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Sumant Nigam
,
Isaac M. Held
, and
Steven W. Lyons

Abstract

The quantitative validity of linear stationary wave theory is examined by comparing the results from a linear primitive equation model on the sphere with the stationary eddies produced by a general circulation model (GCM). The GCM simulated has a flat lower boundary, so that the stationary eddies can be thought of as forced by heating (sensible. latent and radiative) and time-averaged transient eddy flux convergences. Orographic forcing is examined in the second part of this study. The distribution of the diabatic heating and transient eddy flux convergences and the zonally symmetric basic state are taken directly from the GCM's climatology for Northern winter (DJF). Strong Rayleigh friction is included in the linear model wherever the zonal mean wind is small, as well as near the surface.

The linear model is found to simulate the stationary eddy pattern of the GCM with considerable skin in both midlatitudes and the tropics. Some deficiencies include the inaccurate simulation of the upper tropospheric geopotential over North America and distortion of the wind field near the low-level zero-wind line in the subtropics. Decomposition of the linear solution shows that 1) the extratropical upper tropospheric eddy pattern generated by tropical forcing is significant but smaller than due to extratropical forcing, 2) the upper-level extratropical pattern deteriorates somewhat when forcing by transients is removed, while the low-level pattern deteriorates dramatically and 3) there is considerable compensation between the effects of low-level thermal transients and extratropical sensible heating, to the point that we argue that this decomposition is not physically meaningful. The sensitivity of the results to the Rayleigh friction.formulation is discussed, as is the effect of replacing the transients with thermal damping.

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Ernesto Muñoz
,
Antonio J. Busalacchi
,
Sumant Nigam
, and
Alfredo Ruiz-Barradas

Abstract

The Caribbean region shows maxima in easterly winds greater than 12 m s−1 at 925 hPa in July and February, herein referred to as the summer and winter Caribbean low-level jet (LLJ), respectively. It is important to understand the controls and influences of the Caribbean LLJ because other LLJs have been observed to be related to precipitation variability. The purpose of this study is to identify the mechanisms of the Caribbean LLJ formation and variability and their association to the regional hydroclimate. Climatological fields are calculated from the North American Regional Reanalysis and the 40-yr ECMWF Re-Analysis from 1979 to 2001. It is observed that the low-level (925 hPa) zonal wind over the Caribbean basin has a semiannual cycle and an interannual variability, with greater standard deviation during boreal summer. The semiannual cycle has peaks in February and July, which are regional amplifications of the large-scale circulation. High mountains to the south of the Caribbean Sea influence the air temperature meridional gradient, providing a baroclinic structure that favors a stronger easterly wind. The boreal summer strengthening of the Caribbean LLJ is associated with subsidence over the subtropical North Atlantic from the May-to-July shift of the ITCZ and the evolution of the Central American monsoon. Additionally, the midsummer minimum of Caribbean precipitation is related to the Caribbean LLJ through greater moisture flux divergence. From May to September the moisture carried by the Caribbean LLJ into the Gulf of Mexico is strongest. The summer interannual variability of the Caribbean LLJ is due to the variability of the meridional pressure gradient across the Caribbean basin, influenced by tropical Pacific variability during summer.

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Sumant Nigam
,
Natalie P. Thomas
,
Alfredo Ruiz-Barradas
, and
Scott J. Weaver

Abstract

The linear trend in twentieth-century surface air temperature (SAT)—a key secular warming signal—exhibits striking seasonal variations over Northern Hemisphere continents; SAT trends are pronounced in winter and spring but notably weaker in summer and fall. The SAT trends in historical twentieth-century climate simulations informing the Intergovernmental Panel for Climate Change’s Fifth Assessment show varied (and often unrealistic) strength and structure, and markedly weaker seasonal variation. The large intra-ensemble spread of winter SAT trends in some historical simulations was surprising, especially in the context of century-long linear trends, with implications for the detection of the secular warming signal.

The striking seasonality of observed secular warming over northern continents warrants an explanation and the representation of related processes in climate models. Here, the seasonality of SAT trends over North America is shown to result from land surface–hydroclimate interactions and, to an extent, also from the secular change in low-level atmospheric circulation and related thermal advection. It is argued that the winter dormancy and summer vigor of the hydrologic cycle over middle- to high-latitude continents permit different responses to the additional incident radiative energy from increasing greenhouse gas concentrations.

The seasonal cycle of climate, despite its monotony, provides an expanded phase space for the exposition of the dynamical and thermodynamical processes generating secular warming, and an exceptional cost-effective opportunity for benchmarking climate projection models.

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Ching-Yee Chang
,
Sumant Nigam
, and
James A. Carton

Abstract

This study makes the case that westerly bias in the surface winds of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 3 (CAM3), over the equatorial Atlantic in boreal spring has its origin in the rainfall (diabatic heating) bias over the tropical South American continent. The case is made by examination of the spatiotemporal evolution of regional precipitation and wind biases and by dynamical diagnoses of the westerly wind bias from experiments with a steady, linearized dynamical core of an atmospheric general circulation model. Diagnostic modeling indicates that underestimating rainfall over the eastern Amazon region can lead to the westerly bias in equatorial Atlantic surface winds.

