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  • The Relationship Between June Precipitation over Mid-Lower Reaches of the Yangtz...
    Date: 04/02/2011
    Author: ZHAN Yanling and LIN Zhaohui
    Source: The C hinese Meteorological So ciety and Springer-Verlag Berlin Heidelb erg 2011
    size: 1.31 MB
    Using the US Climate Prediction Center (CPC) soil moisture dataset and the observed precipitation over China together with the NCEP/NCAR reanalysis wind and air temperature, the relationship between June precipitation over mid-lower reaches of the Yangtze River basin (MLR–YRB) and spring soil moisture over the East Asian monsoon region was explored, with the signal of the ENSO effect on precipitation removed.
    A significant positive correlation was found between the mean June precipitation and the preceding soil moisture over the MRL–YRB. The possible response mechanism for this relationship was also investigated.It is found that when the soil over the MRL–YRB is wetter (drier)
    than normal in April and May, the air temperature in the lower troposphere over this region in May is lower (higher) than normal, and this
    temperature effect leads to a decrease (increase) in the temperature contrast between the land and the sea.Generally, a decrease (increase) in the land-sea temperature contrast leads to weaker (stronger) East Asian ummer monsoon in June. Southerly (northerly) wind anomalies at 850 hPa then show up in the south of the Yangtze River basin while northerly (southerly) wind anomalies dominate in the north. These anomalies lead to the convergence (divergence) of wind and water vapor and hence gives rise to more (less) precipitation in June over the MLR–YRB.
  • The Philippines–Taiwan Oscillation: Monsoonlike Interannual Oscillation of the...
    Date: 03/01/2012
    Author: Y.-L. CHANG L.-Y. OEY
    Source: American Meteorological Society
    size: 7.72 MB
    Tide gauge and satellite data reveal an interannual oscillation of the ocean’s thermoclines east of the Philippines and Taiwan, forced by a corresponding oscillation in the wind stress curl. This so-called Philippines–Taiwan Oscillation (PTO) is shown to control the interannual variability of the circulation of the subtropical and tropical western North Pacific. The PTO shares some characteristics of known Pacific indices,for example, Nin˜ o-3.4. However, unlike PTO, these other indices explain only portions of the western North Pacific circulation. The reason is because of the nonlinear nature of the forcing in which mesoscale (ocean) eddies play a crucial role. In years of positive PTO, the thermocline east of the Philippines rises while east of Taiwan it deepens. This results in a northward shift of the North Equatorial Current (NEC), increased vertical shear of the Subtropical Countercurrent (STCC)/NEC system, increased eddy activity dominated by warm eddies in the STCC, increased Kuroshio transport off the northeastern coast of Taiwan into the East China Sea, increased westward inflow through Luzon Strait into the South China Sea, and cyclonic circulation and low sea surface height anomalies in the South China Sea. The reverse applies in years of negative PTO.
  • The Impact of Finer-Resolution Air–Sea Coupling on the Intraseasonal Oscillati...
    Date: 05/15/2011
    Source: American Meteorological Society
    size: 5.54 MB
    A newly assembled atmosphere–ocean coupled model, called HadKPP, is described and then used to de-termine the effects of subdaily air–sea coupling and fine near-surface ocean vertical resolution on the rep-resentation of the Northern Hemisphere summer intraseasonal illation. HadKPP comprises the Hadley Centre atmospheric model coupled to theK -Profile Parameterization ocean boundary layer model.
    Four 30-member ensembles were performed that vary in ocean vertical resolution between 1 and 10 m and in coupling frequency between 3 and 24 h. The 10-m, 24-h ensemble exhibited roughly 60% of the observed 30–50-day variability in sea surface temperatures and rainfall and very weak northward propagation. En-hancing only the vertical resolution or only the coupling frequency produced modest improvements in var-iability and just a standing intraseasonal oscillation. Only the 1-m, 3-h configuration generated organized,northward-propagating convection similar to observations. Subdaily su rface forcing produced stronger upper-ocean temperature anomalies in quadrature with anomalous nvection, which likely affected lower-atmospheric stability ahead of the convection, causing propagation. Well-resolved air–sea coupling did not improve the eastward propagation of the boreal summer intraseasonal oscillation in this model.
    Upper-ocean vertical mixing and diurnal variability in coupled models must be improved to accurately resolve and simulate tropical ubseasonal variability. In HadKPP, the mere presence of air–sea coupling was not sufficient to generate an intraseasonal oscillation resembling servations.
  • The Asian Monsoon in the Superparameterized CCSM and Its Relationship to Tropica...
    Date: 03/07/2011
    size: 6.95 MB
    Three general circulation models (GCMs) are used to analyze the impacts of air–sea coupling and super-parameterized (SP) convection on the Asian summer monsoon: Community Climate System Model (CCSM) (coupled, conventional convection), SP Community Atmosphere odel (SP-CAM) (uncoupled, SP con-vection), and SP-CCSM (coupled, SP). In SP-CCSM, coupling improves the basic-state climate relative to SP-CAM and reduces excessive tropical variability in SP-CAM. Adding SP improves tropical variability, the simulation of easterly zonal shear over the Indian and western Pacific Oceans, and increases negative sea surface temperature (SST) biases in that region.
