A13B-0235
Intensified convection over East Africa during shifts in Walker circulation 140-60 ka BP
Thick (up to 5m) diatomite beds deposited during the penultimate interglacial 140-60 ka document intervals where deep freshwater lakes filled large parts of the Central Kenya Rift, East Africa. Palaeohydrological reconstructions of the lake highstands are based on diatom analyses and δ18Odiatom. The chronology of the diatomite deposits is established through precise 40Ar/39Ar dating of intercalated pumice tuffs. The paleolakes experienced multiple hydrological fluctuations on sub-orbital (~1,500 to 2,000 years) time scales. The magnitude of the δ18Odiatom change (±3 permil) and significant changes in the plankton-littoral ratio of the diatom assemblage (±25%) suggest that the paleolake records can be interpreted in the context of long-term climate shifts in East Africa. Using 40Ar/39Ar age control and nominal diatomite-sedimentation rates, we suggest that paleohydrological conditions in equatorial East Africa during the late Pleistocene were primarily influenced by the latitudinal displacement of the inter-tropical convergence zone (ITCZ). Extreme insolation at the eccentricity maximum, and weakened zonal air-pressure gradients in the tropics, favored intensified convection over East Africa and deep freshwater lake conditions. Considering that these scenarios may relate to present-day, ENSO-related shifts in the Walker circulation, we discuss a possible connection between long-term El-Niño-like climate forcing and tropical hydrological change on glacial-Interglacial timescales.
A13B-0236
A Verified Estimation of the El Nino Index NINO3.4 Since 1877
Decadal and longer time scale variability of our best known El Nino Southern Oscillation (ENSO) indices are poorly correlated before 1950 and so our knowledge of ENSO and its interdecadal variability and trend is dubious, especially before 1950. To address this problem, we constructed and compared physically related monthly ENSO indices. Our base index was El Niño index NINO3.4, the sea surface temperature (SST) anomaly averaged over the equatorial box bounded by 5N, 5S, 170W and 120W; we also constructed indices based on the Nighttime Marine Air Temperature over the NINO3.4 region (NMAT3.4) and an Equatorial Southern Oscillation Index (ESOI). Our NINO3.4 index used the HadSST2 monthly data set (Rayner et al. 2006), a data set with smaller uncertainty and better geographical coverage than others. In constructing the index, data at each point for a given month were weighted to take into account the typical considerable spatial variation of the SST anomaly over the NINO3.4 box as well as the number of observations at that point for that month. Missing monthly data were interpolated and "noise" was reduced by using the result that NINO3.4 has essentially the same calendar month amplitude structure every year. This 12-point calendar month structure from April to March was obtained by an EOF analysis over the last 58 years, and then was fitted to the entire monthly time series using a least square approach. Equivalent procedures were followed for NMAT3.4 and ESOI. The new ESOI index uses Darwin atmospheric pressure in the west and is based on theory that allows for variations of the atmospheric boundary layer depth across the Pacific. The new NINO3.4 index was compared with NMAT3.4, the new ESOI and with a record of delta O18 from a coral at Palmyra, an atoll inside the region NINO3.4 (Cobb et al. 2003). Correlation coefficients between NINO3.4 and the three monthly indices mentioned above before 1950 are 0.84, 0.87, 0.73 and 0.93, 0.86, 0.73 for decadal time scales. These relatively high correlation coefficients between physically related but independent monthly time series suggest that we have improved our knowledge of low-frequency variability and trend. All four indices are consistent with a rise in NINO3.4 SST since the beginning of the record and the weakening of the equatorial Trade winds, especially since about 1970.
A13B-0237
ENSO from 1908-2007 in an Ocean GCM
Reconstructed SST records have been used to argue that the intensity and frequency of ENSO events has increased over the last 100 years. However the number of observations used in these reconstructions is limited in the beginning of the 20th century relative to the end of the 20th Century. A recently completed atmospheric reanalysis (C20r) opens up the opportunity to run an ocean general circulation model of the first half of the 20th century to explore the evolution of ENSO events over the last 100 years. The C20r winds are generated from an atmospheric model that assimilates a consistent set of surface pressure observations available from historic datasets. A global version of the POP model (the same as used in the SODA reanalysis) is used in simulation mode (SIMU 3.0.2, no assimilation) to hindcast the state of the tropical Pacific Ocean for the period from 1908 through 1958. To explore ENSO events in the later part of the 20th Century the SODA reanalysis (SODA 2.0.2/2.0.4) is used. SST anomalies are calculated by constructing independent climatologies for the two model products. Analysis of the resulting SST anomalies is then used to explore ENSO structure and evolution for the period from 1908 to 2007. In contrast with the reconstructed SST anomalies, the model NINO 3.4 SST anomalies show ENSO events in the early part of the 20th Century which are just as strong as those of the latter part of the 20th Century.
