Leonard M. Druyan

Forcings and chaos in interannual to decadal climate change

We investigate the roles of climate forcings and chaos (unforced variability) in climate change via ensembles of climate simulations in which we add forcings one by one. The experiments suggest that most interannual climate variability in the period 1979–1996 at middle and high latitudes is chaotic. But observed SST anomalies, which themselves are partly forced and partly chaotic, account for much of the climate variability at low latitudes and a small portion of the variability at high latitudes. Both a natural radiative forcing (volcanic aerosols) and an anthropogenic forcing (ozone depletion) leave clear signatures in the simulated climate change that are identified in observations. Pinatubo aerosols warm the stratosphere and cool the surface globally, causing a tendency for regional surface cooling. Ozone depletion cools the lower stratosphere, troposphere and surface, steepening the temperature lapse rate in the troposphere. Solar irradiance effects are small, but our model is inadequate to fully explore this forcing. Well-mixed anthropogenic greenhouse gases cause a large surface wanning that, over the 17 years, approximately offsets cooling by the other three mechanisms. Thus the net calculated effect of all measured radiative forcings is approximately zero surface temperature trend and zero heat storage in the ocean for the period 1979–1996. Finally, in addition to the four measured radiative forcings, we add an initial (1979) disequilibrium forcing of +0.65 W/m2. This forcing yields a global surface warming of about 0.2°C over 1979–1996, close to observations, and measurable heat storage in the ocean. We argue that the results represent evidence of a planetary radiative imbalance of at least 0.5° W/m2; this disequilibrium presumably represents unrealized wanning due to changes of atmospheric composition prior to 1979. One implication of the disequilibrium forcing is an expectation of new record global temperatures in the next few years. The best opportunity for observational confirmation of the disequilibrium is measurement of ocean temperatures adequate to define heat storage.

A Pinatubo Climate Modeling Investigation

Global cooling of the Earth’s surface has been observed following the largest volcanic eruptions of the past century, although the average cooling is perhaps less than expected from simple energy balance considerations. The Mount Pinatubo eruption, with both the climate forcing and response observed better than previous volcanoes, allows a more quantitative analysis of the sensitivity of climate to a transient forcing. We describe the strategy and preliminary results of a comprehensive investigation of the Pinatubo case.

Sources of Sahel precipitation for simulated drought and rainy seasons

Abstract The sources of sub-Saharan precipitation are studied using diagnostic procedures integrated into the code of the GISS climate model. Water vapor evaporating from defined source region is “tagged,” allowing the determination of the relative contributions of each evaporative source to the simulated July rainfall in the Sahel. Two June–July simulations are studied to compare the moisture sources, moisture convergence patterns and the spatial variations of precipitation for rainy and drought conditions. Results for this eau study indicate that patterns of moisture convergence and divergence over northern Africa had a stronger influence on model rainfall over the sub-Sahara than did evaporation rates over the adjacent oceans or moisture advection from ocean to continent. While local continental evaporation contributed significant amounts of water to sahelian precipitation in the “rainy” simulation, moisture from the Indian Ocean did not precipitate over the Sahel in either case.

A Modification to the NOAH LSM to Simulate Heat Mitigation Strategies in the New York City Metropolitan Area

Abstract A new approach to simulating the urban environment with a mesocale model has been developed to identify efficient strategies for mitigating increases in surface air temperatures associated with the urban heat island (UHI). A key step in this process is to define a “global” roughness for the cityscape and to use this roughness to diagnose 10-m temperature, moisture, and winds within an atmospheric model. This information is used to calculate local exchange coefficients for different city surface types (each with their own “local roughness” lengths); each surface’s energy balances, including surface air temperatures, humidity, and wind, are then readily obtained. The model was run for several summer days in 2001 for the New York City five-county area. The most effective strategy to reduce the surface radiometric and 2-m surface air temperatures was to increase the albedo of the city (impervious) surfaces. However, this caused increased thermal stress at street level, especially noontime thermal str...

Variability and Predictability of West African Droughts: A Review on the Role of Sea Surface Temperature Anomalies

AbstractThe Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from seasonal to decada...

Dynamic Downscaling of Seasonal Climate Predictions over Brazil

Abstract Climate projections for March–April–May (MAM) 1985 and 1997 made with the NASA Goddard Institute for Space Studies (GISS) GCM over South America on a 4° latitude by 5° longitude grid are “downscaled” to 0.5° grid spacing. This is accomplished by interpolating the GCM simulation product in time and space to create lateral boundary conditions (LBCs) for synchronous nested simulations by the regional climate model (RCM) of the GISS/Columbia University Center for Climate Systems Research. Both the GCM and the RCM simulations use sea surface temperature (SST) predictions based on persisted February SST anomalies. Each downscaled prediction is evaluated from an ensemble of five simulations and each is compared to a control ensemble of five RCM simulations driven by synchronous NCEP reanalysis data. An additional five-run control ensemble for MAM 1997 tests the impact of “perfect” SST predictions on the RCM forecast. Results are compared to observational evidence that includes NCEP reanalysis data, Clim...

