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DeAngelis, Anthony M.; Schubert, Siegfried D.; Chang, Yehui; Lim, Young-Kwon; Koster, Randal D.; Wang, Hailan; Marquardt Collow, Allison B.
2023 Journal of Climate
AbstractMuch of Siberia experienced exceptional warmth during the spring of 2020, which followed an unusually warm winter over the same region. Here, we investigate the drivers of the spring warmth from the perspective of atmospheric dynamics and remote influences, focusing on monthly-time-scale features of the event. We find that the warm anomalies were associated with separate quasi-stationary Rossby wave trains emanating from the North Atlantic in April and May. The wave trains are shown to be extreme manifestations of the dominant modes of spring subseasonal meridional wind variability over the Northern Hemisphere. Using a large ensemble of simulations from NASA’s GEOS atmospheric model, in which the model is constrained to remain close to observations over selected regions, we further elucidate the remote drivers of the unusual spring temperatures in Siberia. In both April and May, the wave trains were likely forced from an upstream region including eastern North America and the western North Atlantic. Analysis with a stationary wave model shows that transient vorticity flux forcing over and downwind of the North Atlantic, which is strongly related to storm activity caused by internal variability, is key to generating the wave trains, suggesting limited subseasonal predictability of the Rossby waves and hence the exceptional Siberian warmth. Our observational and model analyses also suggest that anomalous tropical atmospheric heating contributed to the unusual warmth in Siberia through a teleconnection involving upper-troposphere dynamics and the mean meridional circulation. This tropical–extratropical teleconnection offers a possible physical mechanism by which anthropogenic climate change influenced the extreme Siberian warmth.
Komatsu, Kensuke K.; Takaya, Yuhei; Toyoda, Takahiro; Hasumi, Hiroyasu
2023 Journal of Climate
AbstractSnow cover (SC) is an important contributor to atmospheric predictability on subseasonal to seasonal time scales. This paper evaluates the submonthly scale cause-and-effect relationship between SC and surface air temperature (SAT) in Eurasia, which has been typically overlooked in previous statistical analyses of subseasonal to seasonal atmospheric predictability. We focus on the November east–west dipolar SC pattern, a dominant large-scale SC phenomenon. We use an information flow analysis, based on information theory, to infer causal relations not revealed by conventional correlation analysis. This analysis indicates a one-way causality from SAT to SC around the west pole (Europe) in boreal autumn (November), implying that SC has little influence on the time evolution of SAT. In contrast, causality from SC to SAT is significant around the east pole (the Mongolian Plateau). An atmospheric model experiment suggests that the SAT response to SC can persist for a month via snow–albedo feedback, although the response of the upper atmosphere in the model is small. Furthermore, the subseasonal hindcasts show that the contribution of SC may affect the predictability of SAT for up to four weeks around the east pole. We suggest that the geographical and climatological atmospheric conditions are favorable for generating a positive albedo feedback as a “hotspot” of SC–SAT coupling around the east pole in autumn. The agreement of our causality analysis with other analytical and modeling approaches underscores the cause-and-effect relationship between SC and SAT and its contribution to the subseasonal predictability over autumnal Eurasia.
Wang, Yunhe; Yuan, Xiaojun; Bi, Haibo; Ren, Yibin; Liang, Yu; Li, Cuihua; Li, Xiaofeng
2023 Journal of Climate
AbstractThe Arctic sea ice decline and associated change in maritime accessibility have created a pressing need for sea ice thickness (SIT) predictions. This study developed a linear Markov model for the seasonal prediction of model-assimilated SIT. It tested the performance of physically relevant predictors by a series of sensitivity tests. As measured by the anomaly correlation coefficient (ACC) and root-mean-square error (RMSE), the SIT prediction skill was evaluated in different Arctic regions and across all seasons. The results show that SIT prediction has better skill in the cold season than in the warm season. The model performs best in the Arctic basin up to 12 months in advance with ACCs of 0.7–0.8. Linear trend contributions to model skill increase with lead months. Although monthly SIT trends contribute largely to the model skill, the model remains skillful up to 2-month leads with ACCs of 0.6 for detrended SIT predictions in many Arctic regions. In addition, the Markov model’s skill generally outperforms an anomaly persistence forecast even after all trends were removed. It also shows that, apart from SIT itself, upper-ocean heat content (OHC) generally contributes more to SIT prediction skill than other variables. Sea ice concentration (SIC) is a relatively less sensitive predictor for SIT prediction skill than OHC. Moreover, the Markov model can capture the melt-to-growth season reemergence of SIT predictability and does not show a spring predictability barrier, which has previously been observed in regional dynamical model forecasts of September sea ice area, suggesting that the Markov model is an effective tool for SIT seasonal predictions.
Hu, Shineng; Xie, Shang-Ping; Seager, Richard; Cane, Mark A.
