Showing papers in "Journal of Climate in 2009"

Indian Ocean Capacitor Effect on Indo–Western Pacific Climate during the Summer following El Niño

Abstract: Significant climate anomalies persist through the summer (June-August) after El Nino dissipates in spring over the equatorial Pacific. They include the tropical Indian Ocean (TIO) sea surface temperature (SST) warming, increased tropical tropospheric temperature, an anomalous anticyclone over the subtropical northwest Pacific, and increased mei-yu-baiu rainfall over East Asia. The cause of these lingering El Nino effects during summer is investigated using observations and an atmospheric general circulation model (GCM). The results herein indicate that the TIO warming acts like a capacitor anchoring atmospheric anomalies over the Indo-western Pacific Oceans. It causes tropospheric temperature to increase by a moist-adiabatic adjustment in deep convection, emanating a baroclinic Kelvin wave into the Pacific. In the northwest Pacific, this equatorial Kelvin wave induces northeasterly surface wind anomalies, and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone. The GCM results support this Kelvin wave-induced Ekman divergence mechanism. In response to a prescribed SST increase over the TIO, the model simulates the Kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest Pacific. An additional experiment further indicates that the north Indian Ocean warming is most important for the Kelvin wave and northwest Pacific anticyclone, a result corroborated by observations. These results have important implications for the predictability of Indo-western Pacific summer climate: the spatial distribution and magnitude of the TIO warming, rather than simply whether there is an El Nino in the preceding winter, affect summer climate anomalies over the Indo-western Pacific and East Asia.

Contrasting Eastern-Pacific and Central-Pacific Types of ENSO

Abstract: Surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of El Nino–Southern Oscillation (ENSO) in the tropical Pacific: an eastern-Pacific (EP) type and a central-Pacific (CP) type. An analysis method combining empirical orthogonal function (EOF) analysis and linear regression is used to separate these two types. Correlation and composite analyses based on the principal components of the EOF were performed to examine the structure, evolution, and teleconnection of these two ENSO types. The EP type of ENSO is found to have its SST anomaly center located in the eastern equatorial Pacific attached to the coast of South America. This type of ENSO is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical Indian Ocean. In contrast, the CP type of ENSO has most of its surface wind, SST, and subsurface anomalies confined in the central Pacific and tends to onset, develop, and decay in situ. ...

Two Types of El Niño Events: Cold Tongue El Niño and Warm Pool El Niño

Abstract: In this study, two types of El Nino events are classified based on spatial patterns of the sea surface temperature (SST) anomaly. One is the cold tongue (CT) El Nino, which can be regarded as the conventional El Nino, and the other the warm pool (WP) El Nino. The CT El Nino is characterized by relatively large SST anomalies in the Nino-3 region (5°S–5°N, 150°–90°W), while the WP El Nino is associated with SST anomalies mostly confined to the Nino-4 region (5°S–5°N, 160°E–150°W). In addition, spatial patterns of many atmospheric and oceanic variables are also distinctively different for the two types of El Nino events. Furthermore, the difference in the transition mechanism between the two types of El Nino is clearly identified. That is, the discharge process of the equatorial heat content associated with the WP El Nino is not efficient owing to the spatial structure of SST anomaly; as a result, it cannot trigger a cold event. It is also demonstrated that zonal advective feedback (i.e., zonal adve...

Toward Optimal Closure of the Earth's Top-of-Atmosphere Radiation Budget

Abstract: Despite recent improvements in satellite instrument calibration and the algorithms used to determine reflected solar (SW) and emitted thermal (LW) top-of-atmosphere (TOA) radiative fluxes, a sizeable imbalance persists in the average global net radiation at the TOA from satellite observations. This imbalance is problematic in applications that use earth radiation budget (ERB) data for climate model evaluation, estimate the earth’s annual global mean energy budget, and in studies that infer meridional heat transports. This study provides a detailed error analysis of TOA fluxes based on the latest generation of Clouds and the Earth’s Radiant Energy System (CERES) gridded monthly mean data products [the monthly TOA/surface averages geostationary (SRBAVG-GEO)] and uses an objective constrainment algorithm to adjust SW and LW TOA fluxes within their range of uncertainty to remove the inconsistency between average global net TOA flux and heat storage in the earth–atmosphere system. The 5-yr global mean...

