A23B-0778 1340h
Coupled Physical and Chemical Study of the Subarctic Snowpack: Feedback of Metamorphic Intensity on Climate Change
Temperature gradients in snowpacks induce metamorphism via sublimation/condensation cycles that physically alter snow crystals and promote the release or uptake of atmospheric trace gases. A change in the intensity of metamorphism caused by global warming will thus modify both the physics of the snowpack (albedo, heat transfer, permeability) and its chemical exchanges with the atmosphere. These modifications could exert strong feedbacks on global change. To identify these feedbacks, we performed a coupled study of snowpack physics and chemistry during an entire winter in Fairbanks, central Alaska, where cold ambient temperatures and a thin snowpack generate an intense metamorphism. We also monitored the evolution of the same snowpack subjected to low grade metamorphism by letting snow accumulate on tables, under which air circulation prevented the establishment of any significant temperature gradient. Variables measured regularly included: air and snow temperatures, the density, specific surface area and permeability of snow, crystal sizes and shapes and concentrations of major ions, aldehydes and O and D isotopes. The evolution of the natural snowpack is markedly different from that on tables. Snow permeability is about 20 times greater, density is lower and the specific surface area decreases faster with time in the natural snowpack. This results in a lower albedo, stronger soil/atmosphere gas exchanges and different soil/atmosphere heat transfers. The high grade metamorphism in the natural snowpack also leads to enhanced release of trace gases from the snowpack to the atmosphere, as observed for formaldehyde. The effects of a changing climate on snow physics and chemistry will be considered in light of the extensive dataset obtained during the winter of 2003-2004. The following feedbacks will be discussed: rate of decrease of SSA and snow albedo, permeability and heat transfer, input to the atmosphere and atmospheric chemistry-climate feedbacks, release of nutrients and pollutants to the biosphere after melting.
A23B-0779 1340h
Modeling glacial inception with GENIE
Glacial inception is difficult to capture accurately in Earth-system models. It has been suggested that inception can occur either when a wide area gains snow cover which persists from year to year, enhanced by the albedo feedback effect, or when mountain glaciers merge to form larger volumes of ice (the mass balance-elevation effect). Insufficient horizontal resolution may explain the difficulty of simulating the latter process, though it is unclear how it would impact on the former. In this study, we consider the problem in the context of a new, modular Earth-system model (GENIE). In the present configuration, GENIE is run with an intermediate-complexity, fully-dynamical atmosphere (T21, seven levels), a high-resolution thermo-mechanical ice-sheet model, a slab ocean and sea-ice model. A novel feature of the ice-sheet model is that it may be configured to run on any number of arbitrary domains simultaneously, and at different resolutions. This enables small-scale topography to be captured within the ice model over particular regions of the Earth's surface, without compromising performance unduly. Because of these difficulties, and specifically because it requires an accurate calculation of mass-balance, the modelling of glacial inception is a stringent test of the model. It also exploits its particular strengths, namely the flexible coupling of a high-resolution ice sheet model with a dynamical atmosphere. The coupling is achieved via a degree-day method mass-balance scheme, forced by daily temperatures and precipitation from the atmospheric model. The ice model performs a lapse-rate correction to account for the high-resolution topography, and returns albedo and topography to the global model annually. The study considers the effects on modelled inception of orbital parameters and CO$_2$ levels, with conditions 115kyr ago serving as a starting-point. A range of sensitivity studies are performed; these results are presented and interpreted in the light of the characteristics of the model.
A23B-0780 1340h
Impacts of anomalous ocean heat transport on the NAO
Coupled atmosphere-ocean dynamics in the north Atlantic is studied by means of a simple yet realistic model, featuring a baroclinic three dimensional atmosphere coupled to a slab ocean. Anomalous oceanic heat transport due to wind driven circulation is parameterized in terms of a delayed response to the change in wind stress curl due to the North Atlantic oscillation (NAO). Climate variability for different strengths of ocean heat transport efficiency is analyzed. Two types of behavior are found depending on timescale. At interdecadal and longer timescales a negative feedback is found that leads to a reduction in the spectral power of the NAO. By greatly increasing the efficiency of heat transport, the NAO in the model can be made to completely vanish. At decadal timescales a coupled oscillation is found in which SST and geopotential height fields covary. The relevance of these phenomena to the observed climate is discussed.
A23B-0781 1340h
Non-normal Amplification of the Thermohaline Circulation
A simple coupled ocean-atmosphere model, continuous in the meridional direction, is used to examine physical mechanisms leading to transient amplification of thermohaline circulation (THC) anomalies. It is found that in a stable regime, in which small perturbations eventually decay, there are optimal initial conditions leading to a dramatic amplification of initial temperature and salinity anomalies in addition to the THC amplification. The maximum amplification is found after about forty years, and the eventual decay is on a centennial time scale. The initial temperature and salinity anomalies are considerably amplified by factors of a few hundreds and twenty, respectively. The initial conditions leading to this amplification are characterized by mutually canceling initial contributions of temperature and salinity anomalies, such that the initial THC anomaly vanishes. The mechanism of amplification is analyzed and found to be the result of an interaction between a few damped (oscillatory and non-oscillatory) modes with decay time scales of twenty to eight hundred years. The amplification mechanism is also found to be distinct from the advective feedback leading to THC instabilities for large fresh water forcing.