2023 Journal of Climate
AbstractTropical rainfall variations are of direct societal relevance and drive climate variations worldwide via teleconnections. The convective rainfall tends to occur when sea surface temperature (SST) exceeds a threshold, SSTthr, usually taken to be constant in time and space. We analyze 40-yr monthly observations and find that SSTthr varies by up to 4°C in space and with season. Based on local convective instability, we develop a quantitative theory that largely explains the SSTthr variations using the climatological state of the tropical atmosphere. Although it is often assumed that spatial variations of tropical upper-tropospheric temperature are small and can be neglected, it is shown that lower climatological values favor a lower SSTthr. Similarly, a small increase in climatological surface relative humidity also leads to a decrease in SSTthr, as does a lower climatological air–sea temperature difference. Consequently, efforts to understand and predict natural or forced variations in tropical rainfall must account for, in addition to SST, the temperatures aloft and the near-surface humidity and temperature and requires improved understanding of what controls their distribution in space and time.
Ahmed, Fiaz; Neelin, J. David; Hill, Spencer A.; Schiro, Kathleen A.; Su, Hui
2023 Journal of Climate
AbstractTropical areas with mean upward motion—and as such the zonal-mean intertropical convergence zone (ITCZ)—are projected to contract under global warming. To understand this process, a simple model based on dry static energy and moisture equations is introduced for zonally symmetric overturning driven by sea surface temperature (SST). Processes governing ascent area fraction and zonal mean precipitation are examined for insight into Atmospheric Model Intercomparison Project (AMIP) simulations. Bulk parameters governing radiative feedbacks and moist static energy transport in the simple model are estimated from the AMIP ensemble. Uniform warming in the simple model produces ascent area contraction and precipitation intensification—similar to observations and climate models. Contributing effects include stronger water vapor radiative feedbacks, weaker cloud-radiative feedbacks, stronger convection-circulation feedbacks, and greater poleward moisture export. The simple model identifies parameters consequential for the inter-AMIP-model spread; an ensemble generated by perturbing parameters governing shortwave water vapor feedbacks and gross moist stability changes under warming tracks inter-AMIP-model variations with a correlation coefficient ∼0.46. The simple model also predicts the multimodel mean changes in tropical ascent area and precipitation with reasonable accuracy. Furthermore, the simple model reproduces relationships among ascent area precipitation, ascent strength, and ascent area fraction observed in AMIP models. A substantial portion of the inter-AMIP-model spread is traced to the spread in how moist static energy and vertical velocity profiles change under warming, which in turn impact the gross moist stability in deep convective regions—highlighting the need for observational constraints on these quantities.Significance StatementA large rainband straddles Earth’s tropics. Most, but not all, climate models predict that this rainband will shrink under global warming; a few models predict an expansion of the rainband. To mitigate some of this uncertainty among climate models, we build a simpler model that only contains the essential physics of rainband narrowing. We find several interconnected processes that are important. For climate models, the most important process is the efficiency with which clouds move heat and humidity out of rainy regions. This efficiency varies among climate models and appears to be a primary reason for why climate models do not agree on the rate of rainband narrowing.
2023 Journal of Climate
AbstractThe spring land surface temperature (LST) over western Eurasia, which is critical for ensuring food security, shows a clear interannual variability. Based on reanalysis data and numerical simulations, we investigated the potential influencing factors and the related mechanisms of spring LST variability in mid-to-high latitudes of western Eurasia (MHWEA). The results show that the North Atlantic tripole sea surface temperature anomalies (SSTAs) in February, which persist into spring, can significantly affect the spring LST variability over MHWEA. Analyses indicate that the positive phase of the North Atlantic tripole SSTAs pattern tends to increase the meridional SST gradient between positive SSTAs over the midlatitude North Atlantic and negative SSTAs over the south of Greenland, which strengthens the low-level atmospheric baroclinicity and thus induces more active transient eddy activities. Correspondingly, a Rossby wave train triggered by the eddy-mediated processes originates from the North Atlantic and propagates downstream, thereby causing anomalous anticyclonic circulation over MHWEA. Meanwhile, the westerly anomalies over the subpolar North Atlantic accelerate the polar front jet and provide a favorable thermodynamical condition for the tropospheric warming over the Barents–Kara Seas by bringing warm and moist oceanic air. The polar warming tends to weaken the poleward temperature gradient at mid-to-high latitudes and then decelerate the Eurasian midlatitude westerlies, thus dynamically contributing to the circulation changes that can affect spring LST over MHWEA. Model results suggest that the link can be generally reproduced. Therefore, the late-winter North Atlantic tripole SSTAs may act as a precursor for the prediction of spring LST over western Eurasia.Significance StatementThe purpose of this study is to better understand the mechanisms underlying the interannual variability of the spring land surface temperature over western Eurasia, which is of great significance in ensuring food security. Here we show that the positive phase of the North Atlantic tripole sea surface temperature anomalies in February can modulate the tropospheric warming over the Barents–Kara Seas and further decelerate the Eurasian midlatitude westerlies in spring, which benefits local surface warming over western Eurasia. Our results provide a precursor for forecasting spring land surface temperature over western Eurasia.
