Previous studies documented a dramatic weakening of the tropical easterly jet (TEJ) since the mid-20th century and attributed it to the decreasing meridional temperature gradient (MTG) between the Tibetan Plateau and the Indian Ocean. It has recently been reported that the TEJ core intensity experienced a recovery around the late 1990s, but the underlying causes remain unclear. The present study shows that this recovery arises from a combination of multi factors, with enhanced convective activity over the Indian Peninsula (INP) and sea surface temperature (SST) variability in the tropical Pacific as two prominent contributors. Idealized experiments with a baroclinic model demonstrate that the INP convective heating can excite an upper-tropospheric Rossby-wave response that accelerates the easterly wind in TEJ core region. Ocean pacemaker experiments further indicate that the tropical Pacific SST variability exerts a dominant influence on the large-scale TEJ changes through modulating the large-scale MTG and divergent circulations, whereas the tropical Indian Ocean warming acts to inhibit the strengthening and the Atlantic SST variability has negligible effects. These results point out the joint influence of multi factors on driving the decadal recovery of the TEJ intensity around the late 1990s, which can help advance our understanding of mechanisms governing the decadal variability of the TEJ.
Abstract The mechanisms responsible for the extreme 1959 tropical cyclone (TC) season over the western North Pacific (WNP) are investigated. Observational analysis shows that the enhanced basin-mean lifetime maximum intensity (LMI) is jointly contributed by the shift of TC genesis toward a high-LMI region and the more frequent rapid intensification (RI). These TC features are linked to large-scale circulation anomalies, including an enhanced monsoon trough, a weakened subtropical high, and an eastward-shifted tropical upper-tropospheric trough, which together create favorable environmental conditions. Genesis potential indices verify that increased vorticity, moisture, upward motion, and reduced vertical wind shear over the open basin favor TC formation in the high LMI region. Composite analysis of TC nearby environments reveals that the enhanced southwesterly flow and moisture to the south of TCs, which are associated with the monsoon trough, might play a role in increasing RI frequency. To assess the role of the sea surface temperature (SST) pattern, we conducted ensemble simulations with the 28-km Nonhydrostatic Icosahedral Atmospheric Model (NICAM). Sensitivity experiments reproduce the intensified TCs, shifted genesis locations, and anomalous circulations, with the proportion of extreme TCs doubling compared to the climatological run. Similar responses in the realistic 1959 and tropical North Pacific warming experiments suggest that tropical North Pacific SST anomalies contributed to circulation changes broadly consistent with a Matsuno–Gill-type atmospheric response, thereby modulating TC intensity. These results highlight the importance of a basin-wide SST pattern in shaping extreme TC seasons.
Abstract It is well-known that fronts, atmospheric rivers (ARs), and mesoscale convective systems (MCSs) are important weather phenomena linked to hydrological events. Less has been discussed on frontal precipitation when fronts co-occur with other two. This study uses objective algorithms applied to reanalysis and observational datasets to detect these three weather phenomena and categorizes midlatitude fronts based on whether they co-occur with ARs and MCSs. A large proportion of fronts co-occur with ARs. Fronts with ARs and MCSs are common within the storm tracks. Front number varies by type, category, and latitude, as do front intensity, size and related precipitation. Using satellite-derived precipitation estimates, we explore the frontal precipitation distribution and the relationship between precipitation area and intensity of front objects. Results show that fronts with ARs only are frequently associated with light precipitation and tend to have large precipitation areas, whereas fronts with MCSs only are frequently associated with moderate-to-intense precipitation. Fronts with both are more often associated with high rates and produce substantial precipitation amounts within large precipitation areas. Investigation of environmental factors, including frontogenesis, total column water vapor (TCWV), and convective available potential energy (CAPE), over the selected ocean regions suggests a general positive relationship with frontal precipitation, but varying by region. Fronts with ARs and with MCSs have different environmental features, the former of which frequently have high TCWV and the latter have strong CAPE. Peak frontal precipitation is more likely related to convective updrafts of MCSs, rather than frontogenetical forcing or ARs.
Abstract While the ensemble mean of AMIP6 simulations reasonably reproduces the observed features of boreal summer precipitation, substantial intermodel spread persists. To investigate the mechanism underlying this spread, we apply empirical orthogonal function (EOF) analysis to identify the leading mode of global summer precipitation deviations and examine its dynamical impacts on atmospheric circulation. Our results reveal that the dominant mode features an out-of-phase precipitation pattern between the western North Pacific (WNP) and its surrounding regions, which modulates atmospheric circulation through two distinct pathways associated with key precipitation regions. First, enhanced precipitation over the WNP excites a Pacific–Japan (PJ) teleconnection pattern along the East Asian coast through the perturbations of the monsoon trough. Meanwhile, it also generates a meridional wave train extending from the South Indian Ocean to the Ross Sea through perturbations to the subtropical jet. Second, suppressed precipitation over North Africa - the Arabian Sea (NAF-AS), triggers two nearly equatorially symmetric zonal wave trains in the mid- to high latitudes of both hemispheres through disruptions of the North and South African subtropical jets. Sensitivity experiments using a linear baroclinic model (LBM) further support the distinct roles of WNP and NAF–AS diabatic heating in driving circulation variability, supporting the robustness of the proposed pathways. This study may provide a physical explanation for the ‘engine’ role of WNP precipitation deviations in shaping the global pattern of monsoon precipitation intermodel variability identified by Wen and Huan (2025).
Abstract Under global warming, land surfaces have warmed substantially faster than the oceans, leading to widespread drying across most continental regions. However, across South Asia, an anomalous trend of moistening, indicated by an increase in surface relative humidity (RH), has been observed in recent decades despite continued land-surface warming. We find that this moistening pattern is consistently evident in multiple observational, reanalysis, and station-based datasets and is also present in the CMIP6 multi-model mean, indicating that the observed moistening trend is broadly consistent with the externally forced response simulated by CMIP6. These concurrent warming and moistening trends have important implications for humid-heat conditions relevant to human health, as higher RH under a warming climate can significantly reduce heat dissipation and exacerbate heat stress. We further evaluate these humid-heat implications using wet-bulb temperature and heat index. This study further discusses the possible mechanisms behind the anomalous moistening trend, focusing on the monsoon season, and highlights enhanced moisture convergence as a dynamically consistent and physically plausible contributor to the increase in surface RH under warming. The broad consistency between the observed trends and the CMIP6 multi-model mean suggests an important role for the externally forced climate response and provides a basis for future assessments of whether the South Asian moistening trend may persist or intensify under continued anthropogenic forcing. Such assessments should nevertheless account for the influences of internal variability, regional processes, and remaining model biases.
