The northern hemisphere circumglobal teleconnection in a seasonal forecast model and its relationship to European summer forecast skill

Abstract

Forecasting seasonal variations in European summer weather represents a considerable challenge. Here, we assess the performance of a seasonal forecasting model at representing a major mode of northern hemisphere summer climate variability, the circumglobal teleconnection (CGT), and the implications of errors in its representation on seasonal forecasts for the European summer (June, July, August). Using seasonal hindcasts initialised at the start of May, we find that the model skill for forecasting the interannual variability of 500 hPa geopotential height is poor, particularly over Europe and several other "centres of action" of the CGT. The model also has a weaker CGT pattern than is observed, particularly in August, when the observed CGT wavetrain is strongest. We investigate several potential causes of this poor skill. First, model variance in geopotential height in west-central Asia (an important region for the maintenance of the CGT) is lower than observed in July and August, associated with a poor representation of the link between this region and Indian monsoon precipitation. Second, analysis of the Rossby wave source shows that the source associated with monsoon heating is both too strong and displaced to the northeast in the model. This is related to errors in monsoon precipitation over the Bay of Bengal and Arabian Sea, where the model has more precipitation than is observed. Third, the model jet is systematically shifted northwards by several degrees latitude over large parts of the northern hemisphere, which may affect the propagation characteristics of Rossby waves in the model.

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, ).…”

The typhoon-induced drying of the Maritime Continent

“…may help in forecasting the onset and duration of the dry season over the Maritime Continent. Consequently, forecasting TC activity can help improve, at least partially, the forecasts for all of the processes associated with the Maritime Continent circulation (55)(56)(57)(58)(59)(60).…”

Dominant patterns of interaction between the tropics and mid-latitudes in boreal summer: Causal relationships and the role of time-scales

“…The SAM has been connected to subtropical arid conditions in the North African region via the so-called monsoon -desert mechanism, creating reinforced descending motions over the Sahara during strong SAM phases (Rodwell and Hoskins, 1996;Stephan et al, 2019). This mechanism is fairly well captured by both climate (Cherchi et al, 2014) and seasonal forecast models (Beverley et al, 2019).The SAM is also connected to mid-latitude circulation via its interaction with the circumglobal teleconnection pattern (CGT), a wave pattern with 5 centres of action encircling the northern mid-latitudes and affecting temperature and precipitation there (Ding and Wang, 2005;Kripalani et al, 1997). This wave-5 like CGT pattern can be identified through interannual to intraseasonal (weekly) time-scales and it is likely connected with the SAM via twoway causal links (Di Capua et al, 2020).…”

“…This wave-5 like CGT pattern can be identified through interannual to intraseasonal (weekly) time-scales and it is likely connected with the SAM via twoway causal links (Di Capua et al, 2020). Seasonal forecast models are biased in their representation of the CGT, with typically a too weak CGT signal (Beverley et al, 2019). Therefore, seasonal forecasts miss an important source of predictability on intraseasonal time-scales, primarily in summer (Weisheimer and Palmer, 2014).…”

“…Such analyses can however be dominated by spurious correlations and therefore can give only limited insight into cause-effect relationships. On the other hand, model-based studies can be affected by biases in the representation of circulation and precipitation characteristics (Beverley et al, 2019;Schubert et al, 2011;Weisheimer and Palmer, 2014), which can feed back on each other. Also, although perturbation and sensitivity experiments can point towards potential causal relationships, they do not necessarily reveal the causal links between tropical and mid-latitudinal features, since the uncovered relationship may not be the dominant one.…”