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Tamaki Suematsu and Hiroaki Miura

Abstract

The eastward movement of a convectively active region is a distinguishing characteristic of the Madden–Julian oscillation (MJO). However, knowledge about the mechanisms that determine the eastward movement speed remains limited. This study investigates how the background environment modulates the speed of the boreal winter MJO and describes an intrinsic relationship between the MJO and background atmospheric circulation. We calculated the speed of the MJO events from the daily tracking of the locations of the minimum values of the outgoing longwave radiation anomaly in the time–longitude space. These speeds were then used to analyze systematic differences in the sea surface temperature (SST) distribution associated with the MJO speed. The analysis revealed a deceleration of the MJO under low-frequency (>90 days) SST distributions that increased toward the western Pacific from both the Indian Ocean and the eastern Pacific. In contrast, the dependency on SST variability in intraseasonal frequencies (20–90 days) was small. Subsequently, the relationship between the MJO speed and background circulation, which is largely determined by the lower boundary condition set by the low-frequency SST distribution, was analyzed. The analysis counterintuitively revealed that the MJO tends to decelerate when the large-scale zonal circulation with low-level westerlies and upper-level easterlies from the Indian Ocean to the Maritime Continents is strong. The results suggest a novel view that the MJO is an integral component of the Walker circulation and that its eastward movement is modulated by the state of the large-scale flow of the Walker circulation.

Open access
Xingchao Chen, L. Ruby Leung, Zhe Feng, and Fengfei Song

Abstract

Convective vertical transport is critical in the monsoonal overturning, but the relative roles of different convective systems are not well understood. This study used a cloud classification and tracking technique to decompose a convection-permitting simulation of the South Asian summer monsoon (SASM) into subregimes of mesoscale convective systems (MCSs), non-MCS deep convection (non-MCS), congestus, and shallow convection/clear sky. Isentropic analysis is adopted to quantify the contributions of different convective systems to the total SASM vertical mass, water, and energy transports. The results underscore the crucial roles of MCSs in the SASM vertical transports. Compared to non-MCSs, the total mass and energy transports by MCSs are at least 1.5 times stronger throughout the troposphere, with a larger contributing fraction from convective updrafts compared to upward motion in stratiform regions. Occurrence frequency of non-MCSs is around 40 times higher than that of MCSs. However, per instantaneous convection features, the vertical transports and net moist static energy (MSE) exported by MCSs are about 70–100 and 58 times stronger than that of non-MCSs. While these differences are dominantly contributed by differences in the per-feature MCS and non-MCS area coverage, MCSs also show stronger transport intensities than non-MCSs over both ocean and land. Oceanic MCSs and non-MCSs show more obvious top-heavy structures than their inland counterparts, which are closely related to the widespread stratiform over ocean. Compared to the monsoon break phase, MCSs occur more frequently (~1.6 times) but their vertical transport intensity slightly weakens (by ~10%) during the active phases. These results are useful for understanding the SASM and advancing the energetic framework.

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Robin Clancy, Cecilia M. Bitz, Edward Blanchard-Wrigglesworth, Marie C. McGraw, and Steven M. Cavallo

Abstract

Arctic cyclones are an extremely common, year-round phenomenon, with substantial influence on sea ice. However, few studies address the heterogeneity in the spatial patterns in the atmosphere and sea ice during Arctic cyclones. We investigate these spatial patterns by compositing on cyclones from 1985 to 2016 using a novel, cyclone-centered approach that reveals conditions as functions of bearing and distance from cyclone centers. An axisymmetric, cold-core model for the structure of Arctic cyclones has previously been proposed; however, we show that the structure of Arctic cyclones is comparable to those in the midlatitudes, with cyclonic surface winds, a warm, moist sector to the east of cyclones and a cold, dry sector to the west. There is no consensus on the impact of Arctic cyclones on sea ice, as some studies have shown that Arctic cyclones lead to sea ice growth and others to sea ice loss. Instead, we find that sea ice decreases to the east of Arctic cyclones and increases to the west, with the greatest changes occurring in the marginal ice zone. Using a sea ice model forced with prescribed atmospheric reanalysis, we reveal the relative importance of the dynamic and thermodynamic forcing of Arctic cyclones on sea ice. The dynamic and thermodynamic responses of sea ice concentration to cyclones are comparable in magnitude; however, dynamic processes dominate the response of sea ice thickness and are the primary driver of the east–west difference in the sea ice response to cyclones.

