Search Results

You are looking at 1 - 10 of 30 items for

  • Author or Editor: Chia Chou x
  • All content x
Clear All Modify Search
Chia Chou

Abstract

The anomalous low-level anticyclone over the western North Pacific presents a link between El Niño and the western North Pacific–East Asian climate. During a La Niña event, however, the low-level wind anomalies over the western North Pacific reverse sign. The low-level wind anomalies move eastward from South Asia and are established over the western North Pacific in the fall of the El Niño (La Niña) developing year. The anomalous low-level anticyclone associated with El Niño is a response to suppressed convection. This suppressed convection is mainly induced by a cooling tendency associated with the vertical average of the anomalous horizontal advection of moist static energy that is defined as the mechanism of the horizontal advection of moist static energy. The El Niño–related sea surface temperature (SST) anomalies, over the eastern Pacific in particular, create negative meridional gradients of temperature and moisture over the western North Pacific. When the winter monsoon starts to dominate the Asian region, the northerly component of the winter monsoon transports low moist static energy air into South Asia that suppresses convection and induces the anomalous low-level anticyclone. Associated with the mean state of temperature and moisture, the meridional components of the anomalous low-level anticyclone transports cold and dry air to the southward branch and warm and moist air to the northward branch of the anomalous low-level anticyclone. This cooling–warming pattern of the anomalous horizontal advections of moist static energy tends to move the anomalous low-level anticyclone eastward. The mechanism of the horizontal advection of moist static energy implies a phase-locking behavior of the anomalous low-level anticyclone with the seasonal cycle of the mean state over the western North Pacific–East Asian region.

Full access
Chia Chou and Chia-Wei Lan

Abstract

The annual range of precipitation, which is the difference between maximum and minimum precipitation within a year, is examined in climate model simulations under global warming. For global averages, the annual range of precipitation tends to increase as the globe warms. On a regional basis, this enhancement is found over most areas of the world, except for the bands along 30°S and 30°N. The enhancement in the annual range of precipitation is mainly associated with larger upward trends of maximum precipitation and smaller upward trends or downward trends of minimum precipitation. Based on the moisture budget analysis, the dominant mechanism is vertical moisture advection, both on a global average and on a regional scale. The vertical moisture advection, moisture convergence induced by vertical motion, includes the thermodynamic component, which is associated with increased water vapor, and the dynamic component, which is associated with changes in circulation. Generally, the thermodynamic component enhances the annual range of precipitation, while the dynamic component tends to reduce it. Evaporation has a positive contribution to both maximum and minimum precipitation, but very little to the annual range of precipitation. Even though evaporation and horizontal moisture advection are small for a global average, they could be important on a regional basis.

Full access
Guanghua Chen and Chia Chou

Abstract

A composite study is performed to examine the differences in equatorial wave behaviors and large-scale background patterns during tropical cyclone (TC) genesis. After removing TC contamination, Madden–Julian oscillation (MJO), equatorial Rossby (ER) wave, mixed Rossby–gravity (MRG) wave, and tropical depression (TD)-type disturbance (jointly referred to as the MT wave) are quantified to evaluate the attribution of TC geneses. Given that TC geneses are attributed to a single wave or multiple waves, the eight categories are specified based on the moderate thresholds. The TC geneses related to multiple waves are roughly twice as many as those related to a single wave. The MT wave alone accounts for a minor proportion of TC geneses without collaboration with other larger-scale waves. The mean TC genesis location related to ER wave shifts to higher latitudes, and the TC geneses attributed to both of MJO and MT waves are more concentrated at the west. The single-wave categories are characterized by a zonally propagating component with a large spatial scale. In contrast, the joint contribution from more than one wave type favors creating a coherent environment with enhanced low-level cyclonic vorticity, horizontal convergence, or vertical easterly shear in a preferred region. Consequently, the waves have a more robust structure and a more northwestward-propagating component. Correspondingly, the TC geneses in the MJO–ER category tend to occur within a monsoon trough dominated by cyclonic circulation. For the MJO–MT category, the background field exhibits a confluence pattern with a monsoon trough to the west and easterly flows to the east. The collaboration of the ER and MT waves facilitates TC geneses within an easterly environment in the southern flank of the subtropical high.

