Asymmetry of tropical precipitation change under global warming

Nature Geoscience, 2013

Rainfall in the tropics is largely focused in a narrow zonal band near the Equator, known as the intertropical convergence zone. On average, substantially more rain falls just north of the Equator 1. This hemispheric asymmetry in tropical rainfall has been attributed to hemispheric asymmetries in ocean temperature induced by tropical landmasses. However, the ocean meridional overturning circulation also redistributes energy, by carrying heat northwards across the Equator. Here, we use satellite observations of the Earth's energy budget 2 , atmospheric reanalyses 3 and global climate model simulations to study tropical rainfall using a global energetic framework. We show that the meridional overturning circulation contributes significantly to the hemispheric asymmetry in tropical rainfall by transporting heat from the Southern Hemisphere to the Northern Hemisphere, and thereby pushing the tropical rain band north. This northward shift in tropical precipitation is seen in global climate model simulations when ocean heat transport is included, regardless of whether continents are present or not. If the strength of the meridional overturning circulation is reduced in the future as a result of global warming, as has been suggested 4 , precipitation patterns in the tropics could change, with potential societal consequences. One of the defining features of the Earth's climate is the fact that tropical rainfall primarily maximizes within the Northern Hemisphere (Fig. 1a,b). This is especially evident in the Atlantic and eastern Pacific oceans, where precipitation near 5 • S is much weaker than at the equivalent locations north of the Equator, which are some of the rainiest regions on Earth in the intertropical convergence zone. Throughout the tropics, societies as diverse as subsistence farmers in the Sahel and the technology-based cities of India are reliant, directly or indirectly, on monsoon rains. How tropical rainfall evolves in the future will have important and widespread social consequences. Precipitation in the tropics exists largely within narrow zonal bands owing to the large-scale atmospheric overturning known as the Hadley circulation 5. Within the lower branch of this circulation, the trade winds converge near the Equator, drying the subtropics and bringing moisture into the rain bands. This circulation is thermally direct, and thus transports energy in the directions of its upper branch. The Hadley circulation also plays a primary role in determining the hemispheric asymmetry of tropical precipitation because it transports copious amounts of moisture from the Southern Hemisphere to the Northern Hemisphere. A northward cross-equatorial mass transport in the moist lower branch of the

Terrestrial, Atmospheric and Oceanic Sciences

Regional changes of precipitation intensity, convective structure and column water vapor (CWV) in the Tropics are examined using climate model simulation outputs archived in the Coupled Model Intercomparison Project, phase 5 (CMIP5) for the representative concentration pathway 8.5 (RCP8.5) experiments. Under global warming, CWV increases homogeneously in space following the Clausius-Clapeyron scaling. In contrast, precipitation changes exhibit marked regional discrepancies with a mix of positive and negative anomalies in both mean ascending and descending regions. A moisture budget analysis indicates that changes of tropical precipitation are controlled mainly by the dynamic effect (q p 2-l), with a secondary contribution from the thermodynamics effect (q p 2-l). Precipitation changes comply with the "wet-get-wetter" or "dry-get-drier" mechanism over the regions where dynamic and thermodynamic effects work together, accounting for about two thirds of the total tropical area. Examining changes of convection structure further show that regions with negative precipitation anomalies must be accompanied by the weakening of convection in the entire troposphere. Convection structure changes attributing to positive precipitation anomalies, nonetheless, appear to be very different depending on the regions. In the mean ascending region, the positive precipitation anomaly is associated with the deepening of convection. In the mean descending region, the positive precipitation anomaly is a result of proportionally enhanced convection within the troposphere. The above findings are based on area-mean results. Further details may emerge when viewing convection and precipitation changes at local scales.

Science Advances, 2020

Under global warming, ENSO-driven precipitation anomalies over central eastern Pacific shift eastward because of narrowing ENSO SSTA.

