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Li Li and Mark Wimbush

Abstract

Bottom temperature time series recorded beneath the Gulf Stream at 265 and 589 m depth off the Georgia coast are compared with simultaneous time series of main thermocline depth determined from inverted echo sounder and bottom pressure gauge records at the same sites. Bottom temperature is found to be coherent with vertical displacement of the thermocline, suggesting that bottom temperature under the Gulf Stream front is a potentially useful indicator of Gulf Stream displacement. Additional evidence is provided by the similarity of bottom temperature and thermocline depth coherences with longshore current at the shelf break. Bottom temperature at the deeper station appears to be the better indicator of Gulf Stream meandering for periods longer than five days.

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Jiangnan Li and Tim Li

Abstract

The structure and evolution characteristics of atmospheric entropy production associated with the climatologic monsoon onset and evolution were investigated using the National Centers for Environmental Prediction (NCEP) reanalysis data. The entropy balance equation contains two parts. The first part is internal entropy production that corresponds to natural dissipation. The second part is external entropy production that is associated with lower-boundary entropy supply. It is shown that the dissipation process represented by internal entropy production can be used to describe the thermal and dynamical structures of the monsoon. The thermal dissipation due to turbulent vertical diffusion and convection is highly correlated to precipitation. The dynamic dissipation due to wind stress becomes very strong over the Arabian Sea and southwestern part of India in boreal summer, and dynamic dissipation can describe the monsoon structure more clearly than variables such as wind shear. The correlation between surface entropy supply and internal entropy production is so large that the surface entropy supply can also be used to evaluate the monsoon. Over the desert region of Rajasthan, the dissipation is relatively weaker than its surroundings owing to descending large-scale eddy flow and a weak convective flux. The analysis of atmospheric entropy provides a new way to describe the monsoon development characteristics, which differs from those derived from a traditional analysis method.

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Li Yan and Gen Li

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The southern subtropical dipole modes (SSDMs) and southern annular mode (SAM) are important climate modes, which are dominant in the southern middle and high latitudes, respectively, with considerable regional climatic impacts. However, the relationship between the two modes remains unclear. A close inspection reveals that the SAM was significantly correlated with the SSDMs during the austral summer before the mid-1980s. However, the correlations have degraded since then. This decadal shift in the relationship between these two southern dominant modes is due to a weakened connection between the SAM and the subtropical highs that control the SSDMs. This decadal change could be traced back to a poleward shift in the southern westerly belt. El Niño–Southern Oscillation (ENSO) typically plays a moderate role in influencing the precipitation in Australia and a minor role in influencing the precipitation in Africa and South America. Nevertheless, the two southern modes could still affect the austral summer rainfall in the midlatitudes, even though the ENSO signal is absent. All these links between the two southern modes and southern land precipitation may be attributable to the associated transport of moisture in the lower-level circulation.

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Chen Li and Shuanglin Li

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The correlations among the summer, low-level, cross-equatorial flows (CEFs) over the Indian–west Pacific Ocean region on the interannual time scale are investigated by using both the NCEP–NCAR reanalysis and 40-yr ECMWF Re-Analysis (ERA-40) datasets. A significant negative correlation (seesaw) has been illustrated between the Somali CEF and the three CEFs north of Australia (the South China Sea, Celebes Sea, and New Guinea; they are referred to in combination as the Australian CEF). A seesaw index is thus defined with a higher (lower) value representing an intensified (weakened) Somali CEF but a weakened (intensified) Australian CEF. The connection of the seesaw with the East Asian summer monsoon (EASM) is then investigated. The results suggest that an enhanced seesaw corresponds to an intensified EASM with more rainfall in north China, the Yellow River valley, and the upper reach of the Yangtze River. The seesaw reflects the opposite covariability between the two atmospheric action centers in the Southern Hemisphere, Mascarene subtropical high, and Australian subtropical high. Whether the seesaw–EASM connection is influenced by El Niño–Southern Oscillation (ENSO) or the Indian Ocean SST dipole mode (IOD) is analyzed. The results remain unchanged when the ENSO- or IOD-related signals are excluded, although ENSO exerts a significant influence. This implies an additional predictability for the EASM from the CEF seesaw.