Observational and modelling studies of Amazonia interannual climate variability

The interannual variability of climate in the Amazon basin is studied using precipitation and river level anomalies observed near the March/April rainy season peak for the period 1980–86, supported by satellite imagery of tropical convection. Evaluation of this data in conjunction with the corresponding circulation and sea-surface temperature (SST) anomaly patterns indicates that abundant rainy seasons in Northern Amazonia are characterized by anomalously cold surface waters in the tropical eastern Pacific, and negative/positive SST anomalies in the tropical North/South Atlantic, accelerated Northeast trades and a southward displaced Intertropical Convergence Zone (ITCZ) over the Atlantic sector. Years with deficient rainfall show broadly opposite patterns.General circulation model (GCM) experiments using observed SST in three case studies were aimed at testing the teleconnections between SST and Amazon climate implied by the empirical analysis. The GCM-generated surface fields resemble the corresponding observers fields most closely over the tropical Pacific and, with one exception, over the tropical Atlantic as well. The modeled precipitation features, along the Northwest coast of South America, anomalies of opposite sign to the North and South of the equator, in agreement with observations and results from a different GCM. Similarities in simulations run from different initial conditions, but using the same global SST, indicate broad consistency in response to common boundary forcing.

An Analysis of the Potential for Extreme Temperature Change Based on Observations and Model Simulations

Abstract The study analyzes observational climate data for June–August 1977–2004 and simulations of current and future climate scenarios from a nested GCM/regional climate model system to assess the potential for extreme temperature change over the eastern United States. Observational evidence indicates that anomalously warm summers in the eastern United States coincide with anomalously cool eastern Pacific sea surface temperatures, conditions that are conducive to geopotential ridging over the east, less frequent precipitation, and lower accumulated rainfall. The study also found that days following nighttime rain are warmer on average than daytime rain events, emphasizing the importance of the timing of precipitation on the radiation balance. Precipitation frequency and eastern Pacific sea surface temperature anomalies together account for 57% of the 28-yr variance in maximum surface temperature anomalies. Simulation results show the sensitivity of maximum surface air temperature to the moist convection...

Studies of African wave disturbances with the GISS GCM

Abstract Simulations made with the general circulation model of the NASA/Goddard Institute for Space Studies (GISS GCM) run at 4° latitude by 5° longitude horizontal resolution are analyzed to determine the models representation of African wave disturbances. Waves detected in the models lower troposphere over northern Africa during the summer monsoon season exhibit realistic wavelengths of about 2200 km. However, power spectra of the meridional wind show that the waves propagate westward too slowly, with periods of 5–10 days, about twice the observed values. This sluggishness is most pronounced during August, consistent with simulated 600-mb zonal winds that are only about half the observed speeds of the midtropospheric jet. The modeled wave amplitudes are strongest over West Africa during the first half of the summer but decrease dramatically by September, contrary to observational evidence. Maximum amplitudes occur at realistic latitudes, 12°–20°N, but not as observed near the Atlantic coast. Spectral...

A GCM investigation of global warming impacts relevant to tropical cyclone genesis

Two approaches that consider how greenhouse warming might impact the frequency of tropical cyclone (TC) genesis are explored. Results are based on GCM experiments with the q-flux version global climate model of the NASA/Goddard Institute for Space Studies (GISS); one set representing contemporary atmospheric concentrations of CO2, contrasting with the second set representing the global climate in double CO2 equilibrium. July–September means of climate parameters relevant to TC genesis are computed from the simulations and combined to formulate a seasonal genesis parameter (SGP), as suggested in an empirical study by Gray (in Shaw, D.B. (ed.), Meteorology Over the Tropical Oceans, 1979, pp. 155–218). The spatial distribution of the July–September SGP based on the control simulations is compared with the observed distribution and results using other models. The corresponding spatial distribution of the July–September SGP derived from the double CO2 simulations, when compared with the control results, projects a 50% increase in the genesis frequency of TC over the western North Atlantic/Gulf of Mexico basin, but 100–200% increases over the North Pacific Ocean. The increases, most of which are attributable to enhanced ocean temperatures, may be exaggerated, suggesting that the original SGP formulation requires tuning or other revisions. For example, it is noted that SGP computed from the NCEP 1982–1994 re-analysis climatology do not accurately reflect the known spatial distributions of TC genesis frequency. The second approach detects easterly waves over the eastern North Atlantic Ocean by spectral analysis of vorticity and wind component time trends, comparing wave activity in the control and double CO2 simulations. Results indicate a southward shift in future trajectories of easterly waves over West Africa and significant increases in their average amplitude as they cross the African coast and begin to traverse the Eastern Atlantic along 14°N. Copyright