A13B-0238
Late 20th Century Warming and Freshening in the Central Tropical Pacific
Tropical Pacific climate variability explains a significant fraction of global temperature and precipitation variability, yet how the tropical Pacific might evolve under greenhouse forcing is still uncertain. Climate model projections and analyses of instrumental climate data support both a weakening and as well as a strengthening of the tropical Pacific zonal SST gradient1. Published modern corals from the central tropical Pacific (CTP) exhibit trends toward depleted coral oxygen isotopic (δ18O) values over the late 20th century2,3, suggesting that warming and/or seawater freshening has recently occurred in this region. It is important to understand the physical mechanisms that underlie these trends in order to better predict changes in tropical Pacific climate over the next decades. Here, we apply coral δ18O and Sr/Ca to coral cores from Palmyra, Fanning and Christmas Islands (2°N-6°N, 157°W- 162°W) located in the CTP in order to reconstruct the histories of SST and sea-surface salinity (SSS) over the period 1972-1998. The three islands span strong gradients in SST and SSS. The islands are aligned in a NW-SE fashion, with Christmas bathed by the South Equatorial Current and dominated by upwelling variability at the southern end; while Palmyra as the northern-most island, lies in the core of the North Equatorial Counter Current and is heavily influenced by ITCZ variability. We measure Sr/Ca ratio as a proxy for SST with an analytical precision of better than ± 0.3% (1σ). We measure coral δ18O with an analytical precision of ± 0.05‰ (1σ), and derive δ18O of seawater (δ18Osw – a proxy for SSS) by removing the Sr/Ca-derived SST contribution from coral δ18O. The Sr/Ca-derived SST reconstructions reveal warming trends ranging from 0.94 ± 1.55°C (1σ) at Palmyra to 1.65 ± 1.26°C (1σ) at Christmas that increase towards the equator, inferring a reduction in equatorial upwelling over the last three decades. The corals also reveal seawater freshening trends (δ18Osw trends of -0.12 to -0.33 ± 0.09‰, 1σ) that increase towards the ITCZ, coherent with an equatorward shift of the convergent zone that is dynamically consistent with warmer equatorial temperatures. Together, the coral reconstructions support a late 20th century trend towards a weakening of the Pacific zonal SST gradient, which is in agreement with projections of the majority of coupled global climate models under increased greenhouse forcing.
A13B-0239
Variation of Global Mean SST and Modulation of ENSO Activity in the 20th Century
Evidence is presented that the dominant non-trend mode of interdecadal global SST variations is linked to significant modulation of ENSO variance during the past 150 years. The mode resembles the interhemispheric SST pattern linked to Sahel rainfall changes, with colder northern hemisphere conditions co-incident with higher ENSO variance. Simulations of an intermediate coupled global climate model demonstrate that this global SST pattern can drive the interdecadal changes to ENSO variance. The influence on the ENSO dynamics comes primarily from the portion of the SST change local to the tropical Pacific, which is warm over the entire equatorial Pacific particularly in the southeast during times of stronger ENSO activity. We speculate that the warming equatorial SST causes ENSO events occur earlier and thus have longer time to develop, as well as enhancing the vertical temperature gradient and thus increasing the effectiveness of oceanic entrainment.
A13B-0240
Stronger Solar-ENSO connections than ENSO-rainfall connections in the Indian Monsoon
Evidence presented in this paper shows that connections of ENSO with solar activity parameters are statistically stronger than those between ENSO and Indian summer monsoon rainfall. The global wavelet cross spectrum between solar irradiance and (a) monsoon rainfall and (b) the global ENSO index, show significant power around the dominant period of the 11 year solar cycle, passing the chi-squared test of significance proposed by Torrence and Compo (1998) at levels exceeding 95% and 97.5% respectively. In particular, the confidence level for the solar irradiance--ENSO index is as high as 97.5%. On the other hand the global wavelet cross power spectrum between the ENSO index and the all India summer monsoon rainfall (AISMR) shows a peak in the 3-4 year period range at significance levels of only 70%. The computed cross- spectrum shows that the average rainfall is higher and the average ENSO index lower during a test period (1933-1964) of greater solar activity; the z-test confidence levels for the solar irradiance-sunspot-AISMR- ENSO connections exceed 95%. Even over the slightly longer period of 1850-1998 over which the AISMR has been reconstructed, statistical analysis reveals significant influence by sunspots and solar irradiance, particularly over the two test-periods of 1878-1913 and 1933-1964, respectively representing three complete cycles of lowest and highest solar activity in the 148 y record. An increase in solar activity is accompanied by a decrease in ENSO and an increase in rainfall at the 8-16 y band. The link between ENSO and solar activity (95%) is weaker than the link between solar activity and rainfall (97.5%) but stronger than that between ENSO and rainfall (70%). The effect of Nino 3.4 positive tendencies on the monsoon rainfall is to decrease the rainfall. Both these effects occur in the test periods of lower solar activity. The global cross spectrum for ENSO and solar activity index exhibits peaks around the 11 y period at confidence levels of 90% or higher. This confirms that there is a stronger connection between ENSO and solar activity as compared to the ENSO- rainfall connection. Greater solar activity is associated with lower ENSO activity and vice-versa, supporting the well-known inverse relationship between ENSO and rainfall. Over the two test periods an increase in solar activity is associated with a decrease in the ENSO index and an increase in the monsoon rainfall in the 8-16 y period band, but has the opposite effect in the 2-7 y period band. The net effect of solar processes on rainfall thus appears to be the result of counteracting influences on short and long periods, the latter on the whole dominating over the former. The present results suggest that Indian rainfall is affected by solar processes in part indirectly through ENSO. (1) S. Bhattacharyya, R. Narasimha, 2005, Possible association between Indian monsoon rainfall and solar activity, GRL, 32, doi: 10.1029/2004GL021044, 2005. (2) Torrence C. and Webster P. J., 1999: Interdecadal Changes in the ENSO-Monsoon System, Journal of Climate, 12, 2679-2690. (3) Bhattacharyya, S., and R. Narasimha (2007), Regional differentiation in multidecadal connections between Indian monsoon rainfall and solar activity, J. Geophys. Res., 112, D24103, doi:10.1029/2006JD008353.