Changes in Continental Freshwater Discharge from 1948 to 2004

Abstract: A new dataset of historical monthly streamflow at the farthest downstream stations for the world’s 925 largest ocean-reaching rivers has been created for community use. Available new gauge records are added to a network of gauges that covers ∼80 × 106 km2 or ∼80% of global ocean-draining land areas and accounts for about 73% of global total runoff. For most of the large rivers, the record for 1948–2004 is fairly complete. Data gaps in the records are filled through linear regression using streamflow simulated by a land surface model [Community Land Model, version 3 (CLM3)] forced with observed precipitation and other atmospheric forcings that are significantly (and often strongly) correlated with the observed streamflow for most rivers. Compared with previous studies, the new dataset has improved homogeneity and enables more reliable assessments of decadal and long-term changes in continental freshwater discharge into the oceans. The model-simulated runoff ratio over drainage areas with and witho...

The summer North Atlantic Oscillation: past, present, and future.

Abstract: Summer climate in the North Atlantic‐European sector possesses a principal pattern of year-to-year variability that is the parallel to the well-known North Atlantic Oscillation in winter. This summer North Atlantic Oscillation (SNAO) is defined here as the first empirical orthogonal function (EOF) of observed summertime extratropical North Atlantic pressure at mean sea level. It is shown to be characterized by a more northerly location and smaller spatial scale than its winter counterpart. The SNAO is also detected by cluster analysis and has a near-equivalent barotropic structure on daily and monthly time scales. Although of lesser amplitude than its wintertime counterpart, the SNAO exerts a strong influence on northern European rainfall, temperature, and cloudiness through changes in the position of the North Atlantic storm track. It is, therefore, of key importance in generating summer climate extremes, including flooding, drought, and heat

Simulations of Global Hurricane Climatology, Interannual Variability, and Response to Global Warming Using a 50-km Resolution GCM

Abstract: A global atmospheric model with roughly 50-km horizontal grid spacing is used to simulate the interannual variability of tropical cyclones using observed sea surface temperatures (SSTs) as the lower boundary condition. The model’s convective parameterization is based on a closure for shallow convection, with much of the deep convection allowed to occur on resolved scales. Four realizations of the period 1981–2005 are generated. The correlation of yearly Atlantic hurricane counts with observations is greater than 0.8 when the model is averaged over the four realizations, supporting the view that the random part of this annual Atlantic hurricane frequency (the part not predictable given the SSTs) is relatively small (<2 hurricanes per year). Correlations with observations are lower in the east, west, and South Pacific (roughly 0.6, 0.5, and 0.3, respectively) and insignificant in the Indian Ocean. The model trends in Northern Hemisphere basin-wide frequency are consistent with the observed trends i...

Evaluating the “Rich-Get-Richer” Mechanism in Tropical Precipitation Change under Global Warming

Abstract: Examining tropical regional precipitation anomalies under global warming in 10 coupled global climate models, several mechanisms are consistently found. The tendency of rainfall to increase in convergence zones with large climatological precipitation and to decrease in subsidence regions—the rich-get-richer mechanism—has previously been examined in different approximations by Chou and Neelin, and Held and Soden. The effect of increased moisture transported by the mean circulation (the “direct moisture effect” or “thermodynamic component” in respective terminology) is relatively robust, while dynamic feedback is poorly understood and differs among models. The argument outlined states that the thermodynamic component should be a good approximation for large-scale averages; this is confirmed for averages across convection zones and descent regions, respectively. Within the convergence zones, however, dynamic feedback can substantially increase or decrease precipitation anomalies. Regions of negative...

