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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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Mingmei Xie
,
Bo Wu
,
Jia-Zhen Wang
,
Chunzai Wang
, and
Xiubao Sun

Abstract

On decadal time scales, a zonal SST dipole dominates the tropical Indian Ocean in boreal late summer and fall, called the decadal Indian Ocean dipole (D-IOD). The D-IOD has a spatial pattern different from the traditional interannual IOD, with its eastern pole located off Java, rather than the whole Sumatra–Java coasts as the latter. Here, we show that the D-IOD is generated by both the remote tropical Pacific decadal variability (TPDV) forcing and the decadal modulation of interannual IODs, but with its distinctive spatial pattern and seasonality mainly shaped by the former. In August–September (AS), due to the seasonal strengthening of trade winds, the descending branch of TPDV-induced Walker circulation moves westward into the eastern Indian Ocean relative to June–July, which stimulates equatorial easterly anomalies and oceanic upwelling Kelvin waves, causing subsurface cooling off Java. The subsurface cooling just occurs within the time window of climatological coastal upwelling so that subsurface cold anomalies are brought into the surface by mean upwelling and further transported offshore by mean flows, forming the D-IOD eastern pole. The subsurface cooling is only generated near Java but not Sumatra, because the former is closer to the exit of the Indonesian Throughflow (ITF). Weakened ITF during positive TPDV inhibits the growth of subsurface warming off Java prior to the establishment of AS equatorial easterly anomalies, whereas this ITF effect is not observed off Sumatra. Moreover, warming of the D-IOD western pole might be associated with off-equatorial Rossby waves induced by TPDV-related wind stress curls.

Significance Statement

The decadal Indian Ocean dipole (D-IOD) is one of the leading decadal modes of the tropical Indian Ocean SST variations. Its formation mechanisms, especially those related to its spatiotemporal characteristics, are not well understood. Based on observations and reanalysis, we show that the tropical Pacific decadal variability (TPDV) mainly accounts for the distinctive spatial pattern and seasonality of the D-IOD via the seasonal migration of the anomalous Walker circulation and ensuing oceanic subsurface dynamics. Our results highlight that the TPDV is an important source of D-IOD’s predictability, and it might be beneficial for operational decadal predictions for the tropical Indian Ocean.

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Lucas R. Vargas Zeppetello
,
Lily N. Zhang
,
David S. Battisti
, and
Marysa M. Laguë

Abstract

Interannual fluctuations in average summertime temperatures across the western United States are captured by a leading empirical orthogonal function that explains over 50% of the total observed variance. In this paper, we explain the origins of this pattern of interannual temperature variability by examining soil moisture–temperature coupling that acts across seasons in observations and climate models. We find that a characteristic pattern of coupled temperature–soil moisture climate variability accounts for 34% of the total observed variance in summertime temperature across the region. This pattern is reproduced in state-of-the-art global climate models, where experiments that eliminate soil moisture variability reduce summertime average temperature variance by a factor of 3 on average. We use an idealized model of the coupled atmospheric boundary layer and underlying land surface to demonstrate that feedbacks between soil moisture, boundary layer relative humidity, and precipitation can explain the observed relations between springtime soil moisture and summertime temperature. Our results suggest that antecedent soil moisture conditions and subsequent land–atmosphere interactions play an important role in interannual summertime temperature variability in the western United States; soil moisture variations cause distal temperature anomalies and impart predictability at time scales longer than one season. Our results indicate that 40% of the observed warming trend across the western United States since 1981 has been driven by wintertime precipitation trends in the U.S. southwest.

Significance Statement

Year-to-year fluctuations in summertime temperatures have a large impact on drought, wildfire, and extreme heat across the Western United States. We find strong evidence that soil moisture deficits in the preceding spring may be the primary driver of higher-than-average summer temperatures in this region. Our results suggest that memory in the water cycle may lead to greater predictability in the climate system from season to season and that trends in southwest precipitation have exerted a considerable influence on observed warming over the past 40 years.

