toward the Atlantic. These anomalous precursor blocking patterns are interpreted in terms of the anomalous zonal wave-1 or wave-2 planetary wave fluxes into the stratosphere that are known to influence the vortex position and strength. In addition, the polar vortex shift events are further classified into events with small and large polar vortex deformation, since the two types of events are likely to have a different impact at the surface. A significant difference in the zonal wave-2 heat flux into the lower stratosphere exists prior to the two types of events and this is linked to anomalous blocking patterns. This study further defines three types of tropospheric blocking events in which the spatial patterns of blocking frequency anomalies are similar to the blocking patterns prior to EUR, NA, and ATL events, respectively, and our reanalysis reveals that the polar vortex is indeed more likely to shift toward Eurasia, North America, and the Atlantic in the presence of the above three defined tropospheric blocking events. These shifts of the polar vortex toward Eurasia, North America, and the Atlantic lead to statistically significant negative height anomalies near the tropopause and corresponding surface cooling anomalies over these three regions. [ABSTRACT FROM AUTHOR]

The relative impacts of tropical diabatic heating and stratospheric circulation anomalies on wintertime extratropical tropospheric variability are investigated in a linear inverse model (LIM) derived from the observed zero lag and 5-day lag covariances of 7-day running mean departures from the annual cycle. The model predicts the covariances at all other lags. The predicted and observed lag covariances are generally found to be in excellent agreement, even at the much longer lag of 21 days. This validates the LIM’s basic premise that the dynamics of weekly averages are effectively linear and stochastically driven, which justifies further linear diagnosis of the system. Analysis of interactions among the LIM’s variables shows that tropical diabatic heating greatly enhances persistent variability over most of the Northern Hemisphere, especially over the Pacific Ocean and North America. Stratospheric effects are largely confined to the polar region, where they ensure that the dominant pattern of sea level pressure variability is the annular Arctic Oscillation rather than the more localized North Atlantic Oscillation. Over the North Atlantic, both effects are important, although some of the stratospheric influence is ultimately traceable to tropical forcing. In general, the tropically forced anomalies extend through the depth of the troposphere and into the stratosphere, whereas stratospherically generated anomalies tend to be largest at the surface and relatively weak at midtropospheric levels. Some persistent variability is, however, found even in the absence of these “external” forcings, especially near the amplitude maxima of the leading eigenmodes of the internal extratropical tropospheric evolution operator. One of these eigenmodes has a circumglobal zonal wavenumber-5 structure with maxima over the Arabian Sea and the central Pacific, and two others are associated with north–south dipole variations across the North Atlantic jet. Overall, tropical influences are generally found to be larger than stratospheric influences on extratropical tropospheric variability and have a pronounced impact on the persistent, and therefore the potentially predictable, portion of that variability. [ABSTRACT FROM AUTHOR]

Coupling between tropospheric and stratospheric leading modes in anomaly fields is investigated. By using daily data at many levels in addition to monthly mean data, the transition of spatial patterns and the direction and speed of the vertical propagation are examined in detail. Results show that the North Atlantic Oscillation mode (NAO) and the Pacific–North American mode (PNA) dominating in the troposphere couple with the annular mode (AM) and a wavenumber-1 mode (W1) dominating in the stratosphere, respectively, with significant temporal correlations. The transition of the patterns occur at about 150 hPa. The couple of NAO–AM (first mode) amplifies almost simultaneously from the surface to the 10-hPa level. Sometimes amplifications are repeated a few times, in which maxima of the amplification move to the lower atmosphere. Viewing these sequences from a relatively long time scale, the first mode slowly propagates to the lower atmosphere. The coupling of PNA–W1 (second mode) propagates from the troposphere into the stratosphere in about 1 week. There is some relationship between the first and second modes. After the second mode propagates from the troposphere into the stratosphere, the first mode develops with a lag of 10–15 days. However, this relationship has different characteristics between positive and negative phases. Since large-amplitude AM with one phase corresponds to the stratospheric sudden warming, the sudden warming is caused by the following sequence. The PNA with a negative anomaly over the Pacific amplifies in the troposphere, exciting the W1 with a positive anomaly over North America in the stratosphere, which causes the sudden warming. Thus, amplification of the PNA leads to the sudden warming. [ABSTRACT FROM AUTHOR]

The author reflects on the climate change over North America in January 2014. Topics discussed include the recorded low temperatures between January 5 and 7 in many parts of the U.S. such as New York City and Chicago, Illinois, the deviation of surface temperatures, and the pattern of atmospheric circulation. Also mentioned are the impact of global warming, variations in solar activity, and occurrences of sudden stratospheric warming (SSW).