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Xi Lu
,
Shijian Hu
, and
Janet Sprintall

Abstract

Multidecadal variability of the Indonesian Throughflow (ITF) is crucial for the Indo-Pacific and global climate due to significant interbasin exchanges of heat and freshwater. Previous studies suggest that both wind and buoyancy forcing may drive ITF variability, but the role of precipitation and salinity effect in the variability of ITF on multidecadal time scales remains largely unexplored. Here, we investigate the multidecadal changes and long-term trend of the ITF transport during the past six decades, with a focus on the role of precipitation and salinity effect. The diverse datasets consistently indicate a substantial upward trend in the halosteric component of geostrophic transport of ITF in the outflow region at 114°E during the six decades. We find that the meridional differences of the salinity trend in the outflow region explain the increasing trend of the halosteric component of ITF transport. On a larger scale, the tropical western Pacific Ocean and Indonesian seas have experienced significant freshening, which has strengthened the Indo-Pacific pressure gradient and thus enhanced the ITF. In contrast, the equatorial trade wind in the western Pacific Ocean has weakened over recent decades, implying that changes in wind forcing have contributed to weakening the ITF. The combined effect of strengthened halosteric and weakened thermosteric components has resulted in a weak strengthening for the total ITF with large uncertainties. Although both the thermosteric and halosteric components are associated with natural climate modes, our results suggest that the importance of salinity effect is likely increasing given the enhanced water cycle under global warming.

Open access
Kaighin A. McColl
and
Lois I. Tang

Abstract

There is no simple explanation for the spatial structure of near-surface relative humidity over land. We present a diagnostic theory for zonally and temporally averaged near-surface relative humidity (RH) over land based on energy budgets of an atmospheric column in radiative–convective equilibrium. The theory analytically relates RH to the surface evaporative fraction (EF), has no calibrated parameters, and is quantitatively accurate when compared with RH from a reanalysis, and with cloud-permitting simulations over an idealized land surface. The theory is used to answer two basic questions. First, why is RH never especially low (e.g., 1%)? The theory shows that established lower bounds on EF over land and ocean are equivalent to lower bounds on RH that preclude particularly low values, at least for conditions typical of the modern Earth. Second, why is the latitudinal profile of RH over land shaped like the letter W, when both specific humidity and saturation specific humidity essentially decline monotonically from the equator to the poles? The theory predicts that the latitudinal profile of RH should look more like that of water stored in the soil (which also exhibits a W-shaped profile) than in the air (which does not).

Open access
Rachael N. Isphording
,
Lisa V. Alexander
,
Margot Bador
,
Donna Green
,
Jason P. Evans
, and
Scott Wales

Abstract

Presently, there is no standardized framework or metrics identified to assess regional climate model precipitation output. Because of this, it can be difficult to make a one-to-one comparison of their performance between regions or studies, or against coarser-resolution global climate models. To address this, we introduce the first steps toward establishing a dynamic, yet standardized, benchmarking framework that can be used to assess model skill in simulating various characteristics of rainfall. Benchmarking differs from typical model evaluation in that it requires that performance expectations are set a priori. This framework has innumerable applications to underpin scientific studies that assess model performance, inform model development priorities, and aid stakeholder decision-making by providing a structured methodology to identify fit-for-purpose model simulations for climate risk assessments and adaptation strategies. While this framework can be applied to regional climate model simulations at any spatial domain, we demonstrate its effectiveness over Australia using high-resolution, 0.5° × 0.5° simulations from the CORDEX-Australasia ensemble. We provide recommendations for selecting metrics and pragmatic benchmarking thresholds depending on the application of the framework. This includes a top tier of minimum standard metrics to establish a minimum benchmarking standard for ongoing climate model assessment. We present multiple applications of the framework using feedback received from potential user communities and encourage the scientific and user community to build on this framework by tailoring benchmarks and incorporating additional metrics specific to their application.

Significance Statement

We introduce a standardized benchmarking framework for assessing the skill of regional climate models in simulating precipitation. This framework addresses the lack of a uniform approach in the scientific community and has diverse applications in scientific research, model development, and societal decision-making. We define a set of minimum standard metrics to underpin ongoing climate model assessments that quantify model skill in simulating fundamental characteristics of rainfall. We provide guidance for selecting metrics and defining benchmarking thresholds, demonstrated using multiple case studies over Australia. This framework has broad applications for numerous user communities and provides a structured methodology for the assessment of model performance.

