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This study, using the peak electron density of Ionospheric F‐region from the Global‐scale Observations of the Limb and Disk reports, for the first time, a unique phenomenon: the rapid reversal of the intensity of the Equatorial Ionization Anomaly (EIA) crests between the hemispheres during the main phase of 23 April 2023, geomagnetic storm. The Prompt Penetration Electric Field amplified the intensity of both EIA crests. However, the enhancement at the southern crest began to decay within an hour, while the northern crest began to strengthen. Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model simulations indicate that trans‐equatorial wind played a key role in these variations. Wavelet periodograms of ground‐based Total Electron Content measurements confirmed the presence of Traveling Ionospheric Disturbances (TIDs). Storm‐induced winds and TIDs likely changed the altitude of the F‐peaks and the recombination rates between crests, and plasma transport by trans‐equatorial winds also contributed to this rare phenomenon.

The impact of tropical Atlantic Ocean variability modes in the variability of the upper-ocean circulation has been investigated. For this purpose, we use three oceanic reanalyses, an interannual forced-ocean simulation, and satellite data for the period 1982–2018. We have explored the changes in the main surface and subsurface ocean currents during the emergence of Atlantic meridional mode (AMM), Atlantic zonal mode (AZM), and AMM–AZM connection. The developing phase of the AMM is associated with a boreal spring intensification of North Equatorial Countercurrent (NECC) and a reinforced summer Eastern Equatorial Undercurrent (EEUC) and north South Equatorial Current (nSEC). During the decaying phase, the reduction of the wind forcing and zonal sea surface height gradient produces a weakening of surface circulation. For the connected AMM–AZM, in addition to the intensified NECC, EEUC, and nSEC in spring, an anomalous north-equatorial wind curl excites an oceanic Rossby wave (RW) that is boundary-reflected into an equatorial Kelvin wave (KW). The KW reverses the thermocline slope, weakening the nSEC and EUC in boreal summer and autumn, respectively. During the developing spring phase of the AZM, the nSEC is considerably reduced with no consistent impact at subsurface levels. During the autumn decaying phase, the upwelling RW-reflected mechanism is activated, modifying the zonal pressure gradient that intensifies the nSEC. The NECC is reduced in boreal spring–summer. Our results reveal a robust alteration of the upper-ocean circulation during AMM, AZM, and AMM–AZM, highlighting the decisive role of ocean waves in connecting the tropical and equatorial ocean transport.

This study uses two reanalysis datasets and ocean model experiments to examine the dynamics of the interannual variability of the equatorial currents in the Indian Ocean, and to quantify the effects of the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) climate modes on the currents. Strong interannual variability of the equatorial currents mainly occurs in the upper-central basin (UCB) where the Wyrtki jets are located, and in the subsurface-eastern basin (SEB) where the equatorial undercurrent is located. Equatorial waves directly forced by equatorial winds dominate the interannual current anomalies in both the UCB and the SEB, and the reflected waves have a secondary role. The reflected waves tend to weaken the current anomalies in the UCB but intensify the currents in the SEB. In general, ENSO and the IOD have a comparable effect on the interannual current anomalies in the SEB, but the IOD has a larger role than ENSO in the UCB. In some years, either ENSO or the IOD may play a dominant role. Composite analysis suggests that the interannual current anomalies occur about two months prior to the peaks the climate modes. As ENSO and the IOD have apparent seasonality, the current anomalies mainly occur during August to October for pure IOD years, and during October to December for pure ENSO years. The co-occurrence of ENSO and the IOD enhances their respective impact, resulting in the surface current anomalies lasting from July to December and the subsurface current anomalies from July to March of the following year.

Teleconnections between the Quasi Biennial Oscillation (QBO) and the Northern Hemisphere zonally averaged zonal winds, mean sea level pressure (mslp) and tropical precipitation are explored. The standard approach that defines the QBO using the equatorial zonal winds at a single pressure level is compared with the empirical orthogonal function approach that characterizes the vertical profile of the equatorial winds. Results are interpreted in terms of three potential routes of influence, referred to as the tropical, subtropical and polar routes. A novel technique is introduced to separate responses via the polar route that are associated with the stratospheric polar vortex, from the other two routes. A previously reported mslp response in January, with a pattern that resembles the positive phase of the North Atlantic Oscillation under QBO westerly conditions, is confirmed and found to be primarily associated with a QBO modulation of the stratospheric polar vortex. This mid-winter response is relatively insensitive to the exact height of the maximum QBO westerlies and a maximum positive response occurs with westerlies over a relatively deep range between 10 and 70 hPa. Two additional mslp responses are reported, in early winter (December) and late winter (February/March). In contrast to the January response the early and late winter responses show maximum sensitivity to the QBO winds at ∼ 20 and ∼ 70 hPa respectively, but are relatively insensitive to the QBO winds in between (∼ 50 hPa). The late winter response is centred over the North Pacific and is associated with QBO influence from the lowermost stratosphere at tropical/subtropical latitudes in the Pacific sector. The early winter response consists of anomalies over both the North Pacific and Europe, but the mechanism for this response is unclear. Increased precipitation occurs over the tropical western Pacific under westerly QBO conditions, particularly during boreal summer, with maximum sensitivity to the QBO winds at 70 hPa. The band of precipitation across the Pacific associated with the Inter-tropical Convergence Zone (ITCZ) shifts southward under QBO westerly conditions. The empirical orthogonal function (EOF)-based analysis suggests that this ITCZ precipitation response may be particularly sensitive to the vertical wind shear in the vicinity of 70 hPa and hence the tropical tropopause temperatures.