A13B-0241
Non Stationary And Non Gaussian Character Of ENSO: The Role Of Climate Shifts And Nonlinearities
ENSO is the dominant climate mode of variability in the Pacific, having socio-economical impacts on surrounding regions. ENSO exhibits a significant modulation at decadal to interdecadal timescales which is associated to changes of its characteristics (onset, amplitude, frequency, propagation, and predictability). Among these characteristics, some of them are generally ignored in ENSO studies, such as its asymmetry and the deviation of its statistics from those of the Gaussian distribution. These properties could be related to the ability of the current generation of coupled models to predict ENSO and its modulation. Here, the non-Gaussian nature and asymmetry of ENSO is diagnosed from in situ data and the outputs of a variety of models (from intermediate complexity models to full physics coupled general circulation models) using robust statistical tools. In particular alpha-stable laws are used as theoretical background material to quantify the non-Gaussian character of ENSO time series. It is shown that the Alpha-stable character of ENSO may result from the presence of climate shifts inducing non stationnarity in the time series. Also, cool (warm) periods are associated with ENSO statistics having a larger (weaker) tendency towards Gaussianity and a weaker (larger) asymmetry. This supports the hypothesis of ENSO being rectified by changes in mean state through non-linear processes. The relationship between change in mean state and non-linearity is investigated both in the Zebiak and Cane model and the IPCC models, which indicate that the propensity of a model to reproduce extreme events is related to its tendency to emphasize the non-linear interaction between mean state and ENSO variability. More particularly, high statistical moments i.e. high order nonlinearities seem to be involved in the feedback between extreme events occurrence and mean state shift triggering through inverse energy cascade, emphasizing the ENSO multifractal nature.
A13B-0242
Pre and post 1997/1998 Westerly Wind Events and equatorial Pacific cold tongue warming
Westerly Wind Events (WWEs) in the western equatorial Pacific have previously been shown to cause significant warming of sea surface temperature (SST) in the eastern equatorial Pacific. Observational statistics compiled during and prior to the large El Nino event of 1997/1998 link WWEs to substantial (up to 3C) warming in the eastern Pacific cold-tongue region. Since 1998, however, relatively little WWE-related cold tongue warming has been observed and warm equatorial Pacific SST anomalies (SSTAs) have tended to be trapped near the dateline rather than extending to the American coast as in a classical El Nino-Southern Oscillation (ENSO) composite. Here, we revisit the relationship between WWEs and cold-tongue warming using in situ and operational forecast winds and in situ and satellite-based SST. We find significant differences in the basin scale zonal wind anomalies associated with WWEs that occurred before and after 1997/1998. Although the post 1997/1998 composite WWE westerly anomalies are very similar to their predecessors within the WWE regions, conditions east of the WWE regions are different; there are enhanced equatorial easterlies in the post 1997/1998 cases. General ocean circulation model experiments are conducted to explore the extent to which the observed changes in the character of post 1997/1998 WWEs can explain the recent behavior of cold tongue SSTAs. We find that the wind differences can account for the changes in the average cold tongue warming associated with pre and post 1997/1998 WWEs.
