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Haijun Yang and Qiong Zhang

Abstract

A revisit on observations shows that the tropical El Niño–Southern Oscillation (ENSO) variability, after removing both the long-term trend and decadal variation of the background climate, has been enhanced by as much as 50% during the past 50 yr. This is inconsistent with the changes in the equatorial atmosphere, which shows a slowdown of the zonal Walker circulation and tends to stabilize the tropical coupling system. The ocean role is highlighted in this paper. The enhanced ENSO variability is attributed to the strengthened equatorial thermocline that acts as a destabilizing factor of the tropical coupling system. To quantify the dynamic effect of the ocean on the ENSO variability under the global warming, ensemble experiments are performed using a coupled climate model [Fast Ocean Atmosphere Model (FOAM)], following the “1pctto2x” scenario defined in the Intergovernmental Panel on Climate Change (IPCC) reports. Term balance analyses on the temperature variability equation show that the anomalous upwelling of the mean vertical temperature gradient (referred as the “local term”) in the eastern equatorial Pacific is the most important destabilizing factor to the temperature variabilities. The magnitude of local term and its change are controlled by its two components: the mean vertical temperature gradient T z and the “virtual vertical heat flux” −wT ′. The former can be viewed as the background of the latter and these two components are positively correlated. A stronger T z is usually associated with a bigger upward heat flux −wT ′, which implies a bigger impact of thermocline depth variations on SST. The T z is first enhanced during the transient stage of the global warming with a 1% yr−1 increase of CO2, and then reduced during the equilibrium stage with a fixed doubled CO2. This turnaround in T z determines the turnaround of ENSO variability in the entire global warming period.

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Xianglin Dai, Yang Zhang, and Xiu-Qun Yang

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Low-frequency (LF) transient eddies (intraseasonal eddies with time scales longer than 10 days) are increasingly found to be important in large-scale atmospheric circulation, high-impact climate events, and subseasonal-to-seasonal forecasts. In this study, the features and maintenance of available potential energy of LF eddies (LF EAPE), which denote LF temperature fluctuations, have been investigated. Our study shows that wintertime LF EAPE, with greater amplitude than that of the extensively studied high-frequency (HF) eddies, exhibits distinct horizontal and vertical structures. Different from HF eddies, whose action centers are over midlatitude oceans, the LF EAPE is most active in the continents in the midlatitudes, as well as the subpolar region with shallower vertical structure. By diagnosing the derived energy budget of LF EAPE, we find that, with the strong background temperature gradient in mid- and high-latitude continents (e.g., coast regions along the Greenland, Barents, and Kara Seas), baroclinic generation is the major source of LF EAPE. The generated LF EAPE in the subpolar region is transported downstream and southward to midlatitude continents via background flow. The generated LF EAPE is also dissipated by HF eddies, damped by diabatic effects, and converted to LF EKE via vertical motions. The above energy budget, together with the barotropic dynamics revealed by previous works, suggests multiple energy sources and thus complicated dynamics of LF variabilities.

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Qi Zhang, Kunde Yang, and Qiulong Yang

Abstract

An analysis is conducted for the first time to statistically quantify the relationship between the evaporation duct and oceanic evaporation. Through sensitivity analysis, under unstable conditions (air–sea temperature difference less than zero), evaporation duct and evaporation are found to maintain a similar trend with variations in air–sea variables, indicating a possible inherent connection. Furthermore, scatterplots of relevant historical data reveal that the evaporation duct generally increases in a power-law manner with evaporation. Therefore, logarithmic transformation is performed on the data, and then linear regression is adopted to derive the analytical expression of the linear trend. Additionally, based on this analytical expression, a three-parameter empirical model is proposed to estimate the temporal clustering, and the estimated result shows good agreement with the real distribution. The spatial variations of the parameters modeled over different focus areas reflect the influence of geophysical parameters.

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Yang Zhang and Gregory R. Carmichael

Abstract

A detailed gas-phase chemistry mechanism is combined with dust surface uptake processes to explore possible impacts of mineral dust on tropospheric chemistry. The formations of sulfate and nitrate on dust are studied along with the dust effects on the photochemical oxidant cycle for the long-range-transported particles with a diameter of 0.1–40 μm.