Over the past two decades, El Niño events have weakened on average and their sea surface temperature (SST) anomalies shifted westward towards the central Pacific. Moreover, the intertropical convergence zone (ITCZ), which typically migrates southward from its northerly position during El Niño events, has not crossed the Equator since 1998. The causes of these changes remain under debate1–5. Here, using in situ, satellite and atmospheric reanalysis data, we show they can be related to a multidecadal strengthening of cross-equatorial winds in the eastern Pacific. This gradual strengthening of meridional winds is unlikely to be caused by El Niño/Southern Oscillation (ENSO) changes, and contains signals forced both locally and from outside the tropical Pacific, probably from the tropical North Atlantic. Coupled model simulations in which the observed cross-equatorial wind strengthening is superimposed successfully reproduce the key features of the recent changes in tropical climate. In particular, the tropical mean state experiences a ‘La Niña-like’ change, the ENSO amplitude weakens by about 20%, the centre of the SST anomalies shifts westward and the ITCZ now rarely crosses the Equator. Thus, cross-equatorial winds are found to modulate tropical Pacific mean state and variability, with implications for quantifying projected changes in ENSO under anthropogenic warming.

We describe the baseline model configuration and simulation characteristics of the Geophysical Fluid Dynamics Laboratory (GFDL)'s Atmosphere Model version 4.1 (AM4.1), which builds on developments at GFDL over 2013–2018 for coupled carbon‐chemistry‐climate simulation as part of the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL's AM4.0 development effort, which focused on physical and aerosol interactions and which is used as the atmospheric component of CM4.0, AM4.1 focuses on comprehensiveness of Earth system interactions. Key features of this model include doubled horizontal resolution of the atmosphere (~200 to ~100 km) with revised dynamics and physics from GFDL's previous‐generation AM3 atmospheric chemistry‐climate model. AM4.1 features improved representation of atmospheric chemical composition, including aerosol and aerosol precursor emissions, key land‐atmosphere interactions, comprehensive land‐atmosphere‐ocean cycling of dust and iron, and interactive ocean‐atmosphere cycling of reactive nitrogen. AM4.1 provides vast improvements in fidelity over AM3, captures most of AM4.0's baseline simulations characteristics, and notably improves on AM4.0 in the representation of aerosols over the Southern Ocean, India, and China—even with its interactive chemistry representation—and in its manifestation of sudden stratospheric warmings in the coldest months. Distributions of reactive nitrogen and sulfur species, carbon monoxide, and ozone are all substantially improved over AM3. Fidelity concerns include degradation of upper atmosphere equatorial winds and of aerosols in some regions. Plain Language Summary GFDL has developed a coupled chemistry‐climate Atmospheric Model (AM4.1) as part of its fourth‐generation coupled model development activities. AM4.1 includes comprehensive atmospheric chemistry for representing ozone and aerosols and has been developed for use in chemistry and air quality applications, including advanced land‐atmosphere‐ocean coupling. With fidelity near to that of AM4.0, AM4.1 features vastly improved representation of climate mean patterns and variability from previous GFDL atmospheric chemistry‐climate models. Key Points A new atmospheric chemistry‐climate model (AM4.1) has been developed for the Geophysical Fluid Dynamics Laboratory (GFDL)'s fourth‐generation model suite AM4.1 includes an advanced dynamical core and physical parameterizations, with enhanced vertical resolution and revised aerosol and chemistry interactions The AM4.1 model exhibits substantially enhanced fidelity compared to previous‐generation GFDL atmospheric models

By analyzing in situ observations and conducting a series of ocean general circulation model experiments, this study investigates the physical processes controlling intraseasonal variability (ISV) of the Equatorial Undercurrent (EUC) of the Indian Ocean. ISV of the EUC leads to time-varying water exchanges between the western and eastern equatorial Indian Ocean. For the 2001–14 period, standard deviations of the EUC transport variability are 1.92 and 1.77 Sv (1 Sv ≡ 10 6 m 3 s −1 ) in the eastern and western basins, respectively. The ISV of the EUC is predominantly caused by the wind forcing effect of atmospheric intraseasonal oscillations (ISOs) but through dramatically different ocean dynamical processes in the eastern and western basins. The stronger ISV in the eastern basin is dominated by the reflected Rossby waves associated with intraseasonal equatorial zonal wind forcing. It takes 20–30 days to set up an intraseasonal EUC anomaly through the Kelvin and Rossby waves associated with the first and second baroclinic modes. In the western basin, the peak intraseasonal EUC anomaly is generated by the zonal pressure gradient force, which is set up by radiating equatorial Kelvin and Rossby waves induced by the equatorial wind stress. Directly forced and reflected Rossby waves from the eastern basin propagate westward, contributing to intraseasonal zonal current near the surface but having weak impact on the peak ISV of the EUC.

