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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2020-14
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-14
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Apr 2020

Submitted as: research article | 23 Apr 2020

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This preprint is currently under review for the journal WCD.

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

Giorgia Di Capua1,2, Jakob Runge3, Reik V. Donner1,4, Bart van den Hurk2,5, Andrew G. Turner6,7, Ramesh Vellore8, Raghavan Krishnan8, and Dim Coumou1,2 Giorgia Di Capua et al.
  • 1Potsdam Institute for Climate Impact Research, Potsdam, Germany
  • 2VU University of Amsterdam, Institute for Environmental Studies, Amsterdam, Netherlands
  • 3German Aerospace Centre, Institute of Data Science, Jena, Germany
  • 4Magdeburg–Stendal University of Applied Sciences, Magdeburg, Germany
  • 5Deltares, Delft, Netherlands
  • 6Department of Meteorology, University of Reading, Reading, United Kingdom
  • 7National Centre for Atmospheric Science, University of Reading, Reading, United Kingdom
  • 8Indian Institute for Tropical Meteorology, Pune, India

Abstract. Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemisphere. In winter, the El Niño–Southern Oscillation (ENSO) is the dominant source of predictability in the tropics and extra-tropics, but its role in summer is much less pronounced and the exact teleconnection pathways are not well understood. Here, we assess how tropical convection interacts with mid-latitude summer circulation at different intraseasonal time-scales and how ENSO affects these interactions. First, we apply maximum covariance analysis (MCA) between tropical convective activity and mid-latitude geopotential height fields to identify the dominant modes of interaction. The first MCA mode connects the South Asian monsoon with the mid-latitude circumglobal teleconnection pattern. The second MCA mode connects the western North Pacific summer monsoon in the tropics with a wave-5 pattern centred over the North Pacific High in the mid-latitudes. We show that the MCA patterns are fairly insensitive to the selected intraseasonal time-scale from weekly to 4-weekly data. To study the potential causal interdependencies between these modes and with other atmospheric fields, we apply causal effect networks (CEN) at different time-scales. CENs extend standard correlation analysis by removing the confounding effects of autocorrelation, indirect links and common drivers. In general, there is a two-way causal interaction between the tropics and mid-latitudes but the strength and sometimes sign of the causal link are time-scale dependent. We introduce causal maps that plot the regionally specific causal effect from each MCA mode. Those maps confirm the dominant patterns of interaction and in addition, highlight specific mid-latitude regions that are most strongly connected to tropical convection. In general, the identified causal teleconnection patterns are only mildly affected by ENSO and the tropical-mid-latitude linkages remain similar. Still, La Niña strengthens the South Asian monsoon generating a stronger response in the mid-latitudes, while during El Niño years, the Pacific pattern is reinforced. This study paves the way for process-based validation of boreal summer teleconnections in (sub-)seasonal forecast models and climate models and therefore helps to improve sub-seasonal and climate projections.

Giorgia Di Capua et al.

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Giorgia Di Capua et al.

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Short summary
We study the interactions between the tropical convective activity and the mid-latitude circulation in the Northern Hemisphere during boreal summer. We identify two circumglobal wave patterns with phase shifted corresponding to the South Asian and the western North Pacific monsoon systems at intraseasonal time scale. These patterns show a two-way interactions in a causal framework at weekly time scale and assess how El Niño affects these interactions.
We study the interactions between the tropical convective activity and the mid-latitude...
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