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Siegfried D. Schubert
,
Yehui Chang
,
Anthony M. DeAngelis
,
Young-Kwon Lim
,
Natalie P. Thomas
,
Randal D. Koster
,
Michael G. Bosilovich
,
Andrea M. Molod
,
Allison Collow
, and
Amin Dezfuli

Abstract

In late December of 2022 and the first half of January 2023, an unprecedented series of atmospheric rivers (ARs) produced near-record heavy rains and flooding over much of California. Here, we employ the NASA GEOS AGCM run in a “replay” mode, together with more idealized simulations with a stationary wave model, to identify the remote forcing regions, mechanisms, and underlying predictability of this flooding event. In particular, the study addresses the underlying causes of a persistent positive Pacific–North American (PNA)-like circulation pattern that facilitated the development of the ARs. We show that the pattern developed in late December as a result of vorticity forcing in the North Pacific jet exit region. We further provide evidence that this vorticity forcing was the result of a chain of events initiated in mid-December with the development of a Rossby wave (as a result of forcing linked to the MJO) that propagated from the northern Indian Ocean into the North Pacific. As such, both the initiation of the event and the eventual development of the PNA depended critically on internally generated Rossby wave forcings, with the North Pacific jet playing a key role. This, combined with contemporaneous SST (La Niña) forcing that produced a circulation response in the AGCM that was essentially opposite to the positive PNA, underscores the fundamental lack of predictability of the event at seasonal time scales. Forecasts produced with the GEOS-coupled model suggest that useful skill in predicting the PNA and extreme precipitation over California was in fact limited to lead times shorter than about 3 weeks.

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Shaohua Chen
,
Haikun Zhao
,
Philip J. Klotzbach
,
Jian Cao
,
Jia Liang
,
Weican Zhou
, and
Liguang Wu

Abstract

On interannual time scales, there is significant meridional migration of the boreal summer (May–October) synoptic-scale wave (SSW) train relative to the summer monsoon trough line over the western North Pacific (WNP) during 1979–2021. The associated plausible physical reasons for the SSW meridional migration are investigated by comparing analyses between two distinct groups: atypical SSW years where SSWs tend to prevail northward of the summer monsoon trough line and typical SSW years where SSWs largely occur along the summer monsoon trough line. During typical SSW years, SSWs originate primarily from equatorial mixed Rossby-gravity (MRG) waves and then develop into off-equatorial tropical depression (TD) waves in the lower troposphere of the monsoon region. During atypical SSW years, SSWs appear to be sourced from upper-level easterlies, propagating downward to the lower troposphere in the monsoon region, with a prevailing TD wave structure. A budget analysis of barotropic eddy kinetic energy suggests that interannual meridional SSW migration is closely related to changes in the vorticity distribution along the summer monsoon trough over the WNP, especially the western part of the summer monsoon trough. These changes cause low-frequency zonal convergence and shear differences, changing barotropic conversion around the monsoon trough and modulating interannual SSW meridional movement. In response to these changes, there are corresponding differences in SSW sources: a predominate MRG–TD wave pattern in typical SSW years and a predominate TD wave pattern in atypical SSW years. These results improve our understanding of the interannual variability of large-scale circulation and tropical cyclones.

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Valentina Ortiz-Guzmán
,
Martin Jucker
, and
Steven C. Sherwood

Abstract

The Southern Hemisphere climate and weather are affected by several modes of variability and climate phenomena across different time and spatial scales. An additional key component of the atmosphere dynamics that greatly influences weather is quasi-stationary Rossby waves, which attract particular interest as they are often associated with synoptic-scale extreme events. In the Southern Hemisphere extratropical circulation, the most prominent quasi-stationary Rossby wave pattern is the zonal wavenumber 3 (ZW3), which has been shown to have impacts on meridional heat and momentum transport in mid- to high latitudes and on the Antarctic sea ice extent. However, little is known about its impacts outside of polar regions. In this work, we use ERA5 reanalysis data on monthly time scales to explore the influence of phase and amplitude of ZW3 on temperature and precipitation across the Southern Hemisphere midlatitudes. Our results show a significant impact in various regions for all seasons. One of the most substantial effects is observed in precipitation over southeastern Brazil during austral summer, where different phases of the ZW3 force opposite anomalies. When using the ZW3 phase and amplitude as prior information, the probability of occurrence of precipitation extremes in this region increases up to three times. Additionally, we find that this ZW3 weather signature is largely independent of the zonally symmetric Southern Annular Mode (SAM); neither does it seem to be linked to El Niño–Southern Oscillation (ENSO) or Indian Ocean dipole (IOD) signal.