    SP-CCSM is the only model to reasonably simulate the eastward-, westward-, and northward-propagating components of the Asian monsoon. CCSM and SP-CCSM mimic the observed phasing of northward-propagating intraseasonal oscillation (NPISO), SST, precipitation, and surface stress anomalies, while SP-CAM is limited in this regard. SP-CCSM produces a variety of tropical waves with spectral characteristics similar to those in observations. Simulated equatorial Rossby (ER) and mixed Rossby–gravity (MRG) waves may lead to different simulations of the NPISO in each model. Each model exhibits some northward prop-agation for ER waves but only SP-CCSM produces northward-propagating MRG waves, as in observations. The combination of ER and MRG waves over the Indian Ocean influences the spatiotemporal structure of the
    NPISO and contributes to the differences seen in each model.
    The role of ocean coupling must be considered in terms of the time scale of the SST response compared to the time scale of tropical riability. High-frequency disturbances experience coupling via its changes to the basic state, while lower-frequency disturbances may respond directly to SST fluctuations.
  • The impact of d eep convection on the West African summer monsoon climate: a reg...
    Date: 07/01/2011
    Author: M. B. Sylla, F. Giorgi,P. M. Ruti,S. Calmantiand A. Dell’Aquila
    Source: Royal Meteorological Society
    size: 4.26 MB
    The role of the representation of deep convection on key elements of the West African summer monsoon climate is addressed using the egional Climate Model RegCM3. Two simulations in which a scheme of deep convection is activated and then turned off are performed and intercompared. Results show that the presence of deep convective heating along the intertropical convergence zone sustains increased lower-level baroclinicity favoring inte nsification of the jet core and leading to a more realistic African easterly jet. In addition, although the isentropic potential vorticity (IPV) is lower when the convection scheme is switched off, African easterly waves (AEWs) are still generated and propagate westwards but they dissipate around the west coast. Substantial differences b etween the two simulations occur mainly at the 6- to 9-day ime-scale over land, when much weaker activity is simulated in the absence of convection. This indicates that orographic friction and low-level large-scale moisture convergence, gener ating high values of latent heat and IPV,may play the dominant role in the genesis and growth of EWs and that deep convection acts to strengthen the overall wave activity and to favor their west coast development. Copyright c2011 Royal Meteorological Society
  • Impacts of the Tropical Pacific/Indian Oceans on the Seasonal Cycle of the West ...
    Date: 01/19/2011
    Source: American Meteorological Society
    size: 4.3 MB
    century as a result of remote effects of oceanic anomalies amplified by local land–atmosphere interactions. This paper focuses on the impacts of oceanic anomalies upon West African climate and specifically aims to identify those from SST anomalies in the Pacific/Indian Oceans during spring and summer seasons, when they were significant. Idealized sensitivity experiments are performed with four atmospheric general circulation models (AGCMs). The prescribed SST patterns used in the AGCMs are based on the leading mode of covariability between SST anomalies over the Pacific/Indian Oceans and summer rainfall over West Africa. The results show that such oceanic anomalies in the Pacific/Indian Ocean lead to a northward shift of an anomalous dry belt from the Gulf of Guinea to the Sahel as the season advances. In the Sahel, the magnitude of rainfall anomalies is comparable to that obtained by other authors using SST anomalies confined to the proximity of the Atlantic Ocean. The mechanism connecting the Pacific/Indian SST anomalies with West African rainfall has a strong seasonal cycle. In spring (May and June), anomalous subsidence develops over both the Maritime Continent and the equatorial Atlantic in response to the enhanced equatorial heating. Precipitation increases over continental West Africa in association with stronger zonal convergence of moisture. In addition, precipitation decreases over the Gulf of Guinea. During the monsoon peak (July and August), the SST anomalies move westward over the equatorial Pacific and the two regions where subsidence occurred earlier in the seasons merge over West Africa. The monsoon weakens and rainfall decreases over the Sahel, especially in August.
  • Decadal Variability of Asian–Australian Monsoon–ENSO–TBO Relationships
    Date: 09/15/2011
    Source: National Center for Atmospheric Research, National Center for Atmospheric Research
    size: 4.04 MB
    A set of dynamically coupled ocean–atmosphere mechanisms has previously been proposed for the Asia–Pacific tropics to produce a dominant biennial component of interannual variability [the tropospheric biennial oscillation (TBO)]. Namely, a strong Asian–Australian monsoon is often associated with negative SST anomalies in the equatorial eastern Pacific and a negative Indian Ocean dipole in northern fall between the strong Indian monsoon and strong Australian monsoon, and tends to be followed by a weak monsoon and positive SST anomalies in the Pacific the following year and so on. These connections are communicated through the large-scale east–west (Walker) circulation that involves the full depth of the troposphere.
