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Wenhong Li and Rong Fu

Abstract

Using 15-yr instantaneous European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA) data, the authors have examined the large-scale atmospheric conditions and the local surface fluxes through the transition periods from the dry to wet seasons over the southern Amazon region (5°–15°S, 45°–75°W). The composite results suggest that the transition can be divided into three phases: initiating, developing, and mature. The initiating phase is dominated by the local buildup of the available potential energy. This begins about 90 days prior to the onset of the wet season by the increase of local land surface fluxes, especially latent heat flux, which increases the available potential energy of the lower troposphere. The cross-equatorial flow and upper-tropospheric circulation remain unchanged from those of the dry season. The developing phase is dominated by the seasonal transition of the large-scale circulation, which accelerates by dynamic feedbacks to an increase of locally thermal-driven rainfall, starting about 45 days before the onset of the wet season. During this stage, the reversal of the low-level, cross-equatorial flow in the western Amazon increases moisture transport from the tropical Atlantic Ocean and leads to net moisture convergence in the southern Amazon region. In the upper troposphere, the divergent kinetic energy begins to be converted into rotational kinetic energy, and geopotential height increases rapidly. These processes lead to the onset of the wet season and the increase of anticyclonic vorticity at the upper troposphere. After onset, the lower-tropospheric potential energy reaches equilibrium, but the conversion from divergent to rotational kinetic energy continues to spin up the upper-tropospheric anticyclonic circulation associated with the Bolivian high until it reaches its full strength.

This analysis suggests that a weaker (stronger) increase of land surface latent (sensible) heat flux during the dry season and the initiating phase tends to delay the large-scale circulation transition over the Amazon. The influence of land surface heat fluxes becomes secondary during the developing and mature phases after the transition of the large-scale circulation begins. A later northerly reversal and/or weaker cross-equatorial flow, a later southerly withdrawal of the upper-tropospheric westerly wind, and a stronger subsidence could delay and prolong the developing phase of the transition and consequently delay the onset of the Amazon wet season.

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Hui Wang and Rong Fu

Abstract

The characteristics of winter monthly mean extratropical circulation associated with El Niño, including precipitation and surface temperature over the United States, are examined for nine El Niño events during 1950–94. Precipitation and surface temperature over the United States, also the 500-mb geopotential height and sea level pressure over the North Pacific and North America, are significantly different between early winter (November and December) and late winter (January to March). The typical El Niño-related U.S. precipitation and surface temperatures identified in many previous studies, as well as the Pacific–North American (PNA) circulation pattern, emerge in January and persist through February and March. The PNA patterns during these late winter months are coupled both with the tropical El Niño sea surface temperature (SST) variation and with the North Pacific SST variation. In contrast, the PNA patterns in the early winter months correlate only with the North Pacific SST. The tendency for the PNA pattern to occur during El Niño years is much less in early winter months than in late winter months. An ensemble analysis of 12 45-yr (1950–94) integrations of the National Center for Atmospheric Research Community Climate Model forced by the observed time-varying SST shows that the model 500-mb heights display a PNA-like pattern in both early and late winters of El Niño. The ensemble model response to the El Niño SST is thus unable to reproduce the observed differences in the extratropical atmospheric circulation between early and late winter months.

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Wenhong Li and Rong Fu

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Using 15-yr data from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-15), the authors found that rapid southeastward expansion of the rainy area from the western Amazon to southeastern Brazil is a result of midlatitude cold air intrusions. During austral spring, as the large-scale thermodynamic structure over Amazonia becomes destabilized, the incursions of extratropical cold air can trigger intense rainfall along the leading edge of northwest–southeast-oriented cold fronts east of the Andes. As these fronts penetrate into Amazonia, the northerly or northwesterly wind transports warm, moist air from the western Amazon to southeast Brazil. Moisture convergence consequently intensifies, resulting in northwest–southeast-elongated rainy areas. The latter contribute to the observed rapid, southeastward expansion of rainy areas shown in rainfall climatology during austral spring.

