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Renu Joseph
and
Sumant Nigam

Abstract

This study focuses on the assessment of the spatiotemporal structure of ENSO variability and its winter climate teleconnections to North America in the Intergovernmental Panel on Climate Change’s (IPCC) Fourth Assessment Report (AR4) simulations of twentieth-century climate. The 1950–99 period simulations of six IPCC models are analyzed in an effort to benchmark models in the simulation of this leading mode of interannual variability: the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model version 2.1 (CM2.1), the coupled ocean–atmosphere model of the Goddard Institute for Space Studies (GISS-EH), the NCAR Community Climate System Model version 3 (CCSM3), the NCAR Parallel Coupled Model (PCM), the Hadley Centre Coupled Atmosphere–Ocean General Circulation Model version 3 (HadCM3), and version 3.2 of the Model for Interdisciplinary Research on Climate at high resolution [MIROC3.2 (hires)].

The standard deviation of monthly SST anomalies is maximum in the Niño-3 region in all six simulations, indicating progress in the modeling of ocean–atmosphere variability. The broad success in modeling ENSO’s SST footprint—quite realistic in CCSM3—is however tempered by the difficulties in modeling ENSO evolution: for example, the biennial oscillation in CCSM3 and the lack of regular warm-to-cold phase transition in the MIROC model. The spatiotemporal structure, including seasonal phase locking, is, on the whole, well modeled by HadCM3; but there is room for improvement, notably, in modeling the SST footprint in the western Pacific.

ENSO precipitation anomalies over the tropical Pacific and links to North American winter precipitation are also realistic in the HadCM3 simulation and, to an extent, in PCM. Hydroclimate teleconnections that lean on a stationary component of the flow, such as surface air temperature links, are however not well modeled by HadCM3 since the midlatitude ridge in the ENSO response is incorrectly placed in the simulation; PCM fares better.

The analysis reveals that climate models are improving but are still unable to simulate many features of ENSO variability and its circulation and hydroclimate teleconnections to North America. Predicting regional climate variability/change remains an onerous burden on models.

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Stephen Baxter
and
Sumant Nigam

Abstract

The Pacific–North American (PNA) teleconnection is a major mode of Northern Hemisphere wintertime climate variability, with well-known impacts on North American temperature and precipitation. To assess whether the PNA teleconnection has extended predictability, comprehensive data analysis is conducted to elucidate PNA evolution, with an emphasis on patterns of PNA development and decay. These patterns are identified using extended empirical orthogonal function (EEOF) and linear regression analyses on pentad-resolution atmospheric circulation data from the new Climate Forecast System Reanalysis (CFSR). Additionally, dynamical links between the PNA and another important mode of wintertime variability, the North Atlantic Oscillation (NAO), are analyzed both in the presence and absence of notable tropical convections, for example, the Madden–Julian oscillation (MJO), which is known to be influential on both. The relationship is analyzed using EEOF and regression techniques.

It is shown that the PNA structure is similar in both space and time when the MJO is linearly removed from the dataset. Furthermore, there is a small but significant lag between the NAO and PNA, with the NAO leading a PNA of opposite phase on time scales of one to three pentads. It is suggested from barotropic vorticity analysis that this relationship may result in part from excitation of Rossby waves by the NAO in the Asian waveguide.

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Sumant Nigam
and
Isaac M. Held

Abstract

A nondivergent barotropic model on a sphere is used to study the effects of a critical latitude on stationary atmospheric waves forced by topography. Linear and “quasi-linear” calculations are performed with an idealized wavenumber 3 mountain and with realistic topography. Quasi-linear dynamics, where mean flow changes are due to momentum flux convergence, “form drag” and relation to a prescribed climatological mean flow, produces an S-shaped kink in the zonal mean absolute vorticity gradient near the critical latitude, resulting in enhanced reflection. The component of the quasi-linear solution resulting from enhanced reflection at the critical latitude is computed by taking the difference between the linear and the quasi-linear solutions. In a calculation with realistic topography and zonal flow, this reflected component is found to be dominated by a wave train emanating from the western tropical Pacific and propagating northward and then eastward across the Pacific 0cean and the North American continent. This wave train results from the reflection of the Himalayan wave train at the zero-wind latitude in the tropical winter troposphere.

The vorticity gradients in the monthly mean statistics of Oort (1983) show structure near the critical latitude similar to that produced in our quasi-linear model, suggesting that some reflection of incident Rossby waves is likely in the atmosphere, at least in the western Pacific, and that the wind structure responsible for this reflection may be created in part by the stationary Rossby waves themselves.