A13B-0243
CMIP3 Multi-model Comparison on the Interannual Variability of the Upper Ocean Heat Content in the Tropical Pacific
Interannual variability of the upper ocean heat content (OHC) in the tropical Pacific in the 20th century simulations and the 21st century of A1B scenario experiments in the WCRP CMIP3 multi-model data set. We focus on the propagation of OHC anomalies in the tropical Pacific related to the El Nino-Southern Oscillation (ENSO). In both 20th and 21st century simulations, cyclic and anticlockwise propagations of the OHC anomaly in the tropical North Pacific are recognized in almost all models. However, OHC propagation in the off-equator is different from that in the observation. This is due to a different atmospheric response to the equatorial SST and a different forcing of wind stress curl to the upper ocean. Furthermore, it is suggested that the models with shorter ENSO period tend to be associated with OHC anomalies that contribute to ENSO phase transition. On the other hand, the models with longer ENSO period, OHC anomalies could promote ENSO growth through the thermocline feedback. Changes in the frequency and amplitude of ENSO from 20th to 21st century differ from model to model. ENSO frequency does not depend on change in the mean state and amplitude of Nino3 SST or zonal mean equatorial OHC. In the models with ENSO period longer in the 21st century than that in the 20th century, time lag of the propagation of OHC anomalies between the western and eastern equatorial Pacific is longer. It is suggested that a change in ENSO frequency from 20th to 21st century is caused by that in the period of warm water accumulation in the western equatorial Pacific due to a change in wind forcing in the off- equatorial North Pacific.
A13B-0244
Sensitivity of the Mean State and ENSO Variability in the NCEP Climate Forecast System to Changes in SST Bias
Coupled general circulation models (CGCMs) tend to exhibit large tropical biases, including the Southeast Pacific (SEP) warm SST bias. There is growing evidence that mean state errors could substantially limit ENSO variability and predictability in current CGCMs, and therefore reduce reliability of these models' projections of climate change effect on ENSO. In this study, we investigate the influence of the mean SEP SST and Tropics-wide surface wind stress errors in the NCEP Climate Forecast System (CFS) on the quality of its tropical climate and ENSO simulation. We use empirical correction approach to constrain long-term means of these quantities to observations. Several experiments have been conducted with either surface heat or momentum flux adjustments at the air-sea interface. Our results show that both mean ocean-atmosphere circulation and interannual variability in CFS is highly sensitive to the SEP SST bias. Moreover, its reduction exposes and amplifies other systematic errors in CFS, such as an excessive cross-equatorial flow in the eastern Pacific, which appear to be related to an overly active Inter-Tropical Convergence Zone (ITCZ). These ITCZ errors to a large extent are also present in the SST-forced simulation with the atmospheric component of CFS and probably not directly linked to the SST. SEP warm bias in CFS therefore acts as a means to compensate for the ITCZ errors. And as our results suggest, the latter could lead to the development of a cold SST bias at the equator, deepening of the equatorial thermocline and considerable reduction of ENSO amplitude in this model. On the other hand, more realistic mean wind stress forcing of the ocean in CFS markedly improves a number of ENSO characteristics (e.g., its amplitude, dominant timescale, irregularity and duration of warm events), although the SEP SST bias is amplified as a result. To get better insight into the mean state influence on the ENSO cycle in CFS, results from coupled model experiments are analyzed within the framework of the recharge oscillator model of Jin.
A13B-0245
Probability Distribution Function of the Upper Equatorial Pacific Current Speeds
The probability distribution function (PDF) of the upper (0-50 m) tropical Pacific current speeds (w),
constructed from hourly ADCP data (1990-2007) at six stations for the TOGA-TAO project, satisfies the two-
parameter Weibull distribution reasonably well with different characteristics between El Nino and La Nina
events: In the western Pacific, the PDF of w has a larger peakedness during the La Nina events than during
the El Nino events; and vice versa in the eastern Pacific. However, the PDF of w for the lower layer (100-200
m) does not fit the Weibull distribution so well as the upper layer. This is due to the different stochastic
differential equations between upper and lower layers in the tropical Pacific. For the upper layer, the
stochastic differential equations, established on the base of the Ekman dynamics, have analytical solution,
i.e., the Rayleigh distribution (simplest form of the Weibull distribution), for constant eddy viscosity K.
Knowledge on PDF of w during the El Nino and La Nina events will improve the ensemble horizontal flux
calculation, which contributes to the climate studies.
http://faculty.nps.edu/pcchu
A13B-0246
Dynamical Impacts of Tropical Upwelling Cell on the Meridional Scales of ENSO Anomalies
El Nino/Southern Oscillation (ENSO) originates in the equatorial Pacific through interactions between the ocean and the atmosphere. Its sea surface temperature (SST) anomalies can be transported towards sides of the equator with the upwelling effects. The SST anomalies meridional extent during La Nina years is wider than that during El Nino years, because strong trade winds strengthen upwelling and meridional currents. With global warming, there exhibits abrupt weakening for the Walker Circulation in 1976/77. That would reduce trade winds, upwelling and meridional currents. The meridional widths both reduced in El Nino and La Nina events from the observations. However, the La Nina widths are more sensitive to zonal winds than El Nino's and the changes of La Nina widths are much larger than that of El Nino between pre- and post-1976.