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Xiaodan Yang, Yajuan Song, Meng Wei, Yuhuan Xue, and Zhenya Song

Abstract

In this paper, the different effects of the eastern equatorial Pacific (EP) and central equatorial Pacific (CP) Ocean El Niño–Southern Oscillation (ENSO) events on interannual variation in the diurnal sea surface temperature (SST) are explored in both the Niño-3 and Niño-4 regions. In the Niño-3 region, the diurnal SST anomaly (DSSTA) is negative during both EP and CP El Niño events and becomes positive during both EP and CP La Niña events. However, the DSSTA in the Niño-4 region is positive in El Niño years and negative in La Niña years, which is opposite to that in the Niño-3 region. Further analysis indicates that the incident shortwave radiation (SWR), wind stress (WS), and upward latent heat flux (LHF) are the main factors causing the different interannual variations in the DSST. In the Niño-3 region, decreased SWR and increased LHF lead to a negative DSSTA in EP El Niño years, and enhanced WS and increased LHF cause a negative DSSTA in CP El Niño years. Conversely, in that same region, increased SWR and decreased LHF lead to a positive DSSTA in EP La Niña years, and reduced WS and decreased LHF cause a positive DSSTA in CP La Niña years. In the Niño-4 region, the reduced trade wind plays a key role in producing the positive DSSTA, whereas the decreased SWR has an opposite effect that reduces the range of the DSSTA during both EP and CP El Niño events, and conversely the enhanced trade wind plays a key role in producing the negative DSSTA, whereas the increased SWR has an opposite effect that increases the range of the DSSTA during both EP and CP La Niña events.

Open access
Biqi Liu, Guixing Chen, Wenxin Zeng, Lanqiang Bai, and Huiling Qin

Abstract

Monsoon southerlies can be particularly active for days and produce substantial rainfall over East Asia. These multiday episodes of southerly monsoon surge may exhibit distinct diurnal variations due to regional forcings under given large-scale conditions. This study categorizes the southerly surges into two types with different wind diurnal variations to clarify their influence on rainfall over East Asia. In the summers of 1998–2019, there are 63 episodes of southerly surges with large wind diurnal cycles and 55 episodes with small diurnal cycles. The first type of southerly surges usually occurs with anomalous low-level warming over southeastern China related to the westward extension of the western Pacific subtropical high. The second type of southerly surges instead occurs with anomalous cooling due to the deepened midlatitude trough. They thus represent the different mechanisms downscaling from large-scale conditions to regional diurnal forcings. After the onset of the first type, the intensified monsoon southerlies at night lead to the northward displacement of large-scale ascent and northward water vapor transport with warm moist energy. The monsoon rainband tends to move to the north of 35°N with a robust response in precipitation systems, especially in the meso-α-scale rain events from midnight to morning. As a comparison, the rainband stays at 30°–35°N after the onset of the second type when the strengthened large-scale ascent and water vapor convergence are located relatively south. These differences between the two types of southerly monsoon surges highlight that the multiday large-scale conditions interact with subdaily regional forcings and greatly regulate the detailed evolution of summer rainband over East Asia.

Open access
Rui Shi, Xinyu Guo, Ju Chen, LiLi Zeng, Bo Wu, and Dongxiao Wang

Abstract

The responses of surface wind stress to the mesoscale sea surface temperature (SST) anomalies associated with the SST front in the northern South China Sea (NSCS) are studied using satellite observations and reanalysis data. Both satellite and reanalysis data explicitly show the linear relationships between the spatial-high-pass filtered wind stress perturbation derivatives and the underlying SST gradient field. However, the noise in the linear relationships is much smaller in the reanalysis data than in the satellite observations. This result is rarely reported in other frontal areas. The wavelet analysis shows that the satellite scatterometer observed numerous high wavenumber perturbations within 100 km in the NSCS, but these perturbations were absent in the reanalysis data. The linear relationship between the perturbation SST gradient and derivative wind stress fields is not significant at this scale, which enhances the noise in the linear relationship. The spatial bandpass-filtered perturbation between 100 and 300 km can give reasonable estimates of the coupling coefficients between the wind stress divergence and downwind SST gradient (α d) and between the wind stress curl and crosswind SST gradient (α c) in the NSCS, with values of 1.33 × 10−2 and 0.95 × 10−2 N m−2 °C−1, respectively.

Open access
Xianghui Kong, Aihui Wang, Xunqiang Bi, Biyun Sun, and Jiangfeng Wei

Abstract

The sensitivity of hourly precipitation to cumulus parameterization and radiation schemes is explored by using the tropical-belt configuration of the Weather Research and Forecasting (WRF) Model. The domain covers the entire tropical region from 45°S to 45°N with a grid spacing of about 45 km. A series of 5-yr simulations with four cumulus parameterization schemes [new Tiedtke (NT), Kain–Fritsch (KF), new SAS (NS), and Tiedtke (TK)] and two radiation schemes (RRTMG and CAM) are carried out. We focus on the frequencies of hourly precipitation above three thresholds (0.02 mm h−1 = light drizzle rate; 0.2 mm h−1 = moderate rate; and 2 mm h−1 = heavy rate) between the observed CMORPH products and simulations. The sensitivity is higher for precipitation frequency than amount, and frequency is dominated by the cumulus parameterization. Frequencies above the moderate rate are well reproduced, whereas frequencies above the other two rates present large deviations. No combination of physical schemes is found to perform best in reproducing the frequencies above all thresholds. Simulations using the NT and NS schemes show higher precipitation frequencies above the light drizzle rate and lower precipitation frequencies above the heavy rate than those simulations using the KF and TK schemes. Precipitation frequency is higher when reproduced by experiments using the RRTMG scheme than those using the CAM scheme, except for frequencies above the light rate over oceans. The overestimation of frequency is mainly caused by too-frequent convective rainfall. The results imply that the triggering based on the vertical velocity may increase the occurrence of a rain event and that CAPE-based closure may increase the heavy precipitation frequency in the cumulus parameterization.