The results show that mineral dust may influence tropospheric sulfate, nitrate, and O3 formation by affecting trace gas concentrations and the tropospheric oxidation capacity through surface processes. The postulated heterogeneous mechanism provides a plausible interpretation for the observed high nitrate and sulfate on dust and the anticorrelation between O3 and dust in East Asia. The presence of dust results in decreases in the concentrations of SO2 (10%–53%), NOpy (16%–100%, defined as NO3 + N2O5 + HNO3), HxOy (11%–59%, defined as OH + HO2 + H2O2), and O3 (11%–40%) under model conditions representative of spring dust storms in East Asia. The decrease in solar actinic flux and the surface uptake of O3 and its precursors contribute to the total O3 decrease for the conditions studied. Nitrate and sulfate, 0.9–2.1 and 0.3–10 μg m−3, respectively, are formed on dust particles, mostly in the size range of 1.5–10 μm. The magnitude of the dust effect strongly depends on the preexisting dust surfaces, the initial conditions, and the selection of model parameters associated with surface uptake processes. The impact of dust reactions on O3 reduction is highly sensitive to the uptake coefficient and to the possible renoxification from the surface reaction of HNO3 on dust.

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Yang Zhang and Peter H. Stone

Abstract

Baroclinic eddy equilibration and the roles of different boundary layer processes in limiting the baroclinic adjustment are studied using an atmosphere–ocean thermally coupled model. Boundary layer processes not only affect the dynamical constraint of the midlatitude baroclinic eddy equilibration but also are important components in the underlying surface energy budget. The authors' study shows that baroclinic eddies, with the strong mixing of the surface air temperature, compete against the fast boundary layer thermal damping and enhance the meridional variation of surface sensible heat flux, acting to reduce the meridional gradient of the surface temperature. Nevertheless, the requirement of the surface energy balance indicates that strong surface baroclinicity is always maintained in response to the meridionally varying solar radiation. With the strong surface baroclinicity and the boundary layer processes, the homogenized potential vorticity (PV) suggested in the baroclinic adjustment are never observed near the surface or in the boundary layer.

Although different boundary layer processes affect baroclinic eddy equilibration differently with more dynamical feedbacks and time scales included in the coupled system, their influence in limiting the PV homogenization is more uniform compared with the previous uncoupled runs. The boundary layer PV structure is more determined by the strength of the boundary layer damping than the surface baroclinicity. Stronger boundary layer processes always prevent the lower-level PV homogenization more efficiently. Above the boundary layer, a relatively robust PV structure with homogenized PV around 600–800 hPa is obtained in all of the simulations. The detailed mechanisms through which different boundary layer processes affect the equilibration of the coupled system are discussed in this study.

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Yang Zhang and Peter H. Stone

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Baroclinic eddy equilibration under a Northern Hemisphere–like seasonal forcing is studied using a modified multilayer quasigeostrophic channel model to investigate the widely used “quick baroclinic eddy equilibration” assumption and to understand to what extent baroclinic adjustment can be applied to interpret the midlatitude climate. Under a slowly varying seasonal forcing, the eddy and mean flow seasonal behavior is characterized by four clearly divided time intervals: an eddy inactive time interval in summer, a mainly dynamically determined eddy spinup time interval starting in midfall and lasting less than one month, and a quasi-equilibrium time interval for the zonal mean flow available potential energy from late fall to late spring, with a mainly external forcing determined spindown time interval for eddy activity from late winter to late spring. The baroclinic adjustment can be clearly observed from late fall to late spring. The sensitivity study of the eddy equilibration to the time scale of the external forcing indicates that the time scale separation between the baroclinic adjustment and the external forcing in midlatitudes is only visible for external forcing cycles one year and longer.

In spite of the strong seasonality of the eddy activity, similar to the observations, a robust potential vorticity (PV) structure is still observed through all the seasons. However, it is found that baroclinic eddy is not the only candidate mechanism to maintain the robust PV structure. The role of the boundary layer thermal forcing and the moist convection in maintaining the lower-level PV structure is discussed. The adjustment and the vertical variation of the lower-level stratification play an important role in all of these mechanisms.

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Yu Nie, Yang Zhang, Gang Chen, and Xiu-Qun Yang

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Observations and climate models have shown that the midlatitude eddy-driven jet can exhibit an evident latitudinal shift in response to lower-tropospheric thermal forcing (e.g., the tropical SST warming during El Niño or extratropical SST anomalies associated with the atmosphere–ocean–sea ice coupling). In addition to the direct thermal wind response, the eddy feedbacks—including baroclinic mechanisms, such as lower-level baroclinic eddy generation, and barotropic mechanisms, such as upper-level wave propagation and breaking—can all contribute to the atmospheric circulation response to lower-level thermal forcing, but their individual roles have not been well explained. In this study, using a nonlinear β-plane multilevel quasigeostrophic channel model, the mechanisms through which the lower-level thermal forcing induces the jet shift are investigated. By diagnosing the finite-amplitude wave activity budget, the baroclinic and barotropic eddy feedbacks to the lower-level thermal forcing are delineated. Particularly, by examining the transient circulation response after thermal forcing is switched on, it is shown that the lower-level thermal forcing affects the eddy-driven jet rapidly by modifying the upper-level zonal thermal wind distribution and the associated meridional wave propagation and breaking. The anomalous baroclinic eddy generation, however, acts to enhance the latitudinal shift of the eddy-driven jet only in the later stage of transient response. Furthermore, the barotropic mechanism is explicated by overriding experiments in which the barotropic flow in the vorticity advection is prescribed. Unlike the conventional baroclinic view, the barotropic eddy feedback, particularly the irreversible PV mixing through barotropic vorticity advection and deformation, plays a major role in the atmospheric circulation response to the lower-level thermal forcing.