A13B-0247
ENSO Feedbacks in a Multi-Model Ensemble: Role of the Near-Annual Mode
The background state of the equatorial Pacific determines the prevalence of the "slow" ENSO mode over a "fast" near-annual mode, the first being controlled to a large extent by the thermocline feedback, and the latter being related to enhanced zonal advective feedback. In this study we investigate the relative importance of these two feedbacks in the Coupled Model Intercomparison Project focusing on the interaction between near-annual and ENSO modes. The analysis reveals that the models exhibit a wide range of behaviour with respect to both the characteristics of the ENSO mode and the near-annual mode: we find that for most models enhanced (resp. diminished) near-annual variability is associated to faster (resp. slower) ENSO and a tendency towards a cooler mean state. The results of a vertical mode decomposition of the mean equatorial stratification and the analysis of the wind variability allow interpreting the models in which reduced zonal advective feedback is not necessarily associated to a slower ENSO mode. By the means of an intermediate coupled model of the tropical Pacific "tuned" from the CGCM outputs, the contribution of the advection terms (vertical versus zonal) to the rate of SST change is estimated, which allows classifying the CGCMs in 4 groups that account for the dominant feedback processes. The results support the interpretation that biases in the mean state are sustained/maintained by the privileged mode of variability associated to the dominant feedback mechanism in the models. In particular, enhanced near-annual mode variability that is proper to the models having a dominant zonal advective feedback is associated to significant SST asymmetry (or negative skewness) in the western equatorial Pacific. Overall our study suggests that interaction between ENSO variability and mean state may operate through the near-annual mode activity.
A13B-0248
ENSO / PDO-Like Variability of Tropical Ocean Surface Energy Fluxes Over the Satellite Era
Recent variations of tropical climate on interannual to near-decadal scales have provided a useful target for studying the nature of climate feedback processes. A strong warm / cold ENSO couplet (e.g. 1997-2000) along with several subsequent weaker events are prominent interannual signals that are part of an apparent longer term strengthening of the Walker circulation during the mid to late 1990's with some weakening thereafter. Decadal scale changes in tropical SST structure during the 1990s are accompanied by focusing of precipitation over the Indo-Pacific warm pool and an increase in tropical ocean evaporation of order 1.0% / decade. Associated with ENSO and PDO-like tropical SST changes are surface freshwater and radiative fluxes which have important implications for heat and energy transport variations. In this study we examine how surface fluxes attending interannual to decadal SST fluctuations, e.g. precipitation (GPCP, TRMM), turbulent fluxes (OAFlux), and radiative fluxes (ERBE / CERES, SRB) are coupled. Using these data we analyze vertically-integrated divergence of moist static energy, divMSE, and its dry static energy and latent energy components. We examine consistency between these data sets and explore relationships between SST variations, flux changes and modulation of tropical Walker and Hadley circulations. Strong signatures of MSE flux transport linking ascending and descending regions of tropical circulations are found. Relative strengths of these fluxes and transports are interpreted as a measure of efficiency in the overall process of tropical heat balance during episodes of warm or cold tropical SST.
A13B-0249
Impact of the Indian and the North Pacific Ocean on ENSO in a Hybrid Coupled Model
We examine the impacts of the Indian Ocean and the North Pacific Ocean on El Nino and the Southern Oscillation (ENSO) statistics through a series of numerical experiments with a hybrid coupled model. In the control run, an atmospheric general circulation model (AGCM) is coupled to the Zebiak-Cane simple ocean model in the tropical Pacific. Outside the tropical Pacific climotological sea surface temperatures are prescribed in the control simulation. In the first (second) experiment, a slab thermodynamic mixed layer model is coupled to the AGCM in the Indian (North Pacific) Ocean. The inclusion of the Indian Ocean has little impact on the ENSO amplitude, however, the Indian Ocean modifies the ENSO frequency via interactions with the Indian monsoon. The power spectrum of the Indian monsoon rainfall has significant biennial time scale around 20~30 months in the first experiment, which may enhance the biennial time scale of ENSO variability through a shift of the horizontal structure of zonal wind stress variability in the central equatorial Pacific. On the other hand, the North Pacific Ocean (i.e., the second experiment) differently acts to modify the ENSO statistics in terms of amplitude and frequency compared to the Indian Ocean. By comparing directly the two idealized experiments we discussed how differently the North Pacific Ocean and the Indian Ocean acts to the ENSO variability.
