Warm conveyor belt activity over the Pacific: Modulation by the Madden-Julian Oscillation and impact on tropical-extratropical teleconnections

Abstract

Abstract. Research in the last decades revealed that rapidly ascending airstreams in extratropical cyclones – so-called warm conveyor belts (WCBs) – play an important role in extratropical atmospheric dynamics. However on the subseasonal time scale, the modulation of their occurrence frequency, henceforth referred to as WCB activity, has so far received little attention. Also, it is not yet clear whether WCB activity may affect tropospheric teleconnection patterns, which constitute a source of predictability on this subseasonal time scale. Using reanalysis data, this study analyzes the modulation of WCB activity by the Madden-Julian Oscillation (MJO). A key-finding is that WCB activity increases significantly over the western North Pacific when the convection of the MJO is located over the Indian Ocean. This increased WCB activity, which is particularly pronounced during La Niña conditions, is related to enhanced poleward moisture fluxes driven by the circulation of subtropical Rossby gyres associated with the MJO. In contrast, when the convection of the MJO is located over the western North Pacific, WCB activity increases significantly over the eastern North Pacific. This increase stems from a southward shift and eastward extension of the North Pacific jet stream. However, while these mean increases are significant, individual MJO events exhibit substantial variability, with some events even exhibiting anomalously low WCB activity. Individual events of the same MJO phase with anomalously low WCB activity over the North Pacific tend to be followed by the known canonical teleconnection patterns in the Atlantic-European region, i.e., the occurrence frequency of the positive phase of the North Atlantic Oscillation (NAO) is enhanced when convection of the MJO is located over the Indian Ocean, and similarly for the negative phase of the NAO when MJO convection is over the western North Pacific. However, the canonical teleconnection patterns are modified when individual events of the same MJO phase are accompanied by anomalously high WCB activity over the North Pacific. In particular, the link between MJO and the negative phase of the NAO weakens considerably. Reanalysis data and experiments with an idealized general circulation model reveal that this is related to anomalous ridge building over western North America favoured by enhanced WCB activity. Overall, our study highlights the potential role of WCBs in shaping tropical-extratropical teleconnection patterns and underlines the importance of representing them adequately in numerical weather prediction models in order to fully exploit the sources of predictability emerging from the tropics.

Why Moist Dynamic Processes Matter for the Sub‐Seasonal Prediction of Atmospheric Blocking Over Europe

“…Interestingly, the North Pacific Rossby wave pattern is also amplified by WCB activity, in line with recent findings by Quinting et al. (2024) who highlighted the potential role of WCBs in shaping tropical‐extratropical teleconnection patterns. If and how the MJO further affects EuBL onset should be a subject of future work.…”

“…Though the Rossby wave packet emerging from the North Pacific is associated with increased skill over the Atlantic European region (Grazzini & Vitart, 2015), forecast errors related to small‐scale processes inside WCBs could affect the Rossby wave pattern and subsequently lead to errors in the prediction of EuBL onsets. Still, the Rossby wave packet seems to represent a window of forecast opportunity for the prediction of EuBL onset into sub‐seasonal timescales which likely depends on an accurate representation of WCB activity conditioned on the MJO in the North Pacific region, too (Quinting et al., 2024).…”

“…That the presence of long‐lived Rossby wave packet emerging from the North Pacific are associated with enhanced predictive skill over the North Atlantic‐European region is in line with Grazzini and Vitart (2015). It overall indicates an enhanced intrinsic predictability in such situations which may be favored by modes of intraseasonal variability such as the Madden‐Julian Oscillation (MJO) that importantly modulates the North Pacific WCB activity (Quinting et al., 2024). With regard to the practical predictability and the case that small‐scale processes in WCBs are only insufficiently reproduced in NWP models, the increased WCB activity on the other hand may reduce this increased intrinsic predictability.…”

“…Other studies also point to the amplification of errors in the WCBs (Pickl et al., 2023) or highlight the generation of errors in potential temperature and potential vorticity in the WCBs, which can lead to downstream errors in the Rossby wave pattern (Berman & Torn, 2019; Martínez‐Alvarado et al., 2016). Moreover, teleconnections from the Caribbean and North Pacific region affect the occurrence of large‐scale weather regimes, including blocking, in the European region (Michel & Rivière, 2011; Michel et al., 2012) and the teleconnection itself is again modulated by WCB activity (Quinting et al., 2024).…”

Intrinsic predictability limits arising from Indian Ocean Madden–Julian oscillation (MJO) heating: effects on tropical and extratropical teleconnections

“…The dominance of the stretching term for moderate scales shown in Fig. A3 is consistent with strong upper-level divergence associated with baroclinic disturbances near the Pacific jet, as expressed by an increase in frequency of warm conveyor belt outflows (Quinting et al, 2023) and also by the behavior of the longitude-time plots of the stretching term that show very consistent eastward propagation (not shown).…”