A23B-0782 1340h
GCM asessment of aerosol-cloud interactions.
Aerosol-cloud interactions are assessed with the Goddard Institute for Space Studies GCM. Aerosol size distribution and chemical composition are resolved online with the TOMAS microphysics model (Adams and Seinfeld, 2002), while the aerosol-cloud interactions are based on an improved version of the Nenes and Seinfeld (2003) parameterization coupled with a new algorithm for estimating subgrid variability of updraft velocity. The aerosol-cloud interaction module is also coupled with autoconversion processes to assess the effects of aerosols on precipitation. The primary focus of this study will be the indirect forcing of sulfate and carbonaceous aerosol. {\bf References} Adams PJ, Seinfeld JH (2002) Predicting global aerosol size distributions in general circulation models, {\it J.Geoph.Res.}, {\bf 107} (D19): Art. No. 4370 Nenes, A. and Seinfeld, J.H. (2003) Parameterization of cloud droplet formation in global climate models, {\it J.Geoph.Res.}, {\bf 108} (D7), 4415, doi: 10.1029/2002JD002911
A23B-0783 1340h
Coupled ocean-atmosphere multi-decadal oscillation in the Southeast Pacific Basin simulated by a climate model
A natural period and related spatial pattern are estimated from integration results of GFDL R15 climate model with no increase of greenhouse gases and sulfate aerosols. The period of 30-40 years in and around the Southeast Pacific Basin is close to those estimated from climate proxies of tree-ring, ice core, and deep-sea sediments. Results of composite analysis show that sea-ice thickness and sea-level pressure could represent strong coupled feature in the model climate through the oscillation in time and space. Strong variability of atmospheric surface wind approximately matches with closed contours of planetary vorticity, f/H, in the basin.
A23B-0784 1340h
Tropical Cyclone-Induced Ocean Mixing and Ocean Heat Transport
Turbulent mixing driven by tropical cyclones (TCs) creates cool sea surface temperature (SST) anomalies in their wakes. Restoration to `normal' SST patterns must be driven by anomalous (with respect to climatological values) surface fluxes. The upward turbulent mixing of cool water and the anomalous post-storm heat fluxes into the ocean should drive a substantial amount of poleward ocean heat transport (OHT) and significantly perturb the meridional overturning circulation. Given the sensitivity of TC activity to SSTs, strong feedbacks may exist that alter SST gradients and link TC activity to the mean climate state through OHT. A recent study estimates the magnitude of the TC-induced OHT to be on the order of 10$^{15}$ Watts, representing the majority of the present-day total annual heat transported by the Earth's oceans (Emanuel, 2001, 2002, 2003). Here we analyze a variety of the latest SST and ocean heat content re-analyses datasets, including ECMWF ERA-40, and calculate SST anomalies for the majority of strong TCs occurring during the last forty years. Using SST anomalies, we attempt to quantify the annually averaged global OHT attributable to TC-induced mixing and compare between datasets and measurements/observations. Surface flux data along storm paths are extracted from ERA-40 data, and radiative energy imbalances within storm wakes are also used to calculate the implied OHT. Results are compared with satellite-based climatologies in the period in which they overlap and differences between reanalysis and satellite-based estimates of TC-induced OHT are described.
A23B-0785 1340h
Response of the North Atlantic Thermohaline Circulation to Atmospheric CO$_{2}$ Concentration Octupling
Recent studies on climate change arising from increases in atmospheric CO$_{2}$ concentrations have demonstrated not only the possibility of a weakened North Atlantic thermohaline circulation (THC), but also a total recovery in its intensity. The present study considers the case of 8 times the present concentration by carrying out a long-term integration of a GFDL low-resolution coupled model. Results are compared with previous quadrupling experiments. It is found that the THC eventually recovers, but the time of recovery is prolonged by about 5000 years in comparison to the 4xCO$_{2}$ case. During the 3000 years prior to complete recovery, little change is seen in the THC whose intensity varies around 8Sv, but there is a distinctive centennial variability, also seen in the time series of the globally averaged sea-surface temperature. The final recovery stage occurs rapidly about 6200 years after the start of integration and is thereafter marked by a large decadal variability. The average value of the final THC intensity is approximately 16Sv, slightly smaller than those of the control and 4xCO$_{2}$ cases. The initial forcing due to changes in the hydrological cycle are mainly amplifications of those in the 4xCO$_{2}$ case. In other words, there are larger increases in net precipitation over the polar regions, the Himalayas and the central tropical Pacific, and larger decreases in the rest of the low latitudes. The globally averaged sea-surface temperature increases from about 17.6$^{o}$C to 26.3$^{o}$C (23.7$^{o}$C for the 4xCO$_{2}$ case), mostly within the first 1500 years. In the deeper ocean (3km), the average temperature increases from 1.1$^{o}$C and gradually reaches 8.9$^{o}$C (6.6$^{o}$C for the 4xCO$_{2}$ case) at the time of complete THC recovery.
