Explore the vast range of titles available.

To what extent the CMIP6 models are capable of simulating the location and amplitude of zonal wind anomalies in tropical Pacific during the mature phase of ENSO is examined. While most of the models capture the southward shift of the maximum zonal wind anomaly, the simulated locations are less southward and more westward and the simulated intensities are weaker compared to the observation. An anti-symmetric momentum budget analysis reveals that the southward shift in the models results from the anomalous meridional advection, consistent with that derived from the observation. An inter-model regression analysis reveals that the longitudinal and latitudinal locations of the model zonal wind response depend on the mean precipitation pattern in the tropical western Pacific (WP). A stronger southward (eastward) mean precipitation gradient leads to a more southward (eastward) location. In addition, the simulated zonal wind intensity is determined by both area-averaged mean precipitation over WP and anomalous SST intensity in the equatorial eastern Pacific. Therefore, an improved mean precipitation simulation in WP may help reproduce realistic atmospheric responses to ENSO. Graphical abstract

This study focuses on the impact of the stratospheric quasi-biennial oscillation (QBO) on the early stage of the Indian summer monsoon (ISM) in May and June, which has thus far been an ambiguous topic of research. It is found that the 50-hPa QBO in the preceding winter and spring is significantly and negatively correlated with precipitation in the southern Arabian Sea and central India in May, which shifts northward to northern India in June. This correlation is nearly the opposite for the 10-hPa and 20-hPa QBO. An easterly phase of the 50-hPa QBO corresponds to a colder and higher tropopause over the subtropical ISM region which is related to vigorous convection over India. Meanwhile, the QBO-related meridional dipole pattern of zonal wind from the stratosphere to troposphere in the subtropics and mid-latitudes connects to an anomalous high in the upper troposphere across the subtropical land and the northern Arabian Sea, which causes an anomalous descent and in situ adiabatic heating. This heating supports an enhanced meridional land-sea thermal contrast and thus an early and strong ISM. The situation for westerly 50-hPa QBO is generally the opposite. The climate models from the Coupled Model Intercomparison Project Phases 6 (CMIP6) can generally reproduce the QBO–ISM relationship in June (but not in May), though with some discrepancies from the observation. Inter-model comparison demonstrates that better representation of the QBO–ISM correlation depends well on a better simulation of the QBO-related meridional dipole of zonal wind in the subtropical ISM region.

We use multi‐year observations of cross‐track winds (u) from the CHAllenging Minisatellite Payload (CHAMP) and the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) to calculate third‐order structure functions in the thermosphere as a function of horizontal separation (s). They are computed using the mean (〈δu3〉) and the median 〈δu3〉med $\\left({\\langle \\delta {u}^{3}\\rangle }_{\\text{med}}\\right)$ and implemented over non‐polar satellite paths in both hemispheres. On height averages, 〈δu3〉 is shown to scale with s2 for s ≃ 80–1,000 km, in agreement with equivalent estimates in the lower atmosphere from aircraft observations. Conversely, 〈δu3〉med ${\\langle \\delta {u}^{3}\\rangle }_{\\text{med}}$ follows an s3 power law for almost the whole s range, consistent with the two‐dimensional turbulence scaling law for a direct enstrophy cascade. These scaling laws appear independent of winds in distinct atmospheric regions. Furthermore, the functions are predominantly positive, indicating a preferential cyclonic motion for the wind. Plain Language Summary The dynamics of upper atmosphere winds differ significantly from those at lower altitudes, with larger magnitudes and increased sensitivity to solar events. Satellites, especially those in polar orbits, offer an effective means of studying these winds, particularly their East‐West component. To mitigate the chaotic nature of individual measurements influenced by various physical processes, it is common to compute wind averages. A particular way of doing this is by calculating the so‐called third‐order structure functions (SFs), a statistical quantity that provides information on the underlying turbulence processes. The third‐order SFs of satellites' zonal wind observations present two main characteristics. First, they are consistently positive, predicting a preferential cyclonic rotational motion. This is, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Second, and more importantly, the functions display the same type of dependence on the horizontal distance as third‐order SFs of winds in the lower atmosphere. This suggests that similar underlying large‐scale turbulence mechanisms may be at play. Key Points Third‐order structure functions of zonal winds in the thermosphere at mid‐ and low‐ to mid‐latitudes are calculated Meso‐ and Synoptic‐scale structures share a preferential cyclonic motion in the thermosphere Measured scaling laws of third‐order structure functions seem to be independent of the atmospheric region

