Chia Chou

Evaluating the “Rich-Get-Richer” Mechanism in Tropical Precipitation Change under Global Warming

Abstract Examining tropical regional precipitation anomalies under global warming in 10 coupled global climate models, several mechanisms are consistently found. The tendency of rainfall to increase in convergence zones with large climatological precipitation and to decrease in subsidence regions—the rich-get-richer mechanism—has previously been examined in different approximations by Chou and Neelin, and Held and Soden. The effect of increased moisture transported by the mean circulation (the “direct moisture effect” or “thermodynamic component” in respective terminology) is relatively robust, while dynamic feedback is poorly understood and differs among models. The argument outlined states that the thermodynamic component should be a good approximation for large-scale averages; this is confirmed for averages across convection zones and descent regions, respectively. Within the convergence zones, however, dynamic feedback can substantially increase or decrease precipitation anomalies. Regions of negative...

Mechanisms of Global Warming Impacts on Regional Tropical Precipitation

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.

The abrupt shift of typhoon activity in the vicinity of Taiwan and its association with western North Pacific-East Asian climate change.

Bayesian analysis is applied to detect changepoints in the time series of seasonal typhoon counts in the vicinity of Taiwan. An abrupt shift in the typhoon count series occurs in 2000. On average, 3.3 typhoons per year have been noted before 2000 (1970‐99), with the rate increasing to 5.7 typhoons per year since 2000 (2000‐06). This abrupt change is consistent with a northward shift of the typhoon track over the western North Pacific‐East Asian region and an increase of typhoon frequency over the Taiwan‐East China Sea region. The northward shift of the typhoon track tends to be associated with typhoon-enhancing environmental conditions over the western North Pacific, namely, the weakening of the western North Pacific subtropical high, the strengthening of the Asian summer monsoon trough, and the enhanced positive vorticity anomalies in the lower troposphere. Based on observational analysis and model simulations, warm sea surface temperature anomalies over the equatorial western and central Pacific appear to be a major factor contributing to a northward-shifted typhoon track.

Estimating the Gross Moist Stability of the Tropical Atmosphere

Recent theoretical studies have indicated that large-scale circulation in deep convective regions evolves subject to an overall static stability—termed the gross moist stability—that takes into account both dry static stability and moist convective effects. The gross moist stability has been explicitly defined for a continuously stratified atmosphere under convective quasi-equilibrium constraints. A subsidiary quantity—the gross moisture stratification—measures the overall effectiveness in producing precipitation subject to these quasi-equilibrium constraints. These definitions are relevant in regions that experience deep convection sufficiently often; criteria based on climatological precipitation and maximum level of convection are used to define a domain of applicability. In this paper, 10-yr monthly mean rawinsonde data, and European Centre for Medium-Range Weather Forecasts (ECMWF) and National Meteorological Center (NMC) analyses are used to estimate the magnitude and horizontal distribution of these two quantities in the Tropics within the domain of applicability. The gross moist stability is found to be positive but much smaller than typical dry static stability values. Its magnitude varies modestly from 200 to 800 J kg21 and exhibits relatively little dependence on sea surface temperature (SST). These values correspond, for instance, to a phase speed change from 8 to 16 m s21 for the Madden‐Julian oscillation. The gross moisture stratification is larger and exhibits strong dependence on SST, varying from 1500 to 3500 J kg21 between cold and warm SST regions. A high degree of cancellation between effects of increasing low-level moisture and maximum level of convection, respectively, tends to keep the gross moist stability values relatively constant. Differences among the ECMWF and NMC analysis products and the rawinsonde data affect the estimate, but there is qualitative agreement. It is encouraging that reasonably robust estimates of a small, positive gross moist stability (as the difference between larger dry static stability and gross moisture stratification quantities) can be obtained. This helps justify use of small, constant moist phase speeds in some simple models of tropical circulation, although it also points out inconsistencies in how such models neglect variations in the height of convection.

