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Lei Fan
,
Hui-Huang Fu
, and
Yu Liang

Abstract

This study identifies two distinct patterns of the summer Indian Ocean Dipole (IOD) — the coastal IOD and the offshore IOD — named based on the proximity of their eastern pole to Sumatra. Their spatial characteristics, evolutionary mechanisms, relationships with ENSO, impacts on precipitation, and the factors controlling the simulation performances of climate models are discussed.

The coastal IOD shares the same eastern pole as the conventional IOD off Sumatra, but its western pole is located in the central southern tropical Indian Ocean (TIO). The offshore IOD shares the conventional western pole off Somalia, but its eastern pole is located in the central southern TIO. Regarding their evolutions, while they initially develop similarly, their later evolutions differ due to their distinct pole locations: the offshore IOD peaks in summer, while the coastal IOD can be sustained into autumn. The coastal IOD correlates to preceding and late ENSO states, but the offshore IOD does not, making it an independent internal mode of TIO. The two IODs affect climate differently, with only the coastal IOD affecting Australian rainfall. Climate models exhibit varied levels of performance in simulating the two IODs. Specifically, a stronger link between spring TIO rainfall and ENSO, as well as stronger southeasterly monsoonal winds in the southern TIO, can enhance the coastal IOD modeling, while a stronger summer Somali jet benefits the simulation of the offshore IOD. Distinguishing these two IODs has implications for accurate diagnosis and prediction of the summer climate surrounding the TIO.

Restricted access
Yang Zhao
,
Jianping Li
,
Yuan Tian
, and
Jiao Li

Abstract

This study investigates the disparity in quantitative moisture contribution and synoptic-scale vertical motion in the lower reaches of the Yangtze River (LYRB) for different extreme precipitation (EP) types, which are categorized as EP associated with atmospheric river (AR&EP) or non-atmospheric river (non-AR&EP). To analyze moisture contribution, a backward tracking using the water accounting model-2layers is performed. In general, the remote moisture contribution is 9.7 times greater than the local contribution, with ocean contribution being 1.67 times stronger than land contribution. However, terrestrial and oceanic contributions obviously increase in the EP types, especially for oceanic contribution being double in magnitude. Notably, the West Pacific (WP) contribution emerges as the dominant differentia between the EP types, playing a crucial role in AR formation. By solving the quasi-geostrophic omega equation, the upper-level jet stream (ULJ) acts as the primary dynamic forcing for transverse vertical motion in AR&EP, while the baroclinic trough exhibits a relatively weaker influence. However, both systems have a nearly equal impact on vertical velocity in non-AR&EP. The enhanced shearwise elevation in the non-AR&EP type is the response of the stronger upper-level ridge over the Tibetan Plateau (TP), which induce enhanced Q-vector divergence pointing towards the LYRB. However, the main dynamic differences is location of ULJ, which serves as the trigger role although weak. Diabatic forcing proves to be the decisive factor for vertical motion development, the difference attributed to the released excessive latent heating with excess moisture contribution from the WP in AR&EP with enhanced precipitation.

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Clara Orbe
,
David Rind
,
Darryn W. Waugh
,
Jeffrey Jonas
,
Xiyue Zhang
,
Gabriel Chiodo
,
Larissa Nazarenko
, and
Gavin A. Schmidt

Abstract

Stratospheric ozone, and its response to anthropogenic forcings, provides an important pathway for the coupling between atmospheric composition and climate. In addition to stratospheric ozone’s radiative impacts, recent studies have shown that changes in the ozone layer due to 4xCO2 have a considerable impact on the Northern Hemisphere (NH) tropospheric circulation, inducing an equatorward shift of the North Atlantic jet during boreal winter. Using simulations produced with the NASA Goddard Institute for Space Studies (GISS) high-top climate model (E2.2), we show that this equatorward shift of the Atlantic jet can induce a more rapid weakening of the Atlantic meridional overturning circulation (AMOC). The weaker AMOC, in turn, results in an eastward acceleration and poleward shift of the Atlantic and Pacific jets, respectively, on longer time scales. As such, coupled feedbacks from both stratospheric ozone and the AMOC result in a two-time-scale response of the NH midlatitude jet to abrupt 4xCO2 forcing: a “fast” response (5–20 years) during which it shifts equatorward and a “total” response (∼100–150 years) during which the jet accelerates and shifts poleward. The latter is driven by a weakening of the AMOC that develops in response to weaker surface zonal winds that result in reduced heat fluxes out of the subpolar gyre and reduced North Atlantic Deep Water formation. Our results suggest that stratospheric ozone changes in the lower stratosphere can have a surprisingly powerful effect on the AMOC, independent of other aspects of climate change.