Open access
Melissa Gervais
,
Lantao Sun
, and
Clara Deser

Abstract

Future Arctic sea ice loss has a known impact on Arctic amplification (AA) and mean atmospheric circulation. Furthermore, several studies have shown it leads to a decreased variance in temperature over North America. In this study, we analyze results from two fully coupled Community Earth System Model (CESM) Whole Atmosphere Community Climate Model (WACCM4) simulations with sea ice nudged to either the ensemble mean of WACCM historical runs averaged over the 1980–99 period for the control (CTL) or projected RCP8.5 values over the 2080–99 period for the experiment (EXP). Dominant large-scale meteorological patterns (LSMPs) are then identified using self-organizing maps applied to winter daily 500-hPa geopotential height anomalies ( Z 500 ) over North America. We investigate how sea ice loss (EXP − CTL) impacts the frequency of these LSMPs and, through composite analysis, the sensible weather associated with them. We find differences in LSMP frequency but no change in residency time, indicating there is no stagnation of the flow with sea ice loss. Sea ice loss also acts to de-amplify and/or shift the Z 500 that characterize these LSMPs and their associated anomalies in potential temperature at 850 hPa. Impacts on precipitation anomalies are more localized and consistent with changes in anomalous sea level pressure. With this LSMP framework we provide new mechanistic insights, demonstrating a role for thermodynamic, dynamic, and diabatic processes in sea ice impacts on atmospheric variability. Understanding these processes from a synoptic perspective is critical as some LSMPs play an outsized role in producing the mean response to Arctic sea ice loss.

Significance Statement

The goal of this study is to understand how future Arctic sea ice loss might impact daily weather patterns over North America. We use a global climate model to produce one set of simulations where sea ice is similar to present conditions and another that represents conditions at the end of the twenty-first century. Daily patterns in large-scale circulation at roughly 5.5 km in altitude are then identified using a machine learning method. We find that sea ice loss tends to de-amplify these patterns and their associated impacts on temperature nearer the surface. Our methodology allows us to probe more deeply into the mechanisms responsible for these changes, which provides a new way to understand how sea ice loss can impact the daily weather we experience.

Open access
Richard E. Chandler
,
Clair R. Barnes
, and
Chris M. Brierley

Abstract

This paper presents a methodology that is designed for rapid exploratory analysis of the outputs from ensembles of climate models, especially when these outputs consist of maps. The approach formalizes and extends the technique of “intermodel empirical orthogonal function” analysis, combining multivariate analysis of variance techniques with singular value decompositions (SVDs) of structured components of the ensemble data matrix. The SVDs yield spatial patterns associated with these components, which we call ensemble principal patterns (EPPs). A unique hierarchical partitioning of variation is obtained for balanced ensembles in which all combinations of factors, such as GCM and RCM pairs in a regional ensemble, appear with equal frequency: suggestions are also proposed to handle unbalanced ensembles without imputing missing values or discarding runs. Applications include the selection of ensemble members to propagate uncertainty into subsequent analyses, and the diagnosis of modes of variation associated with specific model variants or parameter perturbations. The approach is illustrated using outputs from the EuroCORDEX regional ensemble over the United Kingdom.

Open access
Tianying Liu
,
Zhengyu Liu
,
Yuchu Zhao
, and
Shaoqing Zhang

Abstract

Previous studies have indicated that the extratropics can influence ENSO via specific processes. However, it is still unclear to what extent ENSO is influenced by the extratropics in observation. Now we assess this issue by applying the regional data assimilation (RDA) approach in an advanced model, the GFDL CM2.1. Our study confirms a strong extratropical impact on observed ENSO. Quantitatively, the extratropical atmospheric variability poleward of 20° explains 56% of the observed variance of ENSO and greatly influences ∼67% of observed El Niño events during 1969–2008. This extratropical impact is still significant even as far as poleward of 30°. Furthermore, the impact from the southern extratropics is slightly stronger than that from the northern extratropics, partly caused by the Pacific ITCZ location north of the equator and different mixed-layer depth along the northern Pacific meridional mode (NPMM) and the southern Pacific meridional mode (SPMM). Our study further shows that all of three super El Niño events, those in 1972/73, 1982/83, and 1997/98, are influenced greatly by both hemispheric extratropics, with NPMM and SPMM interfering constructively, while most weak and moderate El Niño events are triggered by only one hemispheric extratropics, with NPMM and SPMM interfering destructively. Besides the extratropical Pacific influence on ENSO via NPMM/SPMM, the extratropics also has a potential impact on ENSO by influencing other tropical oceans and then by interbasin interactions.

Open access
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
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
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