The characteristics of anomalous circulations during spring associated with the climate shift of the South China Sea summer monsoon (SCSSM) onset in 1993/1994 and its physical causes are investigated. It is found that the interdecadal shift of SCSSM onset happened in 1993/1994 is related closely to the 850 hPa zonal wind anomalies over the area around Kalimantan Island. Easterly (westerly) anomalies over Kalimantan Island enhance (weaken) subtropical high over the western North Pacific, leading to the late (early) onset of SCSSM in 1979–1993 (1994–2013). The sea surface temperature anomalies (SSTAs) in the key region 140°–150° E, 5° S–2.5° N influence the interdecadal change of zonal winds over Kalimantan Island. The positive SSTAs over this key region in 1994–2013 force convergence toward the region at low-level and form significant westerly anomalies near Kalimantan Island located to the west of the key region. The negative anomalies of meridional gradient of zonal winds over the South China Sea region increase the atmospheric vorticity over there significantly and result in the weakening and retreating eastward of the subtropical high over the western North Pacific, which is conducive to the early onset of SCSSM.

The western North Pacific summer monsoon (WNPSM) is an important subcomponent of the Asian summer monsoon. The equatorial zonal wind (EZW) in the lower troposphere over the western Pacific may play a critical role in the evolution of the El Niño-Southern Oscillation (ENSO). The possible linkage between the EZW over the western Pacific and the off-equatorial monsoonal winds associated with the WNPSM and its decadal changes have not yet been fully understood. Here, we find a non-stationary relationship between the WNPSM and the western Pacific EZW, significantly strengthening their correlation around the late 1980s/early 1990s. This observed shift in the WNPSM–EZW relationship could be explained by the changes in the related sea surface temperature (SST) configurations across the tropical oceans. The enhanced influence from the springtime tropical North Atlantic, summertime tropical central Pacific, and maritime continent SST anomalies may be working together in contributing to the recent intensified WNPSM–EZW co-variability. The observed recent strengthening of the WNPSM–EZW relationship may profoundly impact the climate system, including prompting more effective feedback from the WNPSM on subsequent ENSO evolution and bolstering a stronger biennial tendency of the WNPSM–ENSO coupled system. The results obtained herein imply that the WNPSM, EZW, ENSO, and the tropical North Atlantic SST may be closely linked within a unified climate system with a quasi-biennial rhythm occurring during recent decades, accompanied by a reinforcement of the WNPSM–ENSO interplay quite possibly triggered by enhanced tropical Pacific–Atlantic cross-basin interactions. These results highlight the importance of the tropical Atlantic cross-basin influences in shaping the spatial structure of WNPSM-related wind anomalies and the WNPSM–ENSO interaction.

The central Indian Ocean (CIO) mode, an intrinsic coupled mode, plays an important role in the intraseasonal variabilities over the Indian monsoon region. Besides the intraseasonal variabilities, the CIO mode also has pronounced seasonal and interannual variabilities. The CIO mode is active during boreal summer but suppressed during boreal winter. The seasonality is mainly attributable to the barotropic instability, which is caused by the large meridional shear of zonal winds. By decomposing the temporal tendency of the meridional gradient of zonal winds, it is found that the zonal wind shear mainly follows the variation of the horizontal eddy flux, which indicates the importance of the multiscale interaction in tropical dynamics. The interannual variability of the CIO mode also depends on the energy transfer associated with the barotropic instability. The influences of El Niño or La Niña and Indian Ocean dipole–zonal mode (IODZM) on the CIO mode are analyzed. El Niño and La Niña have moderate impacts on the CIO mode. El Niño weakens the CIO mode and La Niña strengthens it via the changes in the low-level zonal wind shear.IODZMdoes not significantly change the amplitude of the CIO mode but can shift its latitudinal position by modifying the meridional shear of the zonal winds. The low-frequency variabilities of the CIO mode at seasonal and interannual time scales unveil the impacts of the background circulations at the intraseasonal variabilities during the Indian summer monsoon in a multiscale framework. While the low-frequency variabilities of this mode will clearly have an implication for monsoon variability and prediction, further studies are needed to quantify the impacts.