Full access
Chia Chou and Jien-Yi Tu

Abstract

Similarities and differences between El Niño and global warming are examined in hemispherical and zonal tropical precipitation changes of the ECHAM5/Max Planck Institute Ocean Model (MPI-OM) simulations. Similarities include hemispherical asymmetry of tropical precipitation changes. This precipitation asymmetry varies with season. In the boreal summer and autumn (winter and spring), positive precipitation anomalies are found over the Northern (Southern) Hemisphere and negative precipitation anomalies are found over the Southern (Northern) Hemisphere. This precipitation asymmetry in both the El Niño and global warming cases is associated with the seasonal migration of the Hadley circulation; however, their causes are different. In El Niño, a meridional moisture gradient between convective and subsidence regions is the fundamental basis for inducing the asymmetry. Over the ascending branch of the Hadley circulation, convection is enhanced by less effective static stability. Over the margins of the ascending branch, convection is suppressed by the import of dry air from the descending branch. In global warming, low-level moisture is enhanced significantly due to warmer tropospheric temperatures. This enhances vertical moisture transport over the ascending branch of the Hadley circulation, so convection is strengthened. Over the descending branch, the mean Hadley circulation tends to transport relatively drier air downward, so convection is reduced.

Full access
Chia Chou and J. David Neelin

Abstract

Mechanisms that determine the tropical precipitation anomalies under global warming are examined in an intermediate atmospheric model coupled with a simple land surface and a mixed layer ocean. To compensate for the warm tropospheric temperature, atmospheric boundary layer (ABL) moisture must increase to maintain positive convective available potential energy (CAPE) in convective regions. In nonconvective regions, ABL moisture is controlled by different balances and does not increase as much, creating a spatial gradient of ABL moisture anomalies. Associated with this spatial pattern of the ABL moisture anomalies are two main mechanisms responsible for the anomalous tropical precipitation. In the “upped-ante mechanism,” increases in ABL moisture are opposed by imported dry air wherever inflow from nonconvective regions over margins of convective regions occurs. The ABL moisture is not enough to meet the higher “convective ante” induced by the warmer tropospheric temperature, so precipitation is decreased. In the “anomalous gross moist stability mechanism,” gross moist stability is reduced due to increased ABL moisture. As a result, convection is enhanced and precipitation becomes heavier over convective regions. While the upped-ante mechanism induces negative precipitation anomalies over the margins of convective regions, the anomalous gross moist stability mechanism induces positive precipitation anomalies within convective regions. The importance of variation in gross moist stability, which is likely to differ among climate models, is suggested as a potential factor causing discrepancies in the predicted regional tropical precipitation changes.

Full access
Chia Chou and Yu-Chien Hsueh

Abstract

Mechanisms of northward-propagating intraseasonal oscillations (ISOs) over the Indian Ocean (IO) and the western North Pacific (WNP) are examined for the possibility of their existence in observations. They include the following: 1) the vorticity advection effect, which is associated with the advection of anomalous baroclinic vorticity by mean baroclinic meridional winds; 2) the vertical wind shear effect, which is the vertical advection associated with the meridional gradient of baroclinic divergence and mean easterly vertical wind shear; 3) the moisture advection effect induced by mean flow; and 4) the air–sea interaction via surface latent heat flux. Because of differences in mean state, the influence of each mechanism on the northward-propagating ISOs is different between the IO and the WNP. The vorticity advection effect is consistently found over both the IO and the WNP, while the air–sea interaction has different impacts on the northward-propagating ISOs over the IO and the WNP. The vertical wind shear effect and the moisture advection effect are relatively important over the IO but not over the WNP. Processes to determine changes in SST are also different between the IO and the WNP. Over the IO, SST is mainly associated with surface solar radiation. Wind-stirring effects, surface latent heat flux, and subsurface water entrainment are secondary. Over the WNP, wind-stirring effects become important, but surface solar radiation is secondary.

Full access
Chia Chou and Chao-An Chen

Abstract

Anthropogenic forcings, such as greenhouse gases and aerosols, are starting to show their influence on the climate, as evidenced by a global warming trend observed in the past century. The weakening of tropical circulation, a consequence of global warming, has also been found in observations and in twenty-first-century climate model simulations. It is a common belief that this weakening of tropical circulation is associated with the fact that global-mean precipitation increases more slowly than water vapor. Here, a new mechanism is proposed for this robust change, which is determined by atmospheric stability associated with the depth of convection. Convection tends to extend higher in a warmer climate because of an uplifting of the tropopause. The higher the convection, the more stable the atmosphere. This leads to a weakening of tropical circulation.

