The Impact of Tropical Precipitation on Summertime Euro-Atlantic Circulation via a Circumglobal Wave Train

Abstract

The influence of tropical precipitation variability on summertime seasonal circulation anomalies in the Euro-Atlantic sector is investigated. The dominant mode of the maximum covariance analysis (MCA) between the Euro-Atlantic circulation and tropical precipitation reveals a cyclonic anomaly over the extratropical North Atlantic, contributing to anomalously wet conditions over western Europe and dry conditions over eastern Europe and Scandinavia (in the positive phase). The related mode of tropical precipitation variability is associated with tropical Pacific SST anomalies and is closely linked to the El Niño–Southern Oscillation (ENSO). The second MCA mode consists of weaker tropical precipitation anomalies but with a stronger extratropical signal that reflects internal atmospheric variability. The teleconnection mechanism is tested in barotropic model simulations, which indicate that the observed link between the dominant mode of tropical precipitation and the Euro-Atlantic circulation anomalies is largely consistent with linear Rossby wave dynamics. The barotropic model response consists of a circumglobal wave train in the extratropics that is primarily forced by divergence anomalies in the eastern tropical Pacific. Both the eastward and westward group propagation of the Rossby waves are found to be important in determining the circulation response over the Euro-Atlantic sector. The mechanism was also analyzed in an operational seasonal forecasting system, ECMWF’s System 4. While System 4 is well able to reproduce and skillfully forecast the tropical precipitation, the extratropical circulation response is absent over the Euro-Atlantic region, which is likely related to biases in the Asian jet stream.

Contrasting Mechanisms of Summer Blocking Over Western Eurasia

“…In the eastern area, blocking events are preceded by a statistically significant low‐frequency wave train (Figure c). This is in good agreement with Schneidereit et al (), who showed that the 2010 blocking highs over Russia were associated with a low‐frequency wave train likely forced by a La Niña event in the tropical Pacific (see also O'Reilly et al, ). We go further by showing that this low‐frequency Rossby wave train was not only present during the 2010 events, as we obtain the same results when the blocking events of summer 2010 are removed (Figure S2 in the supporting information; the same holds for 2003 in the western area).…”

“…Problems representing the large‐scale low‐frequency wave train forcing in general circulation models (e.g., O'Reilly et al, ) could lead to an underestimation of blocking events in the eastern area. This could explain the negative bias observed in twentieth century coupled runs from general circulation models from CMIP5 (see Figure 5 from Masato et al, ) at 30–50°E, 60°N.…”

The importance of stratospheric initial conditions for winter North Atlantic Oscillation predictability and implications for the signal‐to‐noise paradox

“…Nonetheless, this study highlights the importance of evaluating the representation of the QBO – and its connection to the NAO – in operational seasonal forecasting systems. A similar signal‐to‐noise issue has been found in skilful seasonal forecasts of summertime European precipitation (Dunstone et al ) and the QBO is unlikely to be responsible for the signal‐to‐noise issue in these summer hindcasts; however, it could be that another relevant teleconnection mechanism, such as from the tropical Pacific (O'Reilly et al ), could be too weakly represented in these summer hindcasts. Nonetheless, the results of the present study suggest that seasonal forecasting models which capture predictable relationships too weakly may be able to exhibit substantial ensemble mean skill with a large number of ensemble members, but will have a signal that is somewhat too weak.…”

How confident are predictability estimates of the winter North Atlantic Oscillation?

“…Understanding the predictability of extratropical circulation anomalies on seasonal time‐scales is essential for making better decisions in societal sectors that rely crucially on weather and climate information for the seasons to come. Whereas our forecast models' abilities to predict the evolution of tropical sea‐surface temperature (SST), especially in the El Niño–Southern Oscillation (ENSO) relevant Pacific Ocean, have improved over the recent decades and led to demonstrated skill (Weisheimer et al, ; Barnston et al, ), forecasting the extratropical tropospheric circulation variability remains a major challenge in seasonal prediction systems (Weisheimer et al, ; Lavers et al, ; Kim et al, ; Sigmond et al, ; Weisheimer and Palmer, ; Molteni et al, ; Befort et al, ; Beverley et al, ; O'Reilly et al, ).…”