A New Moist Turbulence Parameterization in the Community Atmosphere Model

Abstract: A new moist turbulence parameterization is presented and implemented in the Community Atmosphere Model (CAM). It is derived from Grenier and Bretherton but has been heavily modified to improve its numerical stability and efficiency with the long time steps used in climate models. A goal was to provide a more physically realistic treatment of marine stratocumulus-topped boundary layers than in the current CAM. Key features of the scheme include use of moist-conserved variables, an explicit entrainment closure for convective layers, diagnosis of turbulent kinetic energy (TKE) for computation of turbulent diffusivities, an efficient new formulation of TKE transport as a relaxation to layer-mean TKE, and unified treatment of all turbulent layers in each atmospheric column. The scheme is compared with the default turbulence parameterizations in the CAM using three singlecolumn modeling cases, using both operational and high vertical and time resolution. Both schemes performed comparably well on the dry convective boundary layer case. For a stable boundary layer case, the default CAM overdeepens the boundary layer unless its free-tropospheric mixing length is greatly reduced, whereupon the new scheme and default CAM again both perform well at both tested resolutions. A nocturnal stratocumulus case was much better simulated by the new scheme than the default CAM, with much less resolution sensitivity. Global climate simulations with the new scheme in tandem with a new shallow cumulus parameterization are presented in a companion paper.

Forced and Internal Twentieth-Century SST Trends in the North Atlantic*

Abstract: In recent years, two alarming trends in North Atlantic climate have been noted: an increase in the intensity and frequency of Atlantic hurricanes and a rapid decrease in Greenland ice sheet volume. Both of these phenomena occurred while a significant warming took place in North Atlantic sea surface temperatures (SSTs), thus sparking a debate on whether the warming is a consequence of natural climate variations, anthropogenic forcing, or both; and if both, what their relative roles are. Here models and observations are used to detect and attribute long-term (multidecadal) twentieth-century North Atlantic (NA) SST changes to their anthropogenic and natural causes. A suite of Intergovernmental Panel on Climate Change (IPCC) twentieth-century (C20C) coupled model simulations with multiple ensemble members and a signal-to-noise maximizing empirical orthogonal function analysis are used to identify a model-based estimate of the forced, anthropogenic component in NA SST variability. Comparing the results to observations, it is argued that the long-term, observed, North Atlantic basin-averaged SSTs combine a forced global warming trend with a distinct, local multidecadal ‘‘oscillation’’ that is outside of the range of the model-simulated, forced component and most likely arose from internal variability. This internal variability produced a cold interval between 1900 and 1930, followed by 30 yr of relative warmth and another cold phase from 1960 to 1990, and a warming since then. This natural variation, referred to previously as the Atlantic Multidecadal Oscillation (AMO), thus played a significant role in the twentieth-century NA SST variability and should be considered in future, near-term climate projections as a mechanism that, depending on its behavior, can act either constructively or destructively with the region’s response to anthropogenic influence, temporarily amplifying or mitigating regional climate change.

The University of Washington Shallow Convection and Moist Turbulence Schemes and Their Impact on Climate Simulations with the Community Atmosphere Model

Abstract: This paper describes a new version of the University of Washington shallow cumulus parameterization. The new version includes improved treatments of lateral mixing rates into cumulus updrafts, the evaporation of precipitation and of the interaction of cumuli with the underlying subcloud layer, and a treatment of the convective inhibition-based mass-flux closure that is more numerically stable and is suitable for the long time steps of global climate models. The paper also documents its performance when combined with a new moist turbulence parameterization in simulations with version 3.5 of the Community Atmosphere Model (CAM3.5). A single-column simulation of nonprecipitating trade cumulus shows considerable improvements in vertical thermodynamic structure and less resolution sensitivity in the new schemes compared to CAM3.5. In global simulations, the new schemes, combined with an increase of vertical resolution from 26 to 30 levels, produce a significant (7%) reduction in overall climate bias, ...