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Leilane Passos
,
Helene R. Langehaug
,
Marius Årthun
, and
Fiammetta Straneo

Abstract

Decadal thermohaline anomalies carried northwards by the North Atlantic Current are an important source of predictability in the North Atlantic region. Here, we investigate whether these thermohaline anomalies influence surface-forced water mass transformation (SFWMT) in the eastern Subpolar gyre using the reanalyses EN4.2.2 for the ocean and ERA5 for the atmosphere. In addition, we follow the propagation of thermohaline anomalies along two paths: in the Subpolar North Atlantic and the Norwegian Sea. We use observation-based data sets (HadISST, EN4.2.2, and Ishii) between 1947 and 2021 and apply Complex Empirical Orthogonal functions. Our results show that when a warm anomaly enters the eastern Subpolar gyre, more SFWMT occurs in light-density classes (27.0-27.2 kg m−3). In contrast, when a cold anomaly enters the eastern Subpolar gyre, more SFWMT occurs in denser classes (27.4-27.5 kg m−3). Following the thermohaline anomalies in both paths, we find alternating warm-salty and cold-fresh subsurface anomalies, repeating throughout the 74-year-long record with 4 warm-salt and cold-fresh periods after the 50s. The cold-fresh anomaly periods happen simultaneously with the Great salinity anomaly events. Moreover, the propagation of thermohaline anomalies is faster in the SPNA than in the Norwegian Sea, especially for temperature anomalies. These findings might have implications for our understanding of the decadal variability of the lower limb of the Atlantic Meridional Overturning Circulation and predictability in the North Atlantic region.

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Ziming Chen
,
Tianjun Zhou
,
Xiaolong Chen
,
Lixia Zhang
,
Yun Qian
,
Zeyi Wang
,
Linqiang He
, and
L. Ruby Leung

Abstract

Understanding global monsoon (GM) variability and projecting its future changes relies heavily on climate models. However, climate models generally show pronounced biases in GM simulations, and the reasons for this remain unclear. Here, we evaluate the performance of 20 pairs of climate models that participated in both the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6), and identify the sources of their GM simulation biases from an energy transport perspective. The multimodel mean improvement in CMIP6 compared to CMIP5 is demonstrated by the increasing skill scores for various GM metrics from 0.20~0.79 to 0.48~0.83. More specifically, the dry biases in the Northern Hemispheric Summer Monsoon (NHSM) precipitation in CMIP5 (root mean square error, RMSE: 1.85 mm/day) are reduced in CMIP6 (RMSE: 1.66 mm/day). This higher simulation skill is associated with higher skill in simulating the precipitation-solstitial mode, monsoon intensity, and monsoon domains. The improvement in the NHSM precipitation simulation results from that in the meridional transport of atmospheric energy. Atmospheric energy budget analysis shows that the negative biases in downward surface longwave radiation and northward energy transport are smaller in CMIP6 than in CMIP5 in the boreal summer, resulting in a more realistic interhemispheric thermal contrast and meridional gradient of moist static energy. However, a major weakness of the CMIP6 models is found in the Southern Hemisphere Summer Monsoon precipitation simulation due to the positive bias in the top-of-atmosphere downward longwave radiation. This study shows that reasonably reproducing the meridional global atmospheric energy transportation is necessary for skillful GM simulation.

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Free access
Jian-Hua Qian
,
Brian Viner
,
Stephen Noble
,
David Werth
,
Joseph Wermter
,
Steven Chiswell
, and
Cuihua Li

Abstract

Observed precipitation changes in the Southeast U.S. (SEUS) are spatially heterogeneous. Most of the inland SEUS and eastern Gulf coast become drier and the east coast north of Charleston SC and southern Florida become wetter from the old 30-year period of 1961-1990 to the recent period of 1991-2020. The observed climate change is examined from the perspective of daily weather types (WTs). A k-means clustering analysis has been conducted using daily 850 hPa circulation for 1948-2021. The obtained ten weather types (WTs) peak in different seasons, respectively. The frequencies and precipitation intensity of the WTs have been analyzed. A winter WT characterized by a Western Appalachian trough and a summer WT featuring North Atlantic Subtropical High (NASH) have a rising trend of annual frequency from 1948 to 2021. An Appalachian High in the autumn has a decreasing frequency but become drier and stronger. Some precipitation intensity change and small location shift have also been observed. The drying up on the eastern Gulf coast and the inland area of the SEUS is mainly caused by the weakened southwesterly low-level jet on the western flank of the NASH that reduces rain in the spring, the less frequent but stronger and drier Appalachian High in the summer and autumn, and the weaker and more western located Plains trough in the winter, spring, and autumn. The precipitation increase on the east coast and southern Florida is majorly due to more frequent, stronger, and rainier troughs along the western Appalachian as well as the east coast.

