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Jian-Ping Huang and Gerald R. North

Abstract

Due to the variety of periodic or quasi-periodic deterministic forcings (e.g., diurnal cycle, seasonal cycle, Milankovitch cycles, etc.), most climate fluctuations may be modeled as cyclostationary processes since their properties are modulated by these cycles. Difficulties in using conventional spectral analysis to explore the seasonal variation of climate fluctuations have indicated the need for some new statistical techniques. It is suggested here that the cyclic spectral analysis he used for interpreting such fluctuations. The technique is adapted from cyclostationarity theory in signal processing. To demonstrate the usefulness of this technique, a very simple cyclostationarity stochastic climate model is constructed. The results show that the seasonal cycle strongly modulates the amplitude of the covariance and spectrum. The seasonal variation of intraseasonal oscillations in the Tropics has also been studied on a zonally symmetric all-land planet in the absence of external forcing. The idealized planet has no ocean no topography. A 15-year length seasonal run of the atmosphere is analyzed with the NCAR Community Climate Model (CCM2, R15). Analysis of the simulation data indicates the presence of intraseaonal oscillations in the Tropics, which are also localized in the time of year.

Both examples suggest that these techniques might be useful for analysis of fluctuations that exhibit locality in both frequency and season.

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Danqing Huang, Aiguo Dai, and Jian Zhu

Abstract

After a CO2 increase, whether the early transient and final equilibrium climate change patterns are similar has major implications. Here, we analyze long-term simulations from multiple climate models under increased CO2, together with the extended simulations from CMIP5, to compare the transient and equilibrium climate change patterns under different forcing scenarios. Results show that the normalized warming patterns (per 1 K of global warming) are broadly similar among different forcing scenarios (including abrupt 2 × CO2, 4 × CO2, and 1% CO2 increase per year) and during different time periods, except for the first 50 years or so when warming is weaker over the North Atlantic and Southern Ocean but stronger over most continents. During the first 200 years, this consistency is stronger over land than over ocean, but is lower in midlatitudes than other regions. Normalized precipitation change patterns are also similar, albeit to a lesser degree, among different forcing scenarios and across different time periods, although noticeable differences exist during the first few hundred years with smaller increases over the tropical Pacific. Precipitation over many subtropical oceans and land areas decreases consistently under different forcing scenarios and over all time periods. In particular, the transient and near-equilibrium change patterns for both surface air temperature and precipitation are similar over most of the globe, except for the North Atlantic warming hole, which is mainly a transient feature. The Arctic amplification and land–ocean warming contrast are largest during the first 100–200 years after CO2 quadrupling but they still exist in the equilibrium response.

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Huijun Huang, Hongnian Liu, Jian Huang, Weikang Mao, and Xueyan Bi

Abstract

Small-scale turbulence has an essential role in sea-fog formation and evolution, but is not completely understood. This study analyzes measurements of the small-scale turbulence, together with the boundary layer structure and the synoptic and mesoscale conditions over the life cycle of a cold advection fog event and a warm advection fog event, both off the coast of southern China. The measurement data come from two sites: one on the coast and one at sea. These findings include the following: 1) For cold advection fog, the top can extend above the inversion base, but formation of an overlaying cloud causes the fog to dissipate. 2) For warm advection fog, two layers of low cloud can merge to form deep fog, with the depth exceeding 1000 m, when strong advection of warm moist air produces active thermal-turbulence mixing above the thermal-turbulence interface. 3) Turbulence near the sea surface is mainly thermally driven for cold advection fog, but mechanically driven for warm advection fog. 4) The momentum fluxes of both fog cases are below 0.04 kg m−1 s−2. However, the sensible and latent heat flux differ between the cases: in the cold advection fog case, the sensible and latent heat fluxes are roughly upward, averaging 2.58 and 26.75 W m−2, respectively; however, in the warm advection fog case, the sensible and latent heat flux are mostly downward, averaging −6.98 and −6.22 W m−2, respectively. 5) Low-level vertical advection is important for both fogs, but has a larger influence on fog development in the warm advection fog case.

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Shuanglin Li, Jian Lu, Gang Huang, and Kaiming Hu

Abstract

A basin-scale warming is the leading mode of tropical Indian Ocean sea surface temperature (SST) variability on interannual time scales, and it is also the prominent feature of the interdecadal SST trend in recent decades. The influence of the warming on the East Asian summer monsoon (EASM) is investigated through ensemble experiments of several atmospheric general circulation models (AGCMs). The results from five AGCMs consistently suggest that near the surface, the Indian Ocean warming forces an anticyclonic anomaly over the subtropical western Pacific, intensifying the southwesterly winds to East China; and in the upper troposphere, it forces a Gill-type response with the intensified South Asian high, both favoring the enhancement of the EASM. These processes are argued to contribute to the stronger EASM during the summer following the peak of El Niño than monsoons in other years. These model results also suggest that tropical Indian Ocean warming may not have a causal relationship to the synchronous weakening of EASM on interdecadal time scales.