Why the Western Pacific Subtropical High Has Extended Westward since the Late 1970s

Abstract: The western Pacific subtropical high (WPSH) is closely related to Asian climate. Previous examination of changes in the WPSH found a westward extension since the late 1970s, which has contributed to the inter-decadal transition of East Asian climate. The reason for the westward extension is unknown, however. The present study suggests that this significant change of WPSH is partly due to the atmosphere's response to the observed Indian Ocean-western Pacific (IWP) warming. Coordinated by a European Union's Sixth Framework Programme, Understanding the Dynamics of the Coupled Climate System (DYNAMITE), five AGCMs were forced by identical idealized sea surface temperature patterns representative of the IWP warming and cooling. The results of these numerical experiments suggest that the negative heating in the central and eastern tropical Pacific and increased convective heating in the equatorial Indian Ocean/ Maritime Continent associated with IWP warming are in favor of the westward extension of WPSH. The SST changes in IWP influences the Walker circulation, with a subsequent reduction of convections in the tropical central and eastern Pacific, which then forces an ENSO/Gill-type response that modulates the WPSH. The monsoon diabatic heating mechanism proposed by Rodwell and Hoskins plays a secondary reinforcing role in the westward extension of WPSH. The low-level equatorial flank of WPSH is interpreted as a Kelvin response to monsoon condensational heating, while the intensified poleward flow along the western flank of WPSH is in accord with Sverdrup vorticity balance. The IWP warming has led to an expansion of the South Asian high in the upper troposphere, as seen in the reanalysis.

The Response of Northern Hemisphere Snow Cover to a Changing Climate

Abstract: A snowpack model sensitivity study, observed changes of snow cover in the NOAA satellite dataset, and snow cover simulations from the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset are used to provide new insights into the climate response of Northern Hemisphere (NH) snow cover. Under conditions of warming and increasing precipitation that characterizes both observed and projected climate change over much of the NH land area with seasonal snow cover, the sensitivity analysis indicated snow cover duration (SCD) was the snow cover variable exhibiting the strongest climate sensitivity, with sensitivity varying with climate regime and elevation. The highest snow cover–climate sensitivity was found in maritime climates with extensive winter snowfall—for example, the coastal mountains of western North America (NA). Analysis of trends in snow cover duration during the 1966–2007 period of NOAA data showed the largest decreases were concentrated in a zone where seasonal mean air ...

On the Nature of Soil Moisture in Land Surface Models

Abstract: The soil moisture state simulated by a land surface model is a highly model-dependent quantity, meaning that the direct transfer of one model’s soil moisture into another can lead to a fundamental, and potentially detrimental, inconsistency. This is first illustrated with two recent examples, one from the National Centers for Environmental Prediction (NCEP) involving seasonal precipitation forecasting and another from the realm of ecological modeling. The issue is then further addressed through a quantitative analysis of soil moisture contents produced as part of a global offline simulation experiment in which a number of land surface models were driven with the same atmospheric forcing fields. These latter comparisons clearly demonstrate, on a global scale, the degree to which model-simulated soil moisture variables differ from each other and that these differences extend beyond those associated with model-specific layer thicknesses or soil texture. The offline comparisons also show, however, th...

Role of Air-Sea Interaction in the Long Persistence of El Nino-Induced North Indian Ocean Warming*

Abstract: El Nino induces a basin-wide increase in tropical Indian Ocean (TIO) sea surface temperature (SST) with a lag of one season. The north IO (NIO), in particular, displays a peculiar double-peak warming with the second peak larger in magnitude and persisting well through the summer. Motivated by recent studies suggesting the importance of the TIO warming for the Northwest Pacific and East Asian summer monsoons, the present study investigates the mechanisms for the second peak of the NIO warming using observations and general circulation models. This analysis reveals that internal air-sea interaction within the TIO is key to sustaining the TIO warming through summer. During El Nino, anticyclonic wind curl anomalies force a downwelling Rossby wave in the south TIO through Walker circulation adjustments, causing a sustained SST warming in the tropical southwest IO (SWIO) where the mean thermocline is shallow. During the spring and early summer following El Nino, this SWIO warming sustains an antisymmetric pattern of atmospheric anomalies with northeasterly ( northwesterly) wind anomalies north ( south) of the equator. Over the NIO as the mean winds turn into southwesterly in May, the northeasterly anomalies force the second SST peak that persists through summer by reducing the wind speed and surface evaporation. Atmospheric general circulation model experiments show that the antisymmetric atmospheric pattern is a response to the TIO warming, suggestive of their mutual interaction. Thus, ocean dynamics and Rossby waves in particular are important for the warming not only locally in SWIO but also on the basin-scale north of the equator, a result with important implications for climate predictability and prediction.

