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Shyama Mohanty
,
Madhusmita Swain
,
Raghu Nadimpalli
,
K. K. Osuri
,
U. C. Mohanty
,
Pratiman Patel
, and
Dev Niyogi

Abstract

The city of Mumbai, India, frequently receives extreme rainfall (>204.5 mm day−1) during the summer monsoonal period (June–September), causing flash floods and other hazards. An assessment of the meteorological conditions that lead to these rain events is carried out for 15 previous cases from 1980 to 2020. The moisture source for such rain events over Mumbai is generally an offshore trough, a midtropospheric cyclone, or a Bay of Bengal depression. The analysis shows that almost all of the extreme rain events are associated with at least two of these conditions co-occurring. The presence of a narrow zone of high sea surface temperature approximately along the latitude of Mumbai over the Arabian Sea can favor mesoscale convergence and is observed at least 3 days before the event. Anomalous wind remotely supplying copious moisture from the Bay of Bengal adds to the intensity of the rain event. The presence of midtropospheric circulation and offshore trough, along with the orographic lifting of the moisture, give a unique meteorological setup to bring about highly localized catastrophic extreme rainfall events over Mumbai. The approach adopted in this study can be utilized for other such locales to develop location-specific guidance that can aid the local forecasting and emergency response communities. Further, it also provides promise for using data-driven/machine learning–based pattern analysis for developing warning triggers.

Significance Statement

We have identified the meteorological conditions that lead to extreme heavy rains over Mumbai, India. They are that 1) at least two of these rain-bearing systems, offshore trough, midtropospheric circulation, and Bay of Bengal depression moving north-northwestward are concurrently present, 2) an anomalous high SST gradient is present along the same latitude as Mumbai, and 3) the Western Ghats orography favors the rainfall extreme to be highly localized over Mumbai.

Restricted access
Roland Walz
,
Hella Garny
, and
Thomas Birner

Abstract

The stratospheric polar vortex is dynamically coupled to the tropospheric circulation. Therefore, a better mechanistic understanding of this coupled system is important to interpret past and future circulation changes correctly. Previously, idealized simulations with a dry dynamical-core general circulation model and imposed tropical upper-tropospheric warming (TUTW) have shown that a critical warming level exists at which the polar vortex transitions from a weak and variable to a strong and stable regime. Here, we investigate the dynamical mechanism responsible for this regime transition and its influence on the troposphere by performing similar idealized experiments with (REF) and without a polar vortex (NPV). According to the critical-layer control mechanism, the strengthened upper flank of the subtropical jet in response to TUTW leads to an accelerated wave-driven residual circulation in both experiments. For the REF experiment, the stronger residual circulation is associated with changes in the lower-stratospheric thermal structure that are consistent with an equatorward shift of the polar vortex. At a certain threshold of TUTW in the REF experiment, the tropospheric jet and the stratospheric polar vortex form a confined waveguide for planetary-scale waves that presumably favors downward wave coupling events. Consistently, the polar vortex strengthens in combination with an enhanced poleward shift of the tropospheric jet compared to the NPV experiment. Overall, these idealized experiments suggest that a polar vortex strengthening can be caused by greenhouse gas–induced warmings via modifications of the waveguide. This mechanism might also be relevant to understand the polar vortex changes in more complex models.

Open access
Yeon-Woo Choi
and
Elfatih A. B. Eltahir

Abstract

For millennia, Mesopotamia was blessed by enough water supplied by the Tigris and Euphrates Rivers. However, the dwindling freshwater resource is no longer enough. In the future, climate change coupled with a growing population could considerably exacerbate the current water deficit. Based on simulations by carefully selected global and regional climate models, we conclude that these river basins may possibly face further water shortages (mainly due to a reduction in spring-season precipitation) in the next few decades (2021–50) under a scenario of high emissions of greenhouse gases. However, there is no consensus among models regarding these near-term (2021–50) projections of change in precipitation, and society is likely to face the challenge of how to prepare for this uncertain future. The story is different for the late decades of this century: we project, with significantly more confidence, a robust decrease in wet-season (winter to spring) precipitation over the headwaters of these river basins, worsening future water deficits and implying a century-long drying trend over Mesopotamia. Possible physical mechanisms are proposed and discussed. As global warming progresses, higher sea level pressure, centered on the Mediterranean Sea, will likely make upstream storms less frequent and weaker, leading to drying over Mesopotamia. Further, projections show a poleward migration of the fewer Mediterranean storm tracks, decreasing the frequency of storms that penetrate into Mesopotamia. Implementing a global net-zero carbon emissions policy by midcentury could mitigate the severity of the projected droughts in this region.

