Extratropical-Tropical interAction: A unified view on the extratropical impact on the subtropics and tropics at weather timescales

Projektinformationen

ExTrA

ID Finanzhilfevereinbarung: 101075826

DOI

10.3030/101075826

EK-Unterschriftsdatum 16 November 2022

Startdatum 1 Dezember 2022

Enddatum 30 November 2027

Finanziert unter

• European Research Council (ERC)

Gesamtkosten

€ 1 687 470,00

EU-Beitrag

€ 1 687 470,00

Koordiniert durch

WEIZMANN INSTITUTE OF SCIENCE
Israel

Periodic Reporting for period 1 - ExTrA (Extratropical-Tropical interAction: A unified view on the extratropical impact on the subtropics and tropics at weather timescales)

Berichtszeitraum: 2022-12-01 bis 2025-05-31

Zusammenfassung vom Kontext und den Gesamtzielen des Projekts

An immediate, critical challenge facing humanity is the impact of weather extremes under a changing climate across the globe. Accurate weather forecasts and reliable climate projections rely on understanding the complex interactions between the atmospheric circulation and the water cycle on the weather timescale. Largely governed by deep clouds, tropical weather cannot be forecasted more than a day in advance, leaving almost half of humanity living in these regions even more vulnerable. Although extratropical (outside the tropics) flows can drastically affect tropical clouds and precipitation, this effect has not been systematically quantified or understood. The novelty of this proposal is in asking a new question: How do extratropical dynamics influence the (sub)tropics? We address this question by diagnosing flow from the extratropics in the form of dry, cold air intrusions into the tropics, which, as our preliminary results suggest, dramatically modify the atmospheric conditions and air-sea interaction in the tropics.
We will use a new global Lagrangian identification approach to study this extratropical-tropical interaction for the first time, and uniquely combine state-of-the-art data from multiple sources, including a measurement flight campaign, diagnostic tools, and multi-scale modelling. We will quantify the global spatiotemporal occurrence, variability and trends of these events; understand their underlying dynamics and precursors in midlatitudes and their impact on the water cycle; and assess their suitability as predictors of high-impact tropical weather. Our findings will advance not only the meteorological community but also the oceanographic, cloud physics and climate dynamics scientific communities, by providing a unified, global view of the interacting large-scale atmospheric dynamics, clouds and precipitation through their common weather timescales, paving the way for timely, accurate weather warnings.

Arbeit, die ab Beginn des Projekts bis zum Ende des durch den Bericht erfassten Berichtszeitraums geleistet wurde, und die wichtigsten bis dahin erzielten Ergebnisse

Six major scientific and technical achievements:
1. Quantifying and understanding the relevance of cross-equatorial dry intrusions for Indian summer monsoon precipitation. We revealed a novel driver of extreme monsoon precipitation being the feature in focus in the ERC project, i.e. dry intrusions (DIs) from the extratropics into the tropical latitudes. In this regional study, the DIs cross over from the southern Indian Ocean into the Arabian Sea in the northern hemisphere, which is a major moisture source to monsoon precipitation. By triggering intense ocean evaporation, the DIs activate a new moisture source, enhancing precipitation over the Indian subcontinent and causing floods (Rai and Raveh-Rubin 2023).
2. We established the definition of the Lagrangian DI feature for global climatological analyses. The DIs that are previously defined for the extratropics (i.e. descend at least 400 hPa in their first 48 h) now involve an additional requirement to cross equatorward of latitude 25° and remain in the lower troposphere (pressure larger than 700 hPa).
3. We quantified the association of DIs to ocean evaporation in the tropical latitude band within ±25° and find that they are indeed associated with the most extreme evaporation hotspots (see example snapshot in the attached figure).
4. We quantified the occurrence of wind and cold temperature extremes conditioned by DIs. The probability for cold extremes is over 4-fold in the presence of DIs in a global average, peaking in the subtropics (Klaider and Raveh-Rubin 2023), with ongoing work quantifying the global relevance to wind extremes, and compound (co-occurring wind and cold) extremes, suggesting an even higher conditional frequency compared to temperature alone.
5. We understand the dynamical drivers for the Lagrangian evolution of temperature anomalies of DIs, including temperature extremes. Using a Lagrangian-based temperature anomaly decomposition and budget, we achieved a novel compilation of >3 million DI trajectories, quantifying the leading role of adiabatic warming, compensated by cold advection and diabatic cooling.
6. The North Atlantic Waveguide, Dry Intrusion, and Downstream Impact Campaign (NAWDIC) is an international airborne field campaign planned for January-February 2026. In the framework of the ERC project we have prepared a DI diagnostic to be used in the campaign for flight planning. These tools, and others in preparation, are currently implemented into the mission support system.

Fortschritte, die über den aktuellen Stand der Technik hinausgehen und voraussichtliche potenzielle Auswirkungen (einschließlich der bis dato erzielten sozioökonomischen Auswirkungen und weiter gefassten gesellschaftlichen Auswirkungen des Projekts)

Not applicable at the moment.

A snapshot illustrating the close correspondence of ocean evaporation (shaded) to DIs (dots)