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Nora L. S. Fahrenbach
,
Massimo A. Bollasina
,
BjØrn H. Samset
,
Tim Cowan
, and
Annica M. L. Ekman

Abstract

Observations show a significant increase in Australian summer monsoon (AUSM) rainfall since the mid-twentieth century. Yet the drivers of this trend, including the role of anthropogenic aerosols, remain uncertain. We addressed this knowledge gap using historical simulations from a suite of Coupled Model Intercomparison Project phase 6 (CMIP6) models, the CESM2 Large Ensemble, and idealized single-forcing simulations from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Our results suggest that Asian anthropogenic aerosol emissions played a key role in the observed increase in AUSM rainfall from 1930 to 2014, alongside the influence of internal variability. Sulfate aerosol emissions over Asia led to regional surface cooling and strengthening of the climatological Siberian high over eastern China, which altered the meridional temperature and sea level pressure gradients across the Indian Ocean. This caused an intensification and southward shift of the Australian monsoonal westerlies (and the local Hadley cell) and resulted in a precipitation increase over northern Australia. Conversely, the influence of increased greenhouse gas concentrations on AUSM rainfall was minimal due to the compensation between thermodynamically induced wettening and transient eddy-induced drying trends. At a larger scale, aerosol and greenhouse gas forcing played a key role in the climate response over the Indo-Pacific sector and eastern equatorial Pacific, respectively (coined the “tropical Pacific east–west divide”). These findings contribute to an improved understanding of the drivers of the multidecadal trend in AUSM rainfall and highlight the need to reduce uncertainties in future projections under different aerosol emission trajectories, which is particularly important for northern Australia’s agriculture.

Significance Statement

Australian summer monsoon (AUSM) rainfall plays a vital role in sustaining northern Australia’s unique biodiversity and extensive agricultural industry. While observations show a significant increase in AUSM rainfall since the mid-twentieth century, the causes remain uncertain. We find that anthropogenic aerosol emissions from Asia played a key role in driving this multidecadal AUSM rainfall trend by inducing dynamic adjustments over the Indo-Pacific sector. These findings highlight the need to consider different aerosol emission trajectories when assessing future projections of AUSM rainfall.

Open access
Hongjie Huang
,
Zhiwei Zhu
, and
Juan Li

Abstract

During July and August of 2022, the Yangtze River Basin (YRB) experienced its most extreme high temperature (EHT) event since 1979, resulting in large numbers of human casualties and severe economic losses. This paper reveals the spatial and temporal features of the EHT over the YRB (YRB-EHT) in 2022 and disentangles its extreme nature from a historical perspective. Results showed the following: 1) The record-breaking YRB-EHT was directly caused by the adiabatic heating associated with an anomalous barotropic high pressure (or heat dome) and descending motion in the region. The intensified and westward-shifted western North Pacific subtropical high and eastward-extended South Asian high played critical roles in the formation of the heat dome and descending motion anomaly. 2) Convection anomalies over the tropical Atlantic and Pacific induced by the reintensified La Niña–like Pacific sea surface temperature anomaly pattern, along with the strong positive North Atlantic Oscillation (NAO), were the key contributing factors to the formation of the barotropic high pressure anomaly and YRB-EHT. 3) A physics-based empirical simulation model constructed using the factors of the NAO and tropical convection successfully reproduced the historical year-to-year variation of YRB temperatures, as well as the extreme in 2022, implying that the unprecedented 2022 YRB-EHT had universal dynamic origins. This study highlights the importance of the combined impacts of tropical and extratropical forcings in the record-breaking YRB-EHT in 2022, and thus may provide useful clues for seasonal predictions of summer mean or extreme temperatures in the YRB.

Significance Statement

Extreme high temperatures hit the Yangtze River Basin (YRB) in summer 2022, causing large numbers of human casualties and severe economic losses. Here, from a historical perspective, we disentangle the dynamic origins of these high temperatures over the YRB. Based on evidence from both observations and model simulations, the present study highlights the importance of the combined impacts of tropical and extratropical routes of influence on these extremely high temperatures in the YRB, and thus may provide useful clues for seasonal predictions of YRB summer mean or extreme temperatures.

