José M. Castanheira

The Hadley circulation is the primary large-scale meridional circulation in the tropics and is conventionally seen as axisymmetric. However, meridional dynamics in the tropics are far from zonally uniform and recent developments have highlighted the importance of contributions from regional and time confined meridional overturning circulations to the global Hadley regime.In the present work, we decompose the Hadley circulation into axisymmetric modes (AMs) which retain the linearized dynamics of an axisymmetric atmospheric circulation. Such modes are the normal mode solutions of the linearized axisymmetric equations of horizontal atmospheric motion, which coincide with the zonal wavenumber zero (k = 0) normal mode solutions to the linearized equations of horizontal atmospheric motion (Laplace tidal equations). We propose a method for the decomposition into AMs which draws similarities to previously developed local identification methods for equatorial waves ([1] and [2]).The diagnostic potential of the decomposition is shown by analysing the preferred AMs of the Hadley circulation and recalling their physical underpinnings. In the literature, axisymmetric theory and constraints are frequently employed in the study of zonally confined meridional circulations ([3]). Therefore, we also analyse the validity and applicability of the decomposition into AMs in the case of zonally confined regional overturning circulations. Our work aims to be a contribution to the study of different regional meridional overturning regimes and the analysis of the regional contributions to the global Hadley circulation. References:[1] Cruz, J.B., Castanheira, J.M. & DaCamara, C.C. (2024) Local identification of equatorial Kelvin waves in real-time operational forecasts. Quarterly Journal of the Royal Meteorological Society, 150(761), 2440–2457. https://doi.org/10.1002/qj.4717[2] Cruz, J.B., DaCamara, C.C. & Castanheira, J.M. (2025) Local identification of equatorial mixed Rossby–gravity waves. Quarterly Journal of the Royal Meteorological Society, 151(770), e4978. https://doi.org/10.1002/qj.4978[3] Geen, R., Bordoni, S., Battisti, D. S., & Hui, K. (2020). Monsoons, ITCZs, and the concept of the global monsoon. Reviews of Geophysics, 58, e2020RG000700. https://doi.org/10.1029/2020RG000700 This work is supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020- https://doi.org/10.54499/LA/P/0068/2020 , UID/50019/2025, https://doi.org/10.54499/UID/PRR/50019/2025 , UID/PRR2/50019/2025, UID/50017/2025 (doi.org/10.54499/UID/50017/2025) and LA/P/0094/2020 (doi.org/10.54499/LA/P/0094/2020).

Precipitation over the Iberian Peninsula (IP) is highly sensitive to atmospheric rivers (ARs)—long corridors of anomalous horizontal and poleward moisture transport from lower latitudes. This study aims to deepen the understanding of the mechanisms behind Iberian ARs and to quantify their contribution to precipitation over a long‐term period. We identify ARs affecting the IP in the ERA5 reanalysis with a 31‐km horizontal resolution and covering a 42‐year period (1979–2020). Additionally, we explore their association with the fronts of extratropical cyclones. Impact on IP precipitation is evaluated using regional observational gridded products (E‐OBS, IB01) and global reanalysis (ERA5). Three AR events (unnamed storm in October 1979, Storm Gong in January 2013, and Storm Ana in December 2017) were selected as case studies and analyzed in detail. During 1979–2020, the IP was affected by 580 persistent ARs (lasting at least 18 hr) with an annual mean of 13.8 ± 2.8 ARs and the northern IP coast being the most affected region. The year 2013 is marked by the highest total number of persistent ARs (20). Cold fronts were found more often in the vicinity of ARs than warm fronts. Case studies highlight changes in the relationship between ARs and fronts throughout their lifecycle. Precipitation associated with persistent ARs exhibits a strong north‐south gradient in the western IP with the highest contribution to annual precipitation occurring in the northern region (∼30%). Our study demonstrates the important role of ARs in the Iberian precipitation using the latest high‐resolution reanalysis and observational data.

