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Jinlin Zha, Cheng Shen, Deming Zhao, Jinming Feng, Zhongfeng Xu, Jian Wu, Wenxuan Fan, Meng Luo, and Liya Zhang

Abstract

Summer mean (June, July, and August) surface air temperature (SSAT) in East Asia during the period from 1958 to 2001 has shown a warming. However, the relative contributions of external forcing (EF) and internal climate variability (ICV) to the SSAT changes over East Asia remain unclear. In this study, a new approach is applied to estimate the changes in the SSAT determined by the effects of EF and ICV over East Asia during the period from 1958 to 2001. Reanalysis data as well as simulated results from both global atmosphere–ocean coupled model outputs and a regional climate model (RCM) are used for this approach. The observed SSATs over East Asia have undergone a decreasing trend from 1958 to 1972 (−0.14°C decade−1, p < 0.01) and an increasing trend after 1972 (0.24°C decade−1, p < 0.01). While these features are not captured by the reanalysis studied here, they are reproduced when the reanalysis output is downscaled using an RCM. The effects of the EF and the ICV on the SSAT can be separated based on the RCM downscaling simulation. The results show that the SSAT with EF displayed significant warming over most regions of East Asia, whereas the SSAT with ICV mainly exhibited cooling over East Asia. Furthermore, EF mainly influenced the decadal changes of the SSAT, whereas the ICV mainly influenced the interannual changes in the SSAT over East Asia. The interannual changes of the SSAT over East Asia that were influenced by the ICV are mainly manifested as the combined effects of the large-scale ocean–atmosphere circulations, which expressed 79% explanatory power on the SSAT changes.

Open access
Louis Clément, E. L. McDonagh, J. M. Gregory, Q. Wu, A. Marzocchi, J. D. Zika, and A. J. G. Nurser

Abstract

Warming of the climate system accumulates mostly in the ocean and discrepancies in how this is modeled contribute to uncertainties in predicting sea level rise. In this study, regional temperature changes in an atmosphere–ocean general circulation model (HadCM3) are partitioned between excess (due to perturbed surface heat fluxes) and redistributed (arising from changing circulation and perturbations to mixing) components. In simulations with historical forcing, we first compare this excess–redistribution partitioning with the spice and heave decomposition, in which temperature anomalies enter the ocean interior either along isopycnals (spice) or across isopycnals (heave, without affecting the temperature–salinity curve). Second, heat and salinity budgets projected into thermohaline space naturally reveal the mechanisms behind temperature change by spice and heave linked with water mass generation or destruction. Excess warming enters the ocean as warming by heave in subtropical gyres whereas it mainly projects onto warming by spice in the Southern Ocean and the tropical Atlantic. In subtropical gyres, Ekman pumping generates excess warming as confirmed by Eulerian heat budgets. In contrast, isopycnal mixing partly drives warming and salinification by spice, as confirmed by budgets in thermohaline space, underlying the key role of salinity changes for the ocean warming signature. Our study suggests a method to detect excess warming using spice and heave calculated from observed repeat profiles of temperature and salinity.