A23B-0786 1340h
Simulation of observed changes in the North Atlantic Oscillation and surface climate in the latter half of the 20th Century.
The North Atlantic Oscillation (NAO) is the dominant pattern of winter variability in the climate of the Atlantic region. The NAO also shows a large positive trend between the 1960s and 1990s which is manifested as a rapid increase in winter surface temperature and rainfall in Northern Europe. Despite this, the cause of the trend remains a source of much debate and it is not reproduced in simulations of the 20th century with global climate models. Constraining lower boundary conditions using historical sea-surface temperature and sea-ice distributions allows models to reproduce a weak trend in the NAO and land surface temperature. However, we find that the full observed trend can be reproduced when conditions in the lower stratosphere are relaxed towards observations. This result indicates that upper level circulation changes and the strength of the downward coupling to the surface are strong enough to explain a large fraction of the recent change in North European and North American surface climate. It also suggests that upper level variability should be reproduced in models to fully simulate surface climate variations, and that the response of the Atlantic region to anthropogenic forcing is strongly influenced by natural variability on decadal timescales.
A23B-0787 1340h
On the Role of Pacific-Indian Basin Upwelling in Maintaining the Global Meridional Overturning Circulation
An idealized, ocean only general circulation model is used to investigate the importance of diapycnal mixing in different basins to the global meridional overturning circulations. We find that when mixing is concentrated in the tropical Pacific-Indian basin, a global circulation can be maintained with sinking in the North Atlantic and Antarctic and upwelling throughout the Pacific and Indian Oceans. The magnitude of the northward heat flux in the North Atlantic is similar regardless of whether mixing is spread evenly across the globe or concentrated in the tropical Pacific. The northward heat flux in the South Atlantic, however, is close to that observed only when mixing is concentrated in the tropical Pacific. When diapycnal mixing is instead concentrated in the tropical Atlantic, a single-hemisphere, single-basin cell in the North Atlantic is maintained, but most of the deep water formed is upwelled in the Atlantic leaving the Pacific largely quiescent. Thus a reasonably strong heat flux in the North Atlantic is maintained, but the heat flux reverses sign in the South Atlantic and nearly vanishes in the Pacific. In summary, the intensity of the overturning circulation in the North Atlantic increases with the intensity of mixing in either the Atlantic or the Pacific, but the connections between basins are maintained only when mixing is confined to the tropical Pacific or spread uniformly over the globe.
A23B-0788 1340h
Southern Ocean response to high-frequency winds
The Southern Ocean displays variability on many spatial and temporal scales, the sources of which are not always clear. For example, many studies have addressed the relation between Drake Passage transport variations and wind stress variability, but the complexity of the Southern Ocean dynamics and the sparseness of data in the region make it difficult to establish a consistent picture. This study addresses the response of the Southern Ocean to high-frequency wind forcing. Results will be presented of numerical simulations with a barotropic model of the Southern Ocean, forced with a stochastic wind stress. For periods longer than roughly 5 days, the response is well described by the so-called Southern Mode. For shorter periods the response is dominated by the excitation of barotropic normal modes. A main result is that the excitation of resonant modes leads to a complicated phase relationship between the transport and the wind stress, suggesting that the classical notion of transport passively lagging forcing may be inadequate when eigenmodes come into play. Furthermore, it will be shown that form stress, rather than friction, balances the wind stress in the dynamics of the Southern Mode.
A23B-0789 1340h
What Controls Planetary Albedo and its Interannual Variability?
Climatological planetary albedo obtained from the ISCCP D-series flux dataset is broken down into contributions from the atmosphere and surface for various regions of the world. With the exception of Antarctica, the atmosphere accounts for much more of planetary albedo ($>$75%) than the surface. This is because surface albedo is generally small compared to atmospheric albedo and because the atmosphere attenuates the surface contribution.In Antarctica, the surface accounts for more of planetary albedo($\sim$70%) than the atmosphere during all seasons with sunshine. High surface albedo and a small atmospheric attenuation effect in the relatively transparent southern hemisphere polar atmosphere account for the large contribution of the surface. The observed poleward increase in planetary albedo was also examined. The atmosphere and the surface contribute about equally to this increase in the northern hemisphere. However,in the southern hemisphere, the surface contribution is three times larger than that of the atmosphere. This hemispheric asymmetry is largely due to the high Antarctic surface albedo. The ISCCP data sets were also used to determine what controls interannual planetary albedo variability. In a global sense, more than 90% of it can be linearly related to fluctuations in surface albedo, cloud cover and the logarithm of cloud optical depth. The atmosphere contributes more to planetary albedo variability over ice-free ocean and snow-free land areas. However, the surface is dominant in snow and ice-covered regions, accounting for more than 50% at nearly all times of year. The large effect of snow and ice variations on interannual planetary albedo variability suggests that if cloud fields do not change much in a future warmer climate, a retreat of snow cover, or sea ice would lead to a significant increase in net incoming solar radiation, resulting in an enhancement of high latitude climate sensitivity.