We propose a local identification method for mixed Rossby–gravity (MRG) waves, which include both eastward mixed Rossby–gravity (EMRG) waves, also known as eastward inertia–gravity (EIG1) waves, and westward mixed Rossby–gravity (WMRG) waves. The method identifies MRG waves at specific longitudes using only a pair of EMRG and WMRG meridional structures. A criterion is proposed to estimate WMRG wave dominance within the identified MRG wave field. Propagation of WMRG waves can then be studied at specific longitudes. The method has the main advantages of (1) identifying MRG waves in data from a single longitude and pressure level without need for global data and (2) requiring very low computational cost to apply. Finally, it is found that MRG waves alone explain more than 90% of the variance of the latitudinal average of the equatorial meridional wind at 850 hPa and more than 95% at 200 hPa.

Equatorial waves (EWs) are synoptic to planetary-scale disturbances in the tropical atmosphere and are associated to a variety of tropical atmospheric phenomena. For instance, EWs can couple with convection, modulating a substantial fraction of cloud and rainfall variability in the tropics. Space-time filtering techniques that rely on the projection of data onto the structures of EWs are widely used in the literature. Such projection methods are employed with multiple purposes, including the unravelling of physical mechanisms underlying tropical atmospheric phenomena and the evaluation of numerical weather predictions in the tropics. However, most projection techniques rely on the global structures of these waves and, to our knowledge, there have not been efforts toward developing methodologies that identify EWs locally, i.e. over regions covering specific longitude ranges. This type of approach would highly decrease both the amount of data required and the computational power needed to identify EWs. Furthermore, it could potentially reduce the artificial effects local forcings may have on global projections.This work makes use of the meridional and zonal structures of the solutions to the free Laplace tidal equations, known as Hough vector harmonics. By exploiting the properties of these solutions, we propose a methodology that allows for the identification of EWs over specific longitude ranges and perform a local analysis of fundamental wave properties. This work was supported by IDL (UIDB/50019/2020) and CESAM (UIDP/50017/2020+UIDB/50017/2020+LA/P/0094/2020) through national funds by Fundação para Ciência e Tecnologia I.P./MCTES (FCT), Portugal.

Equatorial Kelvin waves (KWs) are associated with a variety of equatorial atmospheric phenomena, namely, tropical convection, cloud and precipitation variability, tropical cyclogenesis, the onset of monsoon season over Africa and India, and even the quasi‐biennial oscillation and the Madden–Julian oscillation. As such, if operational forecasts are to improve near the Tropics, it is important that they correctly represent KWs. This article aims at developing a novel methodology for identifying KWs in real‐time operational forecasts at specific longitudes using only the meridional structures of the solutions to the free Laplace tidal equations, known as Hough vector functions. The main advantages of this newly proposed methodology are that (a) it can identify KWs at a specific longitude and (b) very low computational cost is needed to apply it to real‐time operational forecasts. The application of the method to 2015–2017 European Centre for Medium‐range Weather Forecasts (ECMWF) analysis and operational forecast data reveals a systematic bias in the representation of KWs by the ECMWF operational forecast model.

The normal modes of Laplace's tidal equations seem to be a consolidated subject in the literature. However, up until now, no mode for the zonal wavenumber zero, (i.e., independent of longitude) had been identified as a mixed Rossby–gravity mode with physical meaning. Moreover, the meridional structures of westward gravity modes were not considered to be similar to the meridional structures of the corresponding inertio‐gravity modes for non‐zero wavenumbers (k>0). Here, a mixed Rossby–gravity mode with zero zonal wavenumber () is identified for the first time, and a new classification of the normal modes for the zonal wavenumber zero is proposed. Under the new classification, all the meridional structures corresponding to zero wavenumber modes are similar to the meridional structures of their respective non‐zero wavenumber modes. Moreover, using the new classification, all modes of a given type show smooth frequency variations with the zonal wavenumber, which approach asymptotically the analytic dispersion curves from the ‐plane shallow‐water equations as the fluid height tends to zero.