The study suggests that efforts to reduce coupled model biases, especially seasonal ones, must target continental biases, even in the deep tropics where ocean–atmosphere interaction generally rules.

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Kristopher B. Karnauskas
,
Alfredo Ruiz-Barradas
,
Sumant Nigam
, and
Antonio J. Busalacchi

Abstract

The Palmer drought severity index (PDSI) monitors meteorological and surface hydrological parameters to represent the severity of drought conditions. PDSI datasets are developed for the NCEP North American Regional Reanalysis (NARR) and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) to facilitate North American drought research with these datasets. The drought index calculation, in particular, allows diagnostic assessment of the relative contributions of various surface water balance terms in generation of drought conditions by selectively holding these terms to their climatological value in PDSI computations. The length of the diagnosed PDSI permits analysis of subdecadal time-scale variability, such as ENSO, whose influence on North American drought evolution is investigated. ENSO’s considerable drought impact is potentially predictable, especially in the southern half of the United States.

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Ching-Yee Chang
,
James A. Carton
,
Semyon A. Grodsky
, and
Sumant Nigam

Abstract

The Community Climate System Model version 3 (CCSM3) has a dipolelike pattern with a cold bias in the northern Tropics and a warm bias in the southeastern Tropics, which is reminiscent of the observed pattern of climate variability in boreal spring. Along the equator, in contrast, in boreal spring CCSM3 exhibits striking westerly winds with easterly winds in the upper troposphere, in turn reminiscent of the observed pattern of climate variability in boreal summer. The westerly winds cause a deepening of the eastern thermocline that keeps the east warm despite enhanced coastal upwelling. Thus, the bias in the seasonal cycle of the coupled model appears to project at least partially onto the spatial patterns of natural climate variability in this sector.

Information about the origin of the bias in CCSM3 is deduced from a comparison of CCSM3 with a simulation using specified historical SST to force the Community Atmospheric Model version 3 (CAM3). The patterns of bias in CAM3 resemble those apparent in CCSM3, including the appearance of substantially intensified subtropical bands of sea level pressure (SLP), indicating that the problem may be traced to difficulties in the atmospheric component model. Positive SLP bias also appears in the western tropical region, which may be related to deficient Amazonian precipitation. The positive SLP bias seems to be the cause of the anomalous westerly trade winds in boreal spring, and those in turn appear to be responsible for the anomalous deepening of the thermocline in the southeastern Tropics.

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Semyon A. Grodsky
,
James A. Carton
,
Sumant Nigam
, and
Yuko M. Okumura

Abstract

This paper focuses on diagnosing biases in the seasonal climate of the tropical Atlantic in the twentieth-century simulation of the Community Climate System Model, version 4 (CCSM4). The biases appear in both atmospheric and oceanic components. Mean sea level pressure is erroneously high by a few millibars in the subtropical highs and erroneously low in the polar lows (similar to CCSM3). As a result, surface winds in the tropics are ~1 m s−1 too strong. Excess winds cause excess cooling and depressed SSTs north of the equator. However, south of the equator SST is erroneously high due to the presence of additional warming effects. The region of highest SST bias is close to southern Africa near the mean latitude of the Angola–Benguela Front (ABF). Comparison of CCSM4 to ocean simulations of various resolutions suggests that insufficient horizontal resolution leads to the insufficient northward transport of cool water along this coast and an erroneous southward stretching of the ABF. A similar problem arises in the coupled model if the atmospheric component produces alongshore winds that are too weak. Erroneously warm coastal SSTs spread westward through a combination of advection and positive air–sea feedback involving marine stratocumulus clouds.

This study thus highlights three aspects to improve to reduce bias in coupled simulations of the tropical Atlantic: 1) large-scale atmospheric pressure fields; 2) the parameterization of stratocumulus clouds; and 3) the processes, including winds and ocean model resolution, that lead to errors in seasonal SST along southwestern Africa. Improvements of the latter require horizontal resolution much finer than the 1° currently used in many climate models.

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Justin Sheffield
,
Suzana J. Camargo
,
Rong Fu
,
Qi Hu
,
Xianan Jiang
,
Nathaniel Johnson
,
Kristopher B. Karnauskas
,
Seon Tae Kim
,
Jim Kinter
,
Sanjiv Kumar
,
Baird Langenbrunner
,
Eric Maloney
,
Annarita Mariotti
,
Joyce E. Meyerson
,
J. David Neelin
,
Sumant Nigam
,
Zaitao Pan
,
Alfredo Ruiz-Barradas
,
Richard Seager
,
Yolande L. Serra
,
De-Zheng Sun
,
Chunzai Wang
,
Shang-Ping Xie
,
Jin-Yi Yu
,
Tao Zhang
, and
Ming Zhao