Yang, Mengzhou; Shi, Xia; Yuan, Chaoxia; Lu, Xinyu; Liu, Jingchan
2023 Journal of Climate
AbstractThe extreme precipitation (EP) in the early and late rainy seasons in Southern China is investigated from the perspective of moist static energy (MSE). At the synoptic time scale, the EP is accompanied by the charge–discharge paradigm of the vertically integrated MSE (〈MSE〉); the positive 〈MSE〉 anomaly reaches the peak one day before EP and decreases quickly during the event. The charge–discharge paradigm of 〈MSE〉 is dominated by the horizontal and vertical advection, respectively. However, synoptic systems responsible for the 〈MSE〉 charge in the early and late rainy seasons are different due to the different horizontal distributions of climatological MSE in the lower troposphere caused by the northward migration of solar radiation and the monsoon system. At the interannual time scale, more EP in the early (late) rainy season is associated with the higher seasonal-mean 〈MSE〉 that can be caused by the anomalous anticyclone (cyclone) in the western North Pacific induced by the SST anomalies in the tropical Indian Ocean and central North Pacific (the tropical Pacific). The multimodel ensemble mean of CMIP6 models reproduces well the observed 〈MSE〉–EP relationship in both the historical and Shared Socioeconomic Pathway 5–8.5 (SSP5–8.5) runs. Moreover, the mean state of 〈MSE〉 increases in the SSP5–8.5 compared to historical runs along with more frequent occurrence of EP events. Hence, 〈MSE〉 can serve as a useful metric for studying EP in Southern China at various time scales.
Fan, Hanjie; Yang, Song; Wang, Chunzai; Lin, Shuheng
2023 Journal of Climate
AbstractThe Pacific meridional mode (PMM) can modulate El Niño–Southern Oscillation (ENSO) and is also affected by ENSO-related tropical Pacific sea surface temperature anomalies (SSTAs). Two tropical feedbacks on the PMM have been proposed: a positive one of central tropical Pacific SSTAs and a negative one of eastern tropical Pacific (ETP) SSTAs, the latter of which is suggested to be active only during strong eastern Pacific (EP) El Niño events like those in 1982/83 and 1997/98. However, we find that no strong, negative PMM-like SSTAs appeared, although the PMM indices (PMMIs) were strongly negative in spring of 1983 and 1998. Observation and model experiments show that tropical warming in 1983 and 1998 not only occurred in the ETP but also extended to the date line, thus inducing wind anomalies unfavorable for establishing the wind–evaporation–SST feedback for a negative PMM in the subtropics. To understand the discrepancy between the large negative PMMIs and weak PMM-related subtropical cooling during strong EP El Niño events, we isolate the relative contributions of subtropical and tropical SSTAs to the PMMIs by calculating their spatial projections on the PMM. Analysis combined using observation and CMIP6 models shows that despite the large contribution from subtropical SSTAs, the large tropical SSTAs, especially the extreme ETP warming, could cause large negative PMMIs during strong EP El Niño events even without strong, negative subtropical SSTAs. Our study clarifies the impact of ETP warming in causing a negative PMM and indicates the overstatement of negative PMMIs by tropical SSTAs during strong EP El Niño events.Significance StatementThis paper aims to reevaluate the previously proposed effect of strong eastern Pacific El Niño events, like those in 1982/83 and 1997/98, on exciting a negative Pacific meridional mode (PMM). We find that although the PMM indices were strongly negative during the decay of strong eastern Pacific El Niño events, the large negative PMM sea surface temperature anomalies (SSTAs) could not be observed in the subtropical Pacific. Further diagnosis indicates that the PMM index can be large if strong SSTAs occur in eastern tropical Pacific even without subtropical SSTAs, implying that one should be careful when using the PMM index.
Pan, Yuying; Cheng, Lijing; von Schuckmann, Karina; Trenberth, Kevin E.; Li, Guancheng; Abraham, John; Liu, Yuanxin; Gouretski, Viktor; Yu, Yongqiang; Liu, Hailong; Liu, Chunlei
2023 Journal of Climate
AbstractAs a major component of Earth’s energy budget, ocean heat content (OHC) plays a vital role in buffering climate change. The annual cycle is the most prominent change in OHC but has always been removed to study variations and changes in Earth’s energy budget. Here, we investigate the annual cycle of the upper-2000-m OHC at regional to global scales and assess the robustness of the signals using the spread of multiple observational products. The potential drivers are also investigated by comparing the annual OHC signal with the corresponding change in top-of-atmosphere radiation, surface heat flux, ocean heat divergence, and meridional heat transport. Results show that the robust signal of annual OHC change is significant down to a 1000-m depth globally and can reach down to 1500 m in some areas such as the tropical ocean. The global OHC (0–1500 m) changes from positive anomalies within September–February to negative anomalies within March–August, mainly because of the larger ocean area in the Southern Hemisphere and the seasonal migration of solar irradiance. Owing to the huge ocean heat capacity, the annual cycle of OHC dominates that of the global energy budget. The difference among the OHC annual cycles in the three major ocean basins is mainly attributed to ocean heat transport, especially in the tropics. In the upper 1500 m at mid- and high latitudes and in the upper 50 m of the tropics, the net sea surface heat flux dominates the OHC annual cycle, while in the tropics below 50 m, wind-driven Ekman heat transport associated with the geostrophic flow is the main driver.