Restricted access
Yakelyn R. Jauregui
and
Shuyi S. Chen

Abstract

The Madden–Julian oscillation (MJO) and El Niño–Southern Oscillation (ENSO) are the two most important tropical phenomena that affect global weather and climate on intraseasonal and interannual time scales. Although they occur on different time scales, the MJO-induced sea surface temperature (SST) anomalies over the equatorial Pacific have spatial scales similar to SST anomalies prior to El Niño. This study aims to address the question of whether the MJO plays an important role in the warm pool eastward extension (WPEE) leading up to El Niño. We use over 20 years of satellite observations, including optimum interpolated SST, TRMM-GPM precipitation, and the cross-calibrated multiplatform (CCMP) surface winds from 1998 to 2019, to quantify the spatial structure and duration of the MJO-induced warm SST anomalies over the equatorial Pacific (10°S–10°N, 130°E–180°). The intensity of the MJO is measured by the total rain volume and average surface westerly wind speed throughout its convectively active phase. Results show that 1) 61% of the 98 MJO events induced a WPEE over 1000–3000 km along the equator, which can last beyond 15–30 days after the MJO precipitation ended; 2) the MJO events prior to El Niño are generally stronger and produce significant WPEE far beyond its annual cycle and increasing SST warming in the Niño-3.4 region; 3) consecutive MJO events can produce much stronger WPEE prior to El Niño, which are observed in all El Niño events from 1998 to 2019; and 4) more frequent and stronger MJO-induced WPEE occurs in March–May than other seasons. These results can help better understand the MJO–ENSO interaction and, ultimately, improve the prediction of El Niño onset.

Restricted access
Kunpeng Yang
,
Haijun Yang
, and
Yang Li

Abstract

In the first part of our research on self-sustained multicentennial oscillation of the Atlantic meridional overturning circulation (AMOC), we utilized a hemispheric box model considering only the salinity equations. In this study, we consider both thermal and saline processes in the box model to investigate the AMOC multicentennial oscillation and the role of temperature. The thermal processes have mainly two effects, shortening the oscillation period and stabilizing the system, which are caused by the fast surface temperature restoration and negative feedback between temperature advection and AMOC, respectively. Introducing nonlinearity into the system can lead to self-sustained AMOC oscillation that is controlled by ocean internal dynamics, whose mechanism is generalized as a growing oscillation restrained by nonlinearity. The nonlinearity can arise from subpolar vertical mixing, or a nonlinear relation between the AMOC anomaly and the meridional difference of density anomaly. Linear stability analyses reveal that the eigenmode of the system is sensitive to model parameters, including model geometry, mean strength of the AMOC, and the AMOC’s sensitivity to density perturbation, surface virtual salt flux, and meridional temperature gradient. A larger surface virtual salt flux enhances the positive salinity advection feedback, and a smaller meridional temperature gradient weakens the negative temperature advection feedback. Both processes destabilize the AMOC multicentennial oscillation. Such situations may be expected in the future due to more intense warming and freshwater hosing at the high latitudes of the Northern Hemisphere.

Open access
Zixuan Han
and
Gen Li

Abstract

The mid-Pliocene (approximately 3.3–3.0 Ma) was one of the past geological warm periods. Since this past warmer climate was in many respects comparable to near future warming projections, how the regional monsoonal rainfall changed during the mid-Pliocene is an important scientific and socioeconomic concern. Based on phase 2 of the Pliocene Model Intercomparison Project (PlioMIP2), this work examines the simulated changes of Sahel summer rainfall during the mid-Pliocene. The results show the considerable intermodel uncertainty of the simulated mid-Pliocene Sahel rainfall changes in the PlioMIP2 multimodel ensemble, which is due to the uncertainties of both the simulated dynamic and thermodynamic responses to this past warmer climate. In particular, we find that the intermodel spread in the simulated northern North American warming is a major source of the uncertainty of the mid-Pliocene Sahel summer rainfall changes through two processes. One is a direct dynamic process: a stronger warming over northern North America could enhance the meridional temperature gradient between the extratropical and tropical regions, inducing an interhemisphere energy imbalance of the atmosphere. This could lead to a northward shift of the intertropical convergence zone, strengthening the Western African summer monsoon (WASM) circulation and Sahel summer rainfall. Another is an indirect thermodynamic process: the strengthened WASM circulation could further induce anomalous moisture convergence over the Sahel region, increasing local atmospheric moisture at the low-level troposphere, in favor of a wetter Sahel. Our results suggest that an improved warming simulation over northern North America is essential for the hydrological cycle simulation around the Sahel in the mid-Pliocene warmer climate.