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Qingzhe Zhu
,
Yuzhi Liu
, and
Na Xiao

Abstract

Dust is the predominant type of aerosol over the Tibetan Plateau (TP) due to the existence of surrounding significant dust sources. However, the contributions of different dust sources to the distribution and variation of dust over the TP and corresponding mechanisms are still being explored. By separating emissions from different dust sources in a numerical model, this study detected that dust originating from East Asia, the Middle East, and North Africa are the main contributors to spring dust over the TP, accounting for 42%–68%, 13%–27%, and 9%–25% of the total dust concentration, respectively. East Asian dust primarily affects the dust over the central and northern parts of the TP, whereas Middle Eastern and North African dust mainly contributes to the dust over the western and southern parts of the TP. Additionally, the variation in dust over the TP is related to East Asian and North African dust, which contribute 58% and 35% of the total variation, respectively. The mechanism underlying this association is attributable to the SST over the northern North Atlantic and updrafts over the northern slope of the TP: the increased SST enhances westerlies from North Africa to East Asia and northwesterly winds over the northern slope of the TP and combines with the stronger westerlies to promote the transport of East Asian and North African dust to the TP. Consequently, it is necessary to focus on the impact of multiple dust sources on the dust over the TP.

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Hong Wang
,
Liang Gao
,
Lei Zhu
,
Lulu Zhang
, and
Jiahao Wu

Abstract

Accurately assessing cyclone intensity changes due to global warming is crucial for predicting and mitigating sequential hazards. This study develops a high-resolution, fully coupled air-sea model to investigate the impact of global warming on Super Typhoon Mangkhut (2018). A numerical sensitivity analysis is conducted using the Pseudo-Global Warming (PGW) technique based on multiple global climate models (GCMs) from the Coupled Model Intercomparison Project Phases 6 (CMIP6). Under ocean warming scenarios, the increasing average sea surface temperature (SST) by 2.26 °C, 2.44 °C, 3.45 °C, and 4.53 °C result in reductions in minimum sea-level pressure by 9.2 hPa, 10.6 hPa, 15.7 hPa, and 19.4 hPa, respectively, compared to the original state of Typhoon Mangkhut. Rising SST increases turbulent heat flux, to be specific, an average SST increase of 2.26-4.53 °C changes the turbulent heat flux into 177% to 272% of the original value. Besides, stronger winds enhance SST cooling, including upwelling and entrainment, leading to an increase in the mixed layer depth (MLD). Tropical cyclone heat potential (TCHP) tends to be enhanced under the combined influences as the SST rises. An average increase in SST of 2.26 °C, 2.44 °C, 3.45 °C, and 4.53 °C leads to increase in TCHP of 9.94%, 9.85%, 14.67%, and 15.30%, respectively. However, future changes in atmospheric temperature and humidity will moderate typhoon intensification induced by ocean warming. Considering atmospheric conditions, the maximum wind speed decreases by approximately 10% compared to only considering ocean warming. Nevertheless, typhoon intensity is projected to strengthen under future climate change.

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Qingye Min
and
Renhe Zhang

Abstract

The South Pacific Oscillation (SPO), characterized by a north-south dipole-like pattern of sea level pressure anomalies, is one of the key factors in understanding tropical-extratropical interactions in South Pacific. We show that in boreal summer (June–July–August), the center of the northern lobe sea level pressure anomalies in the SPO is shifted to the east gradually after the 1960–70s. This study focuses on the relationship between the boreal summer SPO and following winter El Niño–Southern Oscillation (ENSO) diversity before and after the eastward shift of the SPO’s subtropical lobe. The eastward shift of the SPO’s subtropical lobe altered both the seasonal footprint mechanism and the trade wind charging mechanism associated with the SPO, thus profoundly influenced the ENSO diversity. It is revealed that when the northern lobe of the SPO shifts to the west of its average location, it tends to strengthen the EP El Niño mainly via the seasonal footprint mechanism. but after the SPO’s northern lobe shifts to the east of its average location, it tends to promote the development of CP El Niño mainly via the trade wind charging mechanism. The changes in the spatial structure of convection over the tropical Pacific and Indian Ocean may be one of the possible causes for the eastward shift in the SPO’s northern lobe. The findings in the present study have implications for a better understanding of ENSO diversity.