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Yang Zhang, Xiu-Qun Yang, Yu Nie, and Gang Chen

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Eddy–zonal flow interactions in the annular modes are investigated in this study using a modified beta-plane multilayer quasigeostrophic (QG) channel model. This study shows the different response of high- and low-phase-speed (frequency) eddies to the zonal wind anomalies and suggests a baroclinic mechanism through which the two eddies work symbiotically maintaining the positive eddy feedback in the annular modes. Analysis also indicates that the different roles played by these two eddies in the annular modes are related to the differences in their critical line distributions. Eddies with higher phase speeds experience a low-level critical layer at the center of the jet. They drive the zonal wind anomalies associated with the annular mode but weaken the baroclinicity of the jet in the process. Lower-phase-speed eddies encounter low-level critical lines on the jet flanks. While their momentum fluxes are not as important for the jet shift, they play an important role by restoring the lower-level baroclinicity at the jet center, creating a positive feedback loop with the fast eddies that extends the persistence of the jet shift.

The importance of the lower-level baroclinicity restoration by the low-phase-speed eddies in the annular modes is further demonstrated in sensitivity runs, in which surface friction on eddies is increased to selectively damp the low-phase-speed eddies. For simulations in which the low-phase-speed eddies become inactive, the leading mode of the zonal wind variability shifts from the position fluctuation to a pulsing of the jet intensity. Further studies indicate that the response of the lower-level baroclinicity to the zonal wind anomalies caused by the low-phase-speed eddies can be crucial in maintaining the annular mode–like variations.

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Zhe Li, Dawen Yang, Yang Hong, Jian Zhang, and Youcun Qi

Abstract

Understanding spatiotemporal rainfall patterns in mountainous areas is of great importance for prevention of natural disasters such as flash floods and landslides. There is little knowledge about rainfall variability over historically underobserved complex terrains, however, and especially about the variations of hourly rainfall. In this study, the spatiotemporal variations of hourly rainfall in the Three Gorges region (TGR) of China are investigated with gauge and newly available radar data. The spatial pattern of hourly rainfall has been examined by a number of statistics, and they all show that the rainfall variations are time-scale and location dependent. In general, the northern TGR receives more-intense and longer-duration rainfall than do other parts of the TGR, and short-duration storms could occur in most of the TGR. For temporal variations, the summer diurnal cycle shifts from a morning peak in the west to a late-afternoon peak in the east while a mixed pattern of two peaks exists in the middle. In statistical terms, empirical model–based estimation indicates that the correlation scale of hourly rainfall is about 40 km. Further investigation shows that the correlation distance varies with season, from 30 km in the warm season to 60 km in the cold season. In addition, summer rainstorms extracted from radar rainfall data are characterized by short duration (6–8 h) and highly localized patterns (5–17 and 13–36 km in the minor and major directions, respectively). Overall, this research provides quantitative information about the rainfall regime in the TGR and shows that the combination of gauge and radar data is useful for characterizing the spatiotemporal pattern of storm rainfall over complex terrain.

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Haijun Yang, Zhengyu Liu, and Qiong Zhang

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Oceanic response to stochastic wind forcing is studied in a tropical–extratropical basin using two shallow water models: a periodically forced model and a time-forward model. Consistent with theory, extratropical stochastic wind forces a decadal spectral peak in the tropical and eastern boundary ocean as a resonant response of the planetary wave basin mode. This resonant response is characterized by a rather uniform amplitude and phase in the equatorial and eastern boundary region. In comparison, away from the eastern boundary, the extratropical ocean is affected significantly by the local Ekman pumping forcing, with spectral peaks varying with location. A complex EOF (CEOF) analysis of the time-forward model simulation further suggests that these resonant responses are robust, and can be extracted as the leading CEOF modes. Thus, the resonance of the planetary wave basin mode gives a physically based guideline for the interpretation of decadal oceanic variability in the tropical–extratropical ocean.

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