Open access
Jeff Wilson
,
Thomas Jung
,
Eric Bazile
,
David Bromwich
,
Barbara Casati
,
Jonathan Day
,
Estelle De Coning
,
Clare Eayrs
,
Robert Grumbine
,
Jun Ioue
,
Siri Jodha S. Khalsa
,
Jorn Kristiansen
,
Machiel Lamers
,
Daniela Liggett
,
Steffen M. Olsen
,
Donald Perovich
,
Ian Renfrew
,
Vasily Smolyanitsky
,
Gunilla Svensson
,
Qizhen Sun
,
Taneil Uttal
, and
Qinghua Yang
Restricted access
Zhongwei Liu
,
Jonathan M. Eden
,
Bastien Dieppois
,
W. Stefaan Conradie
, and
Matthew Blackett

CMIP6 models suggest that extreme fire weather associated with the April 2021 Cape Town wildfire has become 90% more likely in a warmer world.

Free access
Jayarathna W. N. D. Sandaruwan
,
Wen Zhou
,
Paxson K. Y. Cheung
,
Yan Du
, and
Xuan Wang

Abstract

Marine heatwaves (MHWs) are extreme climatic events that can have a significant impact on marine ecosystems and their services across the world. We examine the spatiotemporal variation of summer MHWs in the North Indian Ocean (NIO) and find that the whole NIO basin exhibits a pronounced spatial variability as well as a significant increasing trend in MHW frequency. We show that the NIO has two leading MHW modes linked to two distinct sea surface temperature (SST) patterns during summer. The first MHW mode is associated with basin-wide warming, which is preconditioned by a decaying El Niño–Southern Oscillation (ENSO) and sustained throughout the summer by anomalous northeasterlies extending from the anticyclonic circulation of the western North Pacific subtropical high (WNPSH). The combined effect of thermocline warming due to downwelling oceanic planetary waves, decreased wind-induced evaporative cooling, and enhanced insolation cause basin-wide summer MHWs. The second MHW mode exhibits a zonal dipole pattern, which has unfavorable cooling conditions in the previous seasons. The second MHW mode is associated with a phase change of ENSO and is greatly influenced by the formation of an interhemispheric pressure difference (IHPD) due to strengthening of the Australian high (AH) and weakening of the WNPSH. The IHPD induces cross-equatorial southerly winds across the eastern Indian Ocean. These winds favor the transformation of basin-wide cooling conditions into zonal SST patterns via wind-evaporative-SST and thermocline-SST feedback, causing MHWs with a zonal dipole pattern. These MHW modes have a significant influence on the distribution and intensity of summer precipitation in the NIO.

Restricted access
Yong Liu
,
Shui Yu
, and
Huopo Chen

Abstract

Based on the in-situ observations, reanalysis, and model simulation, the variations in glaze dipole pattern in China and its underlying physical mechanism have been explored. The glaze dipole pattern features an out-of-phase relationship between winter glaze in the south of the Yangtze River valley (YRV) and northern China, accompanied by pronounced interdecadal variation around the late 1970s. The results from synoptic analyses suggest that cold air brought by the northerly and warn moist air by the southwesterly, as well as the occurrence of inversion layer are vital to the glaze weather in the south of YRV. Further analyses indicate that the interdecadal shift of the Pacific decadal Oscillation (PDO) contributes largely to variations in glaze dipole pattern. Specifically, the warm PDO provides a beneficial environment for the occurrence of glaze dipole pattern by stimulating the tropical-extratropical circulation configuration with the deepened East Asian trough, strengthened East Asian westerly jet, anomalous anticyclone over the tropical western Pacific and cyclone over the southern Tibetan Plateau at the decadal time scale. Consequently, the enhanced moisture transport brought by southwesterly and cold air intrusion induced by the deepened East Asian trough benefit the glaze weather in the south of YRV, while the decreased precipitation and a much lower temperature in northern China depress the generation of glaze. Moreover, the results from the CAM4 model simulation indicate the atmospheric circulation anomalies forced by PDO-like SST can roughly reproduce the extratropical configuration related to the glaze, but it has difficulties in capturing the tropical circulation anomalies.

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