    However, the Asia–Pacific climate system is characterized by intermittent decadal fluctuations whereby the TBO during some time periods is more pronounced than others. Observations and models are analyzed to identify processes that make the system less biennial at certain times due to one or some combination of the following:
    1) increased latitudinal extent of Pacific trade winds and wider cold tongue;
    2) warmer tropical Pacific compared to tropical Indian Ocean that weakens trade winds and reduces coupling strength;
    3) eastward shift of the Walker circulation;
    4) reduced interannual variability of Pacific and/or Indian Ocean SSTs.
    Decadal time-scale SST variability associated with the interdecadal Pacific oscillation (IPO) has been shown to alter the TBO over the Indo-Pacific region by contributing changes in either some or all of the four factors listed above. Analysis of a multicentury control run of the Community Climate System Model, version 4 (CCSM4), shows that this decadal modulation of interannual variability is transferred via the Walker circulation to the Asian–Australian monsoon region, thus affecting the TBO and monsoon–Pacific connections.
    Understanding these processes is important to be able to evaluate decadal predictions and longer-term climate change in the Asia–Pacific region.
  • Persistent Weakening Trend in the Spring Sensible Heat Source over the Tibetan P...
    Date: 11/01/2011
    Source: DUAN ET AL.
    size: 2.38 MB
    Using a dataset extended by the addition of data for 2004–08, this study reexamined the trend in the sensible heating (SH) flux at 73 meteorological stations over the Tibetan Plateau (TP) during 1980–2008 and investigated its impact on monsoon precipitation in the surrounding region. In ontrast to ongoing climate warming, a weakening trend in SH is persistent over most of the plateau, despite a sharp increase in the ground–air temperature difference in 2004–08. The weakening trend in SH over the TP is primarily a response to the spatial nonuniformity of large-scale warming  over the East Asian continent, which is characterized by much greater warming amplitude at mid- and high latitudes than over the tropics and subtropics. Furthermore, the suppressed air pump effect, which is driven by SH over the TP and acts as a strong forcing source, gives rise to reduced precipitation along the southern and eastern slopes of the plateau, and increased rainfall over northeastern India and the Bay of Bengal. No significantly stable correlation exists between the SH source over the TP and the overall trend or interdecadal variability in the East Asian or South Asian summer monsoon.
  • Characteristics of Precipitating Convective Systems in the South Asian Monsoon
    Date: 02/01/2011
    Source: University of Washington
    size: 10.38 MB
    Eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data show how convective systems of different types contribute to precipitation of the South Asian monsoon. The main factor determining the amount of precipitation coming from a specific system is its horizontal size. Convective intensity and/or number of embedded convective cells further enhance its precipitation production. The precipitation of the monsoon is concentrated in three mountainous regions: the Himalayas and coastal ranges
    of western India and Myanmar. Along the western Himalayas, precipitation falls mainly from small, but highly convective systems. Farther east along the foothills, systems are more stratiform. These small and medium systems form during the day, as the monsoon flow is forced upslope. Nighttime cooling leads to downslope flow and triggers medium-sized systems at lower elevations. At the mountainous western coasts of India and Myanmar, small and medium systems are present throughout the day, as an orographic response to the southwesterly flow, with a slight superimposed diurnal cycle. Medium systems are favored over the eastern parts of the Arabian Sea and large systems are favored over the Bay of Bengal when an enhanced midlevel cyclonic circulation occurs over the northern parts of these regions. The systems forming upstream of coastal mountains over the Bay of Bengal are larger than those over the Arabian Sea, probably because of the moister conditions over the bay. The large systems over the bay exhibit a pronounced diurnal cycle, with systems forming near midnight and maximizing in midday.
  • Impacts of Cumulus Convective Parameterization Schemes on Summer Monsoon Precipi...
    Date: 03/19/2011
    Author: YU Entao1,2,3,WANG Huijun1,2, GAO Yongqi1,4, and SUN Jianqi1
    Source: The Chinese Meteorological Society and Springer-Verlag
    size: 5.55 MB
    By using the Betts-Miller-Janji´c, Grell-Devenyi, and Kain-Fritsch cumulus convective parameterization schemes in theWeather Research and Forecasting (WRF) model, long time simulations from 2000 to 2009 are conducted to investigate the impacts of different cumulus convective parameterization schemes on summer monsoon precipitation simulation over China. The results show that all the schemes have the capability to reasonably reproduce the spatial and temporal distributions of summer monsoon precipitation and the corresponding background circulation. The observed north-south shift of monsoon rain belt is also well simulated by the three schemes. Detailed comparison indicates that the Grell-Devenyi scheme gives a better performance than the others. Deficiency in simulated water vapor transport is one possible reason for the precipitation simulation bias.

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