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Sumant Nigam
and
Richard S. Lindzen

Abstract

A linear, primitive equation stationary wave model having high vertical and meridional resolution is used to examine the sensitivity of orographically forced (primarily by Himalayas) stationary waves at middle and high latitudes to variations in the basic state zonal wind distribution. We find relatively little sensitivity to the winds in high latitude but remarkable sensitivity to small variations in the subtropical jet. Fluctuations well within the range of observed variability in the jet can lead to large variations in the stationary waves of the high latitude stratosphere, and to large changes even in tropospheric stationery waves. Implications for both sudden warmings and large-scale weather are discussed.

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Richard S. Lindzen
and
Sumant Nigam

Abstract

We examine the importance of pressure gradients due to surface temperature gradients to low-level (p ≥ 700 mb) flow and convergence in the tropics over time scales ≳ 1 month. The latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall.

Our approach is to consider a simple one-layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically—consistent with ECMWF analyzed data. The top of the layer is taken at 700 mb. The influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer. Horizontal convergence within the layer is taken up by cumulonimbus mass flux. However, the development of the cumulonimbus mass flux is associated with a short relaxation time [O(½ hr)] (roughly the development time for such convection). During this short time, horizontal convergence acts to redistribute mass so as to reduce horizontal pressure gradients. This effect proves important in the immediate neighborhood of the equator.

Our results show that flows forced directly by surface temperature are often comparable to observed low-level flows in both magnitude and distribution.

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Sumant Nigam
and
Alfredo Ruiz-Barradas

Abstract

The monotony of seasonal variability is often compensated by the complexity of its spatial structure—the case in North American hydroclimate. The structure of hydroclimate variability is analyzed to provide insights into the functioning of the climate system and climate models.

The consistency of hydroclimate representation in two global [40-yr ECMWF Re-Analysis (ERA-40) and NCEP] and one regional [North American Regional Reanalysis (NARR)] reanalysis is examined first, from analysis of precipitation, evaporation, surface air temperature (SAT), and moisture flux distributions. The intercomparisons benchmark the recently released NARR data and provide context for evaluation of the simulation potential of two state-of-the-art atmospheric models [NCAR's Community Atmospheric Model (CAM3.0) and NASA's Seasonal-to-Interannual Prediction Project (NSIPP) atmospheric model].

Intercomparisons paint a gloomy picture: great divergence in global reanalysis representations of precipitation, with the eastern United States being drier in ERA-40 and wetter in NCEP in the annual mean by up to a third in each case; model averages are like ERA-40. The annual means, in fact, mask even larger but offsetting seasonal departures.

Analysis of moisture transport shows winter fluxes to be more consistently represented. Summer flux convergence over the Gulf Coast and Great Plains, however, differs considerably between global and regional reanalyses. Flux distributions help in understanding the choice of rainy season, especially the winter one in the Pacific Northwest; stationary fluxes are key.

Land–ocean competition for convection is too intense in the models—so much so that the oceanic ITCZ in July is southward of its winter position in the both simulations! The overresponsiveness of land is also manifest in SAT; the winter-to-summer change over the Great Plains is 5–9 K larger than in observations, with implications for modeling of climate sensitivity.

The nature of atmospheric water balance over the Great Plains is probed, despite unbalanced moisture budgets in reanalyses and model simulations. The imbalance is smaller in NARR but still unacceptably large, resulting from excessive evaporation in spring and summer. Adjusting evaporation during precipitation assimilation could lead to a more balanced budget.

Global and regional reanalysis will remain of limited use for hydroclimate studies until they comply with the operative water and energy balance constraints.

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Megan E. Linkin
and
Sumant Nigam

Abstract

The North Pacific Oscillation (NPO) in sea level pressure and its upper-air geopotential height signature, the west Pacific (WP) teleconnection pattern, constitute a prominent mode of winter midlatitude variability, the NPO/WP. Its mature-phase expression is identified from principal component analysis of monthly sea level pressure variability as the second leading mode just behind the Pacific–North American variability pattern.

NPO/WP variability, primarily on subseasonal time scales, is characterized by a large-scale meridional dipole in SLP and geopotential height over the Pacific and is linked to meridional movements of the Asian–Pacific jet and Pacific storm track modulation. The hemispheric height anomalies at upper levels resemble the climatological stationary wave pattern attributed to transient eddy forcing. The NPO/WP divergent circulation is thermal wind restoring, pointing to independent forcing of jet fluctuations.

Intercomparison of sea level pressure, geopotential height, and zonal wind anomaly structure reveals that NPO/WP is a basin analog of the NAO, which is not surprising given strong links to storm track variability in both cases.

The NPO/WP variability is influential: its impact on Alaskan, Pacific Northwest, Canadian, and U.S. winter surface air temperatures is substantial—more than that of PNA or ENSO. It is likewise more influential on the Pacific Northwest, western Mexico, and south-central Great Plains winter precipitation.