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Pei-Syuan Liao
,
Chia-Wei Lan
,
Yu-Chiao Liang
, and
Min-Hui Lo

Abstract

The annual range (AR) of precipitation in the Amazon River basin has increased steadily since 1979. This increase may have resulted from natural variability and/or anthropogenic forcing, such as local land-use changes and global warming, which has yet to be explored. In this study, climate model experiments using the Community Earth System Model, version 2 (CESM2), were conducted to examine the relative contributions of sea surface temperatures (SSTs) variability and anthropogenic forcings to the AR changes in the Amazon rainfall. With CESM2, we design several factorial simulations, instead of actual model projection. We found that the North Atlantic SSTs fluctuation dominantly decreases the precipitation AR trend over the Amazon by −85%. In contrast, other factors, including deforestation and carbon dioxide, contributed to the trend changes, ranging from 25% to 35%. The dynamic component, specifically the tendency of vertical motion, made negative contributions, along with the vertical profiles of moist static energy (MSE) tendency. Seasonal-dependent changes in atmospheric stability could be associated with variations in precipitation. It is concluded that surface ocean warming associated with the North Atlantic natural variability and global warming is the key factor in the increased precipitation AR over the Amazon from 1979 to 2014. The continuous local land-use changes may potentially influence the precipitation AR in the future.

Significance Statement

The annual range (AR) in precipitation, the difference between wet- and dry-season precipitation, has increased from 1979 to 2014 in the Amazon. This increase may have resulted from global warming, deforestation, and sea surface temperature variability in North Atlantic and Pacific. To explore the role of each of these factors in altering the Amazon precipitation AR, five experiments were designed in the climate model (CESM). Among these experiment results, the effect of North Atlantic SSTs was the strongest. In the future, deforestation, global warming, and different ocean temperature states in the North Atlantic and Pacific may become increasingly influential on the changes in precipitation. Further investigation is needed to ascertain how the AR of precipitation in the Amazon will change.

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Kouya Nakamura
,
Shoichiro Kido
,
Takashi Ijichi
, and
Tomoki Tozuka

Abstract

The mean vertical advection of anomalous vertical temperature gradient is considered the dominant generation mechanism of positive sea surface temperature (SST) anomalies associated with the canonical El Niño. However, most past studies had a residual term in their heat budget analysis and/or did not quantify the role of vertical mixing even though active vertical turbulent mixing in the upper ocean is observed in the eastern equatorial Pacific. To quantitatively assess the importance of vertical mixing, a mixed layer heat budget analysis is performed using a hindcast simulation forced by daily mean atmospheric reanalysis data. It is found that when the mixed layer depth is defined as the depth at which potential density increases by 0.125 kg m−3 from the sea surface, the development of positive SST anomalies is predominantly governed by reductions in the cooling by vertical mixing, and their magnitude is much larger than those by vertical advection. The anomalous warming by vertical mixing may be partly explained by an anomalous deepening of the thermocline that leads to a decrease in the vertical temperature gradient, giving rise to suppression of the climatological cooling by vertical mixing. Also, an anomalously thick mixed layer reduces sensitivity to cooling by the mean vertical mixing and contributes to the anomalous SST warming. On the other hand, the dominant negative feedbacks are attributed to both anomalous surface heat loss and anomalous deepening of the mixed layer that weakens warming by the mean surface heat flux.

