Max-Planck-Institute for Meteorology: Extremely warm European summers are preceded by North Atlantic Ocean heat accumulation

In a new study, Lara Wallberg, together with Laura Suarez-Gutierrez, Daniela Matei, and Wolfgang Müller have investigated the relationship between extremely warm European summers and changes in the North Atlantic Ocean. They found a mechanism through which extremely warm European summers are preceded by the accumulation of heat in the North Atlantic Ocean on sub-decadal time scales of five to ten years. Thereby, anomalies of the ocean heat transport and associated ocean heat content changes result in ocean–atmosphere heat flux anomalies leading to extremely warm European summers several years later.

Figure1: Schematic sketch illustrating the described mechanism. The blue arrows illustrate the increasing North Atlantic current over the course of three years; the pink crosses indicate the increase in ocean heat content and accumulation of heat, and the orange belt illustrates the jet stream. The thermometer illustrates the extremely warm European summers. Wallberg et al., 2024, CC BY 4.0 DEED

So far, the assessment of drivers for extreme temperatures on sub- to multi-decadal timescales was limited, and their relevance for extreme summers thus uncertain. The authors address this gap and present a comprehensive explanation for the occurrence of extremely warm European summers in sub-decadal warm phases, and their relation to the heat accumulation in the ocean that occurs several years in advance.

Their results show that European summer temperatures are dominated by sub-decadal variability over large parts of the continent. This sub-decadal variability drives a mechanism associated with a forced damped oscillation of the North Atlantic that influences extremely warm summers on sub-decadal timescales. Thereby, the North Atlantic Ocean responds to the atmospheric momentum and heat forcing by redistributing heat and water masses, and delayed feedback back to the atmosphere 5-10 years later. During this process heat is accumulated alongside the North Atlantic Current and releases heat into the atmosphere during summer. Three years prior to extremely warm European summers ocean heat content starts to increase and accumulates heat along the North Atlantic current around 40°N (Fig. 1, first globe). Two to one year prior to extremely warm European summers this heat accumulation further intensifies and the North Atlantic current starts to shift northwards. This results in warm subtropical water masses reaching higher latitudes. Here, high anomalies of the ocean heat transport, which affect the ocean heat content, lead to alterations in the temperature gradients between the ocean and atmosphere and affecting the ocean-atmosphere heat flux (Fig. 1, second and third globe globe). During extremely warm European summers this accumulated heat is released mainly through the subpolar gyre ocean heat transport to the atmosphere, leading to an increased ocean-atmosphere heat flux. The released heat causes then an above-average warming of the atmosphere reaching even high altitudes. This warming of the atmosphere leads via the jet stream displacement and enhanced atmospheric blocking conditions to extremely warm European summers (Fig. 1, fourth globe).

The finding that the North Atlantic Ocean heat accumulation impacts the occurrence of extremely warm summers over Central Europe on sub-decadal timescales is crucial for reducing the uncertainties in both attribution and prediction of high-impact events, which in turn facilitates preparedness and the efficiency of adaptation and mitigation efforts.

Original publication

Wallberg, L., Suarez-Gutierrez, L., Matei, D., and Müller, W. A.: Extremely warm European summers preceded by sub-decadal North Atlantic ocean heat accumulation, Earth Syst. Dynam., 15, 1–14, https://doi.org/10.5194/esd-15-1-2024, 2024.

Contact

Dr. Lara Wallberg
Max Planck Institute for Meteorology
lara.wallberg@we dont want spammpimet.mpg.de

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