A13B-0250
Role of Stochastic Forcing in ENSO Variability in a Coupled GCM
The role of stochastic forcing (SF) in the El Niño – Southern Oscillation (ENSO) variability in a coupled general circulation model (CGCM) is examined. This is achieved by deriving an estimate of SF and using it to force a coupled ocean-atmosphere model of intermediate complexity. Empirical orthogonal functions (EOFs) of observed tropical oceanic and atmospheric surface variables are compared to the singular vectors of covariance matrix between these variables. The analysis suggests that while a large percentage of ocean surface variability is linearly coupled to the atmosphere, a significant amount of variance in the atmosphere is uncorrelated to any contemporaneous changes in ocean surface temperatures. This atmospheric component may act as SF on the coupled ocean-atmosphere system. A statistical model of surface zonal wind anomalies regressed with the sea surface temperature anomalies (SSTA) is constructed. Two versions are constructed – one using data from 163 years run of Bureau of Meteorology Research Center (BMRC) coupled model, and the other using last 29 years of data from NCEP-Department of Energy (DOE) - II reanalysis (NCEP-2). The surface zonal wind anomalies unpredicted by the statistical model are diagnosed for spatial and temporal characteristics, in order to be validated as SF. A coupled ocean-atmosphere model of intermediate complexity (variant of Zebiak-Cane model) is then driven by the derived SF. Experiments are conducted using various values of the ocean-atmosphere coupling strength. The resulting model ENSO is diagnosed for a broad range of characteristics. The diagnoses include statistical and spectral analysis, evolution of warm events, and seasonal variance locking. The results are compared with ENSO variability in the parent dataset of SF. An attempt is made to quantify the role of stochastic forcing in ENSO variability. Analysis suggests that SF plays a lesser role in the ENSO variability in the CGCM, than in reanalysis. Further, the seasonal locking of warm events in stochastically forced model is compared to that in the parent dataset of SF. It is found that the deviations between the two are more in CGCM than in reanalysis. Experiments involving synthetic SF – composed of both CGCM and reanalysis stochastic components – reveal that these deviations can be traced to the unrealistic seasonal variability of Madden Julian Oscillation (MJO) in the CGCM.
A13B-0251
Mechanisms of Decadal and Centennial ENSO Variability
The new convection scheme recently introduced to the Community Climate System Model (CCSM) yields substantial improvement in the simulation of the modern El Nino/Southern Oscillation (ENSO), altering ENSO behavior away from a simple delayed oscillator and toward a more stochastic system involving both atmospheric and oceanic feedbacks. The new convection scheme has a significant impact on ENSO dynamics, which exhibit a range of behavior even within a single model run. In particular, the role of pre- existing conditions in the eastern Pacific on amplifying El Nino events is extremely important in the new model. Interesting and unexpected correlations are seen between zonal wind/thermocline depth anomalies and variations in NINO3 index, a direct indication of the importance of stochastic forcing. Our results also indicate that ENSO forcing changes dramatically on decadal and centennial timescales within the model run; this implies that representative ENSO statistics require a much longer instrumental record than previously thought. Future work will focus on quantifying the ability of the model to identify the effects of stochastic forcing within Holocene and other paleorecords.
A13B-0252
ENSO and the North Pacific Gyre Oscillation: a dynamical framework to understand present and future climate
Climate change predictions from large-scale coupled ocean-land-atmosphere models (e.g. IPCC) are
potentially useful to predict changes in the climate statistics of ENSO and of decadal modes of Pacific
variability as global warming progresses. However, it still unclear if these models adequately represent the
fundamental dynamics controlling these modes of variability and their future changes.
This talk presents an improved dynamical framework of Pacific Climate Variability that allows us to test
and diagnose the coupled dynamics of ENSO and Decadal Variability. Such a framework is based on the
recently discovered link between ENSO and the North Pacific Gyre Oscillation (NPGO), a mode of decadal
climate variability that appears to intensity under global warming. Using this dynamical framework it is
possible to test the degree of realism of the coupled climate models during the modern period and of their
climate predictions.
http://www.o3d.org/npgo
A13B-0253
Time-mean and Seasonal Response of the Equatorial Pacific to Global Warming: Implications for ENSO-like Variability
The time-mean response of the equatorial Pacific to CO2 doubling (2xCO2) is investigated in an ensemble of coupled climate models. Robust responses are identified and linked with changes in the heat budget of the ocean surface layer. The heat budget changes in response to the anomalous 2xCO2 clear-sky downward radiation are dominated by enhanced vertical ocean heat transport associated with anomalous near-surface thermal stratification and by changes in surface fluxes associated with negative cloud and evaporation feedbacks. These ocean and atmospheric processes stabilize the sea surface temperature (SST) response in the cold tongue and the warm pool respectively. As a result, the ensemble-mean SST response shows a rather uniform basin-wide increase. Despite this absence of changes in the zonal SST gradient, the multi- model ensemble does exhibit additional robust responses that could influence ENSO-like variability. The annual cycle of the cold tongue strengthens due to increased ocean dynamical cooling by climatological upwelling driven by the anomalous near-surface thermal stratification. As a result, the amplitude of the seasonal cycle increases by about 0.5 K in the eastern Pacific. Furthermore, the models simulate a sharpening and shoaling of the equatorial thermocline which results both from the ocean thermodynamical and dynamical response to the anomalous surface warming and to the weakening of the trade winds. The models also simulate robust reductions in ocean zonal currents, Ekman divergence and equatorial upwelling in response to weaker trade winds. To conclude, all these robust responses in the mean-state and the seasonal cycle of the equatorial Pacific are linked with inter-model changes in ENSO-like variability through changes in the strength of the Bjerknes feedbacks.