Abstract

This is the second part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the twentieth-century simulations of intraseasonal to multidecadal variability and teleconnections with North American climate. Overall, the multimodel ensemble does reasonably well at reproducing observed variability in several aspects, but it does less well at capturing observed teleconnections, with implications for future projections examined in part three of this paper. In terms of intraseasonal variability, almost half of the models examined can reproduce observed variability in the eastern Pacific and most models capture the midsummer drought over Central America. The multimodel mean replicates the density of traveling tropical synoptic-scale disturbances but with large spread among the models. On the other hand, the coarse resolution of the models means that tropical cyclone frequencies are underpredicted in the Atlantic and eastern North Pacific. The frequency and mean amplitude of ENSO are generally well reproduced, although teleconnections with North American climate are widely varying among models and only a few models can reproduce the east and central Pacific types of ENSO and connections with U.S. winter temperatures. The models capture the spatial pattern of Pacific decadal oscillation (PDO) variability and its influence on continental temperature and West Coast precipitation but less well for the wintertime precipitation. The spatial representation of the Atlantic multidecadal oscillation (AMO) is reasonable, but the magnitude of SST anomalies and teleconnections are poorly reproduced. Multidecadal trends such as the warming hole over the central–southeastern United States and precipitation increases are not replicated by the models, suggesting that observed changes are linked to natural variability.

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Siegfried Schubert
,
David Gutzler
,
Hailan Wang
,
Aiguo Dai
,
Tom Delworth
,
Clara Deser
,
Kirsten Findell
,
Rong Fu
,
Wayne Higgins
,
Martin Hoerling
,
Ben Kirtman
,
Randal Koster
,
Arun Kumar
,
David Legler
,
Dennis Lettenmaier
,
Bradfield Lyon
,
Victor Magana
,
Kingtse Mo
,
Sumant Nigam
,
Philip Pegion
,
Adam Phillips
,
Roger Pulwarty
,
David Rind
,
Alfredo Ruiz-Barradas
,
Jae Schemm
,
Richard Seager
,
Ronald Stewart
,
Max Suarez
,
Jozef Syktus
,
Mingfang Ting
,
Chunzai Wang
,
Scott Weaver
, and
Ning Zeng

Abstract

The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land–atmosphere feedbacks on regional drought. The runs were carried out with five different atmospheric general circulation models (AGCMs) and one coupled atmosphere–ocean model in which the model was continuously nudged to the imposed SST forcing. This paper provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Niño–Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic multidecadal oscillation (AMO), and a global trend pattern.

One of the key findings is that all of the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific forcing pattern, with a cold Pacific leading to reduced precipitation and a warm Pacific leading to enhanced precipitation over most of the United States. While the response to the Atlantic pattern is less robust, there is general agreement among the models that the largest precipitation response over the United States tends to occur when the two oceans have anomalies of opposite signs. Further highlights of the response over the United States to the Pacific forcing include precipitation signal-to-noise ratios that peak in spring, and surface temperature signal-to-noise ratios that are both lower and show less agreement among the models than those found for the precipitation response. The response to the positive SST trend forcing pattern is an overall surface warming over the world’s land areas, with substantial regional variations that are in part reproduced in runs forced with a globally uniform SST trend forcing. The precipitation response to the trend forcing is weak in all of the models.

It is hoped that these early results, as well as those reported in the other contributions to this special issue on drought, will serve to stimulate further analysis of these simulations, as well as suggest new research on the physical mechanisms contributing to hydroclimatic variability and change throughout the world.

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Justin Sheffield
,
Andrew P. Barrett
,
Brian Colle
,
D. Nelun Fernando
,
Rong Fu
,
Kerrie L. Geil
,
Qi Hu
,
Jim Kinter
,
Sanjiv Kumar
,
Baird Langenbrunner
,
Kelly Lombardo
,
Lindsey N. Long
,
Eric Maloney
,
Annarita Mariotti
,
Joyce E. Meyerson
,
Kingtse C. Mo
,
J. David Neelin
,
Sumant Nigam
,
Zaitao Pan
,
Tong Ren
,
Alfredo Ruiz-Barradas
,
Yolande L. Serra
,
Anji Seth
,
Jeanne M. Thibeault
,
Julienne C. Stroeve
,
Ze Yang
, and
Lei Yin

Abstract

This is the first part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the historical simulations of continental and regional climatology with a focus on a core set of 17 models. The authors evaluate the models for a set of basic surface climate and hydrological variables and their extremes for the continent. This is supplemented by evaluations for selected regional climate processes relevant to North American climate, including cool season western Atlantic cyclones, the North American monsoon, the U.S. Great Plains low-level jet, and Arctic sea ice. In general, the multimodel ensemble mean represents the observed spatial patterns of basic climate and hydrological variables but with large variability across models and regions in the magnitude and sign of errors. No single model stands out as being particularly better or worse across all analyses, although some models consistently outperform the others for certain variables across most regions and seasons and higher-resolution models tend to perform better for regional processes. The CMIP5 multimodel ensemble shows a slight improvement relative to CMIP3 models in representing basic climate variables, in terms of the mean and spread, although performance has decreased for some models. Improvements in CMIP5 model performance are noticeable for some regional climate processes analyzed, such as the timing of the North American monsoon. The results of this paper have implications for the robustness of future projections of climate and its associated impacts, which are examined in the third part of the paper.

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