A23B-0790 1340h
Nonlinear dynamics in a simple two-timescale vegetation-atmosphere system and its implications for North China drought
Based on the dynamic vegetation model of a GCM, a conceptual model was constructed to describe the vegetation-atmosphere system. As the dynamic vegetation model has two timescales for vegetation growth, it revealed some mechanisms that were not revealed in one timescale models. It is indicated that areas with large atmospheric internal variability or strongly influenced by outside forcing are more vulnerable to drought. In areas with sparse initial vegetation, it is easier to fall into long term drought for vegetation with longer growth or loss timescales; while it is converse in areas with dense vegetation. GCM experiments shows that neither northern desertification nor farming experiment can simulate the drought happened in North China after the end of the 1970s. But when we combined the influences of the two, the pattern of the drought was almost simulated. This supports the conceptual model results. Farming alone can't lead to the drought because there isn't enough outside forcing to cause the drought although there is SST forcing, but it makes the system more vulnerable to drought by increasing the vegetation growth timescales. Northern desertification is an additional perturbation to drag the system to drought.
A23B-0791 1340h
Westerly Wind Bursts: ENSO's tail rather than the dog?
Westerly wind bursts (WWBs) in the equatorial Pacific occur during the development of most El Ni{\~n}o events and are therefore believed to be a major factor in ENSO's dynamics. Because of their short time scale, WWBs are normally considered part of a stochastic forcing of ENSO, completely external to the interannual ENSO variability. Recent observational studies, however, suggest that the occurrence and characteristics of WWBs may depend to some extent on the state of ENSO components, implying that WWBs are modulated by ENSO itself. Satellite observations are used here to show that WWBs are significantly more likely to occur when the warm pool is extended eastward. Based on these observations, we include an idealized representation of WWBs in an intermediate coupled ocean-atmosphere ENSO model, such that the occurrence of WWBs is modulated by the warm pool extent. The resulting model run is compared with a run in which the WWBs are stochastically applied. The modulation of WWBs by ENSO results in an increase in the slow frequency component of the WWBs. This, in turn, makes the amplitude of ENSO events forced by the modulated WWBs twice as large as the amplitude of ENSO events forced by stochastic WWBs. We therefore suggest that the modulation of WWBs by the equatorial Pacific SST is a critical element of ENSO's dynamics, and that WWBs should not be regarded as purely stochastic forcing. It is further shown that WWB modulation by the large scale equatorial SST field is equivalent to an increase in the ocean-atmosphere coupling strength, making the coupled equatorial Pacific effectively self-sustained.
A23B-0792 1340h
Spin-Up Procedures for a Coupled Climate System Model
Natural climate variability arises both from the variability within the components of the climate system, and from the feedbacks between them. Coupled climate system models aim to represent both of these sources of variability. In order to use a coupled model to study natural variability on decadal and longer timescales, the simulated climate should be both realistic and stable over time. In the case of the coarse-resolution models used for millennial-scale climate studies, this can generally only be achieved through the use of flux adjustments. Both the control climate of such a model, and the magnitude of the flux adjustments required to eliminate drift, depend on the manner in which the model is "spun up", or initialised. The traditional approach used to initialise a coupled model, whereby the atmospheric and oceanic components are spun up independently, has a number of problems. Extensive use of flux adjustments is required in order to prevent the atmosphere and ocean models from exhibiting unrealistic drift upon coupling. The use of a relaxation boundary condition to spin up the ocean model also causes it to exhibit both a phase lag and a modulation of the seasonal cycle relative to observations. This paper assesses a variety of modifications to the traditional procedure for initialising a coarse-resolution atmosphere-sea ice-ocean general circulation model. As well as spinning up the atmosphere and ocean models independently, we assess the relative merits of spinning up the coupled model as a whole. We also attempt to modify the surface temperatures and salinities used to spin up the ocean model, in order to bring the model response into line with observations. In general, we find that the spin-up procedure involves a compromise between the degree of realism of the simulated climate and the magnitude of the flux adjustments required to eliminate drift in the coupled model.
A23B-0793 1340h
Study of the interannual Variability of the Thermohaline Circulation of the Mediterranean Sea With a Numerical Model
The Mediterranean thermohaline circulation is mostly controlled by the density gradient between the Atlantic Ocean and the Mediterranean Sea, which is triggered by deep water formation. The purpose of this work is to investigate the mechanisms driving the variability of deep water formation, a major phenomenon that can modify the whole thermohaline circulation. We thus performed numerical simulations using a 1/8° resolution model of the Mediterranean Sea, MED8. In a first stage, we characterize interannual variability under present climate conditions. Two simulations of 50 years are realized under different forcing conditions, either a climatogical or a random interannual forcing, that allow to isolate the intrinsic ocean variability from that induced by the atmospheric forcing. As expected a strong variability in the intermediate and deep water formation process was observed under random atmospheric forcing. Moreover some anomalous convective events arose in the Aegean basin after successive cold winters. There, intermediate water (250-1000m) became denser than the underlying deeper waters leading to new dense water formation. Nevertheless the impact of these intense events on the averaged thermohaline properties was weak. Indeed similar internal trend for the yearly averaged temperature and salinity, under both atmospheric conditions, was obtained showing the stability of the thermohaline circulation with respect to intense events. These results are eventually discussed in the light of the past 20 years in situ observations including the Big Transient event and its subsequent evolution.