Projected Changes in Mean and Extreme Precipitation in Africa under Global Warming. Part I: Southern Africa

Abstract: Probable changes in mean and extreme precipitation in East Africa are estimated from general circulation models (GCMs) prepared for the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4). Bayesian statistics are used to derive the relative weights assigned to each member in the multimodel ensemble. There is substantial evidence in support of a positive shift of the whole rainfall distribution in East Africa during the wet seasons. The models give indications for an increase in mean precipitation rates and intensity of high rainfall events but for less severe droughts. Upward precipitation trends are projected from early this (twenty first) century. As in the observations, a statistically significant link between sea surface temperature gradients in the tropical Indian Ocean and short rains (October–December) in East Africa is simulated in the GCMs. Furthermore, most models project a differential warming of the Indian Ocean during boreal autumn. This is favorable for an increa...

Seasonality and Interannual Variability of the Westerly Jet in the Tibetan Plateau Region

Abstract: In this study, 40-yr ECMWF Re-Analysis (ERA-40) data are used for the description of the seasonal cycle and the interannual variability of the westerly jet in the Tibetan Plateau region. To complement results based on the analysis of monthly mean horizontal wind speeds, an occurrence-based jet climatology is constructed by identifying the locations of the jet axes at 6-hourly intervals throughout 1958–2001. Thus, a dataset describing the highly transient and localized features of jet variability is obtained. During winter and summer the westerly jet is located, respectively, to the south and north of the Tibetan Plateau. During the spring and autumn seasons there are jet transitions from south to north and vice versa. The median dates for these transitions are 28 April and 12 October. The spring transition is associated with large interannual variations, while the fall transition occurs more reliably within a 3-week period. The strength of the jet exhibits a peculiar seasonal cycle. During northw...

ENSO and Extreme Rainfall Events in South America

Abstract: The influence of the opposite phases of ENSO on the frequency of extreme rainfall events over South America is analyzed for each month of the ENSO cycle on the basis of a large set of daily station rainfall data and compared with the influence of ENSO on the monthly total rainfall. The analysis is carried out with station data and their gridded version and the results are consistent. Extreme events are defined as 3-day mean precipitation above the 90th percentile. The mean frequencies of extreme events are determined for each month and for each category of year (El Nino, La Nina, and neutral), and the differences between El Nino and neutral years and La Nina and neutral years are computed. Changes in the mean intensity of extreme events are also investigated. Significant ENSO signals in the frequency of extreme events are found over extensive regions of South America during different periods of the ENSO cycle. Although ENSO-related changes in intensity show less significance and spatial coherence...

Application of MJO Simulation Diagnostics to Climate Models

Abstract: The ability of eight climate models to simulate the Madden‐Julian oscillation (MJO) is examined using diagnostics developed by the U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group. Although the MJO signal has been extracted throughout the annual cycle, this study focuses on the boreal winter (November‐April) behavior. Initially, maps of the mean state and variance and equatorial space‐time spectra of 850-hPa zonal wind and precipitation are compared with observations. Models best represent the intraseasonal space‐time spectral peak in the zonal wind compared to that of precipitation. Using the phase‐ space representation of the multivariate principal components (PCs), the life cycle properties of the simulated MJOs are extracted, including the ability to represent how the MJO evolves from a given subphase and the associated decay time scales. On average, the MJO decay (e-folding) time scale for all models is shorter (;20‐ 29 days) than observations (;31 days). All models are able to produce a leading pair of multivariate principal components that represents eastward propagation of intraseasonal wind and precipitation anomalies, although the fraction of the variance is smaller than observed for all models. In some cases, the dominant time scale of these PCs is outside of the 30‐80-day band. Several key variables associated with the model’s MJO are investigated, including the surface latent heat flux, boundary layer (925 hPa) moisture convergence, and the vertical structure of moisture. Low-level moisture convergence ahead (east) of convection is associated with eastward propagation in most of the models. A few models are also able to simulate the gradual moistening of the lower troposphere that precedes observed MJO convection, as well as the observed geographical difference in the vertical structure of moisture associated with the MJO. The dependence of rainfall on lower tropospheric relative humidity and the fraction of rainfall that is stratiform are also discussed, including implications these diagnostics have for MJO simulation. Based on having the most realistic intraseasonal multivariate empirical orthogonal functions, principal component power spectra, equatorial eastward propagating outgoing longwave radiation (OLR), latent heat flux, low-level moisture convergence signals, and vertical structure of moisture over the Eastern Hemisphere, the superparameterized Community Atmosphere Model (SPCAM) and the ECHAM4/ Ocean Isopycnal Model (OPYC) show the best skill at representing the MJO.