A13B-0254
ENSO modulation during a global-warming progression
The multi-decadal modulation of the El Nino-Southern Oscillation (ENSO) during a global-warming progression has been analyzed herein by means of diagnostics of IPCC-AR4 coupled general circulation model (CGCMs) and the eigen analysis of a simplified version of an intermediate ENSO model. The amplitude and period of ENSO are fluctuating in a multi-decadal time scale as greenhouse gases increase. The climate system model outputs suggest that the multi-decadal modulation of ENSO is related to the delayed response of the subsurface temperature in the tropical Pacific compared to the response time of the sea surface temperature (SST), which would lead a modulation of the vertical temperature gradient. Furthermore, an eigen analysis considering only two parameters, the changes in the zonal contrast of the mean background SST and the changes in the vertical contrast between the mean surface and subsurface temperatures in the tropical Pacific, exhibits a good agreement with the CGCM outputs. In particular, the change in the vertical contrast in the equatorial eastern Pacific turns out to be more influential on the ENSO modulation than changes in the mean SST itself.
A13B-0255
Changes of the Shallow Pacific Meridional Overturning Circulation Under Global Warming
The equatorial thermocline in the Pacific ocean is known to be ventilated by the subtropical surface waters via the shallow meridional overturning circulation (aka. the subtropical cells, STCs). Both observational and modeling studies have shown such a tropical-subtropical linkage and suggested the variations of both the thermohaline properties of the source waters and the strength of the STCs that carry them can affect the equatorial thermocline, the sea surface temperature in the equatorial upwelling regions, and consequently the tropical climate and its variability. Under global warming the variations of both kinds are expected to happen and to impact the tropical climate in response to the changing wind stress and the surface density patterns. In this study, we diagnose the model output from the IPCC AR4 simulations and identify some robust changes of the STCs under global warming. The surface branch of the STCs (namely the poleward Ekman transport) is weakened, in accord with the weakening of the local easterlies that drive the STCs. The subsurface branch of the STCs (namely the equatorward pycnocline transport) is also weakened, but not so much as the surface branch. This weakening mainly takes place in the Northern Hemisphere, while the STCs transport in the Southern Hemisphere remains stable. In the equatorial regions, both the equatorial undercurrent (EUC) and the equatorial upwelling become shallower, consistent with the shoaling and flattening of the equatorial pycnocline. The EUC transport, however, is nearly invariant. It is found that there is also a weakening of the Indonesian Throughflow (ITF) which compensates the reduction of the equatorward pycnocline transport of the STCs, and leaves the pycnocline transport that actually ventilates the EUC insensitive to global warming. Despite this, the thermohaline properties of the ventilated water do change. A suite of adjoint passive tracer and backtracking Lagrangian trajectory experiments are carried out with an ocean general circulation model to study the changes of the subduction regions and the hydrographic properties of the source waters of the equatorial thermocline under global warming.
A13B-0256
Changes in ENSO Stability Under a Warmer Climate in CGCMs
In this study, a Bjerknes stability (BJ) index, proposed by Jin et al. (2007), is adopted to assess the overall stability of El Niño and Southern Oscillation (ENSO) in state-of-the-art coupled GCMs. The BJ index depicts the dependence of the growth rate of the leading coupled ENSO-like mode on the damping processes and the three dynamical positive feedbacks, specifically zonal advective feedback, thermocline feedback, and Ekman upwelling feedback. Each feedback depends on the model mean states and several other physical parameters, which assess the sensitivity of an atmospheric response to an ENSO SST anomaly and of an oceanic response to an anomalous surface wind. The 12 coupled models from the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) modeling groups are used for validating and estimating the BJ index. Through a comparison of the BJ index derived from the first 100-year monthly data for preindustrial experiments (control runs) and from the first 100-year after stabilization for CO2 increase experiments (CO2 runs), we investigate changes in dynamical feedbacks and atmospheric and oceanic responses associated with ENSO under a future warmer climate, and their impacts on ENSO stability. The models show a wide range of ENSO stability behavior from groups with relatively strongly damped ENSO modes to groups with relatively strongly growing ENSO modes. It is shown that models with a relatively large (small) ENSO amplitude exhibit a relatively large (small) BJ index, which is consistent with theoretical studies. The correlation of the above relation for the control runs and CO2 runs is 0.88 and 0.87, respectively. The diversity of the ENSO stability is attributed to different mean states and the different sensitivities of oceanic and atmospheric responses to a forcing from model to model. However, inconsistency between the models in terms of changes in the model sensitivity of the responses to a forcing and model mean states under a warmer climate makes it difficult to conclude the influence of greenhouse warming on the stability of the ENSO
A13B-0257
ENSO Response To Global Warming As Simulated By ECHO-G/S
Global warming may shift the properties and dynamics of ENSO. We study the changes in ENSO characteristics in a coupled general circulation model, ECHO-G/S. First, we analyse the mean state changes by comparing present day simulation and various high CO2 climates. The model shows a little El Nino-like changes in the sea surface temperature and wind stress in the eastern tropical Pacific. As the mean temperature rises, the ENSO amplitude and the frequency of strong El Ninos and La Nina decrease. The analysis shows that the weakening of the atmospheric and oceanic sensitivities is related to the weakening of ENSO. In addition to the surface changes, the remote subsurface sea temperature response in the western Pacific to the wind stress in the eastern Pacific influences the subsequent ENSO amplitude. However, ENSO amplitude does not show linear response to the greenhouse gas concentrations. Acknowledgement: This research was supported by a project,¢®¡Æmetri-2008-B-5¢®¡¾.