A23B-0794 1340h
A Conceptual Modeling Study on Biosphere-Atmosphere Interactions and its Implications for Physically Based Climate Modeling
This paper presents a conceptual modeling study on the behaviors of terrestrial biosphere-atmosphere system as they relate to multiple equilibrium states and climate variability, and emphasizes their implications for physically based climate modeling. The conceptual biosphere-atmosphere model consists of equilibrium responses of vegetation and precipitation to each other, dynamics of the vegetation system, and stochastic forcing of precipitation representing the impact of atmospheric internal variability. Using precipitation as the atmospheric variable in describing the biosphere-atmosphere interactions, this model pertains to regions where biosphere productivity is limited by water. Low moisture convergence in the atmosphere combined with high sensitivity of the atmospheric climate to vegetation changes provide the most favorable condition for the existence of multiple equilibrium states. In a coupled biosphere-atmosphere system with multiple equilibria, experiments varying the stochastic forcing indicate that atmospheric internal variability is an important factor in the long term variability of the model climate and in its sensitivity to initial conditions. Specifically, the enhancement of low-frequency rainfall variability by vegetation dynamics is most pronounced with a moderate magnitude of atmospheric internal variability, and is less pronounced if internal variability is either too large or too small; detecting the existence of multiple equilibria by examining the sensitivity of the coupled model climate to initial conditions is not always reliable, since too large an internal variability reduces or even eliminates the model sensitivity to initial conditions. Findings from the conceptual model are confirmed using results from a physically based, synchronously coupled biosphere-atmosphere model.
A23B-0795 1340h
Stationarity of the ocean component of the climate signal
Empirical orthogal functions (EOFs) have been used extensively in recent years to explore the primary spatial and temporal distribution of the ocean's variability on both basin and global scales. Such empirical segmentation of time series are sometimes used as a way to extend time series back in time. This paper using both EOFs and Cyclic EOFs [Kim, et al. 1999] explores the spatial distribution of the ocean's variability and its relationship to the dynamic climate signal and how the primary signal patterns are modified in time. Using a relatively high resolution ocean model (0.2 degrees) forced with a 40+ year time series of realistic NCEP atmospheric fluxes, the stationarity of the interannual and longer sea surface height and upper ocean temperature signals are quantified and related to the forces applied at the ocean's surface. (Stationarity: A stationary process has the property that the mean, variance and autocorrelation structure do not change over time. For this purpose, any long term trend is removed.) With the model as an example time series, the stationarity of the ocean's upper ocean quantities is further examined through the use of satellite measurements of the ocean. While a percentage of the ocean's variability is relatively stationary, a significant proportion is not. Regional causes of the non-stationarity in the signal are explored.
A23B-0796 1340h
Behavior of the Thermohaline Circulation as a Function of Climate Sensitivity for Various Increased CO$_{2}$ Scenarios
Using a global coupled model of intermediate complexity (2D atmosphere, 3D ocean), the behavior of the thermohaline circulation is explored for various increased CO$_{2}$ scenarios. A novel feature of the model is our ability to adjust the model's sensitivity to CO$_{2}$ increases by changing the strength of the cloud feedback. By varying the rate and duration of increases in atmospheric CO$_{2}$, the model's sensitivity, and the river runoff scheme for perturbations in freshwater forcing, we are able to obtain and analyze a spectrum of behaviors, ranging from little change in the thermohaline circulation to a full shutdown of sinking in the North Atlantic. Although the initial decrease in overturning strength (i.e., over a centennial time scale) is primarily a function of the rate of CO$_{2}$ increase, whether the North Atlantic sinking ultimately shuts down is governed by the product of the model's prescribed climate sensitivity and the stabilization CO$_{2}$. However, if the rate of increase (for a given stabilization level) is sufficiently slow (e.g., over several hundred years), a recovery can occur for a model sensitivity/stabilization combination that collapses given a more rapid increase. Over a portion of phase space a weak, meta-stable overturning circulation exists; once the circulation crosses this threshold, a collapse rapidly occurs.