The authors’ analysis suggests that cold air intrusions during austral spring collectively assist the transformation of large-scale thermodynamic and dynamic environments to those favorable for the wet season onsets. Each time the cold fronts pass by, they tend to increase the atmospheric humidity and the buoyancy of the lower troposphere, which destabilizes the atmosphere. In the upper troposphere, the cold air intrusions supply kinetic energy for the development of anticyclonic flow. Cold air intrusions in the transitional season are not different from those occurring immediately before the wet season onsets except that the latter occurs under a more humid and unstable atmospheric condition. Thus, cold air intrusions can trigger the wet season onsets only when atmospheric and land surface conditions are “ready” for the onset.

Comparisons among early, normal, and late onsets on an interannual scale further suggest that more frequent and stronger cold air intrusions trigger the early onsets of wet seasons given suitable large-scale thermodynamic conditions. Likewise, less frequent and weaker cold air intrusions could delay the wet season onset even though the large-scale thermodynamic conditions appear to be favorable. Occasionally, strong unstable atmospheric thermodynamic conditions and northerly reversal of cross-equatorial flow can lead to wet season onsets without cold air intrusions. In such cases, enhanced precipitation is centered over central and eastern Amazon, and rainfall increases more gradually compared to the onset with cold air intrusions.

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Hui Wang and Rong Fu

Abstract

This study examines the interannual variability of winter upper-troposphere water vapor over the Northern Hemisphere using the National Aeronautics and Space Administration Water Vapor Project, the International Satellite Cloud Climatology Project data, and the European Centre for Medium-Range Weather Forecasting reanalysis. The El Niño–Southern Oscillation related tropical sea surface temperature (SST) anomalies dominate the upper-troposphere water vapor anomalies south of the climatological jet. The anomalies of baroclinic instability in the storm track regions, which relate to the Pacific–North American and the North Atlantic oscillation patterns, dominate those north of the climatological jet. The upper-troposphere water vapor increases in the eastern tropical Pacific, the Gulf of Mexico, and some areas of the North Atlantic with warmer tropical SST. It decreases in the subtropical and extratropical northeastern Pacific. Deep convection and vertical moisture fluxes dominate these changes. To the north of the climatological jet, stronger upper-level cyclonic flow dries the upper troposphere when the baroclinicity of the storm tracks is enhanced. Both vertical and meridional moisture transport contribute to these water vapor anomalies in the midlatitudes. High clouds, as a possible source/sink of water vapor, respond to the tropical SST anomalies and extratropical circulation in a pattern similar to the upper-troposphere water vapor, and they consequently positively correlate to the latter. In the Tropics and extratropics where high clouds are relatively abundant, water vapor concentration increases with temperature. Thus, the increase of evaporation or sublimation of high clouds probably contributes to the observed moistening of the upper troposphere, in addition to enhanced vapor transport. Conversely, in the subtropics where high clouds appear infrequently, water vapor concentration decreases with temperature, suggesting that the downward advection of drier air associated with subsidence dominates the drying of the upper troposphere.

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Hui Wang and Rong Fu

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By analyzing the 15-yr (1979–93) reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF), it has been found that the seasonal and synoptic time-scale variations of the South American low-level jets (LLJs) are largely controlled by an upper-level trough and associated low-level zonal flow, rather than by horizontal temperature gradients along the slope of the Andes. The northerly LLJs are maintained by strong zonal pressure gradients caused by the upstream trough and westerly flow crossing the Andes through lee cyclogenesis. The process involves both baroclinic development of the upper-level trough and mechanical deflection of the westerly flow by the Andes. When an anticyclonic circulation replaces the trough and westerly flow over the eastern South Pacific, the northerly LLJs tend to diminish or reverse into southerly LLJs. The dependence of the LLJs upon the upstream wind pattern helps to explain how the seasonal variation of the South American LLJs is related to the seasonal changes of the large-scale circulation pattern over the eastern South Pacific. On synoptic time scales, the relation between LLJs and cross-Andes zonal flow is strong in austral winter, spring, and fall. This relation weakens somewhat in summer, when Amazon convection is strongest. The analysis also demonstrated strong connections of the LLJs with South American precipitation, intensity of the South Atlantic convergence zone (SACZ), and low-level cross-equatorial flow. A method for up to 5-day forecasts of the LLJs based on 700-hPa zonal winds over the subtropical eastern South Pacific was also introduced. A cross validation indicates a certain degree of predictability for South American LLJs. The results further suggest that the upstream flow pattern over the South Pacific should be closely monitored to determine the variability of the South American LLJs.