The Warm Water Volume (WWV), a proxy for the equatorial Pacific heat content, is the most widely used oceanic precursor of the El Niño Southern Oscillation (ENSO). The standard interpretation of this lead relation in the context of the recharge oscillator theory is that anomalous easterlies during, e.g. La Niña, favour a slow recharge of the equatorial band that will later favour a transition to El Niño. Here we demonstrate that WWV only works as the best ENSO predictor during boreal spring, i.e. during ENSO onset, in both observations and CMIP5 models. At longer lead times, the heat content in the western Pacific (WWVW) is the best ENSO predictor, as initially formulated in the recharge oscillator theory. Using idealised and realistic experiments with a linear continuously stratified ocean model, and a comprehensive wave decomposition method, we demonstrate that spring WWV mostly reflects the fast Kelvin wave response to wind anomalies early in the year, rather than the longer-term influence of winds during the previous year. WWV is hence not an adequate index of the slow recharge invoked in the recharge oscillator. The WWVW evolution before spring is dominated by forced Rossby waves, with a smaller contribution from the western boundary reflection. WWVW can be approximated from the integral of equatorial wind stress over the previous ~ 10 months, thus involving a longer-term time scale than WWV main time scale (~ 3 months). We hence recommend using WWVW rather than WWV as an index for the slow recharge before the spring predictability barrier.

Coupled climate models used for long-term future climate projections and seasonal or decadal predictions share a systematic and persistent warm sea surface temperature (SST) bias in the tropical Atlantic. This study attempts to better understand the physical mechanisms responsible for the development of systematic biases in the tropical Atlantic using the so-called Transpose-CMIP protocol in a multi-model context. Six global climate models have been used to perform seasonal forecasts starting both in May and February over the period 2000–2009. In all models, the growth of SST biases is rapid. Significant biases are seen in the first month of forecast and, by 6 months, the root-mean-square SST bias is 80% of the climatological bias. These control experiments show that the equatorial warm SST bias is not driven by surface heat flux biases in all models, whereas in the south-eastern Atlantic the solar heat flux could explain the setup of an initial warm bias in the first few days. A set of sensitivity experiments with prescribed wind stress confirm the leading role of wind stress biases in driving the equatorial SST bias, even if the amplitude of the SST bias is model dependent. A reduced SST bias leads to a reduced precipitation bias locally, but there is no robust remote effect on West African Monsoon rainfall. Over the south-eastern part of the basin, local wind biases tend to have an impact on the local SST bias (except in the high resolution model). However, there is also a non-local effect of equatorial wind correction in two models. This can be explained by sub-surface advection of water from the equator, which is colder when the bias in equatorial wind stress is corrected. In terms of variability, it is also shown that improving the mean state in the equatorial Atlantic leads to a beneficial intensification of the Bjerknes feedback loop. In conclusion, we show a robust effect of wind stress biases on tropical mean climate and variability in multiple climate models.

The regional sea level variability and its projection amidst the global sea level rise is one of the major concerns for coastal communities. The dynamic sea level plays a major role in the observed spatial deviations in regional sea level rise from the global mean. The present study evaluates 27 climate model simulations from the sixth phase of the Coupled Model Intercomparison Project (CMIP6) for their representation of the historical mean states, variability and future projections for the Indian Ocean. Most models reproduce the observed mean state of the dynamic sea level realistically; however, consistent positive bias is evident across the latitudinal range of the Indian Ocean. The strongest sea level bias is seen along the Antarctic Circumpolar Current (ACC) regime owing to the stronger than observed south Indian Ocean westerlies and its equatorward bias. This equatorward shift of the wind field also results in a stronger positive windstress curl across the southeasterly trade wind regime in the southern tropical basin and an easterly wind bias along the equatorial waveguide. Owing to the anomalous easterly equatorial winds, the thermocline in the eastern tropical basin is shallower in the models than observed, resulting in enhanced variability there. Such spurious variability in the eastern part of the basin causes models to become biased towards the dipole zonal mode or Indian Ocean dipole patterns in the tropics. In the north Indian Ocean, the summer monsoon winds are weak in the model leading to weaker coastal upwelling and positive sea level bias along the western Arabian Sea. Further, it is noted that the high-resolution models compare better in simulating the sea level variability, particularly in the eddy-dominated regions like the ACC regime in interannual timescale. However, these improved variabilities do not necessarily produce a better mean state likely due to the spurious enhanced mixing driven by parametrizations set in these high-resolution models. Finally, the overall pattern of the projected dynamic sea level rise is similar for the mid (SSP2-4.5) and high-end (SSP5-8.5) scenarios, except that the magnitude is higher under the high emission situation. Notably, the projected dynamic sea level change is milder when only the best-performing models are used compared to the complete ensemble.