Climate Dynamics, 2011

December–January–February (DJF) rainfall variability in southeastern South America (SESA) is studied in 18 coupled general circulation models from the WCRP/CMIP3 dataset, for present climate and the SRES-A1B climate change scenario. The analysis is made in terms of properties of the first leading pattern of rainfall variability in the region, characterized by a dipole-like structure with centers of action in the SESA and South Atlantic Convergence Zone (SACZ) regions. The study was performed to address two issues: how rainfall variability in SESA would change in a future climate and how much of that change explains the projected increasing trends in the summer mean rainfall in SESA identified in previous works. Positive (negative) dipole events were identified as those DJF seasons with above (below) normal rainfall in SESA and below (above) normal rainfall in the SACZ region. Results obtained from the multi-model ensemble confirm that future rainfall variability in SESA has a strong projection on the changes of seasonal dipole pattern activity, associated with an increase of the frequency of the positive phase. In addition, the frequency increase of positive dipole phase in the twenty first century seems to be associated with an increase of both frequency and intensity of positive SST anomalies in the equatorial Pacific, and with a Rossby wave train-like anomaly pattern linking that ocean basin to South America, which regionally induces favorable conditions for moisture transport convergence and rainfall increase in SESA.

Journal of Climate, 2012

Global 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 pre...

Environmental Research Letters, 2010

Current changes in tropical precipitation from satellite data and climate models are assessed. Wet and dry regions of the tropics are defined as the highest 30% and lowest 70% of monthly precipitation values. Observed tropical ocean trends in the wet regime (1.8%/decade) and the dry regions (−2.6%/decade) according to the Global Precipitation Climatology Project (GPCP) over the period including Special Sensor Microwave Imager (SSM/I) data , where GPCP is believed to be more reliable, are of smaller magnitude than when including the entire time series ) and closer to model simulations than previous comparisons. Analysing changes in extreme precipitation using daily data within the wet regions, an increase in the frequency of the heaviest 6% of events with warming for the SSM/I observations and model ensemble mean is identified. The SSM/I data indicate an increased frequency of the heaviest events with warming, several times larger than the expected Clausius-Clapeyron scaling and at the upper limit of the substantial range in responses in the model simulations.

Earth System Dynamics Discussions

We revisit the issue of the response of the precipitation characteristics to global warming 15 based on analyses of global and regional climate model projections for the 21st century. The 16 prevailing response we identify can be summarized as follows: increase in the intensity of 17 precipitation events and extremes, with the occurrence of events of "unprecedented" 18 magnitude, i.e. magnitude not found in present day climate; decrease in the number of light 19 precipitation events and in wet spell lengths; increase in the number of dry days and dry spell 20 lengths. This response, which is mostly consistent across the models we analized, is tied to the 21 difference between precipitation intensity responding to increases in local humidity 22 conditions, especially for heavy and extreme events, and mean precipitation responding to 23 slower increases in global evaporation. These changes in hydroclimatic characteristics have 24 multiple and important impacts on the Earth's hydrologic cycle and on a variety of sectors, 25 and as examples we investigate effects on the potential stress due to increases in dry and wet 26 extremes, changes in precipitation interannual variability and changes in potential 27 predictability of precipitation events. We also stress how the understanding of the 28 hydroclimatic response to global warming can shed important insights into the fundamental 29 behavior of precipitation processes, most noticeably tropical convection.