Will Extratropical Storms Intensify in a Warmer Climate

Abstract: Extratropical cyclones and how they may change in a warmer climate have been investigated in detail with a high-resolution version of the ECHAM5 global climate model. A spectral resolution of T213 (63 km) is used for two 32-yr periods at the end of the twentieth and twenty-first centuries and integrated for the Intergovernmental Panel on Climate Change (IPCC) A1B scenario. Extremes of pressure, vorticity, wind, and precipitation associated with the cyclones are investigated and compared with a lower-resolution simulation. Comparison with observations of extreme wind speeds indicates that the model reproduces realistic values. This study also investigates the ability of the model to simulate extratropical cyclones by computing composites of intense storms and contrasting them with the same composites from the 40-yr ECMWF Re-Analysis (ERA-40). Composites of the time evolution of intense cyclones are reproduced with great fidelity; in particular the evolution of central surface pressure is almost ex...

The Soil Moisture–Precipitation Feedback in Simulations with Explicit and Parameterized Convection

Abstract: Moist convection is a key aspect of the extratropical summer climate and strongly affects the delicate balance of processes that determines the surface climate in response to larger-scale forcings. Previous studies using parameterized convection have found that the feedback between soil moisture and precipitation is predominantly positive (more precipitation over wet soils) over Europe. Here this feedback is investigated for one full month (July 2006) over the Alpine region using two different model configurations. The first one employs regional climate simulations performed with the Consortium for Small-Scale Modeling Model in Climate Mode (CCLM) on a grid spacing of 25 km. The second one uses the same model but integrated on a cloud-resolving grid of 2.2 km, allowing an explicit treatment of convection. Each configuration comprises one control and two sensitivity experiments. The latter start from perturbed soil moisture initial conditions. Comparison of the simulated soil moisture–precipitatio...

Drought in the Southeastern United States: Causes, Variability over the Last Millennium, and the Potential for Future Hydroclimate Change*

Abstract: An assessment of the nature and causes of drought in the southeastern United States is conducted as well as an assessment of model projections of anthropogenically forced hydroclimate change in this region The study uses observations of precipitation, model simulations forced by historical SSTs from 1856 to 2007, tree-ring records of moisture availability over the last millennium, and climate change projections conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change From the perspective of the historical record, the recent drought that began in winter 2005/06 was a typical event in terms of amplitude and duration Observations and model simulations are used to show that dry winter half-years in the Southeast are weakly associated with La Ninas in the tropical Pacific but that this link varies over time and was possibly of opposite sign from about 1922 to 1950 Summer-season precipitation variability in the Southeast appears governed by purely internal atmosph

Seasonally evolving dominant interannual variability modes of East Asian climate.

