Metrics of the Hadley circulation strength and associated circulation trends

Pikovnik, Matic; Zaplotnik, Žiga; Boljka, Lina; Žagar, Nedjeljka

doi:10.5194/wcd-3-625-2022

Cited by 12 publications

(7 citation statements)

References 39 publications

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“…11) . Consequently, analyzing a spatially averaged metric for the HC strength 48 , 49 (Ψ avg , the averaged streamfunction between 10° and 25° and between 1000 mb and 100 mb) yields similar results; the unprecedented human-induced weakening of the HC reverses a multi-centennial forced strengthening of the circulation (Supplementary Figs. 12 – 14) .…”

Section: Methodsmentioning

confidence: 79%

Anthropogenic forcings reverse a simulated multi-century naturally-forced Northern Hemisphere Hadley cell intensification

Hess,

Chemke

2024

Nat Commun

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The Hadley circulation plays a central role in determining the location and intensity of the hydrological cycle in tropical and subtropical latitudes. Thus, the human-induced historical and projected weakening of the Northern Hemisphere Hadley circulation has considerable societal impacts. Yet, it is currently unknown how unparalleled this weakening is relative to the response of the circulation to natural forcings in past centuries. Here, using state-of-the-art climate models, we show that in contrast to the recent and future human-induced Hadley circulation weakening, natural forcings acted to intensify the circulation by cooling the climate over the last millennium. The reversal of a naturally-forced multi-centennial trend by human emissions highlights their unprecedented impacts on the atmospheric circulation. Given the amplifying effect of natural forcings on the Hadley circulation, our analysis stresses the importance of adequately incorporating natural forcings in climate model projections to better constrain future tropical climate changes.

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Section: Methodsmentioning

confidence: 79%

Anthropogenic forcings reverse a simulated multi-century naturally-forced Northern Hemisphere Hadley cell intensification

Hess,

Chemke

2024

Nat Commun

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“…9. Following previous studies (Oort and Yienger, 1996;Cook, 2003;Pikovnik et al, 2022), the ψ function is defined by a vertical integration of the zonally-averaged meridional wind v as…”

Section: Discussion Of the Resultsmentioning

confidence: 99%

Reply on RC2

Hollowed

2024

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“…4. In fact, the unbalanced zonal-mean flow is dominated by the Hadley circulation (Pikovnik et al, 2022). It weakens with positive SST bias (EXP_POS, EXP_10N, and EXP_IOD), whereas it strengthens with monopolar negative SST bias (EXP_NEG).…”

Section: Total Biasesmentioning

confidence: 97%

“…The TROP and EXTR balanced biases exhibit teleconnections, which can be demonstrated by using the stationary wave activity flux (WAF; Plumb, 1985) and the Rossby wave source (RWS; Sardeshmukh and Hoskins, 1988). The WAF is an indicator of the propagation of the Rossby wave activity, and the RWS denotes the wave forcing, which takes the form −∇ • v χ (ζ + f ), with v χ representing the divergent wind, ζ the relative vorticity, and f the Coriolis parameter.…”

Section: Mechanism Of Bias Teleconnectionsmentioning

confidence: 99%

Atmospheric bias teleconnections in boreal winter associated with systematic sea surface temperature errors in the tropical Indian Ocean

Zhao,

Žagar,

Lunkeit

et al. 2023

Weather Clim. Dynam.

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Abstract. Coupled climate models suffer from significant sea surface temperature (SST) biases in the tropical Indian Ocean (TIO), leading to errors in global climate predictions. In this study, we investigate the local and remote effects of the TIO SST bias on the simulated atmospheric circulation and spatio-temporal variability – bias teleconnections. A set of century-long simulations forced by idealized SST perturbations, which resemble various (monopolar or dipolar, positive or negative) TIO SST biases in coupled climate models, are conducted with an intermediate-complexity atmospheric model. Bias teleconnections with a focus on boreal wintertime are analysed using the normal-mode function (NMF) decomposition, which can differentiate between balanced and unbalanced flows across spatial scales. The results show that the atmospheric circulation biases caused by the TIO SST bias have the Gill–Matsuno-type pattern in the tropics and Rossby-wave-train structure in the extratropics, similar to the steady-state response to tropical heating perturbations. The teleconnections between the tropical and extratropical biases are set up by Rossby wave activity flux emanating from the subtropics. Over 90 % of the bias variance (i.e. the square of the bias amplitude) is contained in zonal wavenumbers k≤5. The northward shift of the SST bias away from the Equator weakens the amplitude but does not change the spatial structure of the atmospheric response. Besides, the positive SST bias produces stronger bias teleconnections than the negative one of the same size and magnitude. In the NMF framework, the change in the spatial variance of the time-mean state (i.e. energy) is equal to the sum of the bias variance and the covariance between the circulation bias and the reference state (i.e. bias covariance). Due to the TIO SST biases, the global unbalanced zonal-mean (k=0) flow energy decreases, whereas its balanced counterpart increases. These changes primarily arise from the strong bias covariance. For k>0, both the global unbalanced and the tropical balanced energies increase in the case of a monopolar SST bias and decrease in the case of a dipolar SST bias. The increase appears mainly as the bias variance, whereas the decrease is associated with a strong negative bias covariance at k=1 and 2. In contrast, the extratropical balanced wave energy decreases (increases) when the TIO SST bias is positive (negative), which is mainly associated with the bias covariance at k=1. The change in the interannual variance (IAV) is contingent upon the sign of the TIO SST bias. A positive bias reduces, whereas a negative one increases, the IAV in both balanced and unbalanced flows. Geographically, large IAV changes are observed in the tropical Indo-West Pacific region, Australia, South and Northeast Asia, the Pacific-North America region, and Europe, where the background IAVs are strong.

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Metrics of the Hadley circulation strength and associated circulation trends

Cited by 12 publications

References 39 publications

Anthropogenic forcings reverse a simulated multi-century naturally-forced Northern Hemisphere Hadley cell intensification

Anthropogenic forcings reverse a simulated multi-century naturally-forced Northern Hemisphere Hadley cell intensification

Reply on RC2

Atmospheric bias teleconnections in boreal winter associated with systematic sea surface temperature errors in the tropical Indian Ocean

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