Journal of Geophysical Research, 2008

Associations between global and regional precipitation and surface temperature anomalies on inter-annual and longer time scales are explored for the period of 1979-2006 using the GPCP precipitation product and the NASA-GISS surface temperature data set. Positive (negative) correlations are generally confirmed between these two variables over tropical oceans (lands). ENSO is the dominant factor in these interannual, tropical relations. Away from the tropics, particularly in the Northern Hemisphere mid-high latitudes, this correlation relationship becomes much more complicated with positive and negative values of correlation tending to appear over both ocean and land, with a strong seasonal variation in the correlation patterns. Relationships between long-term linear changes in global precipitation and surface temperature are also assessed. Most intense long-term, linear changes in annual-mean rainfall during the data record tend to be within the tropics. For surface temperature however, both the strongest linear changes are observed in the Northern Hemisphere mid-high latitudes, with much weaker temperature changes in the tropical region and Southern Hemisphere. Finally, the ratios between the linear changes in zonal-mean rainfall and temperature anomalies over the period are estimated. Globally, the calculation results in a +2.3%/C precipitation change, although the magnitude is sensitive to small errors in the precipitation data set and to the length of record used for the calculation. The long-term temperature-precipitation relations are also compared to the inter-annual variations of the same ratio in a zonally-averaged sense, and are shown to have similar profiles, except for over tropical land areas.

2012

1] The Earth's climate system was subject to two multidecadal warming trends in the beginning and end of the 20th century, having been interrupted only by a cooling trend in mid-century . The spatio-temporal distribution of surface temperature during this time, especially the land-ocean warming contrast in recent decades, has been the subject of many climate change detection studies. The focus of this study is the south-to-north warming asymmetry and we observed a similar Latitudinal Asymmetry of Temperature Change (LATC) for the two warming sub-periods and the cooling sub-period. Basically, the temperature change was low in the Southern Hemisphere extra-tropics (60 S) and increased monotonically to peak values (0.15 C/decade for warming trends) in the Northern Hemisphere extra-tropics (60 N). We hypothesized that the LATC is a fundamental characteristic of the planet's transient response to global forcing. We tested this hypothesis using climate model simulations of CO 2 and aerosol forcing, and the simulations revealed very similar LATC as seen in the observations. In the simulations, the LATC did not depend on the asymmetry of the forcing and furthermore weakened significantly in equilibrium simulations, leading to the deduction that the LATC was caused by a corresponding asymmetry in the land-ocean fraction, i.e., the analyses of model simulations supported the hypothesis of LATC being a fundamental characteristic of the planet's transient response. If LATC is preserved as the planet warms beyond 2 C, precipitation patterns can be drastically disrupted in the tropics and sub-tropics, with major implications for regional climate. Citation: Xu, Y., and V. Ramanathan (2012), Latitudinally asymmetric response of global surface temperature: Implications for regional climate change,

Journal of Climate, 2013

Orbital precession changes the seasonal distribution of insolation at a given latitude but not the annual mean. Hence, the correlation of paleoclimate proxies of annual-mean precipitation with orbital precession implies a nonlinear rectification in the precipitation response to seasonal solar forcing. It has previously been suggested that the relevant nonlinearity is that of the Clausius-Clapeyron relationship. Here it is argued that a different nonlinearity related to moisture advection by the atmospheric circulation is more important. When perihelion changes from one hemisphere's summer solstice to the other's in an idealized aquaplanet atmospheric general circulation model, annual-mean precipitation increases in the hemisphere with the brighter, warmer summer and decreases in the other hemisphere, in qualitative agreement with paleoclimate proxies that indicate such hemispherically antisymmetric climate variations. The rectification mechanism that gives rise to the precipitation changes is identified by decomposing the perturbation water vapor budget into ''thermodynamic'' and ''dynamic'' components. Thermodynamic changes (caused by changes in humidity with unchanged winds) dominate the hemispherically antisymmetric annual-mean precipitation response to precession in the absence of land-sea contrasts. The nonlinearity that enables the thermodynamic changes to affect annual-mean precipitation is a nonlinearity of moisture advection that arises because precessioninduced seasonal humidity changes correlate with the seasonal cycle in low-level convergence. This interpretation is confirmed using simulations in which the Clausius-Clapeyron relationship is explicitly linearized. The thermodynamic mechanism also operates in simulations with an idealized representation of land, although in these simulations the dynamic component of the precipitation changes is also important, adding to the thermodynamic precipitation changes in some latitudes and offsetting it in others.