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  • Author or Editor: Henk A. Dijkstra x
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Elian Vanderborght
,
Jonathan Demaeyer
,
Georgy Manucharyan
,
Woosok Moon
, and
Henk A. Dijkstra

Abstract

In recent theory trying to explain the origin of baroclinic low-frequency atmospheric variability, the concept of eddy memory has been proposed. In this theory, the effect of synoptic-scale heat fluxes on the planetary-scale mean flow depends on the history of the mean meridional temperature gradient. Mathematically, this involves the convolution of a memory kernel with the mean meridional temperature gradient over past times. However, the precise shape of the memory kernel and its connection to baroclinic wave dynamics remains to be explained. In this study we use linear and proxy response theory to determine the shape of the memory kernel of a truncated two-layer quasigeostrophic atmospheric model. We find a memory kernel that relates the eddy heat flux to the zonal mean meridional temperature gradient on time scales greater than 2 days. Although the shape of the memory kernel is complex, we show that it may be well approximated as an exponential, particularly when reproducing baroclinic low-frequency intraseasonal modes of variability. By computing the terms in the Lorenz energy cycle, we find that the shape of the memory kernel can be linked to the finite time that growing baroclinic instabilities require to adapt their growth properties to the local zonal mean atmospheric flow stability. Regarding the explanation for observed baroclinic annular modes in the Southern Hemisphere, our results suggest that it is physical for these modes to be derived directly from the thermodynamic equation by considering an exponentially decaying memory kernel, provided accurate estimates of the necessary parameters are incorporated.

Significance Statement

The goal of this study was to derive the memory of the zonal mean temperature field contained in eddy heat fluxes. To do this we used recent developments in a theory stemming from statistical mechanics, called proxy response theory. This theory facilitated direct numerical computations of the parameterization that links eddy heat fluxes to the zonal mean temperature field. Notably, this parameterization incorporates a crucial memory component, which we demonstrated to be essential in explaining the periodicity of low-frequency modes of variability, specifically the baroclinic annular mode (BAM). Understanding the role of memory as a driver of this variability holds great significance, as the BAM constitutes a dominant pattern of large annular variability within the Southern Hemisphere circulation. Enhanced comprehension of this driver, which is memory, can lead to improved understanding and predictive capabilities concerning observed annular weather patterns.

Open access
Paul C. F. van der Vaart
,
Henk A. Dijkstra
, and
Fei Fei Jin

Abstract

The equatorial tropical Pacific climate system is a delicate coupled system in which winds driven by gradients of sea surface temperature (SST) within the basin interact with the ocean circulation to maintain SST gradients. This results in a time mean state having a strong zonal temperature contrast along the equator with an eastern cold tongue and a western warm pool. By the same coupled processes, interannual variability, known as the El Niño–Southern Oscillation (ENSO), is present in the Pacific. This variability can be attributed to an oscillatory coupled mode, the ENSO mode, in the equatorial ocean–atmosphere system. Using a Zebiak–Cane-type intermediate coupled model, the coexistence of an eastern cold tongue in the annual mean state and ENSO in the Pacific climate system is investigated. The ENSO mode arises as a robust oscillatory mode on a coupled mean state and becomes unstable if the cold tongue of the mean state is sufficiently strong. The origin of this mode, its propagation mechanism, its sensitivity to parameters, and its relation to the spatial structure of the annual mean state are considered.

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