Strengthening gradients in the tropical west Pacific connect to European summer temperatures on sub-seasonal timescales
Abstract. Recent work has shown that (sub-)seasonal variability in tropical Pacific convection, closely linked to ENSO, relates to summertime circulation over the Euro-Atlantic. The teleconnection is non-stationary, probably due to long-term changes in both the tropical Pacific and extra-tropical Atlantic. It also appears imperfectly captured by numerical models. In a previous study we found that the best predictor of errors in sub-seasonal forecasts of European temperature, is a dipole in tropical west Pacific sea surface temperatures (SSTs). In this diagnostic study we use reanalysis data to further investigate the teleconnection pathway and the processes behind its non-stationarity. We show that SST gradients associated with the dipole represent a combination of ENSO variability and west Pacific warming, and have become stronger since 1980. Associated patterns of suppressed and enhanced tropical heating are followed by quasi-stationary waves that linger for multiple weeks. Situations with La Niña-like gradients are followed by high pressure centers over eastern Europe and Russia, three to six weeks later. Inverted situations are followed by high pressure over western Europe, three to six weeks later. The latter situation is however also conditional on a strong meridional tripole in north Atlantic SST and a co-located jet stream. Overall, the sub-seasonal pathway diagnosed in this study connects to patterns detected at seasonal scales, and confirms earlier findings that the summertime connectivity between the Pacific and Europe has shifted in recent decades. It also partly explains the increased occurrence of high sea level pressures and summer temperatures over the European continent.
Chiem van Straaten et al.
Status: open (until 06 Apr 2023)
- RC1: 'Comment on wcd-2023-6', Anonymous Referee #1, 21 Mar 2023 reply
Chiem van Straaten et al.
Chiem van Straaten et al.
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This work examines the process that anomalous heating over the tropical-subtropical western North Pacific can affect European temperature variability, with the modulation from the North Atlantic SST, on subseasonal timescales in the summer season. The authors defined a West Pacific Dipole (WPD) index to characterize the combined zonal and meridional heating contrast in the western North Pacific. They found that positive WPD can trigger teleconnections towards Europe, leading to warm T2m anomalies over western Europe in 3-6 weeks. They also found that the negative phase of WPD has become more frequent since 1980, consistent with the “Western Pacific warming mode”. However, the summertime temperature over western Europe has not shown any cooling trend since 1980. The authors argued that the North Atlantic SST also plays an important role in modulating the teleconnections.
The results in this manuscript potentially present valuable improvement to our understanding of subseasonal climate predictability and prediction. I have some major comments and clarification questions, hopefully could help to improve the manuscript.
I guess I am confused the connection here: one is change in the mean (shift in the distribution) and another one is change in the standard deviation (change in the shape of the distribution). For example, in L285-289, the authors stated: though negative WPD has occurred more frequently, western Europe has not experienced a relative cooling as expected. Based on the results shown here, WPD affects the “subseasonal variability” in western European t2m. More frequent of this event is not necessarily equal to change in seasonal mean temperature, if the standard deviation has become larger (e.g., more frequent cold subseasonal events, but also more frequent and stronger warm subseasonal events).
In short, it would be helpful if the authors clarify the different timescales across the phenomena they have mentioned in the manuscript. Also, in L290-296, the authors examined that the positive WPD phase has become more likely to result in positive t2m anomalies in western Europe. It is also helpful to shown the negative WPD phase: has negative WPD phase become less likely to result in negative t2m anomalies in western Europe?
L94-97: Please include the longitudinal ranges of Nino3 and Nino4 regions. Also, how about the climatology? Or SST anomalies are relative to zonal mean (between 20N-20S), rather than relative to any climatology?
L122: “eastern” edge of the Nino4 area: “western”
L124-125: please provide the lon/lat ranges of the components 1 and 2.
L194: “Both types of heating contrasts were found to be important by Ding et al. (2011)” -> important to what?
L208-209 & L283: why not include t2m anomalies over Eastern Europe and Russia in Figure 5?
L250-254 & Fig. 7: it would be helpful to also include wave patterns (Z300 anomalies) in Figure 7.
L281-284: Are there any evidences supporting that the more frequent warming & high pressure over Eastern Europe and Russia are statistically significant related to the increasing WPD negative phase?