In this communication, we will present results from an analysis of the variability of the vertically averaged (i.e., barotropic) atmospheric circulation simulated by climate models, which integrate the current Coupled Model Intercomparison Project (CMIP6). The variabilities in two ensembles of Atmospheric Model Intercomparison Project (AMIP) simulations were compared with the variabilities in two ensembles of fully coupled simulation counterparts of the current CMIP6 (Castanheira and Marques, 2022).The atmospheric models simulate less variability of the barotropic atmospheric circulation over the Northern Atlantic and more variability over the North Pacific when compared with the corresponding variabilities in the ERA5 reanalysis (“observations”), at intraseasonal and interannual scales. When integrated over the whole globe, the variability in the coupled climate simulations is smaller than the variability in the corresponding AMIP simulations. The smaller global variability of the coupled simulations results in no mean overestimation of the subtropical jet variability in the North Pacific, but further underestimation of the jet stream variability in the Northern Atlantic. The results suggest that the reduction of the biases in the barotropic atmospheric variability over the North Pacific, in the coupled climate simulations, is achieved through compensating biases in the mean Sea Surface Temperatures (SSTs). Moreover, the reduction of the positive biases in the North Pacific seems to be associated with a reduction of the excitation of the most unstable barotropic mode of the atmospheric circulation, which contributes also to a reduction of the barotropic atmospheric variability in the North Atlantic region. Acknowledgements: The CESAM is supported by FCT/MCTES through the project UIDP/50017/2020+ UIDB/ 50017/20201+LA/P/0094/2020.ReferencesCastanheira, J. M., Marques, C. A. F. (2022). Biases of the Barotropic Atmospheric Circulation Variability in CMIP6 Models. Journal of Climate, Vol. 35, 5071–5085, DOI: 10.1175/JCLI-D-21-0581.1.

Equatorial waves are essential features of the tropical atmosphere dynamics, and their diagnosis is relevant for both operational and climate analysis. Here, we present a method for the identification of equatorial waves using the multivariate spatial structures of the eigensolutions of the linearized shallow‐water equations over the sphere, named Hough function vectors. The method does not require any a priori space–time filtering to separate westward‐ and eastward‐moving waves, and it can be applied to instantaneous or daily circulation fields. The wave types (Kelvin, Rossby, mixed Rossby–gravity, inertio‐gravity) are unambiguously identified by the intrinsic dynamical relationship between the horizontal wind components and geopotential fields represented in the multivariate meridional structures of the Hough function vectors. Both free and convectively coupled equatorial waves can be diagnosed. Moreover, different time filterings can be applied a posteriori, depending on the purposes of the analysis. This article is written as a proof of concept, and direct comparisons are made with results of other methods recently reviewed in a work published in this journal.

Two ensembles of Atmospheric Model Intercomparison Project (AMIP) simulations, in the scope of the current Coupled Model Intercomparison Project (CMIP6), are compared with their fully coupled counterparts. The atmospheric models simulate less barotropic atmospheric circulation variability over the North Atlantic and more barotropic atmospheric circulation variability over the North Pacific when compared with reanalysis variability, at intraseasonal and interannual scales. The coupled climate simulations have smaller global barotropic variability than the corresponding AMIP simulations. The smaller variability of the coupled simulations results in no mean overestimation of the subtropical jet variability in the North Pacific, but further underestimation of the jet stream variability in the North Atlantic. The results suggest that the reduction of the biases, in the North Pacific barotropic atmospheric variability, of coupled climate simulations is achieved through compensating biases in the mean sea surface temperatures (SSTs). Moreover, the reduction of the positive biases in the North Pacific seems to be associated with a reduction of the excitation of the most unstable barotropic mode of the atmospheric circulation, which contributes also to a reduction of the barotropic atmospheric variability in the North Atlantic.

An analysis of the energy equation of the three‐dimensional normal modes of the atmospheric circulation provides evidence on the behaviour of the most important equatorial waves which compose the Madden–Julian Oscillation. They are the equatorial Rossby waves of zonal wavenumber 1 and 3 and the Kelvin waves of zonal wavenumber 1. In the mean, the equatorial Rossby waves gain energy by convective heating and atmospheric column stretching and transfer it to the circulation field, whereas the Kelvin waves draw energy from the circulation field and dissipate it by surface friction. The convergence associated with the Kelvin waves contributes to the moisture fuelling of the convection centre. Therefore, the coupling between equatorial Rossby and Kelvin waves is mediated by convective heating/column stretching excitation of Rossby waves and moisture convergence by Kelvin waves.