Luo, Haolin; Wang, Ziqian; Wu, Huan; Zeng, Zhuoyu; Yu, Wei
2023 Journal of Climate
AbstractNumerous studies have indicated that the atmospheric heat source (AHS) over the Tibetan Plateau (TP) is highly correlated with the western North Pacific anomalous anticyclone (WNPAC) in summer. However, such an interannual relationship has been weakened since the late 1990s. The present work shows that the TP AHS was significantly and positively correlated with the WNPAC during the period 1979–99 (P1), while this relationship became insignificant hereafter [2000–20 (P2)]. From an atmospheric perspective, we identify that the long-term change in the upper-level atmospheric circulation over the TP is an important cause for weakening the relationship. An obvious upper-level anticyclonic trend occurred over the northeastern TP in the past four decades, with an easterly trend on the anticyclone’s southern flank, with anomalous westerlies during P1 but anomalous easterlies during P2 over the main portion of the TP. With the anomalous upper-level westerlies in P1, abnormal high pressure induced by the TP heating (i.e., AHS) extended downstream in the upper troposphere. Subsequently, anomalous descending motions formed over the northwestern Pacific due to the eastward-extended high pressure, together with the vertical transport of negative relative vorticity, favorable for the enhancement of the WNPAC. In P2, the TP heating-induced abnormal high pressure was confined over the southern TP due to the anomalous easterlies, suppressing its downstream influence and finally breaking the connection between the TP AHS and the WNPAC. Modeling results from both linear baroclinic model (LBM) sensitivity experiments and the CESM Large Ensemble dataset further confirm the important role of the change in background circulation in weakening the relationship.Significance StatementThe atmospheric heat source (AHS) over the Tibetan Plateau (TP) is generally believed to closely connect with the western North Pacific anomalous anticyclone (WNPAC) during boreal summer. Previous studies have revealed that a significant interannual correlation exists between the TP AHS and the WNPAC; however, such a relationship was weakened recently, but the causes are unknown. This study highlights the important contribution from the change in background circulation to the weakened relationship. An upper-level easterly trend occurred over the TP in recent summers, under which the TP heating-induced abnormal atmospheric response was confined in the TP area, limiting the downstream influence of the TP heating and finally destroying the connection between the TP AHS and the downstream WNPAC.
Boschat, Ghyslaine; Purich, Ariaan; Rudeva, Irina; Arblaster, Julie
2023 Journal of Climate
AbstractThe Southern Annular Mode (SAM) describes the annular or zonal component of the large-scale atmospheric circulation in the Southern Hemisphere (SH) extratropics and influences surface climate across the SH. Although this annular flow is dominant in austral summer, in other seasons considerable zonal asymmetries are evident, reflecting a zonal wave 3 (ZW3) pattern. We define an index representing asymmetric flow using the first two leading modes of meridional wind variability in the SH. Two orthogonal ZW3 indices are used together to capture longitudinal shifts in the wave train and their connection to tropical convection. We compare the impacts of the SAM and ZW3 on surface climate by examining composites of temperature and precipitation fields during each season. Impacts on mean and extreme surface climates are assessed. We find that the SAM and ZW3 are not clearly separated modes, but rather, ZW3 modulates the impact of the SAM across the midlatitudes. The SAM influence on regional temperature and precipitation is similar for both mean impacts and extremes. The ZW3 influence on extremes is more varied across indices and does not always reflect the ZW3 impact on mean fields. Notably, amplified ZW3 activity has a significant influence on the number of midlatitude fronts and frontal rainfall, highlighting the importance of considering ZW3 when exploring the surface climate impacts of large-scale SH circulation states, particularly for nonsummer seasons.Significance StatementVariations in the strength and position of the midlatitude westerly winds have a strong influence on surface climates. While these winds are predominantly zonally symmetric in the Southern Hemisphere, few studies to date have explored the role of the asymmetric component of this circulation, particularly for seasons outside of summer. By defining two new indices of meridional circulation, this study reveals new important impacts on temperature, rainfall, and the likelihood of extreme climates in regions of southern Australia and South America, and sea ice regions around Antarctica. These findings question the validity of considering only zonal-mean winds for climate studies of the Southern Hemisphere and have important implications for the seasonal forecasting and predictability of extreme climate events in the near future.
Hong, Haixu; Sun, Jianqi; Wang, Huijun
2023 Journal of Climate
AbstractIn this study, the synoptic atmospheric patterns responsible for regional extreme high-temperature events (REHEs) over northern Asia (NA) are investigated. First, a hybrid regionalization approach is applied to the daily maximum temperature (Tmax), and three subregions of NA can be identified: western NA, central NA, and southeastern NA. To better understand the mechanism for the NA REHE formation, the REHE-related synoptic circulation patterns over each subregion are further categorized into two types. These six synoptic circulation patterns influence the NA REHE occurrence through different radiation and advection processes. Generally, the radiation process dominates the NA REHE occurrence, while the horizontal temperature advection plays a more important role in the synoptic dipole patterns than in the monopole high patterns. The heatwaves associated with the six synoptic patterns can last more than 3.8 days, with a maximum of 2 weeks. From the forecasting perspective, six wave trains are explored as the precursors of these six synoptic circulation patterns, separately. The wave trains originate from the North Atlantic Ocean and Europe with at least a 3-day lead and then propagate eastward to NA, exerting influences on the pronounced six synoptic circulation patterns and consequently affecting the NA REHEs. In terms of long-term change, the REHEs over the three subregions show significant increasing trends over 1960–2018 and significant interdecadal increases around the mid-1990s, in which the contribution of each synoptic pattern–related REHE is different.