A23B-0797 1340h
Polar Amplification of Global Warming in Models Without Ice-Albedo Feedbacks
Non-ice-albedo feedback mechanisms leading to polar amplification, as reported by Alexeev (2003), are explored in three aquaplanet climate model systems of different complexity. We analyze this pattern using three different "ghost forcing" experiments (Hansen et al, 1997). In the first one we uniformly add 4W/m2 to the oceanic mixed layer in order to roughly simulate a 2xCO2 forcing at the surface. The second forcing, of the same magnitude, is applied only within the tropics and the third forcing is applied only polewards of 30 degrees (north and south). It turns out that our systems' equilibrium responses are linear with respect to these forcings. Surprisingly, the response to the tropical-only forcing is essentially non-local with quite significant warming at higher latitudes. The response to the high-latitude-only forcing is more local and has higher amplitude near the poles. Our explanation of the polar amplification obtained in the uniform forcing experiment is therefore two-fold. Firstly, the tropics are much more difficult to warm because of the higher sensitivity of the surface budget to SST changes at higher temperatures. Secondly, any extra heat deposited in the tropics is not easily radiated to outer space because of the high opaqueness of the tropical atmosphere. The energy, most of which is latent, needs to be redistributed by transports to the extra-tropics. Consequently, the tropical "ghost forcing" results in an essentially non-local response, while the extra-tropical one yields a more localized response, because the energy in the atmosphere cannot propagate effectively equator-wards from high latitudes. The paper deals with these mechanisms in three climate model systems with no ice-albedo feedbacks - an EBM and two different GCMs - one with cloud feedbacks and the other with cloud feedbacks excluded. References. Alexeev, V.A., (2003) Sensitivity to CO2 doubling of an atmospheric GCM coupled to an oceanic mixed layer: a linear analysis. Climate Dynamics, 20: p.775-787. Hansen, J., Sato M, and R. Ruedy, (1997) Radiative forcing and climate response, JGR, 102, No. D6, 6831-6864.
A23B-0798 1340h
Antarctic Circumpolar Wave dynamics in a simplified ocean- atmosphere coupled model
The Antarctic Circumpolar Wave (ACW) is one of the main pattern of variability in the Ocean-Atmosphere system in the southern Hemisphere extratropics. It involves sea surface temperature (SST), sea level pressure (SLP) and other variables, and consists of a wave train of zonal number 2, travelling around Antarctica at the speed of $6-8 cm s^{-1}$, hence taking around 8 years to complete a circle. A fundamental feature of this observed pattern is that anomalies are eastward propagating and seem to be phase locked: for example SST and SLP are in quadrature (high downstream of warm SST). Nevertheless the atmospheric part of the wave has been questioned by some observational studies. Different analytical and numerical studies have veen proposed, but a convincing theoretical explanation for the ACW is still missing. In this work we study the ACW as simulated by a simple dynamical model, in order to determine the basic physical processes that characterize it. The model used is an atmospheric quasi-geostrophic tridimensional model coupled to an ocean "slab" mixed layer, which includes mean geostrophic advection by the antarctic circumpolar current (ACC). The atmosphere-ocean coupling is obtained via surface sensible heat fluxes. We analyse three configuration of the model, a "passive ocean" one, where the ocean responds to the atmopheric forcing but does not feeds back to the atmosphere; a "passive atmosphere" one, where the stationary reponse of the atmosphere to prescribed SST anomalies; and a fully coupled one. The two forced experiment show separately a positive feedback in the coupled system.The passive ocean experiment shows an ACW-type low frequency variability in the ocean, ie a propagating SST anomaly with 4 years period. SSTa amplitude created were around $0.5^C$ wich is less than observed anomalies ($1.5^oC$). This means that the stochastic focing of the atmosphere is sufficient to substain a variability of the SST whose periodicity is set by the mean advection.The passive atmosphere experiment exhibits an equivalent-barotropic response with high pressure and high air surface temperature roughly 60 to 90 degrees downstream of warm SST anomalies. This response seems to follows SST aomalies if they are made eastward propagative. We next study the fully coupled simulation to measure the effect of hte feedback on the SST anomaly propagation, the dependence on surface fluxes intensity is also studied and the relevance to observed climate discussed.
A23B-0799 1340h
Predicting Climate Sensitivity Using Observations of Fluctuations in a Model with Adjustable Feedbacks
The availability of high spatial and spectral resolution infrared radiances from instruments such as AIRS places renewed emphasis on the development of analytic tools for the comparison of model output and climate date. Recent work by Cionni et al. (2004) and by Haskins et al. (1999) indicates that analysis of short term fluctuations may provide a measure of equilibrium climate sensitivity. Cionni et al. (2004) analyzed only a single model. We extend their work by making use of a model with variable climate sensitivity, to test the ability of this method to predict the equilibrium climate sensivity on the basis of short-term fluctuations. We analyze the behavior of a one-dimensional radiative-convective model subject to various stochastic forcings, and for various climate sensitivities, modulated by either albedo or water vapor feedbacks. Results will be shown for the model's response to solar and CO$_2$ forcing. We compare the equilibrium model sensitivity to these forcings with the sensitivity derived from the Fluctuation Dissipation Theorem, as discussed in Cionni et al. (2004). We use the radiative-convective model of Emmanuel (1991), modified to incorporate parameters that adjust the strength of various feedbacks. Equilibrium climate sensitivity are compared for control, fixed water vapor, and enhanced surface-albedo feedback runs with fixed carbon dioxide and solar constant. The ratio of the climate sensitivities for the control and modified sensitiviy cases was calculated. The same parameter settings are then used in cases where the CO$_2$ or the solar forcing are varied stochastically. The method of Cionni et al. (2004) is used to estimate the climate sensitivity by calculating the lag covariance of the forcing and response for the fluctuating forcing cases.