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Veeshan Narinesingh
,
Huan Guo
,
Stephen T. Garner
, and
Yi Ming

Abstract

Coupled ocean and prescribed sea surface temperature (SST) experiments are performed to investigate the drivers of Northern Hemisphere (NH) midlatitude winter circulation and blocking changes in warmer climates. In coupled experiments, a historical simulation is compared to a simulation following an end of the 21st century SSP5-8.5 emissions scenario. The SSP5-8.5 simulation yields poleward shifted jets and an enhanced stationary wave pattern compared to the historical simulation. In terms of blocking, a reduction is found across North America and over the Pacific Ocean with suggestion of more blocking over parts of Eurasia. Separately, prescribed SST experiments are performed decomposing the SSP5-8.5 SST response into a uniform warming component plus a spatially dependent change in SST pattern. SSP5-8.5 changes in circulation are primarily driven by a uniform warming of SST. Uniform warming is also found to account for most of the SSP5-8.5 blocking reduction over North America and the Pacific Ocean, but not over Eurasia. El Niño like changes to the SST pattern also yield less blocking over the Pacific and North America. However, adding the responses of uniform and pattern experiments yields a non-linear overreduction of blocking compared to the SSP5-8.5 experiment. Regional analyses of block energetics suggest that much of the reductions in blocking in warming simulations are driven by decreased baroclinic conversion in some regions and enhanced dissipation from diabatic sources in others.

Open access
Zizhen Dong
,
Lin Wang
,
Ruowen Yang
, and
Jie Cao

Abstract

This study investigates the propagation and maintenance mechanisms of the dominant intraseasonal oscillation over the western North Pacific in boreal winter, the quasi-biweekly oscillation (QBWO). The wintertime QBWO over the western North Pacific is characterized by the westward-northwestward movement from the tropical western Pacific to the western North Pacific and resembles the n = 1 equatorial Rossby wave. Its westward migration is primarily driven by the seasonal-mean zonal winds that advect vorticity anomalies in the lower–middle troposphere and moisture anomalies in the lower troposphere. Its northward movement is preconditioned by the vorticity dynamics of the beta effect, the low-level vertical moisture variation, and the local air–sea interaction. The latter involves the atmospheric forcing on the underlying ocean by changing the surface heat flux fluctuations and the sea surface temperature feedback on the low-level atmospheric instability. Its maintenance is primarily through atmospheric external energy sources from diabatic heating, which first generates eddy available potential energy and then converts it to eddy kinetic energy.

Significance Statement

Atmospheric quasi-biweekly oscillation (QBWO) is an important climate phenomenon over the western North Pacific in boreal winter. It can trigger significant influences both in the tropical and extratropical regions. Given the importance, it is necessary to investigate the propagation and maintenance mechanisms of the QBWO over the western North Pacific in boreal winter. The QBWO usually propagates northwestward from the tropical western Pacific to the western North Pacific. The westward propagation is driven by the seasonal mean zonal winds that advect vorticity anomalies in the lower–middle troposphere and moisture anomalies in the lower troposphere. The northward propagation is caused jointly by several processes, including the atmospheric forcing on the ocean via the surface heat flux fluctuations, the feedback of ocean on the low-level atmospheric instability, the vorticity dynamics of the beta effect, and the vertical variation of low-level moisture. In addition, converting from eddy available potential energy to eddy kinetic energy is essential in maintaining the QBWO’s development. The disclosure of these crucial processes deepens the dynamics of the QBWO over the western North Pacific in boreal winter.

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Yifei Fan
,
Duo Chan
,
Pengfei Zhang
, and
Laifang Li

Abstract

Despite global warming, sea surface temperature (SST) in the subpolar North Atlantic has decreased since the 1900s. This local cooling, known as the North Atlantic cold blob (North Atlantic cold blob), signifies a unique role of the subpolar North Atlantic in uptaking heat and hence impacts downstream weather and climate. However, a lack of observational records and its constraints on climate models leave the North Atlantic cold blob formation mechanism inconclusive. Using simulations from the Coupled Model Intercomparison Project Phase 6, we assess the primary processes driving the North Atlantic cold blob within individual models and the consistency of mechanisms across models. We show that 11 out of 32 models, which we call “Cold Bold” models, simulate subpolar North Atlantic cooling over 1900–2014. Further analyzing the heat budget of subpolar North Atlantic SST shows that models have distinct mechanisms of cold blob formation. Whereas four out of the 11 Cold Blob models indicate decreased Oceanic Heat Transport Convergence (OHTC) as the key mechanism, another four models suggest changes in radiative processes making predominant contributions. The contribution of OHTC and radiative processes are comparable in the remaining three models. Such a model spread in the mechanism of cold blob formation may be associated with distinct base-state Atlantic Meridional Overturning Circulation (AMOC) strength, which explains about 39% of the inter-model spread in the contribution of OHTC to the simulated cold blob. Models with a stronger base-state AMOC suggest a greater role of OHTC, whereas those with a weaker base-state AMOC indicate radiative processes are more responsible. This model discrepancy suggests that the cold blob formation mechanism diagnosed from single models should be interpreted with caution.

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