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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.

Restricted access
Chong Wang
and
Xiaofeng Li

Abstract

This article developed a deep learning (DL) model for estimating tropical cyclone (TC) 34-, 50-, and 64-kt (1 kt ≈ 0.51 m s−1) wind radii in four quadrants from infrared images in the global ocean. We collected 63 675 TC images from 2004 to 2016 and divided them into three periods (2004–12, 2013–14, and 2015–16) for model training, validation, and testing. First, four DL-based radius estimation models were developed to estimate the TC wind radius for each of the four quadrants. Then, the entire original images and the one-quarter-quadrant subimages were included in the model training for each quadrant. Last, we modified the mean absolute error (MAE) loss function in these DL-based models to reduce the side effect of an unbalanced distribution of wind radii and developed an asymmetric TC wind radius estimation model globally. The comparison of model results with the best-track data of TCs shows that the MAEs of 34-kt wind radius are 18.8, 19.5, 18.6, and 18.8 n mi (1 n mi = 1.852 km) for the northeast, southeast, southwest, and northwest quadrants, respectively. The MAEs of 50-kt wind radius are 11.3, 11.3, 11.1, and 10.8 n mi, respectively, and the MAEs of 64-kt wind radius are 8.9, 9.9, 9.2, and 8.7 n mi, respectively. These results represent a 12.1%–35.5% improvement over existing methods in the literature. In addition, the DL-based models were interpreted with two deep visualization toolboxes. The results indicate that the TC eye, cloud, and TC spiral structure are the main factors that affect the model performance.

Restricted access
David J. Lorenz

Abstract

Many studies have focused on the long-term positive feedback between annular mode zonal wind (U) perturbations and the eddy momentum fluxes (M). Lagged correlation analysis between U and M anomalies, however, shows that a transient period of negative eddy forcing follows the peak in zonal wind anomalies. This negative forcing is more ubiquitous than the positive feedback because it occurs for all U EOFs not just EOF1. It has been hypothesized that this response is either 1) an intrinsic feature of the eddies independent of the U or 2) caused by U-induced changes in Rossby wave reflection. Here it is shown that the response can be reproduced in a GCM by imposing a rapid change in U; therefore, mechanism 1 does not appear to be relevant. Furthermore, the transient response can be generated in a model when there are no turning latitudes; therefore, mechanism 2 does not appear to be relevant. Instead it is shown that the transient response is due to the adjustment of a preexisting eddy field to a change in the background wind. This transient effect is negative when the meridional scale of the U change is small enough compared to the waves, and vice versa. The sign of the initial response depends on the relative size of advection by U versus retrogression by the background vorticity gradient on the meridional tilt of the Rossby waves. Finally, it is shown that this transient response has a large damping effect on U variability.

Restricted access
Junde Li
and
Moninya Roughan

Abstract

Examining eddy–mean flow interactions in western boundary currents is crucial for understanding the mechanisms of mesoscale eddy generation and the role of eddies in the large-scale circulation. However, this analysis is lacking in the East Australian Current (EAC) system. Here we show the detailed three-dimensional structure of the eddy–mean flow interactions and energy budget in the EAC system. The energy reservoirs and conversions are greatest in the upper 500 m, with complex vertical structures. Strong mean kinetic energy is confined within a narrow band (24.5°–32.5°S) in the EAC jet. Most energy is contained in the eddy fields instead of the mean flow in the EAC typical separation and extension regions (south of 32.5°S). Strong barotropic instability is the primary source of eddy kinetic energy north of 36°S, while baroclinic instability dominates the eddy kinetic energy production in the EAC southern extension, which peaks in the subsurface. The mean flow transfers 5.22 GW of kinetic energy and 3.33 GW of available potential energy to the eddy field in the EAC typical separation region. The largest conversion term is from available potential energy conversion from the mean flow to the eddy field through baroclinic instability, dominating between 29° and 35.5°S. Nonlocal eddy–mean flow interactions also play a role in the energy exchange between the mean flow and the eddy fields. This study provides the mean state of the eddy–mean flow interactions in the EAC system, paving the way for further studies exploring seasonal and interannual variability and provides a baseline for assessing the impact of environmental change.

Open access
Kirsten L. Findell
,
Rowan Sutton
,
Nico Caltabiano
,
Anca Brookshaw
,
Patrick Heimbach
,
Masahide Kimoto
,
Scott Osprey
,
Doug Smith
,
James S. Risbey
,
Zhuo Wang
,
Lijing Cheng
,
Leandro B. Diaz
,
Markus G. Donat
,
Michael Ek
,
June-Yi Lee
,
Shoshiro Minobe
,
Matilde Rusticucci
,
Frederic Vitart
, and
Lin Wang

Abstract

The World Climate Research Programme (WCRP) envisions a world “that uses sound, relevant, and timely climate science to ensure a more resilient present and sustainable future for humankind.” This bold vision requires the climate science community to provide actionable scientific information that meets the evolving needs of societies all over the world. To realize its vision, WCRP has created five Lighthouse Activities to generate international commitment and support to tackle some of the most pressing challenges in climate science today. The overarching goal of the Lighthouse Activity on Explaining and Predicting Earth System Change is to develop an integrated capability to understand, attribute, and predict annual to decadal changes in the Earth system, including capabilities for early warning of potential high impact changes and events. This article provides an overview of both the scientific challenges that must be addressed, and the research and other activities required to achieve this goal. The work is organized in three thematic areas: (i) monitoring and modeling Earth system change; (ii) integrated attribution, prediction, and projection; and (iii) assessment of current and future hazards. Also discussed are the benefits that the new capability will deliver. These include improved capabilities for early warning of impactful changes in the Earth system, more reliable assessments of meteorological hazard risks, and quantitative attribution statements to support the Global Annual to Decadal Climate Update and State of the Climate reports issued by the World Meteorological Organization.

Free access