Significance Statement

The Sahel summer rainfall change in a global warmer climate is a widespread scientific and socioeconomic issue because of its important impacts on regional agriculture, ecosystems, food security, water resources, and even cultural environment. However, the present study identifies a nonnegligible intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene (one of the most recent geological warm periods in Earth’s history) in the Pliocene Model Intercomparison Project phase 2 (PlioMIP2) multimodel ensemble. In particular, such an intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene is attributed to that of the simulated northern North America warming via both the direct dynamic process and the indirect thermodynamic process. This is quite different from the uncertainty source of near-future projections of Sahel summer rainfall changes. The present results improve our understanding of the underlying physics of the hydrological cycle change around the Sahel region in the mid-Pliocene warmer climate, with implications for the future projections of regional monsoonal rainfall.

Restricted access
Mengke Zhu
,
Hong-Li Ren
,
Zizhan Hu
,
Jonathon S. Wright
, and
Shiming Xu

Abstract

Several recent studies have highlighted differences in simulated properties of El Niño–Southern Oscillation (ENSO) under transient and equilibrium responses to increasing CO2. However, the reasons behind these disparate responses and the extent to which they are robust to different scales of CO2 forcing remain unclear. In this study, we adopt a climate system model with reduced SST bias in the eastern tropical Pacific and incrementally apply abrupt increases in CO2, analyzing outputs after each simulation reaches quasi-equilibrium with the imposed forcing. The results suggest that ENSO activity under quasi-equilibrium first increases and then decreases with increasing CO2, peaking in simulations with CO2 concentrations similar to the present day. Bjerknes–Jin stability analysis indicates that changes in the ENSO growth rate result primarily from changes in the thermocline feedback and thermodynamic damping terms. While thermodynamic damping increases monotonically with increasing CO2, the positive thermocline feedback varies within the range of internal variability up to twice the preindustrial value of CO2 and then weakens sharply with further increases. The mechanisms behind these changes include weaker mean ocean upwelling and weaker dynamical coupling between the atmosphere and subsurface ocean associated with substantial near-surface freshening at higher levels of CO2. These changes steepen the thermodynamic barrier to mixing between the surface and subsurface, weakening the east–west temperature gradient in the mean state and suppressing variability in the cold tongue. Analysis of similar model simulations from the Coupled Model Intercomparison Project (CMIP6) archive indicates that changes in the Bjerknes–Jin stability index are robust but do not establish a consensus as to the mechanisms behind them.

Significance Statement

This study investigates how El Niño–Southern Oscillation changes with increasing CO2 forcing by using a global model with improved tropical Pacific climate. The simulations roughly correspond to scenarios in which emissions are reduced to maintain CO2 concentrations at near-constant values over a long period, with levels ranging from about 2/3 to more than 5 times the present-day concentration. Analysis of these simulations suggests that ENSO activity is strongest when CO2 concentrations are similar to the present-day and becomes substantially weaker when CO2 is more than double its present-day value. Reduced ENSO activity with further increases in CO2 is caused by weaker interactions between the atmosphere and the subsurface ocean. Both the amplitude of warm (El Niño) events and the occurrence frequency of warm and cold (La Niña) events decrease as ENSO events become first more difficult to grow and then more difficult to trigger with increasing CO2.

Restricted access
Jonathan D. Wille
,
Simon P. Alexander
,
Charles Amory
,
Rebecca Baiman
,
Léonard Barthélemy
,
Dana M. Bergstrom
,
Alexis Berne
,
Hanin Binder
,
Juliette Blanchet
,
Deniz Bozkurt
,
Thomas J. Bracegirdle
,
Mathieu Casado
,
Taejin Choi
,
Kyle R. Clem
,
Francis Codron
,
Rajashree Datta
,
Stefano Di Battista
,
Vincent Favier
,
Diana Francis
,
Alexander D. Fraser
,
Elise Fourré
,
René D. Garreaud
,
Christophe Genthon
,
Irina V. Gorodetskaya
,
Sergi González-Herrero
,
Victoria J. Heinrich
,
Guillaume Hubert
,
Hanna Joos
,
Seong-Joong Kim
,
John C. King
,
Christoph Kittel
,
Amaelle Landais
,
Matthew Lazzara
,
Gregory H. Leonard
,
Jan L. Lieser
,
Michelle Maclennan
,
David Mikolajczyk
,
Peter Neff
,
Inès Ollivier
,
Ghislain Picard
,
Benjamin Pohl
,
F. Martin Ralph
,
Penny Rowe
,
Elisabeth Schlosser
,
Christine A. Shields
,
Inga J. Smith
,
Michael Sprenger
,
Luke Trusel
,
Danielle Udy
,
Tessa Vance
,
Étienne Vignon
,
Catherine Walker
,
Nander Wever
, and
Xun Zou