Mechanisms Limiting the Northward Extent of the Northern Summer Monsoons over North America, Asia, and Africa*

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 dryne...

Mechanisms for Global Warming Impacts on Precipitation Frequency and Intensity

AbstractGlobal warming mechanisms that cause changes in frequency and intensity of precipitation in the tropics are examined in climate model simulations. Under global warming, tropical precipitation tends to be more frequent and intense for heavy precipitation but becomes less frequent and weaker for light precipitation. Changes in precipitation frequency and intensity are both controlled by thermodynamic and dynamic components. The thermodynamic component is induced by changes in atmospheric water vapor, while the dynamic component is associated with changes in vertical motion. A set of equations is derived to estimate both thermodynamic and dynamic contributions to changes in frequency and intensity of precipitation, especially for heavy precipitation. In the thermodynamic contribution, increased water vapor reduces the magnitude of the required vertical motion to generate the same strength of precipitation, so precipitation frequency increases. Increased water vapor also intensifies precipitation due ...

Interannual Variability of the Western North Pacific Summer Monsoon: Differences between ENSO and Non-ENSO Years

Abstract The interannual variability of the western North Pacific (WNP) summer monsoon is examined for the non-ENSO, ENSO developing, and ENSO decaying years, respectively. The ENSO developing (decaying) year is defined as the year before (after) the mature phase of ENSO, and the non-ENSO year is defined as the year that is neither the ENSO developing year nor the ENSO decaying year. A strong (weak) WNP summer monsoon tends to occur during the El Nino (La Nina) developing year and a weak (strong) WNP summer monsoon tends to occur during the El Nino (La Nina) decaying year. In all non-ENSO, ENSO developing, and ENSO decaying years, the strong (weak) WNP summer monsoon is associated with the positive (negative) rainfall anomalies, cold (warm) sea surface temperature anomalies, warm (cold) upper-tropospheric temperature anomalies, low (high) surface pressure anomalies, and a low-level cyclonic (anticyclonic) circulation anomaly over the subtropical WNP. The 850-hPa wave train associated with the WNP and east...

Tropical teleconnection and local response to SST anomalies during the 1997–1998 El Niño

The quasi-equilibrium tropical circulation model (QTCM) is used to examine the response to various sea surface temperature (SST) anomalies in the tropical oceans during the 1997–1998 El Nino. Both local and remote responses are noted. The negative precipitation anomalies to the north and south of the El Nino-Southern Oscillation (ENSO) enhanced precipitation region are largely a response to the warm SST anomalies in the central and eastern Pacific. However, in the western Pacific and maritime continent, reduction of rainfall is mainly caused by local cold SST anomalies. In the winter of the 1997–1998 El Nino, strong warm SST anomalies in the Indian Ocean contributed to the local enhanced rainfall. They affect precipitation anomalies in central, eastern, and southern Africa. The drought in northern South America is clearly a remote response to ENSO warm SST anomalies in the Pacific, while the SST anomalies in the Atlantic also impact the drought. The tropical Pacific cold SST anomalies surrounding the ENSO warm anomalies appear not to be caused by surface flux changes associated with atmospheric teleconnection (in simulations with specified SST in the ENSO warm region and a mixed-layer ocean model elsewhere). Atmospheric circulation tends to spread the warm anomalies, but ocean dynamics appears also to be important for both cold and warm SST anomalies outside the equatorial upwelling region. In both the regional specified SST experiments and mixed-layer experiments, relative subsidence tends to occur within convective zones, and it is generally localized. On the other hand, tropospheric temperature and wind anomalies spread much farther. The typical spatial scale of the remote response in temperature and wind fields tends to be larger than the dominant scale of the precipitation response. The remote response in precipitation anomalies does not appear to be related to temperature and wind anomalies in a simple manner.

Changes in the Annual Range of Precipitation under Global Warming

AbstractThe 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 componen...

Depth of Convection and the Weakening of Tropical Circulation in Global Warming

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-firstcentury 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.