Open access
Johannes Mayer, Michael Mayer, Leopold Haimberger, and Chunlei Liu

Abstract

This study uses the ECMWF ERA5 reanalysis and observationally constrained top-of-the-atmosphere radiative fluxes to infer net surface energy fluxes covering 1985–2018, which can be further adjusted to match the observed mean land heat uptake. Various diagnostics are applied to provide error estimates of inferred fluxes on different spatial scales. For this purpose, adjusted as well as unadjusted inferred surface fluxes are compared with other commonly used flux products. On a regional scale, the oceanic energy budget of the North Atlantic between the RAPID array at 26.5°N and moorings located farther north (e.g., at the Greenland–Scotland Ridge) is evaluated. On the station scale, a comprehensive comparison of inferred and buoy-based fluxes is presented. Results indicate that global land and ocean averages of unadjusted inferred surface fluxes agree with the observed heat uptake to within 1 W m−2, while satellite-derived and model-based fluxes show large global mean biases. Furthermore, the oceanic energy budget of the North Atlantic is closed to within 2.7 (−0.2) W m−2 for the period 2005–09 when unadjusted (adjusted) inferred surface fluxes are employed. Indirect estimates of the 2004–16 mean oceanic heat transport at 26.5°N are 1.09 PW (1.17 PW with adjusted fluxes), which agrees well with observed RAPID transports. On the station scale, inferred fluxes exhibit a mean bias of −20.1 W m−2 when using buoy-based fluxes as reference, which confirms expectations that biases increase from global to local scales. However, buoy-based fluxes as reference are debatable, and are likely positively biased, suggesting that the station-scale bias of inferred fluxes is more likely on the order of −10 W m−2.

Open access
Chan-Pang Ng, Qinghong Zhang, Wenhong Li, and Ziwei Zhou

Abstract

In many countries, thunderstorms are the main contributor to hourly extreme precipitation (HEP). Prior studies have shown that the number of thunderstorms decreased steadily in whole country of China; however, HEP has increased significantly in several areas over the past half-century. The role of thunderstorms in changes in HEP occurrence remains largely unknown in China. In this study, for the first time, we used continuous 32-yr records of hourly precipitation and thunder, and the fifth-generation European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5), to analyze changes in thunderstorms under various vertical wind shear (VWS) environments, and their contribution to HEP occurrence. The number of HEP events associated with thunderstorms (TD-HEP) increased significantly in southern China (SC) but decreased significantly in northeastern China (NEC) and east of the Tibetan Plateau (ETP). Weak VWS thunderstorms accounted for 69.1% of TD-HEP in SC. Changes in the most unstable convective available potential energy and precipitable water (PW) in SC favored an increase in weak-VWS thunderstorms, which resulted in an increase of 2.35 h per warm season in overall “station-mean” TD-HEP events from 1980 to 2011. As the major contributor to HEP in NEC, moderate VWS thunderstorms decreased by 0.37 h per warm season, due mainly to a reduction in PW, leading to a negative trend in TD-HEP events. Similarly, the decreasing TD-HEP occurrence on the ETP was due to a decrease of 1.12 h per warm season of moderate VWS thunderstorms. Studying the VWS environments of thunderstorms, and changes therein under a warming climate, can improve understanding of the changes in HEP in China.

Open access
Xiaojing Li, Youmin Tang, Xunshu Song, and Ting Liu

Abstract

Maritime Continent (MC) rainfall plays an important role in global climate variability, but its prediction remains extremely challenging. Based on a long-term state-of-the-art hindcast product recently completed by the authors’ group, this work investigates the decadal variation of the MC rainfall predictability in the wet season for the first time. The prediction skills were relatively high before 1940 and after 1980, but relatively low between these years. In a diagnostic analysis of the controlling factors of the decadal variation, the signal strength represented by the variance of the rainfall variability was identified as the dominant factor. Further analysis concluded that the phases of El Niño–Southern Oscillation (ENSO) are the key controlling sources. The MC rainfall was more predictable during periods dominated by El Niño events than during periods dominated by La Niña events because El Niño elicits stronger ocean–atmosphere interactions in the tropics, providing a stronger signal of MC rainfall than La Niña events.