Abstract

Between 15 and 19 March 2022, East Antarctica experienced an exceptional heat wave with widespread 30°–40°C temperature anomalies across the ice sheet. This record-shattering event saw numerous monthly temperature records being broken including a new all-time temperature record of −9.4°C on 18 March at Concordia Station despite March typically being a transition month to the Antarctic coreless winter. The driver for these temperature extremes was an intense atmospheric river advecting subtropical/midlatitude heat and moisture deep into the Antarctic interior. The scope of the temperature records spurred a large, diverse collaborative effort to study the heat wave’s meteorological drivers, impacts, and historical climate context. Here we focus on describing those temperature records along with the intricate meteorological drivers that led to the most intense atmospheric river observed over East Antarctica. These efforts describe the Rossby wave activity forced from intense tropical convection over the Indian Ocean. This led to an atmospheric river and warm conveyor belt intensification near the coastline, which reinforced atmospheric blocking deep into East Antarctica. The resulting moisture flux and upper-level warm-air advection eroded the typical surface temperature inversions over the ice sheet. At the peak of the heat wave, an area of 3.3 million km2 in East Antarctica exceeded previous March monthly temperature records. Despite a temperature anomaly return time of about 100 years, a closer recurrence of such an event is possible under future climate projections. In Part II we describe the various impacts this extreme event had on the East Antarctic cryosphere.

Significance Statement

In March 2022, a heat wave and atmospheric river caused some of the highest temperature anomalies ever observed globally and captured the attention of the Antarctic science community. Using our diverse collective expertise, we explored the causes of the event and have placed it within a historical climate context. One key takeaway is that Antarctic climate extremes are highly sensitive to perturbations in the midlatitudes and subtropics. This heat wave redefined our expectations of the Antarctic climate. Despite the rare chance of occurrence based on past climate, a future temperature extreme event of similar magnitude is possible, especially given anthropogenic climate change.

Open access
Jonathan D. Wille
,
Simon P. Alexander
,
Charles Amory
,
Rebecca Baiman
,
Léonard Barthélemy
,
Dana M. Bergstrom
,
Alexis Berne
,
Hanin Binder
,
Juliette Blanchet
,
Deniz Bozkurt
,
Thomas J. Bracegirdle
,
Mathieu Casado
,
Taejin Choi
,
Kyle R. Clem
,
Francis Codron
,
Rajashree Datta
,
Stefano Di Battista
,
Vincent Favier
,
Diana Francis
,
Alexander D. Fraser
,
Elise Fourré
,
René D. Garreaud
,
Christophe Genthon
,
Irina V. Gorodetskaya
,
Sergi González-Herrero
,
Victoria J. Heinrich
,
Guillaume Hubert
,
Hanna Joos
,
Seong-Joong Kim
,
John C. King
,
Christoph Kittel
,
Amaelle Landais
,
Matthew Lazzara
,
Gregory H. Leonard
,
Jan L. Lieser
,
Michelle Maclennan
,
David Mikolajczyk
,
Peter Neff
,
Inès Ollivier
,
Ghislain Picard
,
Benjamin Pohl
,
F. Martin Ralph
,
Penny Rowe
,
Elisabeth Schlosser
,
Christine A. Shields
,
Inga J. Smith
,
Michael Sprenger
,
Luke Trusel
,
Danielle Udy
,
Tessa Vance
,
Étienne Vignon
,
Catherine Walker
,
Nander Wever
, and
Xun Zou