Wang, Weiyi; Liu, Xiaohong; Wu, Chenglai; Lin, Guangxing; Wang, Yong; Lu, Zheng; Zhao, Xi; Wei, Linyi
2023 Journal of Climate
AbstractIn this study, the fast response of East Asian summer precipitation to COVID-19–induced aerosol emission reductions is examined using the Community Earth System Model, version 2.2 (CESM2.2). The emission reductions decreased aerosol optical depth and cloud cover over northern China in June 2020. The troposphere became warmer, strengthening the land–sea thermal contrast and anomalous southerly winds. The subtropical westerly jet accelerated and shifted southward, favoring low-level convergence, upward air motions, and subsequent condensational heating over the Yangtze River basin (YRB). The feedback of condensational heating in return strengthened the convergence and ascent. The western North Pacific subtropical high was intensified, which further enhanced the moisture advection and convergence over the YRB. Both the enhanced moisture convergence and ascent increased precipitation over the YRB during June 2020. Furthermore, local and remote emission reductions show different impacts on convection and moisture transport over the YRB. The emission reductions over China caused stronger convective precipitation (1.15 vs 0.63 mm day−1) but weaker larger-scale precipitation (1.17 vs 2.24 mm day−1) than the emission reductions outside China. In addition to the emission reductions, the sea surface temperature (SST) anomalies in 2020 also play an important role in increasing precipitation over the YRB, contributing about 42.8%. The relative contribution of SST anomalies also increases under the COVID-19–induced emission scenario.
2023 Journal of Climate
AbstractObserved climate records of length, homogeneity, and reliability are the basis of climatological studies on tropical cyclones (TCs). However, such data are scarce for TC size in terms of wind field, particularly over the western North Pacific (WNP). This study demonstrates that deep learning can practically bridge this data gap when applied to satellite data. Using transfer learning, deep learning algorithms were developed to estimate reliable TC sizes from infrared imagery for the WNP TCs. The algorithms were then applied to a homogeneous satellite database to reconstruct a new historical dataset of TC sizes, named DeepTCSize, which covers 37 years (1981–2017) over the WNP. DeepTCSize includes multiple TC size quantities, such as wind radii of 17, 26, and 33 m s−1 and maximum winds (i.e., R17, R26, R33, and RMW), which have high correlations (R = 0.85, 0.84, 0.79, and 0.76, respectively) with postseason quality-controlled best track data. Comparisons with ocean wind observations were made and this further revealed that DeepTCSize has good quality and is free from spurious error trends, providing an advantage over the historical “best estimates” of TC sizes currently available in the best track archives for the WNP. The new reconstructed TC sizes dataset for the WNP TCs shows significant expanding trends in the annual-mean outer circulations (at a rate of 2% decade−1 for R17 and a rate of 2% decade−1 for R26), which are mainly associated with weaker storms, as well as a weak contracting trend in the annual-mean inner-core size (RMW).Significance StatementTropical cyclone (TC) size largely controls the TC-induced hazard and risk. If the size of TC can be determined more efficiently in observations spanning a long-enough period, the climatology and changes in TC can be better modeled and understood. This study applies deep learning methods to reconstruct a new dataset of multiple inner- to outer-core TC size metrics from infrared imagery of satellites for the western North Pacific TCs. The dataset spans 37 years. It is homogenous and has comparable accuracy with the existing “best estimates.” Using the dataset, a significant expanding trend was identified in the outer-core size, while the inner-core size exhibits a weak contracting trend. The dataset can be employed in several applications.
Ibrahim, Hamed D.; Sun, Yunfang
2023 Journal of Climate
AbstractCharacterizing the physical processes that modulate the continuous partitioning of heat between the ocean and overlying atmosphere is important for monitoring the subsequent flow of the heat accumulating in the ocean because of anthropogenic climate change. Oceanic rainfall sensible heat flux (Qp), whereby rainwater cools the sea surface, is computed and compared to the sea surface heat energy balance in the 60°N–60°S region. Contrary to popular belief, the results show that Qp is large at both short and long time scales, accounting for up to 22.5% of sea surface net heat flux around the 5.8°N line of latitude, 10.1% in the tropical 20°N–20°S region, and 5.7% in the global 60°N–60°S region. In the mixed layer of these same regions, area-average temperature change owing to a 10-yr accumulated Qp is up to −2.6° and −1.4°C, respectively. Further analysis reveals a previously unspecified rainfall–evaporation negative feedback between successive evaporation–rainfall cycles at the sea surface. The Qp depresses sea surface temperature and thus inhibits evaporation (latent heat flux), which in turn inhibits rainfall owing to decrease in water vapor supply to the atmosphere. The decrease in sea surface temperature also inhibits heat conduction from the ocean to the atmosphere (sensible heat flux). To compensate for the weaker latent and sensible heat fluxes, sea surface upward longwave radiation flux strengthens. We conclude that Qp acts like a modulator of Earth’s heat energy flow by controlling the partition of upper-ocean heat energy and the cycle of heat flow in the ocean and between the ocean and the atmosphere.Significance StatementUpper-ocean heat energy is partitioned between the ocean and the overlying atmosphere. Characterizing the physical processes that modulate this continuous partitioning is important for monitoring the subsequent flow of the heat energy accumulating in the ocean because of anthropogenic climate change. Here, we identify sea surface cooling by rainwater (oceanic rainfall sensible heat flux) as a modulator of this partition: this cooling depresses sea surface temperature and thus inhibits evaporation (latent heat flux), which in turn inhibits rainfall owing to the decrease in water vapor supply to the atmosphere; depressing sea surface temperature also inhibits heat conduction from the ocean to the atmosphere (sensible heat flux). To compensate for the weaker latent and sensible heat fluxes, sea surface upward longwave radiation flux strengthens.