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Free access
Masahiro Shiozaki
,
Hiroki Tokinaga
, and
Masato Mori

Abstract

Atmospheric teleconnections from the Pacific El Niño are key to determining the East Asian winter climate. Using the database for Policy Decision-making for Future climate change (d4PDF) large ensemble simulations, the present study investigates a mechanism for the warm and cold East Asian winters during El Niño with a focus on atmospheric teleconnections triggered by anomalous sea surface temperature (SST) patterns in the tropical Indo-Pacific. Our results show that the Western Pacific (WP) teleconnection pattern plays a primary role in the warm winters in East Asia. The WP pattern tends to appear in years when both an early El Niño and the positive phase of the Indian Ocean dipole mode (IOD) develop in boreal autumn. In those years, the tropical Indian Ocean (TIO) strongly warms in the following winter, forming a distinct zonal contrast in precipitation anomalies over the tropical Indo-Pacific through a reduced Walker circulation. The Rossby wave source anomalies indicate that the WP pattern is associated with the weakened Indo-Pacific Walker circulation. By contrast, the WP pattern does not dominate in the cold winters due to the absence of strong TIO warming. The present study proposes a mechanism that promotes excitation of the WP pattern through the upper-troposphere divergence in East Asia associated with the Walker circulation modulated by the tropical Indo-Pacific interbasin interaction.

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Carla M. Roesch
,
Andrew P. Ballinger
,
Andrew P. Schurer
, and
Gabriele C. Hegerl

Abstract

Using the past to improve future predictions requires an understanding and quantification of the individual climate contributions to the observed climate change by aerosols and greenhouse gases (GHG), which is hindered by large uncertainties in aerosol forcings and responses across climate models. To estimate historical aerosol responses, we apply detection and attribution methods to attribute a joint change in temperature and precipitation to forcings by combining signals of observed changes in tropical wet and dry regions, the interhemispheric temperature asymmetry, global mean temperature (GMT) and global mean land precipitation (GMLP). Fingerprints representing the climate response to aerosols (AER) and the remaining external forcings (noAER; mostly GHG) are derived from large-ensembles of historical single- and ALL-forcing simulations from three models in phase 6 of the Coupled Model Intercomparison Project and selected using a perfect model study. Results from an imperfect model study and a hydrological sensitivity analysis support combining our choice of temperature and precipitation fingerprints into a joint study. We find that diagnostics including temperature and precipitation slightly better constrain the noAER signal than diagnostics based purely on temperature or GMT-only and allow for the attribution of AER cooling (even when GMT is not included in the fingerprint). These results are robust across, using fingerprints from different climate models. Estimated contributions for AER and noAER agree with estimates from the most recent IPCC report. Finally, we attribute a best estimate of 0.46 K (0.05–0.86 K) of aerosol-induced cooling and of 1.63 K (1.26–2.00 K) of noAER warming in 2010–2019 relative to 1850–1900 using the combined signals of GMT and GMLP.

Open access
Chia-Wei Lan
,
Chao-An Chen
, and
Min-Hui Lo

Abstract

Between 1979 and 2021, global ocean regions experienced a decrease in dry season precipitation, while the trend over land areas varied considerably. Some regions, such as southeastern China, the Maritime Continent, eastern Europe, and eastern North America, showed a slight increasing trend in dry season precipitation. This study analyzes the potential mechanisms behind this trend by using the fifth major global reanalysis produced by ECMWF (ERA5) data. The analysis shows that the weakening of downward atmospheric motions played a critical role in enhancing dry season precipitation over land. An atmospheric moisture budget analysis revealed that larger convergent moisture fluxes lead to an increase in water vapor content below 400 hPa. This, in turn, induced an unstable tendency in the moist static energy profile, leading to a more unstable atmosphere and weakening downward motions, which drove the trend toward increasing dry season precipitation over land. More water vapor in the low troposphere is because of higher moisture convergence and moisture transport from ocean to land regions. In summary, this study demonstrates the intricate elements involved in altering dry season rainfall trends over land, which also emphasizes the importance of comprehending the spatial distribution of the wet-get-wetter and dry-get-drier paradigm.

Significance Statement

This study found that global land precipitation during the dry season slightly increased from 1979 to 2021, while precipitation over oceans declined. Moist static energy analysis showed a trend toward less stability in areas with increased dry season precipitation and the opposite trend in regions with declining precipitation. Water vapor content trends and dynamic components were the primary controlling mechanism for precipitation trends. Furthermore, the hotspots with pronounced increases or decreases in dry season precipitation reflect local circulation changes influenced by anthropogenic or natural factors.

Open access