The occurrence of Indian summer monsoon rainfall (ISMR) during June to September, especially over the eastern Gangetic plain of India, is the lifeline for densely residing people. The intraseasonal and interannual variability in ISMR provokes drought and flood conditions and largely affects agriculture practices. Owing to its importance, several studies on the variability of ISMR over the meteorological subdivisions, namely West Bengal (WB), Jharkhand (JH), Bihar (BR), East Uttar Pradesh (EUP), and West Uttar Pradesh (WUP), respectively, have been conducted; however, the contribution of large-scale precipitation (LSP) and convective precipitation (CP) in ISMR needs to discuss. The LSP is precipitated out from the stratus or nimbostratus clouds, while CP occurs from the cumulus and cumulonimbus clouds, and both of them coexist in ISMR during summer monsoon months. The objective of this paper is to analyze and discuss the climatological characteristics and possible causes of occurrence of LSP and CP over the meteorological subdivisions. For this purpose, the data of LSP, CP, zonal, meridional (u and v components) wind, and relative humidity (rh) at the spatial resolution of 0.25° × 0.25° for the period of 1980–2019 are taken from the European Centre for Medium-Range Weather Forecasts (ECMWF). The outgoing longwave radiation (OLR) data at a surface resolution of 1° × 1° for the same periods are obtained from the National Centre for Environmental Information (of NOAA). The observed rainfall data of the India Meteorological Department at the same resolution and period is considered and compared with ECMWF Reanalysis (ERA5) data. The spatial and temporal distribution of both types of precipitation is analyzed, and their linkage with OLR, zonal winds, and rh at pressure levels of 1000 hPa, 850 hPa, and 700 hPa (in lower troposphere) is examined. The daily climatological values of CP (LSP) are relatively higher (lower) in each meteorological subdivision. The associated lower values of OLR are noticed over the Gangetic West Bengal, Jharkhand, and Bihar, while higher values of OLR are seen over the East and West UP. From the pressure levels of 1000 to 700 hPa, the change in the zonal wind, i.e., easterly to westerly and vice versa, and occurrence of a large amount of rh (>80%) may be possibly initiated moist convective activity for more precipitating out CP over the Gangetic West Bengal, Jharkhand, and Bihar in comparison to East and West Uttar Pradesh.

The anomalous zonal wind moves southward during the ENSO mature phase in boreal winter. Previous studies suggest that it may be caused by the nonlinear interaction of annual cycles or the influence of background mean state changes. In this research, the ECHAM4.6 atmospheric model is used to confirm the mechanism of the anomalous zonal wind southward shifting. The annual cycle of solar radiation and SST are removed in the sensitivity experiments to avoid the interaction between the ENSO and annual cycle. The results show that the north–south asymmetry mode of the ENSO anomalous wind field is not the result of a nonlinear interaction between ENSO and the annual cycle. The mean v-winds in winter motivate the southward shifting of the ENSO anomalous wind field through advection.

Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (∼ 54∘ N) and northern Norway (∼ 69∘ N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLT-SL) using SMR and PRR winds and (2) the mesosphere summer length (M-SL) using the PRR and MLS. Under both definitions, the summer begins around April and ends around middle September. The largest year-to-year variability is found in the summer beginning in both definitions, particularly at high latitudes, possibly due to the influence of the polar vortex. At high latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity as well as large-scale atmospheric influences (e.g., quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO.

This paper describes the comparative study on some features of the tropospheric and lower stratospheric circulation over the tropical Indian region during the monsoon season in the context of deficient and excess monsoon (dry and wet) years. The monthly mean of zonal wind component are computed directly with NCEP reanalysis data for the main rainy season of June–September. The composite anomalies of zonal wind circulation for monsoon season in four meteorologically important regions of lower atmosphere viz., lower troposphere, middle troposphere, upper troposphere and lower stratosphere in respect of wet and dry monsoon years were calculated and compared. The results of our analysis show major contrasts between the characteristics of winds of tropical troposphere and lower stratosphere over Indian region during the two scenarios. In the dry years, the troposphere and lower stratosphere are characterized by weaker wind flows in comparison with the normal flow of low level jet (LLJ) and tropical easterly jet (TEJ) streams . The observed weak LLJ and TEJ support the rainfall deficit in dry years. In contrast, during the wet years, the troposphere is dominated by stronger westerly flows at lower and middle tropospheric levels in concomitant with stronger easterlies in the upper troposphere and lower stratosphere. The long term variability of zonal winds averaged between 20°N and 20°S shows decreasing trend in the absolute values in speed during 1981 to 2010. Over the period of study, both tropical westerlies and easterlies and also jet streams have weakened. The results from this study serve as bases for further work on the tropospheric circulation features over the tropical Indian region for an enhanced understanding of the regional climate.