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Zili Shen
,
Anmin Duan
,
Wen Zhou
,
Yuzhuo Peng
, and
Jinxiao Li

Abstract

Two large ensemble simulations are adopted to investigate the relative contribution of external forcing and internal variability to Arctic sea ice variability on different time scales since 1960 by correcting the response error of models to external forcing using observational datasets. Our study suggests that previous approaches might overestimate the real impact of internal variability on Arctic sea ice change especially on long time scales. Our results indicate that in both March and September, internal variability plays a dominant role on all time scales over the twentieth century, while the anthropogenic signal on sea ice change can be steadily and consistently detected on a time scale of more than 20 years after the 2000s. We also reveal that the dominant mode of internal variability in March shows consistency across different time scales. On the contrary, the pattern of internal variability in September is highly nonuniform over the Arctic and varies across different time scales, indicating that sea ice internal variability in September at different time scales is driven by different factors.

Open access
Hongyu Luo
,
Haipeng Yu
,
Zeyong Hu
,
Jie Zhou
,
Bofei Zhang
,
Yaoxian Yang
,
Shanling Cheng
,
Yongqi Gong
, and
Yu Ren

Abstract

The summer atmospheric heat source (AHS) over the Tibetan Plateau (TP) induces meridional circulations in TP and its surrounding areas. Previous studies mainly focused on the monsoon circulation on the south side of TP, while the formation and maintenance mechanisms of meridional circulation on its north side remain unclear. This study compared three calculation methods of the AHS, analyzed the spatial–temporal variability of the summer AHS over the TP, and discussed its influence on the interannual variability of meridional circulation on the north side of the TP based on the two-dimensional decomposition method of atmospheric circulation and sensitivity experiments. The results indicate that in the positive AHS anomalies years, the diabatic heating of condensation latent release in southeastern TP could motivate anomalous ascending motion. Simultaneously, the increased meridional temperature gradient between the mid- and high latitudes of East Asia leads to an enhanced southward westerly jet. In this context, the region on the north side of TP, located on the north side of the westerly jet entrance, is affected by negative anomalous relative vorticity advection, prevailing anomalous descending motion, which makes the descending branch of meridional circulation significantly presented. Unlike previous studies that considered the descending branch of meridional circulation as the compensation for upward flow, the results of the linear baroclinic model (LBM) verify that the descending branch is mainly influenced by the vorticity advection related to regional scale variability of the westerly jet. This study reveals the physical mechanism of meridional circulation on the north side of TP, which offers valuable implications for seasonal forecasting in TP and Northwest China.

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Emily J. Becker
and
Michael K. Tippett

Abstract

The effect of the El Niño–Southern Oscillation (ENSO) teleconnection and climate change trends on observed North American wintertime daily 2-m temperature is investigated for 1960–2022 with a quantile regression model, which represents the variability of the full distribution of daily temperature, including extremes and changes in spread. Climate change trends are included as a predictor in the regression model to avoid the potentially confounding effect on ENSO teleconnections. Based on prior evidence of asymmetric impacts from El Niño and La Niña, the ENSO response is taken to be piecewise linear, and the regression model contains separate predictors for warm and cool ENSO. The relationship between these predictors and shifts in median, interquartile range, skewness, and kurtosis of daily 2-m temperature are summarized through Legendre polynomials. Warm ENSO conditions result in significant warming shifts in the median and contraction of the interquartile range in central-northern North America, while no opposite effect is found for cool ENSO conditions in this region. In the southern United States, cool ENSO conditions produce a warming shift in the median, while warm ENSO conditions have little impact on the median, but contracts the interquartile range. Climate change trends are present as a near-uniform warming in the median and across quantiles and have no discernable impact on interquartile range or higher-order moments. Trends and ENSO together explain a substantial fraction of the interannual variability of daily temperature distribution shifts across much of North America and, to a lesser extent, changes of the interquartile range.