http://www.atmos.umd.edu/~dankd/FDT.html
A23B-0800 1340h
An Appraisal of Storm Track Depictions in Coupled Simulations of the CCSM Version 3.0
The Community Climate System Model, Version 3.0 is the most recent release of the global coupled climate model produced by the CCSM community, and contains improved model physics particularly with regards to cloud and ice phase processes. Numerous multi-century control runs were conducted with CCSM-3.0 at several resolutions. The purpose of this study is to evaluate the representation of transient atmospheric circulation features in the CCSM-3.0 in comparison to observation. Cyclone climatologies are a useful method for evaluating climate models as they provide details on the synoptic time scale as opposed to monthly means which are more readily available. A feature-tracking algorithm of intermediate complexity is applied to the once-daily output sea level pressure fields of the CCSM-3.0 millennial integration for twenty years of the integration. These calculations are compared to a similar analysis using daily fields of the European Centre for Medium-Range Weather Forecasts 40-Years Re-Analysis (ERA-40). Both cyclonic and anticyclonic features are tracked for both hemispheres, and computed statics include feature trajectory, as well as genesis and lysis frequencies. The results highlight differences in model simulations with observation but also indicate improvement of the newer version of the model to the CCSM-2, which was evaluated in a previous study. A central question to be answered by this study is the overall effects of the higher spatial resolution in the T85 simulation versus T42 horizontal resolution.
A23B-0801 1340h
Arctic Surface and Atmospheric States: NSA Analogs to SHEBA and Interannual Variability
Both winter and summer at the Surface Heat Budget of the Arctic Ocean (SHEBA) site can be described as dynamic equilibria between surface and atmosphere, where, for each season, different mechanisms are used by the constituent media to exchange energy. The wealth of measurements at SHEBA that allowed the processes of energy exchange there to be examined in such detail are only available for a single calendar year. However, a multiple year, high temporal resolution observational record is available nearby, from the Atmospheric Radiation Measurement (ARM) Program's North Slope of Alaska (NSA) Point Barrow installation. Here we consider how representative the Point Barrow site is of the SHEBA site, for both warm and cold season processes, by highlighting common traits of the atmosphere as it interacts with two prevalent Arctic surface types (perennial sea ice at SHEBA and tundra-topped permafrost at NSA). We also investigate the interannual variability of clear and cloudy sky surface air temperature, surface radiation fluxes, and atmospheric temperature and humidity structure at NSA, finding similar preferred modes of behavior there as observed at SHEBA, with clear sky temperature and moisture variations accounting for the bulk of the interannual variability in surface radiative energy exchange.
A23B-0802 1340h
Modes of Variability Under Climate Change Scenarios
This combined observational and modeling study concerns how patterns of variability including the El Niño Southern Oscillation (ENSO), the Arctic Oscillation- North Atlantic Oscillation (AO-NAO), and the Pacific North American Pattern (PNA) may change in the coming decades. Possible modifications to these teleconnection patterns, including changes in amplitude, frequency, duration, and spatial pattern, are analyzed. The signature and influence of these teleconnection patterns on regional diagnostics including surface air temperature, precipitation, zonal and meridional wind, geopotential height, storms, and the fluxes of heat and momentum are also explored. Analytical techniques include correlation and compositing of NCEP reanalysis data from two periods: 1) 1958-1998, and 2) 1979-1998. Model data from the Goddard Institute for Space Studies (GISS) GCM (both an atmosphere only and a coupled version), driven with 1) current and 2) enhanced greenhouse forcing are compared as well.
A23B-0803 1340h
Crop phenology feedback on climate over central US in a regional climate model
The moisture and CO2 fluxes over cropland represent local climate forcing and an important component of atmospheric energy and CO2 budgets. Since observed fluxes, especially for CO2, are rarely available over extensive areas the fluxes are mainly estimated by climate models. The carbon sequestration and water consumption by crops are only crudely represented in the models. For example, most climate models use climatological or static crop growth and development that do not change from year to year, indistinguishable between flood and drought years. To improve the moisture and CO2 fluxes (i.e., photosynthesis) from crops we coupled crop models (CERES for corn and CropGro for soybean) with the regional model (MM5) along with the land surface model (LSM). This crop-climate coupled model with interactive crop phenology can simulate interannual variations in CO2 and water fluxes from the surface. The coupled model was used to simulate CO2 and moisture fluxes in the past couple of growing seasons in the central U.S. Results were compared with available CO2 flux observations at some AmeriFlux sites. It is found that the coupled model gives more realistic seasonal accumulation of CO2 fluxes and that the dynamic crop development in the coupled model has a strong feedback on regional precipitation. The typical climate models using static crop phenology significantly overestimate CO2 fluxes during early growing season because of positive biases in specifying leaf area index.