2023 Journal of Climate
AbstractUncertainties both in the initial condition (IC) and surface wind high-frequency perturbations (HFPs) contribute to the prediction spread of El Niño–Southern Oscillation (ENSO), yet the relative roles of the two are hard to separate, mostly due to the nonlinearity in ENSO dynamics. In this study, we conducted two ensemble experiments using the CESM model to analyze the source of ENSO spread arising from the tropical Pacific. The HFPs were precluded using an online low-pass-filtering scheme in one experiment. Results showed that the uncertainty in ENSO prediction depends on both the IC and HFPs. By comparing simulations with and without HFPs, it is found that the predicted ENSO spread decreased in magnitude when HFPs are removed, especially in the first four prediction months. At integration time longer than that, the additional impact of HFPs cannot be detected on top of that of the uncertainty in IC. To the ENSO spread anomaly, we found that it is tightly associated with uncertainty in the sea surface temperature and surface wind in the central-eastern equatorial Pacific at the short integration time, and associated with the HFPs in the western equatorial Pacific at the long integration time. When the HFPs are removed, the precursor of the ENSO spread anomaly at the long lead time emerges in the northeastern tropical Pacific, the area where the Pacific meridional mode originates from. Our results indicate that the uncertainty in ENSO prediction depends primarily on IC, and the HFPs in the western equatorial Pacific play the secondary role.
Amma, Michinari; Hayasaka, Tadahiro
2023 Journal of Climate
AbstractWe investigated the interannual variations in the annual mean and seasonal cycle of upward shortwave radiation at the top of the atmosphere (TOA SW↑) over the Arctic using the Clouds and the Earth’s Radiant Energy System (CERES) observation data during 2001–20. The annual mean TOA SW↑ over the Arctic showed a decreasing trend from 2001 to 2012 (−2.5 W m−2 decade−1) and had a large interannual variability after 2012. The standard deviation of detrended TOA SW↑ increased from 0.4 W m−2 in 2001–12 to 1.1 W m−2 in 2012–20. Over land, TOA SW↑ variation was related to snow cover in May; snow cover, cloud fraction, and cloud optical depth (COD) in June; and cloud fraction and COD in July. Over ocean, TOA SW↑ variation in June and July was linked to sea ice cover. TOA SW↑ variation over ocean in June and July after 2012 was highly related to the North Atlantic Oscillation (NAO). This study suggests that changes in the large annual mean TOA SW↑ variability after 2012 are explained by the timing of land snow and sea ice melt in spring and summer and cloud variability over land in summer.
Quan, Heng; Zhang, Boer; Bourguet, Stephen; Linz, Marianna; Chen, Gang
2023 Journal of Climate
AbstractStudying temperature probability distributions and the physical processes that shape them is important for understanding extreme temperature events. Previous work has used a conditional mean temperature framework to reveal whether horizontal temperature advection drives temperature to extreme or median values at a specific location as a method to dynamically interpret temperature probability distributions. In this paper, we generalize this method to study how other processes shape temperature probability distributions and explore the diverse effects of horizontal temperature advection on temperature probability distributions at different locations and different temperature percentiles. We apply this generalized method to several representative regions to demonstrate its use. We find that temperature advection drives temperatures toward more extreme values over most land in the midlatitudes (i.e., cold air advection occurs during cold anomalies and warm air advection occurs during warm anomalies). In contrast, we find that horizontal temperature advection dampens temperature anomalies in some coastal summer monsoon regions, where extreme temperatures result from other processes, such as horizontal humidity advection and vertical temperature advection. By calculating the mean of processes conditioned on the temperature percentile, this method enables composite analysis of processes that contribute to events for all percentiles and a range of processes. We show examples of composites at different percentiles for certain processes and regions to illustrate the conditional mean analysis. This general approach may benefit future studies related to temperature probability distributions and extreme events.
Gregory, Catherine H.; Holbrook, Neil J.; Marshall, Andrew G.; Spillman, Claire M.