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Ruping Huang
,
Shangfeng Chen
,
Wen Chen
,
Renguang Wu
,
Zhibiao Wang
,
Peng Hu
,
Liang Wu
,
Lei Wang
, and
Jingliang Huangfu

Abstract

The poleward migration of tropical cyclone (TC) activity in recent years has been linked to the expansion of the Hadley circulation (HC). Here, we investigate the impact of the winter regional HC over the western Pacific (WPHC) on the frequency of following summer landfalling TC (LTC) in China. Results show that interannual variation of the LTC frequency has a very close connection with the northern WPHC edge (WPHCE). After removing the El Niño–Southern Oscillation signal, there still exists a significant correlation between them. When the winter WPHCE shifts poleward, the associated lower-level southwesterly (easterly) wind anomalies over the subtropical western Pacific (tropical central-eastern Pacific) induce sea surface temperature (SST) warming (cooling) anomalies therein via suppressing (enhancing) upward surface heat flux. In turn, the SST warming (cooling) excites an anomalous cyclonic (anticyclonic) circulation to its west via a Rossby wave response, thus maintaining the southwesterly (easterly) wind anomalies. In addition, the negative rainfall anomalies over the tropical central-eastern Pacific induced by negative SST anomalies can stimulate an anomalous intensive Walker circulation with anomalous upward motion around the tropical western Pacific. Through this positive air–sea interaction, the winter WPHCE signal would be preserved in the ocean and maintained to the succeeding summer, then favoring LTC genesis landward by decreasing the vertical wind shear and increasing the low-level vorticity and midlevel humidity. Meanwhile, anomalous midtropospheric easterly winds over the subtropics are favorable for steering more LTCs toward China’s coast. This study suggests that the winter WPHCE variation is a potential predictor for the prediction of the following summer LTC activity over China.

Significance Statement

Tropical cyclone (TC) is one of the most catastrophic high-impact weather events, which may cause great casualties and severe property losses over the coastal areas, particularly when it makes landfall. Previous research studies have related the poleward migration trend of TC locations to the Hadley circulation (HC) expansion. Compared to the long-term trend, the magnitude of the year-to-year change of the HC edge (HCE) is even larger, leading to a stronger impact on the TC activity. A recent study has suggested that the northern HCE over the western Pacific (WPHCE) in boreal winter exhibits a notable interannual variability. In this study, we reveal that the wintertime WPHCE has a very close connection with the landfalling TC (LTC) frequency over China in the following summer. After removing the El Niño–Southern Oscillation (ENSO) signal, there still exists a significant positive correlation between them. Observational evidence and numerical model experiments consistently confirm that this time-lagged association is attributable to the air–sea interaction processes in the tropical Pacific. Thus, the results of this study could provide an additional predictor besides ENSO to improve understanding of the LTC activity in China.

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Wenhao Jiang
,
Huopo Chen
, and
Huijun Wang

Abstract

This study investigates the spatiotemporal variations of the summer frequency of daytime–nighttime compound extreme high-temperature events (FCEHEs) in the mid–high latitudes of Asia (MHA) from 1979 to 2014. Results show that FCEHE has shown an upward trend with fluctuations, especially in Mongolia–Baikal. The descending anomaly caused by the anomalous high pressure over Mongolia–Baikal results in reduced cloud cover, which increases solar radiation reaching the ground, favoring the higher FCEHE. This process is consistent during the daytime and nighttime periods, with relatively limited nighttime solar radiation, potentially compensated by the increased downward longwave radiation to sustain the extreme high temperatures. This benefit process is closely connected with two main factors: the increased sea ice in the Barents Sea during spring and the anomalously warm sea surface temperature (SST) in the Northwest Pacific during summer. The increased sea ice can affect the Eurasia (EU) teleconnection, while the warm SST affects the Pacific-Japan/East Asia–Pacific pattern (PJ/EAP). Subsequently, these factors further modulate the circulation anomalies and then FCEHE.

Significance Statement

This study provides valuable insights into the spatiotemporal variations and the possible underlying mechanisms for change in the frequency of daytime–nighttime compound extreme high-temperature events (FCEHEs) in the mid–high latitudes of Asia. The spring sea ice anomalies over the Barents Sea and summer sea surface temperature anomalies in the Northwest Pacific affect the local anticyclonic circulation in Mongolia–Baikal through Eurasia (EU) and Pacific-Japan/East Asia–Pacific (PJ/EAP) patterns, respectively. The resulting descending anomaly and reduced cloud cover contribute to interannual variations of FCEHE, which is highly similar during the daytime and nighttime periods. During the nighttime, when the solar radiation is relatively limited, the increased downward longwave radiation may compensate to sustain extreme high temperatures.

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