Open access
Qiaoling Ren, Wei Wei, Mengmeng Lu, and Song Yang

Abstract

The wintertime Middle East jet stream (MEJS) is an important upstream signal for the East Asian winter monsoon and the subsequent Asian summer monsoon. Thus, the maintenance and interannual variations of the MEJS as well as its similarities and differences with the East Asian jet stream (EAJS) and the North American jet stream (NAJS) are studied dynamically using the geopotential tendency equation and empirical orthogonal function analysis. Analysis reveals that the MEJS is mainly maintained by tropical diabatic heating and the low-frequency transient eddy (TE) vorticity forcing. It is different from the EAJS, which is maintained by both tropical diabatic heating and high-frequency TE vorticity forcing, and the NAJS, which is mainly sustained by high-frequency TE vorticity forcing. Furthermore, while temperature advection plays a considerable role in the maintenance of EAJS and NAJS, it is less important for the MEJS. On interannual time scales, the meridional shift of the MEJS is related to low-frequency TE heating, while the variation of the jet’s intensity is associated with temperature advection. For both EAJS and NAJS, the interannual variations are mainly contributed by high-frequency TE vorticity forcing, although temperature advection also promotes their meridional shifts. These results suggest that whether or not the maintenance of the jet streams is related to tropical diabatic heating, their interannual variations are not directly induced by this forcing.

Significance Statement

The wintertime Middle East jet stream (MEJS) is a narrow and strong westerly wind belt over the Middle East whose variations in intensity and location can affect the Asian monsoon significantly. However, little effort has been devoted to investigating the MEJS. Thus, dynamical diagnosis and statistical analysis are applied in this study to understand the MEJS and its variability comprehensively. Analysis reveals that low-frequency transient eddies, which are the mobile atmospheric systems with a lifespan longer than 10 days, are important for both the maintenance and the interannual variability of the MEJS. This phenomenon is apparently different from the East Asian and North American jet streams, in which synoptic transient eddies (lifetime shorter than 10 days) play an essential role.

Open access
Masashi Kohma, Masatoshi Mizukoshi, and Kaoru Sato

Abstract

Tropopause folding events (TFs) are characterized by the rapid and deep descent of the tropopause and are considered to play a significant role in mass exchange between the stratosphere and troposphere. In the present study, TFs occurring in the Antarctic coastal region were examined using the ERA5 dataset. First, the climatological distribution of TF frequency in the extratropics of the Southern Hemisphere was examined. Similar to results from previous studies, TFs were found to often occur along the coast of Antarctica, which is located more than 1000 km south of the maximum of the eddy kinetic energy of synoptic-scale disturbances. This result suggests that the climatological pattern of frequency of TFs in the southern high latitudes cannot be explained only by the geographical distribution of storm tracks. Next, a composite analysis of TFs at Syowa Station was performed. When the negative anomaly of the tropopause height was greatest, strong Q-vector divergence and downwelling were observed in the vicinity of the TF locations. The distribution of Q vectors is related to a local westerly jet and strengthening of the frontal structure associated with meridionally contracted synoptic-scale disturbances. The roles of the topography of the Antarctic Plateau and the radiative cooling on the surface of the continent during the contraction of the disturbances are also discussed based on ray-tracing theory.

Open access
Yajuan Song, Fangli Qiao, Jiping Liu, Qi Shu, Ying Bao, Meng Wei, and Zhenya Song

Abstract

The Southern Ocean, characterized by strong westerly winds and a rough sea state, exhibits the most pronounced sea spray effects. Sea spray ejected by ocean surface waves enhances heat and water exchange at the air–sea interface. However, this process has not been considered in current climate models, and the influence of sea spray on the coupled air–sea system remains largely unknown. This study incorporated a parameterization of the sea spray influence on latent and sensible heat fluxes into the First Institute of Oceanography Earth System Model version 2.0 (FIO-ESM v2.0), a climate model coupled with an ocean surface waves component. The results indicate that the spray-mediated enthalpy flux accounted for over 20%–50% of the total value. Sea spray promoted ocean evaporation and heat transport, resulting in air and ocean surface cooling and strengthened westerly winds. Furthermore, a moist and stable atmosphere favored an increase in cloud fraction over the Southern Ocean, particularly low-level clouds. Increased clouds reflected downward shortwave radiation and reduced solar radiation absorption at the surface. At present, the climate models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6) still suffer notable deficiencies in reasonably reproducing the climatological features of the Southern Ocean, including warm SST and underestimated clouds biases with more absorbed shortwave radiation. Our results suggest that consideration of sea spray effects is a feasible solution to mitigate these common biases and enhance the confidence in simulations and predictions with climate models.