Abstract: A season-reliant empirical orthogonal function (S-EOF) analysis is applied to seasonal mean precipitation over East Asia for the period of 1979-2004. The first two dominant modes account for 44% of the total interannual variance, corresponding to post-ENSO and ENSO turnabout years, respectively. The first mode indicates that in El Nino decaying summer, an anomalous anticyclone appears over the western North Pacific (WNP). This anticyclone is associated with strong positive precipitation anomalies from central China to southern Japan. In the following fall, enhanced convection appears over the WNP as a result of the un- derlying warm SST anomalies caused by the increase of the shortwave radiative flux in the preceding sum- mer. A dry condition appears over southeastern China. The anomalous precipitation pattern persists throughout the subsequent winter and spring. The second mode shows that during the El Nino developing summer the anomalous heating over the equatorial central Pacific forces a cyclonic vorticity over the WNP. This strengthens the WNP monsoon. Meanwhile, an anomalous anticyclone develops in the northern Indian Ocean and moves eastward to the South China Sea and the WNP in the subsequent fall and winter. This leads to the increase of precipitation over southeastern China. The anticyclone and precipitation anomalies are maintained in the following spring through local air-sea interactions. The diagnosis of upper-level velocity potential and midlevel vertical motion fields reveals a season- dependent Indian Ocean forcing scenario. The Indian Ocean basinwide warming during the El Nino mature winter and the subsequent spring does not have a significant impact on anomalous circulation in the WNP, because convection over the tropical Indian Ocean is suppressed by the remote forcing from the equatorial central-eastern Pacific. The basinwide warming plays an active role in impacting the WNP anomalous an- ticyclone during the ENSO decaying summer through atmospheric Kelvin waves or Hadley circulation.

Global and Continental Drought in the Second Half of the Twentieth Century: Severity–Area–Duration Analysis and Temporal Variability of Large-Scale Events

Abstract: Using observation-driven simulations of global terrestrial hydrology and a cluster algorithm that searches for spatially connected regions of soil moisture, the authors identified 296 large-scale drought events (greater than 500 000 km2 and longer than 3 months) globally for 1950–2000. The drought events were subjected to a severity–area–duration (SAD) analysis to identify and characterize the most severe events for each continent and globally at various durations and spatial extents. An analysis of the variation of large-scale drought with SSTs revealed connections at interannual and possibly decadal time scales. Three metrics of large-scale drought (global average soil moisture, contiguous area in drought, and number of drought events shorter than 2 years) are shown to covary with ENSO SST anomalies. At longer time scales, the number of 12-month and longer duration droughts follows the smoothed variation in northern Pacific and Atlantic SSTs. Globally, the mid-1950s showed the highest drought a...

Hadley Cell Widening: Model Simulations versus Observations

Abstract: Observations show that the Hadley cell has widened by about 28‐58 since 1979. This widening and the concomitant poleward displacement of the subtropical dry zones may be accompanied by large-scale drying near308Nand308S.Suchdryingposesarisktoinhabitantsoftheseregionswhoareaccustomedtoestablished rainfall patterns. Simple and comprehensive general circulation models (GCMs) indicate that the Hadley cell may widen in response to global warming, warming of the west Pacific, or polar stratospheric cooling. The combinationofthesefactorsmayberesponsiblefortherecentobservations.Butthereisnostudysofarthathas compared the observed widening to GCM simulations of twentieth-century climate integrated with historical changes in forcings. Here the Hadley cell widening is assessed in current GCMs from historical simulations of thetwentiethcenturyaswellasfutureclimateprojectionsandpreindustrialcontrolruns.Theauthorsfindthat observedwideningcannotbeexplainedbynaturalvariability.Thisobservedwideningisalsosignificantlylarger than in simulations of the twentieth and twenty-first centuries. These results illustrate the need for further investigation into the discrepancy between the observed and simulated widening of the Hadley cell.

The Moist Static Energy Budget of a Composite Tropical Intraseasonal Oscillation in a Climate Model

Abstract: The intraseasonal moist static energy (MSE) budget is analyzed in a climate model that produces realistic eastward-propagating tropical intraseasonal wind and precipitation variability. Consistent with the recharge–discharge paradigm for tropical intraseasonal variability, a buildup of column-integrated MSE occurs within low-level easterly anomalies in advance of intraseasonal precipitation, and a discharge of MSE occurs during and after precipitation when westerly anomalies occur. The strongest MSE anomalies peak in the lower troposphere and are, primarily, regulated by specific humidity anomalies. The leading terms in the column-integrated intraseasonal MSE budget are horizontal advection and surface latent heat flux, where latent heat flux is dominated by the wind-driven component. Horizontal advection causes recharge (discharge) of MSE within regions of anomalous equatorial lower-tropospheric easterly (westerly) anomalies, with the meridional component of the moisture advection dominating the...

