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Yuwei Wang and Yi Huang

Abstract

Climate model comparisons show that there is considerable uncertainty in the atmospheric temperature response to CO2 perturbation. The uncertainty results from both the rapid adjustment that occurs before SST changes and the slow feedbacks that occur after SST changes. The analysis in this paper focuses on the rapid adjustment. We use a novel method to decompose the temperature change in AMIP-type climate simulation in order to understand the adjustment at the process level. We isolate the effects of different processes, including radiation, convection, and large-scale circulation in the temperature adjustment, through a set of numerical experiments using a hierarchy of climate models. We find that radiative adjustment triggers and largely controls the zonal mean atmospheric temperature response pattern. This pattern is characterized by stratospheric cooling, lower-tropospheric warming, and a warming center near the tropical tropopause. In contrast to conventional views, the warming center near the tropopause is found to be critically dependent on the shortwave absorption of CO2. The dynamical processes largely counteract the effect of the radiative process that increases the vertical temperature gradient in the free troposphere. The effect of local convection is to move atmospheric energy vertically, which cools the lower troposphere and warms the upper troposphere. The adjustment due to large-scale circulation further redistributes energy along the isentropic surfaces across the latitudes, which cools the low-latitude lower troposphere and warms the midlatitude upper troposphere and stratosphere. Our results highlight the importance of the radiative adjustment in the overall adjustment and provide a potential method to understand the spread in the models.

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Yi-Hui Wang and Gudrun Magnusdottir

Abstract

An objective analysis of tropospheric anticyclonic- and cyclonic-breaking Rossby waves is performed for the Southern Hemisphere in austral summer (December–February) of 1979–2009. The climatology of both anticyclonic and cyclonic Rossby wave breaking (RWB) frequency is presented. The frequency of anticyclonic RWB is highest in an extended region of the Eastern Hemisphere on the anticyclonic side of the jet, while that of cyclonic RWB is highest on the cyclonic side of the jet. A composite analysis of anticyclonic and cyclonic RWB shows how they contribute to a positive and negative southern annual mode (SAM) index, respectively. The time series of austral summer anticyclonic RWB occurrence has a trend that closely matches the trend in the SAM index.

Regions of RWB that are significantly correlated with the SAM index are objectively determined. Even though several such regions are identified, only two regions (anticyclonic and cyclonic) covering 17% of the area of the hemisphere are required in a linear regression model of the SAM index. The anticyclonic RWB region is zonally extended at 45°S and explains 78% of the variability of the summer-mean SAM index. The cyclonic region is located at high latitudes somewhat decoupled from the jet, in the longitudinal sector of the Indian Ocean. On synoptic time scales, transitions of the SAM index respond to RWB without time lag.

ENSO cycles present an interesting zonal asymmetry to the distribution of Southern Hemispheric RWB in the central Pacific. Anticyclonic RWB is increased in the tropical/subtropical central Pacific during La Niña compared to El Niño. This increase is related to the strong local decrease in zonal wind. At the same time, anticyclonic RWB outside the central Pacific is increased in frequency poleward and decreased in frequency equatorward of 42°S, corresponding to a positive SAM index.

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Yi-Hui Wang and Gudrun Magnusdottir

Abstract

Several studies have found an eastward shift in the northern node of the North Atlantic Oscillation (NAO) during the winters of 1978–97 compared to 1958–77. This study focuses on the connection between this shift of the northern node of the NAO and Rossby wave breaking (RWB) for the period 1958–97. It is found that the region of frequent cyclonic RWB underwent a northeastward shift at high latitudes in the latter 20-yr period. On a year-to-year basis, the cyclonic RWB region moves along a southwest–northeast (SW–NE)-directed axis. Both latitude and longitude of the winter maximum frequency of cyclonic RWB occurrence are positively correlated with the NAO index.

To investigate the role of location of cyclonic RWB in influencing the NAO pattern, the geographical location of frequent cyclonic RWB is divided into two subdomains located along the SW–NE axis, to the south (SW domain) and east (NE domain) of Greenland. Two composites are assembled as one cyclonic RWB occurrence is detected in one of the two subdomains in 6-hourly instantaneous data. The forcing of the mean flow due to cyclonic RWB within individual subdomains is found to be locally restricted to where the breaking occurs, which is usually near the jet exit region and far removed from the jet core. The difference in the jet between the NE and SW composites resembles the difference in the mean jet between the 1978–97 and 1958–77 periods, which suggests that the change in cyclonic RWB occurrence in the two subdomains is associated with the wobbling of the jet on the decadal time scale.