Open access
Lijing Cheng, Grant Foster, Zeke Hausfather, Kevin E. Trenberth, and John Abraham

Abstract

The increased concentrations of greenhouse gases in the atmosphere create an increase in Earth’s thermal energy, which is mainly stored in the ocean. Quantification of the rate of increase in ocean heat content (OHC) is vital for understanding the current and future climate of Earth. Linear trend lines have been frequently used to quantify long-term rates of change, but are inappropriate because they cannot capture nonlinearity in trends, have large start- and end-point sensitivity, and the assumption of linearity is nonphysical. Here observed and model-based linear regressions with higher-order polynomial (quadratic), piecewise linear, and locally weighted scatterplot smoothing (LOWESS) are compared. Piecewise linear and LOWESS perform best in depicting multidecadal trends. It is shown that linear rates are valid for up to about 15-yr segments (i.e., it is valid to compute linear rates within a 15-yr time window). Using the recommended methods, ocean warming for the upper 2000 m increases from about 0 to 0.06 ± 0.08 W m−2 for 1958–73 to 0.58 ± 0.08 W m−2 for 2003–18, indicating an acceleration of ocean warming that happens in all four ocean basins and from near the sea surface to 2000 m. There is consistency between multimodel-mean historically forced climate models and observations, which implies that the contribution of internal variability is small for global 0–2000 m OHC. Notable increases of OHC in the upper ocean (i.e., 0–300 m) after about 1980 and the deeper ocean (300–2000 m) after the late 1980s are also evident. This study suggests alternative methods to those currently used to estimate ocean warming rates to provide a more accurate quantification of long-term Earth’s energy changes.

Significance Statement

Quantifying long-term rates of change is needed to understand the time evolution of ocean warming and to assess the changing ocean and Earth’s energy budgets. Linear trend lines have been frequently used but cannot capture nonlinearity in trends, and have large start- and end-point sensitivity. Based on an analysis of the statistical features of ocean heat content time series, this study proposes two alternative methods to quantify the rates of change, including piecewise linear fit and LOWESS. Robust increases in warming for the upper 2000 m detected through observational records and climate models from 1958 to 2020, indicate a robust acceleration of ocean warming. Slow penetration of heat from the upper ocean into the deeper ocean is also evident.

Open access
Yiming Wang, Bo Wu, and Tianjun Zhou

Abstract

The western North Pacific anomalous anticyclone (WNPAC) is the most important interannual circulation anomaly over the western Pacific warm pool. It can persist from boreal winter to the following summer and has great impacts on the East Asian monsoon. The maintenance of the WNPAC in boreal summer was explained by Ekman divergence anomalies over the western North Pacific (WNP) induced by the equatorial easterly Kelvin waves from the tropical Indian Ocean and the Maritime Continent. In this study, we propose a new mechanism for the maintenance of the WNPAC in the season, which we refer to as the “wind-induced moist enthalpy advection” (WIMEA) mechanism. Warm anomalies over the tropical Indian Ocean enhance local deep convection and thus excite atmospheric easterly Kelvin waves to the east. Climatological moist enthalpy decreases eastward over the WNP due to the local northward extension of the intertropical convergence zone (ITCZ) with the establishment of the WNP summer monsoon. Hence, the easterly anomalies transport low moist enthalpy (dry) air westward to the WNP. The induced negative moist enthalpy advection anomalies drive downward motion anomalies and thus suppress deep convection over the WNP under the constraint of the moist static energy budget balance. This moist teleconnection mechanism does not rely on Ekman processes. Numerical experiments using a dry linear baroclinic model indicate that the WIMEA and the conventional Kelvin wave–induced Ekman divergence mechanism have nearly equal relative contributions to the maintenance of the summer WNPAC, with the former mainly driven by the enhanced convection over the tropical Indian Ocean, while the latter is driven by that over the Maritime Continent.

Open access