Finally, and perhaps, most importantly, NPO/WP is strongly linked to marginal ice zone variability of the Arctic seas with an influence that surpasses that of other Pacific modes. Although NPO/WP variability and impacts have not been as extensively analyzed as its Pacific cousins (PNA, ENSO), it is shown to be more consequential for Arctic sea ice and North American winter hydroclimate.

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Scott J. Weaver
and
Sumant Nigam

Abstract

Variability of the Great Plains low-level jet (GPLLJ) is analyzed from the perspective of larger-scale, lower-frequency influences and regional hydroclimate impacts as opposed to the usual analysis of its frequency, diurnal variability, and mesoscale structure. The circulation-centric core analysis is conducted with monthly data from the high spatiotemporal resolution, precipitation-assimilating North American Regional Reanalysis, and the 40-yr ECMWF Re-Analysis (ERA-40) (as necessary) to identify the recurrent patterns of GPLLJ variability and their large-scale circulation links. The links are first investigated from regressions of an index representing meridional wind speed in the climatological jet-core region; the core region itself is defined from analysis of seasonal and diurnal variability of the jet structure and moisture fluxes.

The analysis reveals that GPLLJ variability is, indeed, linked to coherent, large-scale, upper-level height patterns over the Pacific and North Atlantic Oscillation (NAO) variability in the Atlantic. A Rossby wave source analysis shows the Pacific height pattern to be potentially linked to tropical diabatic heating anomalies in the west-central basin and in the eastern Pacific sector. EOF analysis of GPLLJ variability shows it to be composed of three modes that, together, account for ∼75% of the variance. The modes represent the strengthening/expansion of the jet core (38%), with a strong precipitation impact on the northern Great Plains, and linked to post-peak-phase ENSO variability; meridional shift of the GPLLJ (23%), with a Gulf states precipitation focus, and linked to pre-peak-phase ENSO variability; and in-place strengthening of the GPLLJ (12%), with dipolar influence on Great Plains and Gulf states precipitation, and linked to summer NAO variability.

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Alfredo Ruiz-Barradas
and
Sumant Nigam

Abstract

Interannual variability of warm-season rainfall over the Great Plains is analyzed using the recently released North American Regional Reanalysis (NARR). The new dataset differs from its global counterparts in the additional assimilation of precipitation and radiances. This along with the use of a more comprehensive land surface model in generation of NARR offers the prospect of obtaining improved estimates of surface hydrologic and near-surface meteorological fields.

NARR’s representation of hydroclimate is used to weigh in on the authors’ recent finding of the dominance of large-scale moisture flux convergence over evaporation in accounting for Great Plains precipitation variations. Evaporation estimates are notoriously uncertain and, while the NARR ones are not assured to be realistic, they are more constrained than those diagnosed before from inline and offline assessments.

NARR’s portrayal of warm-season hydroclimate variability corroborates the importance of remote water sources in generation of Great Plains precipitation variability and supports the authors’ claim that some state-of-the-art atmosphere/land surface models vigorously recycle precipitation, erroneously, at least in context of Great Plains interannual variability. These very models have been key to recent claims of strong coupling between soil moisture and precipitation.

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Alfredo Ruiz-Barradas
and
Sumant Nigam

Abstract

Interannual variability of Great Plains precipitation in the warm season months is analyzed using gridded observations, satellite-based precipitation estimates, NCEP reanalysis data and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data, and the half-century-long NCAR Community Atmosphere Model (CAM3.0, version 3.0) and the National Aeronautics and Space Administration (NASA) Seasonal-to-Intraseasonal Prediction Project (NSIPP) atmospheric model simulations. Regional hydroclimate is the focus because of its immense societal impact and because the involved variability mechanisms are not well understood.

The Great Plains precipitation variability is represented rather differently, and only quasi realistically, in the reanalyses. NCEP has larger amplitude but less traction with observations in comparison with ERA-40. Model simulations exhibit more realistic amplitudes, which are between those of NCEP and ERA-40. The simulated variability is however uncorrelated with observations in both models, with monthly correlations smaller than 0.10 in all cases. An assessment of the regional atmosphere water balance is revealing: Stationary moisture flux convergence accounts for most of the Great Plains variability in ERA-40, but not in the NCEP reanalysis and model simulations; convergent fluxes generate less than half of the precipitation in the latter, while local evaporation does the rest in models.

Phenomenal evaporation in the models—up to 4 times larger than the highest observationally constrained estimate (NCEP’s)—provides the bulk of the moisture for Great Plains precipitation variability; thus, precipitation recycling is very efficient in both models, perhaps too efficient.

Remote water sources contribute substantially to Great Plains hydroclimate variability in nature via fluxes. Getting the interaction pathways right is presently challenging for the models.

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