Full access
Chia Chou and J. David Neelin

Abstract

Mechanisms determining the poleward extent of summer monsoon convergence zones for North America, Asia, and Africa are examined in an intermediate atmospheric model coupled with a simple land model and a mixed layer ocean. Observations show that thermodynamical factors associated with the net heat flux into the atmospheric column provide favorable conditions for the monsoon convergence zone to extend farther poleward than actually occurs. To understand the discrepancy, a series of experiments are designed to test the importance of mechanisms previously examined in the South American case by the authors, namely, soil moisture, ventilation, and the interactive Rodwell–Hoskins mechanism. The latter refers to the interaction between baroclinic Rossby wave dynamics and convective heating. In North America, experiments suggest that ventilation by both temperature and moisture advection is a leading effect. The interactive Rodwell–Hoskins mechanism tends to favor east coast rainfall and west coast dryness. In Asia, ventilation by moisture advection is particularly important and the interactive Rodwell–Hoskins mechanism tends to favor interior arid regions and east coast precipitation. Overall, these dynamical factors are crucial in setting the poleward extent of the convergence zone over North America and Asia. Africa differs from the other continents because of the high surface albedo over much of northern Africa. Because there is less positive net flux of energy into the atmospheric column, convection is less thermodynamically favored and the dynamical factors, ventilation and the interactive Rodwell–Hoskins mechanism, have a weaker impact on preventing poleward extent of the convergence zone.

Full access
Chia-Chi Wang, Chia Chou, and Wei-Liang Lee

Abstract

The effects of moisture on the intertropical convergence zone (ITCZ) over the eastern Pacific on the synoptic time scale are investigated using an intermediate complexity atmospheric circulation model, the quasi-equilibrium tropical circulation model (QTCM1), on an aquaplanet.

The dry simulation shows results consistent with those of simple dynamic models, except that a slightly stronger heating rate is needed owing to different model designs. In the moist simulations, the most important result is the formation of a tail southwest of a vortex during and after the ITCZ breakdown. This tail may extend zonally more than 60° longitude and last for more than two weeks in an idealized simulation. In the eastern North Pacific, this phenomenon is often observed in cases that involve easterly waves. In a sense, the formation of the tail suggests a possible mechanism that forms an ITCZ efficiently.

This study shows that the surface convergent flow induced by a disturbance initializes a positive wind–evaporation feedback that forms the tail. In the tail, the most important energy source is surface evaporation, and the latent heat is nicely balanced by an adiabatic cooling of the ascending motion. In other words, the energy is redistributed vertically by vertical energy convergence.

The lifespan of the tail is controlled by the propagation of tropical waves that modify the surface wind pattern, leading to a decrease in surface wind speed and corresponding surface fluxes. It may explain the absence of the tail in some of the events in the real atmosphere.

Full access
Chia-Chi Wang, Wei-Liang Lee, and Chia Chou

ABSTRACT

Aerosols are one of the key factors influencing the hydrological cycle and radiation balance of the climate system. Although most aerosols deposit near their sources, the induced cooling effect is on a global scale and can influence the tropical atmosphere through slow processes, such as air–sea interactions. This study analyzes several simulations of fully coupled atmosphere–ocean climate models under the influence of anthropogenic aerosols, with the concentrations of greenhouse gases kept constant. In the cooling simulations, precipitation is reduced in deep convective areas but increased around the edges of convective areas, which is opposite to the “rich-get-richer” phenomenon in global warming scenarios in the first-order approximation. Tropical convection is intensified with a shallower depth, and tropical circulations are enhanced. The anomalous gross moist stability (M′) mechanism and the upped-ante mechanism can be used to explain the dynamic and thermodynamic processes in the changes in tropical precipitation and convection. There is a northward cross-equatorial energy transport due to the cooler Northern Hemisphere in most of the simulations, together with the southward shift of the intertropical convergence zone (ITCZ) and the enhancement of the Hadley circulation. The enhancement of the Hadley circulation is more consistent between models than the changes of the Walker circulation. The change in the Hadley circulation is not as negligible as in the warming cases in previous studies, which supports the consistency of the ITCZ shift in cooling simulations.

Full access