Abstract

Between 15 and 19 March 2022, East Antarctica experienced an exceptional heat wave with widespread 30°–40°C temperature anomalies across the ice sheet. In Part I, we assessed the meteorological drivers that generated an intense atmospheric river (AR) that caused these record-shattering temperature anomalies. Here, we continue our large collaborative study by analyzing the widespread and diverse impacts driven by the AR landfall. These impacts included widespread rain and surface melt that was recorded along coastal areas, but this was outweighed by widespread high snowfall accumulations resulting in a largely positive surface mass balance contribution to the East Antarctic region. An analysis of the surface energy budget indicated that widespread downward longwave radiation anomalies caused by large cloud-liquid water contents along with some scattered solar radiation produced intense surface warming. Isotope measurements of the moisture were highly elevated, likely imprinting a strong signal for past climate reconstructions. The AR event attenuated cosmic ray measurements at Concordia, something previously never observed. Last, an extratropical cyclone west of the AR landfall likely triggered the final collapse of the critically unstable Conger Ice Shelf while further reducing an already record low sea ice extent.

Significance Statement

Using our diverse collective expertise, we explored the impacts from the March 2022 heat wave and atmospheric river across East Antarctica. One key takeaway is that the Antarctic cryosphere is highly sensitive to meteorological extremes originating from the midlatitudes and subtropics. Despite the large positive temperature anomalies driven from strong downward longwave radiation, this event led to huge amounts of snowfall across the Antarctic interior desert. The isotopes in this snow of warm airmass origin will likely be detectable in future ice cores and potentially distort past climate reconstructions. Even measurements of space activity were affected. Also, the swells generated from this storm helped to trigger the final collapse of an already critically unstable Conger Ice Shelf while further degrading sea ice coverage.

Open access
Yiling Zheng
,
Chi-Yung Tam
,
Kang Xu
, and
Matthew Collins

Abstract

The Indian Ocean dipole (IOD) is the dominant mode of interannual variability in the tropical Indian Ocean (TIO), characterized by warming (cooling) in western TIO and cooling (warming) in eastern TIO during its positive (negative) phase. Observed IOD events exhibit distinct amplitude asymmetry in relation to negative nonlinear dynamic heating. Nearly all models in phase 5 of the Coupled Model Intercomparison Project (CMIP) simulate a less-skewed IOD than observed, but 6 out of 20 CMIP6 models can reproduce realistic high skewness. Analysis of less-skewed models indicates that the positive IOD-like biases in the mean state, which can be traced back to their weaker simulations of the preceding Indian summer monsoon, reduce the convective response to positive sea surface temperature anomalies in the western TIO, resulting in a weaker zonal wind response and weaker nonlinear zonal advection during positive IOD events. Besides, ocean stratification in the eastern TIO influences the IOD skewness: stronger stratification leads to larger mixed-layer temperature response to thermocline changes, contributing to larger anomalous vertical temperature gradient, larger nonlinear vertical advection, and thus stronger positive IOD skewness. Our findings underscore the importance of reducing Indian summer monsoon biases and eastern TIO stratification biases, for properly representing the IOD in Earth system models.

Restricted access
Lu Wang
,
Yifeng Cheng
,
Xiaolong Chen
, and
Tianjun Zhou

Abstract

The onset of the summer monsoon associated with global warming is of great concern to the scientific community. While observational data diagnosis has shown the impact of intraseasonal oscillation (ISO) on the monsoon onset, how the ISO may affect the onset of monsoon under global warming remains unknown. Here, by analyzing the onset of the summer monsoon over the South Asian marginal seas projected by models in phase 6 of the Coupled Model Intercomparison Project (CMIP6) under the SSP5–8.5 scenario, we show evidence that the majority of models (>70%) project an earlier onset over the Arabian Sea (ArS) but a delayed onset over the Bay of Bengal (BoB) and the South China Sea (SCS). The temporal shifts of the monsoon onset are attributed to the changes in the premonsoon northward migration of equatorial ISO (NMISO), which is a trigger of monsoon onset and will be advanced (postponed) over the ArS (BoB and SCS). The subtropical upper-level westerly anomaly, inducing delayed occurrence of easterly shear, acts to delay the NMISO over the entire Indian Ocean. However, the intensified low-level southerly wind over the ArS, as well as its induced asymmetric pattern of boundary layer moisture work together to advance the premonsoon NMISO in the area, outweighing the delayed impact from vertical shear. These large-scale circulation changes are driven by tropical warming in the upper troposphere, land warming over the Arabian Peninsula, and ocean warming over the eastern Pacific. This analysis enriches monsoon onset projections by highlighting the role of ISO in influencing the future changes in monsoon onset.

Restricted access