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Hui Wang and Rong Fu

Abstract

The relationship between South American precipitation and cross-equatorial flow over the western Amazon is examined using the 15-yr (1979–93) European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. A meridional wind index, the V index, is constructed to represent the variability of the cross-equatorial flow, based on area-averaged (5°S–5°N, 65°–75°W) daily 925-hPa meridional winds. The V index displays large submonthly, seasonal, and interannual variabilities, and correlates well with precipitation over South America. Two circulation regimes are identified, that is, a southerly regime and a northerly regime. Linear regression shows that when the V index is southerly, precipitation is mainly located to the north of the equator. When the V index is northerly, precipitation shifts toward the Amazon basin and subtropical South America. The V index is predominately southerly in austral winter and northerly in summer. The onset (demise) of the Amazon rainy season is led by an increase in the frequency of the northerly (southerly) V index. The relation between the V index and upper-level circulation is consistent with the seasonal cycle of the South American monsoon circulation. Hence, the V index is a good indicator for precipitation change over tropical and subtropical South America.

The singular value decomposition (SVD) analysis suggests that the V-index-related variation represents 92% of the total covariance between the low-level meridional wind and precipitation over South America. It also represents 37% of the seasonal variance of precipitation. On the seasonal timescale, the V index appears to correlate well with the meridional migration of the Hadley cell globally. On submonthly timescales, the change of V index is not correlated with the meridional wind over the adjacent oceans except in the South Atlantic convergence zone, suggesting a control by more localized and higher-frequency dynamic processes. The SVD analysis also suggests that during spring and fall precipitation changes over the equatorial eastern Amazon are associated with the seasonal variations of sea surface temperature in the Pacific and the Atlantic Oceans.

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Hui Wang and Rong Fu

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Using outgoing longwave radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) daily rain-rate data, systematic changes in intensity and location of the Atlantic intertropical convergence zone (ITCZ) were detected along the equator during boreal spring. It is found that the changes in convection over the tropical Atlantic may be induced by deep convection in equatorial South America. Lagged regression analyses demonstrate that the anomalies of convection developed over the land propagate eastward across the Atlantic and then into Africa. The eastward-propagating disturbances appear to be convectively coupled Kelvin waves with a period of 6–7.5 days and a phase speed of around 15 m s−1. These waves modulate the intensity and location of the convection in the tropical Atlantic and result in a zonal variation of the Atlantic ITCZ on synoptic time scales. The convectively coupled Kelvin wave has substantial signals in both the lower and upper troposphere. Both a reanalysis dataset and the Quick Scatterometer (QuikSCAT) ocean surface wind are used to characterize the Kelvin wave. This study suggests that synoptic-scale variation of the Atlantic ITCZ may be linked to precipitation anomalies in South America through the convectively coupled Kelvin wave. The results imply that the changes of Amazon convection could contribute to the large variability of the tropical Atlantic ITCZ observed during boreal spring.