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Shao-Yi Lee and Chien Wang

Abstract

Previous studies on the response of the South Asian summer monsoon to the direct radiative forcing caused by anthropogenic absorbing aerosols have emphasized the role of premonsoonal aerosol forcing. This study examines the roles of aerosol forcing in both pre- and postonset periods using the Community Earth System Model, version 1.0.4, with the Community Atmosphere Model, version 4. Simulations were perturbed by model-derived radiative forcing applied (i) only during the premonsoonal period (May–June), (ii) only during the monsoonal period (July–August), and (iii) throughout both periods. Soil water storage is found to retain the effects of premonsoonal forcing into succeeding months, resulting in monsoonal central India drying. Monsoonal forcing is found to dry all of India through local responses. Large-scale responses, such as the meridional rotation of monsoon jet during June and its weakening during July–August, are significant only when aerosol forcing is present throughout both premonsoonal and monsoonal periods. Monsoon responses to premonsoonal forcing by the model-derived “realistic” distribution versus a uniform wide-area distribution were compared. Both simulations exhibit central India drying in June. June precipitation over northwestern India (increase) and southwestern India (decrease) is significantly changed under realistic but not under wide-area forcing. Finally, the same aerosol forcing is found to dry or moisten the July–August period following the warm or cool phase of the simulations’ ENSO-like internal variability. The selection of years used for analysis may affect the precipitation response obtained, but the overall effect seems to be an increase in rainfall variance over northwest and southwest India.

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Lei Wang and Jin-Yi Yu

Abstract

The tropospheric biennial oscillation (TBO) is conventionally considered to involve transitions between the Indian and Australian summer monsoons and the interactions between these two monsoons and the underlying Indo-Pacific Oceans. Here it is shown that, since the early 1990s, the TBO has evolved to mainly involve the transitions between the western North Pacific (WNP) and Australian monsoons. In this framework, the WNP monsoon replaces the Indian monsoon as the active Northern Hemisphere TBO monsoon center during recent decades. This change is found to be caused by stronger Pacific–Atlantic coupling and an increased influence of the tropical Atlantic Ocean on the Indian and WNP monsoons. The increased Atlantic Ocean influence damps the Pacific Ocean influence on the Indian summer monsoon (leading to a decrease in its variability) but amplifies the Pacific Ocean influence on the WNP summer monsoon (leading to an increase in its variability). These results suggest that the Pacific–Atlantic interactions have become more important to the TBO dynamics during recent decades.

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Yi Zhang and Wei-Chyung Wang

Abstract

Two 100-yr equilibrium simulations from the NCAR Community Climate Model coupled to a nondynamic slab ocean are used to investigate the activity of northern winter extratropical cyclones and anticyclones under a greenhouse warming scenario. The first simulation uses the 1990 observed CO2, CH4, N2O, CFC-11, and CFC-12 concentrations, and the second adopts the year 2050 concentrations according to the Intergovernmental Panel on Climate Change business-as-usual scenario. Variables that describe the characteristic properties of the cyclone-scale eddies, such as surface cyclone and anticyclone frequency and the bandpassed root-mean-square of 500-hPa geopotential height, along with the Eady growth rate maximum, form a framework for the analysis of the cyclone and anticyclone activity.

Objective criteria are developed for identifying cyclone and anticyclone occurrences based on the 1000-hPa geopotential height and vorticity fields and tested using ECMWF analyses. The potential changes of the eddy activity under the greenhouse warming climate are then examined. Results indicate that the activity of cyclone-scale eddies decreases under the greenhouse warming scenario. This is not only reflected in the surface cyclone and anticyclone frequency and in the bandpassed rms of 500-hPa geopotential height, but is also discerned from the Eady growth rate maximum. Based on the analysis, three different physical mechanisms responsible for the decreased eddy activity are discussed: 1) a decrease of the extratropical meridional temperature gradient from the surface to the midtroposphere, 2) a reduction in the land–sea thermal contrast in the east coastal regions of the Asian and North American continents, and 3) an increase in the eddy meridional latent heat fluxes. Uncertainties in the results related to the limitations of the model and the model equilibrium simulations are discussed.

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Yi-Leng Chen and Jian-Jian Wang

Abstract

The effects of precipitation on the surface temperature and airflow over the island of Hawaii, which are not considered in previous studies, are presented. It is found that clouds and rains can modify the surface thermal fields and result in changes in the intensity of diurnal circulations and the timing of wind shifts from downslope (upslope) to upslope (downslope) flow at the surface in the early morning (late afternoon). The onset of upslope (downslope) flow at the surface is closely related to the virtual temperature anomalies on the slope surface from the adjacent environment for both the rain and dry cases.

Prior to sunrise, the rain cases feature higher surface temperatures in contrast to the dry cases because of a more extensive cloud cover, which reduces the longwave radiation, and the precipitating downdrafts, which bring the warmer air above the nocturnal inversion to the surface. Hence, at the surface a weaker (stronger) downslope flow is observed for the rain (dry) cases, which is consistent with warmer (colder) temperatures on the slope surface. After sunrise, because of reduced insulation by clouds, evaporative cooling of raindrops, and slower heating of the wet surface, the rain cases have a slower surface temperature increase than the dry cases. For the rain cases, the latest turning from downslope to upslope flow at the surface occurs in the Hilo area where the total rainfall is the largest. For the dry cases, the latest upslope flow onset is at the eastern tip of the island where the surface temperature remains colder than the environment after sunrise.