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William B. Rossow, Kenneth R. Knapp, and Alisa H. Young

Abstract

ISCCP continues to quantify the global distribution and diurnal-to-interannual variations of cloud properties in a revised version. This paper summarizes assessments of the previous version, describes refinements of the analysis and enhanced features of the product design, discusses the few notable changes in the results, and illustrates the long-term variations of global mean cloud properties and differing high cloud changes associated with ENSO. The new product design includes a global, pixel-level product on a 0.1° grid, all other gridded products at 1.0°-equivalent equal area, separate satellite products with ancillary data for regional studies, more detailed, embedded quality information, and all gridded products in netCDF format. All the data products including all input data, expanded documentation, the processing code, and an operations guide are available online. Notable changes are 1) a lowered ice–liquid temperature threshold, 2) a treatment of the radiative effects of aerosols and surface temperature inversions, 3) refined specification of the assumed cloud microphysics, and 4) interpolation of the main daytime cloud information overnight. The changes very slightly increase the global monthly mean cloud amount with a little more high cloud and a little less middle and low cloud. Over the whole period, total cloud amount slowly decreases caused by decreases in cumulus/altocumulus; consequently, average cloud-top temperature and optical thickness have increased. The diurnal and seasonal cloud variations are very similar to earlier versions. Analysis of the whole record shows that high cloud variations, but not low clouds, exhibit different patterns in different ENSO events.

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Tao Wang, Wenshou Tian, Tao Lian, Cheng Sun, Fei Xie, Jiankai Zhang, and Qingqing Yin

Abstract

Changes in the meridional position of the sea surface temperature (SST) anomalies (SSTAs) associated with the interannual component (PC1-I) of the principal component 1 (PC1) of the first leading mode of the North Pacific SST (referred to here as PC1-I-related SSTAs) are investigated using reanalysis products and climate model output. It is found that the PC1-I-related SSTAs (or PC1-I anomalies) significantly shift southward at a rate of 1.04° latitude per decade and have moved southward by 4.4° since the 1960s. Our further analysis indicates that the southward shift of the PC1-I-related SSTAs is due to changes in ENSO teleconnections. Compared to the 1950–75 period (PRE era), the meridional width of the ENSO-induced tropical positive geopotential height (GH) anomaly is narrower during the 1991–2016 period (POST era), inducing a southward shift of the subtropical westerly anomaly over the North Pacific through geostrophic wind relations. This southward shift of the westerly anomaly favors the southward shift of the ENSO-induced negative GH anomaly (cyclonic circulation anomaly) over the North Pacific by positive vorticity forcing of the zonal wind shear. The southward-shifting GH anomaly associated with ENSO further forces the PC1-I anomaly to shift southward. Furthermore, the contraction of the ENSO-induced tropical positive GH anomaly is related to the contraction of the meridional width of ENSO. The modeling results support that the decrease in the ENSO meridional width favors the contraction of the ENSO-induced tropical positive GH anomaly and the southward shift of ENSO teleconnections over the North Pacific, contributing to the southward shift of the PC1-I anomaly.

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Aiden Jönsson and Frida A.-M. Bender

Abstract

Despite the unequal partitioning of land and aerosol sources between the hemispheres, Earth’s albedo is observed to be persistently symmetric about the equator. This symmetry is determined by the compensation of clouds to the clear-sky albedo. Here, the variability of this interhemispheric albedo symmetry is explored by decomposing observed radiative fluxes in the CERES EBAF satellite data record into components reflected by the atmosphere, clouds, and the surface. We find that the degree of interhemispheric albedo symmetry has not changed significantly throughout the observational record. The variability of the interhemispheric difference in reflected solar radiation (asymmetry) is strongly determined by tropical and subtropical cloud cover, particularly those related to nonneutral phases of El Niño–Southern Oscillation (ENSO). As ENSO is the most significant source of interannual variability in reflected radiation on a global scale, this underscores the interhemispheric albedo symmetry as a robust feature of Earth’s current annual mean climate. Comparing this feature in observations with simulations from coupled models reveals that the degree of modeled albedo symmetry is mostly dependent on biases in reflected radiation in the midlatitudes, and that models that overestimate its variability the most have larger biases in reflected radiation in the tropics. The degree of model albedo symmetry is improved when driven with historical sea surface temperatures, indicating that the degree of symmetry in Earth’s albedo is dependent on the representation of cloud responses to coupled ocean–atmosphere processes.

Open access