A13B-0258
Re-reading the IPCC Report: Aerosols, Droughts and ENSO Events
The Technical Summary of Working Group One in the IPCC Fourth Assessment Report states that "changes in aerosols may have affected precipitation and other aspects of the hydrologic cycle more strongly than other anthropogenic forcing agents" and that "Simulations also suggest that absorbing aerosols, particularly black carbon, can reduce the solar radiation reaching the surface and can warm the atmosphere at regional scales, affecting the vertical temperature profile and the large-scale atmospheric circulation". Taking these two statements at face value I first identify eight seasonal, anthropogenic, regional scale, aerosol plumes which now occur each year and then report the correlation of the aerosol optical depth (AOD) of some of these plumes with climate anomalies in the higher latitudes and with ENSO events. The eight identified aerosol plumes vary significantly in extent and AOD inter annually. They have also increased in geographic extent and AOD over recent decades as the population in the tropics, the origin of the majority of these plumes, has increased dramatically requiring increased levels of agriculture and commercial activity. I show that: the AOD of the South East Asian Plume, occurring from late July to November, correlates with four characteristics of drought in south eastern Australia; the aerosol index of the Middle East Plume correlates negatively with rainfall in Darfur; and the volume of tephra ejected by volcanoes in south east Asia correlates: negatively with rainfall and water inflows into the Murray River in south eastern Australia; and positively with ENSO events over the period 1890/91 to 2006. I conclude that aerosol plumes over south eastern Asia are the cause of drought in south eastern Australia and ENSO events and confirm the statements made in the IPCC Report with respect to these aerosol plumes. I propose a new component of surface aerosol radiative forcing, Regional Dimming, which interferes with the seasonal movement of the Inter Tropical Convergence Zone and forces the regional Hadley Cells into anomalous seasonal positions producing blocking high pressure systems in the higher latitudes and causing climate change by altering the wind systems. The South East Asian Plume also creates ENSO events by altering or inhibiting the circulation of the Walker Cell which changes the MSL pressure relationship between Darwin and Tahiti creating Southern Oscillation Index events and reducing the wind speed in the central Pacific Ocean causing an increase in the sea surface temperatures in the Nino 3.4 area. Finally I suggest that further research on the effects of these eight regional scale aerosol plumes on the hydrologic cycle and large-scale atmospheric circulation using a global circulation model is crucial to the understanding and attribution of climate change and is urgently required.
A13B-0259
ENSO signals in the middle atmosphere of a multi-model ensemble for twentieth-century simulation to year 2000
ENSO signals in temperature and zonal wind in the middle atmosphere are analyzed for the coupled model simulations participating in the third phase of the Coupled Model Intercomparison Project (CMIP3) and they are compared with those of the obervation (ERA-40 Reanalysis). Analysis period is the recent 50 years from 1951 to 2000 for the simulations, while it is the recent 22 years from 1980 to 2001 for ERA-40. Multiple linear regression analysis is used to isolate the ENSO signals from the zonal-mean anomalies in temperature and zonal wind for the simulations and the observation. Reference (explanatory) variables are the mean value, the linear trend, the QBOs at 20 and 50 hPa, volcanic aerosols of El Chichon and Mount Pinatubo, ENSO (SOI), and the 11-year solar cycles. Each coefficient is expanded by annual, semiannual, and triannual cycles to explain seasonality. None of the simulations reproduces the QBO, but the zonal winds at 20 and 50 hPa over the equator are used as reference variables, similarly to those in ERA-40. The observed annual average ENSO signal for the zonal mean temperature in the tropics is more or less reproduced in terms of the spatial pattern of the tropospheric warming and the stratospheric cooling, although the amplitudes are underestimated. The observed mid-latitude stratospheric warmings in both hemispheres are also qualitatively captured. The ENSO signal of zonal-mean zonal wind, represented as the subtropical jet intensification in the equatorward flank in both hemispheres and its extension into the lower stratosphere is also reproduced.