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Li Deng and Tim Li

Abstract

The interannual variability of the boreal summer intraseasonal oscillation (BSISO) is investigated using observed outgoing longwave radiation (OLR) and ERA-Interim data for the period of 1980–2012. It is found that the interannual variability of BSISO intensity is much stronger in the tropical western Pacific (TWP) than the tropical Indian Ocean (TIO). A BSISO intensity index is defined based on a multivariate EOF analysis in TWP. It is found that strong BSISO years are associated with El Niño–like sea surface temperature anomalies in the tropical Pacific, anomalous easterly shear, and enhanced background moisture condition in the region. Using a 2.5-layer atmospheric model with a specified idealized background mean state, the authors further examine the relative roles of background moisture and vertical shear fields in modulating the BSISO intensity. Sensitivity numerical experiments indicate that the background moisture change is most important in regulating the BSISO intensity, whereas the background vertical shear change also plays a role.

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Li Li and Yaocun Zhang

Abstract

Observational analysis indicates that the East Asian jet stream consists of two separate branches: the East Asian subtropical jet (EASJ) and the East Asian polar front jet (EAPJ). The impacts of different intensity configurations of the EASJ and EAPJ on precipitation during the mei-yu season are investigated using the NCEP–NCAR Reanalysis Project (NNRP) dataset and daily gauge observations in East China. The intensity and location of precipitation are associated with different configurations of the EASJ and EAPJ. Precipitation intensity increases with intensification of the EASJ and EAPJ. The rainband is located to the north of the mei-yu region when the EASJ intensifies and the EAPJ weakens. Further analyses indicate that the intensity changes of the EASJ and EAPJ are linked to the cold and warm airmass activities. For cases with strong EASJ and EAPJ, both the warm-moist and cold air masses are active. When the warm-moist and cold air masses meet near 30°N, abundant precipitation occurs in the Yangtze-Huai River basin (YHRB). For cases with weak EASJ and EAPJ, both the cold and warm-moist air masses are inactive, and no significant precipitation occurs in the YHRB. For cases with strong EASJ and weak EAPJ, the warm-moist air mass moves northward while the cold air mass is weak. Precipitation concentrates to the north of YHRB. For cases with weak EASJ and strong EAPJ, cold air extends farther south while the warm-moist air mass is inactive. Precipitation occurs to the south of YHRB.

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Xiao-Feng Li, Jianping Li, and Yun Li

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The middle–lower valley of the Yangtze River (MLY), located in the middle of eastern China, has been one of the largest economic centers of China since ancient times. Winter precipitation variability over the MLY is important for China because of its significant influence on the local economy. However, few studies have focused on the long-term variability of winter precipitation over the MLY. This study reports a significant wetting trend over the MLY in winter during the three decades since the late 1970s, forming a “mid-east-China winter wetting” pattern, which has become an important feature of precipitation change under the weakening East Asian winter monsoon. This wetting trend in the MLY also implies the poleward extension of the precipitation belts of southern China.

Further investigation reveals that the increasing sea surface temperature (SST) in the tropical Indian Ocean (TIO) is the dominant factor responsible for recent increases in precipitation over the MLY. The thermal forcing driven by warming of the TIO SST gives rise to an anomalous cyclonic circulation along the coast of eastern China. This transports more water vapor onto the Chinese mainland, shifts and causes anomalous convergence over the MLY, and generates the increase in precipitation there. As such, the increasing SST in the TIO induces over 80% of the observed wetting trend over the MLY. This mechanism was verified by results obtained from two sets of sensitivity experiments using a numerical spectral atmospheric general circulation model. Thus, increasing SST in the TIO has made a dominant contribution to the recent winter precipitation increase over the MLY.