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Danqing Huang, Aiguo Dai, Jian Zhu, Yaocun Zhang, and Xueyuan Kuang

Abstract

Global-mean surface temperature has experienced fast warming during 1985–98 but stabilized during 1999–2013, especially in boreal winter. Climate changes over East Asia between the two warming periods and the associated mechanisms have not been fully understood. Analyses of observation and reanalysis data show that winter precipitation has decreased (increased) over southern (northeastern) China from 1985–98 to 1999–2013. Winds at 300 hPa over East Asia strengthened during 1999–2013 around 30°–47.5°N but weakened to the north and south of it. This change pattern caused the East Asian polar front jet (EAPJ) and the East Asian subtropical jet (EASJ) to shift, respectively, equatorward and poleward during 1999–2013. Associated with these jet displacements, the Siberian high enhanced and the East Asian trough shifted westward. The enhanced Siberian high strengthened the East Asian winter monsoon and weakened southwesterly winds over the South China Sea, leading to precipitation decreases over southern China. The westward shift of the East Asian trough enhanced convergence and precipitation over northeastern China. A combination of a negative phase of the interdecadal Pacific oscillation and a positive phase of the Atlantic multidecadal oscillation during 1999–2013 resulted in significant tropospheric warming over the low and high latitudes and cooling over the midlatitudes of East Asia. These changes enhanced the meridional temperature gradient and thus westerlies over the region between the two jets but weakened them to the south and north of it, thereby contributing to the wind change patterns and the jet displacements.

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Fukai Liu, Jian Lu, Yiyong Luo, Yi Huang, and Fengfei Song

Abstract

Climate models project an enhancement in SST seasonal cycle over the midlatitude oceans under global warming. The underlying mechanisms are investigated using a set of partially coupled experiments, in which the contribution from direct CO2 effects (i.e., the response in the absence of wind change) and wind feedbacks can be isolated from each other. Results indicate that both the direct CO2 and wind effects contribute to the enhancement in the SST seasonal cycle, with the former (latter) being more important in the Northern Hemisphere (Southern Hemisphere). Further decomposition of the wind effect into the wind stress feedback and wind speed feedback reveals the importance of the wind stress–driven ocean response in the change of SST seasonal cycle, a result in contrast to a previous study that ascribed the midlatitude SST seasonal cycle change to the thermodynamic wind speed feedback. The direct CO2 effect regulates the SST seasonal cycle through the warming-induced shoaling in the annual mean mixed layer depth (MLD) as well as the MLD difference between winter and summer. Moreover, the surface wind seasonal cycle changes due solely to the direct CO2 effect are found to bear a great resemblance to the full wind response, suggesting that the root cause for the enhancement of the midlatitude SST seasonal cycle resides in the direct CO2 effect. This notion is further supported by an ocean-alone experiment that reproduces the SST seasonal cycle enhancement under a spatially and temporally homogeneous surface thermal forcing.

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Dan-Qing Huang, Jian Zhu, Yao-Cun Zhang, and An-Ning Huang

Abstract

To investigate the concurrent impacts of the East Asian polar front jet (EAPJ) and subtropical jet (EASJ) on the summer rainfall over eastern China, positive (strengthened EAPJ with weakened EASJ) and negative (weakened EAPJ with strengthened EASJ) configurations are identified. In the positive configuration, rainfall decreases in the northern part of eastern China and increases in the southern part, vice versa in the negative configuration. The possible mechanisms maintaining the two jet configurations are further proposed from the perspectives of sea surface temperature (SST) and synoptic-scale transient eddy activities (STEA). In the positive configuration, meridional distributed cold–warm SST anomalies over the eastern North Pacific may induce regional circulation and meridional temperature gradient anomalies, which can strengthen the EAPJ and weaken the EASJ. The central Pacific La Niña–like SST anomalies are related with the Arctic vortexlike anomalies in the stratosphere, which may strengthen the EAPJ. Furthermore, the divergence of Eliassen–Palm vectors and the conversion from eddy kinetic energy to mean kinetic energy over the active region of the EAPJ may strengthen the EAPJ, vice versa for the weakened EASJ. In the negative configuration, associated with the warm SST anomalies over the western North Pacific, the enhanced convective activities may lead to a strengthened EASJ via meridional teleconnection. The teleconnection may be intensified by the strengthened easterly vertical shear. Additionally, eastern Pacific La Niña–like SST anomalies may intensify the Walker circulation, which may strengthen the EASJ via the Hadley circulation. The STEA-related anomalies are almost opposite those in the positive configuration, especially for the weakened EAPJ.