2023 Journal of Climate
AbstractMarine heatwaves (MHWs) can severely impact marine biodiversity, fisheries, and aquaculture. Consequently, there is an increasing desire to understand the drivers of these events to inform their predictability so that proactive decisions may be made to reduce potential impacts. In the Tasman Sea (TS), several relatively intense and broad-scale MHWs have caused significant damage to marine fisheries and aquaculture industries. To assess the potential predictability of these events, we first determined the main driver of each MHW event in the TS from 1993 to 2021. We found that those MHWs driven by ocean advection—approximately 45% of all events—are generally longer in duration and less intense and affected a smaller area compared with the remaining 55%, which are driven by air–sea heat fluxes, are shorter in duration, and are more surface intense. As ocean advection–driven events in the TS have been closely studied and reported previously, we focus here on atmospherically driven MHWs. The predictability of these events is assessed by identifying the patterns of atmospheric pressure, winds, and air–sea heat fluxes in the Southern Hemisphere that coincide with MHWs in the Tasman Sea. We found that atmospherically driven MHWs in this region are more likely to occur during the positive phase of the asymmetric Southern Annular Mode (A-SAM)—which presents as an atmospheric zonal wave-3 pattern and is more likely to occur during La Niña years. These A-SAM events are linked to low wind speeds and increased downward solar radiation in the TS, which lead to increased surface ocean temperatures through the reduction of mixing.Significance StatementThe purpose of this study is to understand factors of the atmosphere that contribute to an accumulation of heat in the upper ocean in the Tasman Sea to better inform predictability. Higher incidences of ocean extreme thermal events (known as marine heatwaves) in this region are becoming increasingly more common and threatening the important marine industries that support the people of both Australia and New Zealand. We need to know the sources of this extra heat to understand whether such events can be predicted. Previous studies have found the East Australian Current to be responsible for around half of these events, and our results show a connection between a known atmospheric pattern and the other half. As we continue to improve our ability to anticipate this pattern, this suggests that we may also be able to predict these extreme heating events.
Li, Jiandong; Geen, Ruth; Mao, Jiangyu; Song, Yajuan; Vallis, Geoffrey K.; Wu, Guoxiong
2023 Journal of Climate
AbstractAsian large-scale orography profoundly influences circulation in the North Hemisphere. Considerable spring top-of-the-atmosphere (TOA) radiative cooling over Southeast China (SEC) is very likely related to upstream orography forcing. Here we investigate the mechanical and thermal forcings of Asian large-scale orography, particularly the Tibetan Plateau (TP), on downstream East Asian cloud amount and atmospheric radiation budget during March–April using the Global Monsoons Model Intercomparison Project simulations. The thermal forcing drives significant surface heating and a low-level cyclone over the TP, pumping low-level air to the middle troposphere. Ascent and water vapor convergence triggered by the thermal forcing favor air condensation, low–middle clouds, and resultant strong spring cloud radiative cooling over SEC. Moreover, the thermal forcing moves the position of cloud radiative cooling westward toward the TP. The TP’s blocking role weakens low-level westerlies over SEC, but its deflecting role increases downstream high-level westerlies, dynamically favoring cloud formation over SEC and the eastward ocean. In addition, the TP can force ascent and increase cloud amounts over the western and central TP. The thermal forcing contributes to 57.1% of total cloud amount and 47.6% of TOA cloud radiative cooling induced by the combined orography forcing over SEC while the mechanical one accounts for 79.4% and 95.8% of the counterparts over the ocean to the east of SEC. Our results indicate that Asian large-scale orography shapes the contemporary geographical distribution of spring East Asian cloud amount and atmospheric radiation budget to a large extent.Significance StatementClouds tied to large-scale topography and circulation exhibit some remarkable geographical distributions. The global strongest cloud radiative cooling, with an intensity of up to −90 W m−2, occurs over Southeast China (SEC) during March–April. The primary purpose of this study is to understand the influences of Asian large-scale orography, particularly the Tibetan Plateau (TP), on this unique climatic phenomenon using the latest climate model simulations. Our results show that Asian large-scale orography forcing significantly increases ascent, low–middle cloud formation, and resultant strong spring cloud radiative cooling over SEC and downstream ocean. The sensible-heat-driven air pump induced by the TP’s thermal forcing maintains strong cloud radiative cooling over SEC. This study provides valuable insights that link Asian large-scale orography forcing to downstream cloud–radiation characteristics.
Liao, Huaxia; Cai, Zhichao; Guo, Jingsong; Song, Zhenya
2023 Journal of Climate
AbstractEl Niño–Southern Oscillation (ENSO) is the most influential interannual climate variability on Earth. The tendency of the mature phase of ENSO, characterized by the strongest sea surface temperature (SST) anomalies, to appear during the boreal winter is known as seasonal phase locking. Climate models are challenged by biases in simulating ENSO seasonal phase locking. Here, we evaluated the ENSO phase-locking simulation performance in 50 models of phase 6 of the Coupled Model Intercomparison Project (CMIP6) and found that the models with the intertropical convergence zone (ITCZ) poleward bias tended to simulate more ENSO events that peaked out of the boreal winter season. The contributions of the ITCZ poleward bias to the ENSO phase-locking bias were also evaluated, yielding a correlation coefficient of 0.55, which can explain approximately 30% of the ENSO seasonal phase-locking bias. The mechanism that influences the simulation of ENSO seasonal phase locking was also assessed. The ITCZ poleward bias induces a dry bias over the equatorial Pacific, especially during the boreal summer. During ENSO events, the meridional movement of the ITCZ is prevented, and the equatorial precipitation and convection anomalies that respond to ENSO events are also restrained. The restrained convection anomaly weakens the ENSO-related zonal wind anomaly, triggering a weaker eastern tropical Pacific thermocline anomaly during the following autumn. The weakened thermocline anomaly cannot sustain further development of ENSO-related SST anomalies. Therefore, ENSO events in models containing the ITCZ poleward bias are restrained during the boreal summer and autumn and, thus, tend to peak out of the winter season.Significance StatementWe aimed to better understand the mechanism that induces bias when simulating ENSO seasonal phase locking, that is, what disturbs the simulated ENSO events peaking during the boreal winter. As previous studies have primarily focused on the South Pacific convergence zone (SPCZ) bias and other biases, this study is the first to propose the effects of the poleward ITCZ latitude bias and clarify the corresponding mechanism. We show that latitudinal bias can explain approximately 30% of the ENSO seasonal phase-locking bias. This is important because the biases in simulating ENSO seasonal phase locking have long hampered the prediction of ENSO. Our study highlights the importance of the latitude of the ITCZ and provides a basis for the future development of climate models.