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Brian J. Soden and Rong Fu

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This paper combines satellite measurements of the upwelling 6.7-μm radiance from TOVS with cloud-property information from ISCCP and outgoing longwave radiative fluxes from ERBE to analyze the climatological interactions between deep convection, upper-tropospheric humidity, and atmospheric greenhouse trapping. The satellite instruments provide unmatched spatial and temporal coverage, enabling detailed examination of regional, seasonal, and interannual variations between these quantities. The present analysis demonstrates that enhanced tropical convection is associated with increased upper-tropospheric relative humidity. The positive relationship between deep convection and upper-tropospheric humidity is observed for both regional and temporal variations, and is also demonstrated to occur over a wide range of space and time scales. Analysis of ERBE outgoing longwave radiation measurements indicates that regions or periods of increased upper-tropospheric moisture are strongly correlated with an enhanced greenhouse trapping, although the effects of lower-tropospheric moisture and temperature lapse rate are also observed to be important. The combined results for the Tropics provide a picture consistent with a positive interrelationship between deep convection, upper-tropospheric humidity, and the greenhouse effect. In extratropical regions, temporal variations in upper-tropospheric humidity exhibit little relationship to variations in deep convection, suggesting the importance of other dynamical processes in determining changes in upper-tropospheric moisture for this region. Comparison of the observed relationships between convection, upper-tropospheric moisture, and greenhouse trapping with climate model simulations indicates that the Geophysical Fluid Dynamics Laboratory (GFDL) GCM is qualitatively successful in capturing the observed relationship between these quantities. This evidence supports the ability of the GFDL GCM to predict upper-tropospheric water vapor feedback, despite the model's relatively simplified treatment of moist convective processes.

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Lan Dai, Jonathon S. Wright, and Rong Fu

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We investigate the physical processes behind summer drought in North China by evaluating moisture and energy budget diagnostics and linking them to anomalous large-scale circulation patterns. Moisture budget analysis reveals that summer drought in North China was caused dynamically by reduced vertical moisture advection due to anomalous subsidence and reduced horizontal moisture advection due to anomalous northeasterly winds. Energy budget analysis shows that reduced latent heating was balanced dynamically by decreased dry static energy (DSE) divergence in the middle-to-upper troposphere. Linking these results to previous work, we suggest that summer drought in North China was predicated on co-occurrence of the positive phases of the Eurasian (EU) and Pacific–Japan (PJ) teleconnection patterns, potentially modulated by the circumglobal teleconnection (CGT). In the typical case, the negative phase of the CGT intensified the positive EU-related upper-level cyclone. Resulting upper-level cooling and positive surface feedback imposed a cold-core surface anticyclone that weakened with height. By contrast, when the positive phase of the CGT occurred in tandem with the positive EU and PJ patterns, the anticyclone had a warm core and intensified with height. The two cases were unified by strong subsidence but exhibited opposite meridional advection anomalies. In the cold-core cases, meridional moisture inflow was reduced but meridional DSE export was enhanced, further limiting precipitation while maintaining negative thermal anomalies. In the warm-core case, which only occurred once, enhanced meridional inflow of water vapor supplied moisture for sporadic precipitation while reduced meridional DSE export helped to maintain strong static stability.

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Yizhou Zhuang, Amir Erfanian, and Rong Fu

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Although the influence of sea surface temperature (SST) forcing and large-scale teleconnection on summer droughts over the U.S. Great Plains has been suggested for decades, the underlying mechanisms are still not fully understood. Here we show a significant correlation between low-level moisture condition over the U.S. Southwest in spring and rainfall variability over the Great Plains in summer. Such a connection is due to the strong influence of the Southwest dryness on the zonal moisture advection to the Great Plains from spring to summer. This advection is an important contributor for the moisture deficit during spring to early summer, and so can initiate warm season drought over the Great Plains. In other words, the well-documented influence of cold season Pacific SST on the Southwest rainfall in spring, and the influence of the latter on the zonal moisture advection to the Great Plains from spring to summer, allows the Pacific climate variability in winter and spring to explain over 35% of the variance of the summer precipitation over the Great Plains, more than that can be explained by the previous documented west Pacific–North America (WPNA) teleconnection forced by tropical Pacific SST in early summer. Thus, this remote land surface feedback due to the Southwest dryness can potentially improve the predictability of summer precipitation and drought onsets over the Great Plains.

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