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Dan-Qing Huang, Jian Zhu, Yao-Cun Zhang, Jun Wang, and Xue-Yuan Kuang

Abstract

Spring persistent rainfall (SPR) over southern China has great impact on its society and economics. A remarkable feature of the SPR is high frequency. However, SPR frequency obviously decreases over the period of 1997–2011. In this study, the possible causes have been investigated from the perspective of the individual and concurrent effects of the East Asian subtropical jet (EASJ) and East Asian polar front jet (EAPJ). A close relationship is detected between SPR frequency and EASJ intensity (but not EAPJ intensity). Associated with strong EASJ, abundant water vapor is transported to southern China by the southwesterly flow, which may trigger the SPR. Additionally, frequencies of both strong EASJ and weak EAPJ events are positively correlated with SPR frequency. Further investigation of the concurrent effect indicates a significant positive correlation between the frequencies of SPR and the strong EASJ–weak EAPJ configuration. Associated with this configuration, southwesterly flow strengthens in the lower troposphere, while northerly wind weakens in the upper troposphere. This provides a dynamic and moist condition, as enhanced ascending motion and intensified convergence of abundant water vapor over southern China, which favors the SPR. All analyses suggest that the EASJ may play a dominant role in the SPR occurrence and that the EAPJ may play a modulation role. Finally, a possible mechanism maintaining the strong EASJ–weak EAPJ configuration is proposed. Significant cooling over the northeastern Tibetan Plateau may induce a cyclone anomaly in the upper troposphere, which could result in an accelerating EASJ and a decelerating EAPJ.

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Gary E. Moore, Christopher Daly, Mei-Kao Liu, and Shi-Jian Huang

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A dry three-dimensional mesoscale model was used to study the diurnal cycle of mountain-valley winds in the southern San Joaquin Valley during a summer day. A scheme for interpolating potential temperature was developed to provide hourly temperature fields to initialize and force the dynamically predicted wind fields. A simplified modeling approach was used to produce steady state solutions that are dynamically consistent with the momentum equation and supplied temperature fields. Model performance was evaluated by comparing observed and predicted surface winds. Some features of the wind field flow aloft were qualitatively examined with regard to their importance in air quality studies.

The morning drainage-upslope transition and the evening reversal of upslope flow were realistically simulated throughout most of the valley. The variation of wind speeds throughout the valley and over the course of the day were simulated with an average bias of 9% of the average wind. Wind directions were simulated with an overall average bias of 5° and midday hourly correlation coefficients of typically r = 0.8. Model performance was below average during the morning and evening transition periods, when thermal forcing is at a minimum and valley winds are light and variable. At midday, the model produces strong upward vertical motions near the ridge crests and divergence-driven subsidences at the foot of the mountains typical of observations made in mountain-valley systems. During the morning, modeled drainage flow down the mountains results in a convergence zone in the southern and narrowest part of the valley, resulting in rising motions; down-valley flow, sometimes observed in mountain-valley systems, also occurs. The model is best suited for applications in mountain-valley regions for which wind observations are sparse and do not adequately reflect thermally driven circulation.

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Danqing Huang, Aiguo Dai, Ben Yang, Peiwen Yan, Jian Zhu, and Yaocun Zhang

Abstract

Recent concurrent shifts of the East Asian polar-front jet (EAPJ) and the East Asian subtropical jet (EASJ) in the boreal winter have raised concerns, since they could result in severe weather events over East Asia. However, the possible mechanisms are not fully understood. In this study, the roles of the interdecadal Pacific oscillation (IPO) and the Atlantic multidecadal oscillation (AMO) are investigated by analyzing reanalysis data and model simulations. Results show that combinations of opposite phases of the IPO and AMO can result in significant shifts of the two jets during 1920–2014. This relationship is particularly evident during 1999–2014 and 1979–98 in the reanalysis data. A combination of a negative phase of the IPO (−IPO) and a positive phase of the AMO (+AMO) since the late 1990s has enhanced the meridional temperature gradient and the Eady growth rate and thus westerlies over the region between the two jets, but weakened them to the south and north of the region, thereby contributing to the equatorward and poleward shifts of the EAPJ and EASJ, respectively. Atmospheric model simulations are further used to investigate the relative contribution of −IPO and +AMO to the jet shifts. The model simulations show that the combination of −IPO and +AMO favors the recent jet changes more than the individual −IPO or +AMO. Under a concurrent −IPO and +AMO, the meridional eddy transport of zonal momentum and sensitive heat strengthens, and more mean available potential energy converts to the eddy available potential energy over the region between the two jets, which enhances westerly winds there.

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