Naha, Rajashree; McGregor, Shayne; Singh, Martin
2023 Journal of Climate
AbstractRecent analysis of pantropical interactions suggests that after 1980 the tropical Atlantic Ocean’s (TAO) influence on the tropical Pacific Ocean (TPO) appears to have become much more pronounced while the tropical Indian Ocean’s (TIO) influence appears to have weakened. This study explores whether and how decadal changes in TAO and TPO SSTs modulate these pantropical connections in an attempt to explain the recent dominance of the TAO. To this end, we carry out a series of idealized atmosphere-only experiments using the ACCESS atmospheric general circulation model where the magnitude and sign of the decadal TAO SST signal are varied, presenting various warm and cool Atlantic scenarios. To understand further if these pantropical connections are influenced by changes in TPO SST, we carry out the above TAO experiments with both warm and cool phases of Pacific decadal variability (PDV). We find that an imposed TAO warming leads to increases in TPO atmospheric temperature and stability, which lead to a decrease in average TPO precipitation, with the most prominent changes occurring in June–August. These changes in TPO precipitation induced by TAO warming are largely mirrored when TAO cooling is added, whereas the TPO rainfall response to TAO anomalies remains relatively unchanged for the different phases of PDV. In contrast to the precipitation response, the wind response did display some asymmetries between different phases of TAO SST variability. Specifically, surface winds in the western half of the Niño-4 region exhibited a significantly different response to positive versus negative Atlantic multidecadal variability (AMV), whereas the surface winds in the western equatorial Pacific were significantly stronger (roughly 40% larger) in the positive phase of PDV than in the negative phase. These results suggest that the phases of PDV and AMV may modulate pantropical interactions through their effect on zonal wind stress.
Crueger, Traute; Schmidt, Hauke; Stevens, Bjorn
2023 Journal of Climate
AbstractEarth’s planetary albedo shows a remarkable hemispheric symmetry. We assess to what extent CMIP models symmetrize the hemispheric clear-sky albedo asymmetry and what the role of clouds is for this. Following Voigt et al., we calculate a reference TOA reflected solar radiation considering the masking of clear-sky asymmetry by symmetric cloud contributions. We use the simple radiation model of Donohoe and Battisti to estimate this benchmark and to separate surface, aerosol, and cloud contributions to the compensation of this benchmark. In CERES, tropical clouds enhance the reference asymmetry while extratropical cloud asymmetries balance the reference asymmetry and the additional asymmetry introduced by tropical clouds. CMIP multimodel means show similar results as CERES. Clouds compensate reference asymmetries by 85% (CMIP3), 65% (CMIP5), and 78% (CMIP6) as compared with 98% for CERES. Spatial distributions of hemispheric differences indicate clear improvements across the CMIP phases. Remaining all-sky reflection asymmetries predominantly result from too-small, partly compensating cloud asymmetries: a too-weak enhancement of the reference asymmetry in the tropical Atlantic and eastern Pacific Oceans is accompanied by a too-weak compensation by extratropical clouds. Thus, tropical clouds and extratropical storm track regions are largely responsible for the compensation of hemispheric clear-sky asymmetries in CERES and CMIP, and for remaining biases in the GCMs. An unexpected result is the magnitude of model biases in the clear-sky asymmetries, which potentially condition systematic cloud biases. Experiments testing cloud-controlling factors influencing hemispheric asymmetry could help us to understand what drives hemispheric cloud differences.
Feng, Xiaofang; Wu, Liguang; Wang, Chao
2023 Journal of Climate
AbstractThe impact of climate change on tropical cyclone (TC) activity is often assessed by various downscaling approaches, statistical–dynamical frameworks, and high-resolution global climate models using the projected changes of environmental factors. Uncertainty in simulating and projecting TC-relevant, large-scale circulation is closely linked to the projection of TC activity in a warming climate. Based on the model output in phase 6 of the Coupled Model Intercomparison Project (CMIP6), this study examines the intermodel biases in simulating the western North Pacific monsoon trough (MT), which is one of the most important large-scale circulation systems for TC activity, especially TC formation. It is found that most CMIP6 models can successfully simulate the climatological mean structure of the MT, although considerable biases remain in its exact location and its simulated historical changes. The mean latitude of the simulated MT spreads between 10° and 20°N, with noticeable differences in its orientation. The multimodel ensemble mean indicates that the MT exhibits no significant long-term zonal and poleward shifts in the future scenarios, consistent with the projection in the selected models in which the simulated MT resembles the observed spatiotemporal characteristics of the counterpart. Further analysis suggests that the intermodel bias in the simulated MT location is closely related to the east–west contrast of sea surface temperature (SST) anomalies in the tropical Pacific. More attention is required on improving the simulation of the basinwide SST distribution and its associated MT to reduce the uncertainty in predicting the future location of TC formation.