In the afternoon, the extensive cloud cover, the evaporative cooling of rain showers, and moist soil conditions contribute to a lower surface temperature and result in the weaker upslope flow at the surface for the rain cases than for the dry cases. During the evening hours, the surface temperature decrease is slower for the rain cases than for the dry cases because of a reduction of longwave radiation heat loss due to a more extensive cloud cover. For the rain cases, the evaporative cooling and precipitation downdrafts cause a low surface temperature and the early downslope flow onset at the surface in the Hilo area, whereas on the upper slope, the orographic clouds reduce the outgoing longwave radiation and delay the turning from the upslope to downslope flow.

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Zhifang Xu, Yi Wang, and Guangzhou Fan

Abstract

The relatively smooth terrain embedded in the numerical model creates an elevation difference against the actual terrain, which in turn makes the quality control of 2-m temperature difficult when forecast or analysis fields are utilized in the process. In this paper, a two-stage quality control method is proposed to address the quality control of 2-m temperature, using biweight means and a progressive EOF analysis. The study is made to improve the quality control of the observed 2-m temperature collected by China and its neighboring areas, based on the 6-h T639 analysis from December 2009 to February 2010. Results show that the proposed two-stage quality control method can secure the needed quality control better, compared with a regular EOF quality control process. The new method is, in particular, able to remove the data that are dotted with consecutive errors but showing small fluctuations. Meanwhile, compared with the lapse rate of temperature method, the biweight mean method is able to remove the systematic bias generated by the model. It turns out that such methods make the distributions of observation increments (the difference between observation and background) more Gaussian-like, which ensures the data quality after the quality control.

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Jian-Jian Wang and Yi-Leng Chen

Abstract

A case study of trade-wind rainbands observed on 22 August 1990 during the Hawaiian Rainband Project is presented. It shows that the interaction between the morning rainbands and the island-induced airflow is important for the evolution of the rainbands. In the early morning of 22 August, there are two convective periods, 0400–0600 and 0700–0900 HST (Hawaii standard time), on the windward side of the island of Hawaii. For both periods, preexisting rain cells are observed in the trade-wind flow at least 40 km upstream of the island and move westward toward the island.

At night and in the early morning, the offshore flow opposes the trade winds resulting in a convergent region over the area immediately upstream of the island. As the first group of rain cells (0400–0600 HST) moves toward the island, the low-level convergent airflow provides a favorable kinematic background for the enhancement of the coming rain cells. These rain cells merge in the convergent zone and become a well-defined rainband. However, after the first rainband meets the offshore flow, the cool air feeds into the lowest levels of the rainband. This is an unfavorable thermal condition for the rainband and is thus partly responsible for the decay of the first rainband over the windward lowlands. After the arrival of the first rainband, the depth of the offshore flow at Hilo increases from about 250 m to over 500 m. Its horizontal extent also extends from approximately 10 km to more than 20 km offshore.

The second group of rain cells (0700–0900 HST) also becomes a well-defined rainband as it moves over the convergent zone. Interacting with a deep and extensive offshore flow resulting from precipitation effects from the first rainband, the rain cells associated with the second rainband are much deeper and stronger than the first rainband. The second rainband moves toward the island during the morning transition, during which the offshore flow retreats and onshore flow begins. After the onset of the onshore flow, the low-level airflow in the Hilo Bay region diverges and splits around the island. This provides an unfavorable dynamic condition for the maintenance of the rainband. Therefore, the second rainband weakens. It dissipates and only reaches the eastern tip of the island.

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Yi-Hui Wang and W. Timothy Liu

Abstract

This study investigates the regional atmospheric response to the Kuroshio Extension (KE) using a combination of multiple satellite observations and reanalysis data from boreal winter over a period of at least a decade. The goal is to understand the relationship between KE variations and atmospheric responses at low frequencies. A climate index is used to measure the interannual to decadal KE variability, which leaves remarkable imprints on the mesoscale sea surface temperature (SST). Clear spatial coherence between the SST signals and frontal-scale atmospheric variables, including surface wind convergence, vertical velocity, precipitation, and clouds, is identified by linear regression analysis. Consistent with previous studies, the penetrating effect of the KE variability on the free atmosphere is found. The westward tilt of the atmospheric response above the KE near 500 hPa is revealed. The difference in the associations of frontal-scale air temperature and geopotential height with the KE variability between the satellite observations and the reanalysis data suggests an imperfect interpretation of frontal-scale oceanic forcing on the overlying atmosphere in the reanalysis assimilation system.

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