An Observed Connection between the North Atlantic Oscillation and the Madden-Julian Oscillation

Abstract: Based on the bivariate Madden–Julian oscillation (MJO) index defined by Wheeler and Hendon and 25 yr (1979–2004) of pentad data, the association between the North Atlantic Oscillation (NAO) and the MJO on the intraseasonal time scale during the Northern Hemisphere winter season is analyzed. Time-lagged composites and probability analysis of the NAO index for different phases of the MJO reveal a statistically significant two-way connection between the NAO and the tropical convection of the MJO. A significant increase of the NAO amplitude happens about 5–15 days after the MJO-related convection anomaly reaches the tropical Indian Ocean and western Pacific region. The development of the NAO is associated with a Rossby wave train in the upstream Pacific and North American region. In the Atlantic and African sector, there is an extratropical influence on the tropical intraseasonal variability. Certain phases of the MJO are preceded by the occurrence of strong NAOs. A significant change of upper zonal ...

Lifetime of Anthropogenic Climate Change: Millennial Time Scales of Potential CO2 and Surface Temperature Perturbations

Abstract: Multimillennial simulations with a fully coupled climate–carbon cycle model are examined to assess the persistence of the climatic impacts of anthropogenic CO2 emissions. It is found that the time required to absorb anthropogenic CO2 strongly depends on the total amount of emissions; for emissions similar to known fossil fuel reserves, the time to absorb 50% of the CO2 is more than 2000 yr. The long-term climate response appears to be independent of the rate at which CO2 is emitted over the next few centuries. Results further suggest that the lifetime of the surface air temperature anomaly might be as much as 60% longer than the lifetime of anthropogenic CO2 and that two-thirds of the maximum temperature anomaly will persist for longer than 10 000 yr. This suggests that the consequences of anthropogenic CO2 emissions will persist for many millennia.

Homogenization of Temperature Series via Pairwise Comparisons

Abstract: An automated homogenization algorithm based on the pairwise comparison of monthly temperature series is described. The algorithm works by forming pairwise difference series between serial monthly temperature values from a network of observing stations. Each difference series is then evaluated for undocumented shifts, and the station series responsible for such breaks is identified automatically. The algorithm also makes use of station history information, when available, to improve the identification of artificial shifts in temperature data. In addition, an evaluation is carried out to distinguish trend inhomogeneities from abrupt shifts. When the magnitude of an apparent shift attributed to a particular station can be reliably estimated, an adjustment is made for the target series. The pairwise algorithm is shown to be robust and efficient at detecting undocumented step changes under a variety of simulated scenarios with step- and trend-type inhomogeneities. Moreover, the approach is shown to yield a lower false-alarm rate for undocumented changepoint detection relative to the more common use of a reference series. Results from the algorithm are used to assess evidence for trend inhomogeneities in U.S. monthly temperature data.

Climate Response of the Equatorial Pacific to Global Warming

Abstract: The climate response of the equatorial Pacific to increased greenhouse gases is investigated using numerical experiments from 11 climate models participating in the Intergovernmental Panel on Climate Change’s Fourth Assessment Report. Multimodel mean climate responses to CO2 doubling are identified and related to changes in the heat budget of the surface layer. Weaker ocean surface currents driven by a slowing down of the Walker circulation reduce ocean dynamical cooling throughout the equatorial Pacific. The combined anomalous ocean dynamical plus radiative heating from CO2 is balanced by different processes in the western and eastern basins: Cloud cover feedbacks and evaporation balance the heating over the warm pool, while increased cooling by ocean vertical heat transport balances the warming over the cold tongue. This increased cooling by vertical ocean heat transport arises from increased near-surface thermal stratification, despite a reduction in vertical velocity. The stratification respo...