A23B-0804 1340h
Optimizing climate model parameter choice using Linear Inverse Modeling (LIM)
Currently, a range of parameterizations exists for physical processes in the climate system, such as evaporation, convection, and cloud formation. These parameterizations are often based on physical principles, but the range of temperature projections among model simulations of future climate, as illustrated in the most recent IPCC report (Climate Change 2001), suggests much room for improvement. Within these parameterizations are coefficients that control the net effects of small-scale processes. Examples include coefficients of drag, precipitation efficiencies and rates of evaporation. These coefficients may be optimized by bringing the statistical and dynamical properties of climate model output into agreement with the statistical and dynamical properties of climate observations. For this purpose the observations must be carefully chosen to possess high information content, coverage, homogeneity, and reproducibility. We demonstrate the application of this technique by optimizing the parameters of a simple model based on observations. The relevant dynamics are extracted directly from space observations by Linear Inverse Modeling (LIM). LIM is a method for extracting the intrinsic linear dynamics that govern the climatology of a complex system directly from observations of the system. Here we apply LIM to observations of spectrally resolved infrared radiances. Spectrally resolved infrared radiances measured at the top of the atmosphere contain information on temperature, water vapor, clouds, and other absorbers, including aerosols and trace gases, at all altitudes from the surface through the stratosphere, covering of the entire earth. Infrared radiances, as they would be observed at the top of the atmosphere, may be calculated from a climate model via a radiative transfer code. The model parameter values are optimized by bringing the dynamical properties of infrared radiances calculated from model output into agreement with the dynamical properties of infrared radiance observations determined by LIM.
A23B-0805 1340h
Comparing the Effects of Sea Ice and Cloud Variability on the Top-of-Atmosphere Albedo in Observations and GCMs
Output from the Goddard Institute for Space Studies (GISS) atmosphere-only general circulation model is compared to satellite observations to assess how the top-of-atmosphere (TOA) albedo responds to changes in sea ice concentration and in cloud properties. The observations used for the comparison include monthly TOA radiative fluxes from the Radiation Budget Experiment, sea ice concentrations from UK Hadley Centre (HadISST1 data set); and cloud properties from the TOVS Polar Pathfinder. The model was run with transient sea surface temperature and sea ice concentration from the HadISST1 data set. We focus on the Northern Hemisphere polar regions during 1950-1998 in the model simulations and 1984-1990 in the observations (limited by the availability of radiation data). The short wave radiative effectiveness (RE) of sea ice concentrations is calculated as the difference in the mean TOA albedo over gridboxes with zero and maximum sea ice cover fraction. The RE of sea ice is 0.29 and 0.22 in the model and observations, respectively. The mean RE of sea ice in the model decreases almost exponentially from 0.33 to 0.1 as the monthly total cloud cover fraction increases from 0.1 to 1.0. In the model, the maximum and minimum total cloud fraction are observed in winter (0.75) and summer (0.55), respectively. On the other hand, observations from the TOVS polar pathfinder and from ground-based observations show maximum total cloud cover fraction in summer and minimum in spring. The analysis will be extended using model outputs from the Atmospheric and Coupled Model Intercomparison Projects.
A23B-0806 1340h
Climate Response to Increased Greenness at High Latitudes in Climate Model ARPEGE-CLIMAT
Satellite-derived normalized difference vegetation index(NDVI) indicates that greenness at high latitudes has increased. This increase may result from climate warming in the polar and subpolar regions, the effects of which may include a lengthened growing season, increased CO2 concentration in the atmosphere, and enhanced nutrient availability in the warmed soils and thawed permafrost. However, the feedbacks of increased greenness on climate have not been well documented. In this study we employed the climate model ARPEGE-CLIMAT, a version of the ARPEGE model developed at Meteo-France modified by the Bergen climate model (BCM) group, to identify these impacts. We first conducted a 10-year model simulation of the present climate with a horizontal resolution of T63 (linear grid) and 31 vertical layers with boundary conditions taken from the NCAR/NCEP reanalysis, which served as our control run. A sensitivity simulation with the same integration time was then performed in which the vegetation coverage and leaf area index (LAI) were increased by 20% poleward of 60oN. Through these parallel model simulations we aim to evaluate the impacts of increased greenness on the climate at high latitudes. The comparison of these two simulations indicates that increased greenness results in a significantly increased local evaporation during summer from May to August, which changes local hydrological cycle and energy budgets. Throughout the entire year, the impacts of increased greenness on atmospheric circulation are complex. Sea level pressure (SLP) and surface air temperature show larger anomalies in winter than in summer over the polar and sub-polar regions. The anomalous atmospheric conditions in winter months from October to March are characterized by unfavorable effects on the establishment of winter circulation systems and favorable effects on the retreat of these systems in spring; surface air temperature anomalies are consistent with changes to the atmospheric circulation. Interplay of atmospheric dynamic and thermodynamic processes, triggered by the increased greenness, and impacts of land surface processes are apparently responsible for such atmospheric circulation anomalies.