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J. Li

Abstract

A hypothesis that the fractional scattering into the forward peak is related to solar zenith angle and single scattering albedo is proposed. Calculations show that this assumption can increase the accuracy of the δ-Eddington approximation. For the scattering conservative case this method can improve the results in the region of thin optical depth. For the scattering nonconservative case this method can reduce the errors for reflection and absorption in the region of small solar zenith angle, where the incoming solar energy is most significant.

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J. Li

Abstract

Various aspects of infrared radiative transfer through clouds are investigated. First, three solutions to the IR radiative transfer equation are presented and assessed, each corresponding to a different approximation for the Planck function. It is shown that the differences in results between solutions with linear and exponential dependence of the Planck source function are small for typical vertical resolutions in climate models. Second, a new perturbation-based approach to solving the IR radiative transfer equation with the inclusion of cloud scattering is presented. This scheme follows the standard perturbation method, and allows one to identify the zeroth-order equation with the absorption approximation and the first-order equation as including IR scattering effects. This enables one solution to accurately treat cloudy layers in which cloud scattering is included, and allows for an improved and consistent treatment of absorbing aerosol layers in the absence of cloud by using the zeroth-order equation. This new scheme is more simple and efficient compared to previous perturbation method work for treating infrared absorption and scattering. Last, a general method is devised for calculating the random, maximum, and slantwise overlap of cloud layers, which conveniently integrates into the two-stream radiative transfer solution in this work. For several random and maximum (or slantwise) overlap cloud cases with a wide variation of cloud fractions, the error in the cooling rate is generally less than 1 K day−1 and the error in the radiative flux is generally less than 3 W m−2.

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Tianming Li

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The eastern Pacific and Atlantic have a curious climatic asymmetry relative to the equator. Whereas the intertropical convergence zone (ITCZ) characterized by persistent and heavy rainfall and the warmest surface waters reside north of the equator, a cold tongue in sea surface temperature (SST) occurs at and south of the equator even though the time-mean solar radiation is approximately symmetric about the equator. In this paper the author investigates the relative role of three types of coupled ocean–atmosphere interaction processes—the meridional wind–SST feedback, the evaporation–wind feedback, and the low-level stratus cloud–SST feedback—in determining the climatic asymmetry relative to the equator.

This study has two components. First, a simple analytical model is constructed in which the aforementioned three positive-feedback mechanisms are all included in a unified dynamic framework. The author’s stability analysis indicates that in a reasonable parameter regime the growth rates associated with the three coupled instabilities are of the same order of magnitude, suggesting that they are all important in contributing to the climatic asymmetry. Because of the dependence of the three feedback mechanisms on the existence of a shallow oceanic mixed layer that, in turn, is a result of equatorial easterlies, the existence of the equatorial easterlies is essential for the amplification of the climatic asymmetry.

Next, a hybrid coupled general circulation model is used in which a realistic continental and coastal geometry is presented. The model starts from an ideal symmetric condition forced only by the annual-mean insolation at the top of the atmosphere which is approximately symmetric about the equator. In the presence of the three air–sea interaction mechanisms, the coupled model is capable of reproducing a realistic asymmetric time-mean state in the eastern Pacific and Atlantic. The fundamental cause of the asymmetry in the eastern Pacific is the tilt of the western coast of the Americas, which perturbs SST in the vicinity of the coastal region through a so-called coastal wind-upwelling mechanism. The asymmetry in the Atlantic, on the other hand, results from the land–ocean thermal contrast between the bulge of northwestern Africa and the ocean to the south. The ocean–atmosphere interactions act as an amplifier to amplify the asymmetry set up by the continental or coastal asymmetry. Numerical experiments presented here demonstrate the importance of the geographic asymmetries and the ocean–atmosphere interactions in determining the preferred climatic position for the ITCZ.

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