Search Results

You are looking at 31 - 40 of 60 items for

  • Author or Editor: Sumant Nigam x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
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.

Full access
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.

Full access
Alfredo Ruiz-Barradas
and
Sumant Nigam

Abstract

The annual cycle of precipitation and the interannual variability of the North American hydroclimate during summer months are analyzed in coupled simulations of the twentieth-century climate. The state-of-the-art general circulation models, participating in the Fourth Assessment Report for the Intergovernmental Panel on Climate Change (IPCC), included in the present study are the U.S. Community Climate System Model version 3 (CCSM3), the Parallel Climate Model (PCM), the Goddard Institute for Space Studies model version EH (GISS-EH), and the Geophysical Fluid Dynamics Laboratory Coupled Model version 2.1 (GFDL-CM2.1); the Met Office’s Third Hadley Centre Coupled Ocean–Atmosphere GCM (UKMO-HadCM3); and the Japanese Model for Interdisciplinary Research on Climate version 3.2 [MIROC3.2(hires)]. Datasets with proven high quality such as NCEP’s North American Regional Reanalysis (NARR), and the Climate Prediction Center (CPC) U.S.–Mexico precipitation analysis are used as targets for simulations.

Climatological precipitation is not easily simulated. While models capture winter precipitation very well over the U.S. northwest, they encounter failure over the U.S. southeast in the same season. Summer precipitation over the central United States and Mexico is also a great challenge for models, particularly the timing. In general the UKMO-HadCM3 is closest to the observations.

The models’ potential in simulating interannual hydroclimate variability over North America during the warm season is varied and limited to the central United States. Models like PCM, and in particular UKMO-HadCM3, exhibit reasonably well the observed distribution and relative importance of remote and local contributions to precipitation variability over the region (i.e., convergence of remote moisture fluxes dominate over local evapotranspiration). However, in models like CCSM3 and GFDL-CM2.1 local contributions dominate over remote ones, in contrast with warm-season observations. In the other extreme are models like GISS-EH and MIROC3.2(hires) that prioritize the remote influence of moisture fluxes and neglect the local influence of land surface processes to the regional precipitation variability.

Full access
Sumant Nigam
and
Steven C. Chan

Abstract

This study revisits the question posed by Hoskins on why the Northern Hemisphere Pacific sea level pressure (SLP) anticyclone is strongest and maximally extended in summer when the Hadley cell descent in the northern subtropics is the weakest. The paradoxical evolution is revisited because anticyclone buildup to the majestic summer structure is gradual, spread evenly over the preceding 4–6 months, and not just confined to the monsoon-onset period, which is interesting, as monsoons are posited to be the cause of the summer vigor of the anticyclone.

Anticyclone buildup is moreover found focused in the extratropics, not the subtropics, where SLP seasonality is shown to be much weaker, generating a related paradox within the context of the Hadley cell’s striking seasonality. Showing this seasonality to arise from, and thus represent, remarkable descent variations in the Asian monsoon sector, but not over the central-eastern ocean basins, leads to the resolution of this paradox.

Evolution of other prominent anticyclones is analyzed to critique the development mechanisms: the Azores high evolves like the Pacific one, but without a monsoon to its immediate west. The Mascarene high evolves differently, peaking in austral winter. Monsoons are not implicated in both cases.

Diagnostic modeling of seasonal circulation development in the Pacific sector concludes this inquiry. Of the three forcing regions examined, the Pacific midlatitudes are found to be the most influential, accounting for over two-thirds of the winter-to-summer SLP development in the extratropics (6–8 hPa), with the bulk coming from the abatement of winter storm-track heating and transients. The Asian monsoon contribution (2–3 hPa) is dominant in the Pacific (and Atlantic) subtropics.

The modeling results resonate with observational findings and attest to the demise of winter storm tracks as the principal cause of the summer vigor of the Pacific anticyclone.

Full access
Alfredo Ruiz-Barradas
and
Sumant Nigam

Abstract

The present study assesses the potential of the U.S. Climate Variability and Predictability (CLIVAR) Drought Working Group (DWG) models in simulating interannual precipitation variability over North America, especially the Great Plains. It also provides targets for the idealized DWG model experiments investigating drought origin. The century-long Atmospheric Model Intercomparison Project (AMIP) simulations produced by version 3.5 of NCAR’s Community Atmosphere Model (CAM3.5), the Lamont-Doherty Earth Observatory’s Community Climate Model (CCM3), and NASA’s Seasonal-to-Interannual Prediction Project (NSIPP-1) atmospheric models are analyzed; CCM3 and NSIPP-1 models have 16- and 14-ensemble simulations, respectively, while CAM3.5 only has 1.

The standard deviation of summer precipitation is different in AMIP simulations. The maximum over the central United States seen in observations is placed farther to the west in simulations. Over the central plains the models exhibit modest skill in simulating low-frequency precipitation variability, a Palmer drought severity index proxy. The presence of a linear trend increases correlations in the period 1950–99 when compared with those for the whole century. The SST links of the Great Plains drought index have features in common with observations over both the Pacific and Atlantic Oceans.

Interestingly, summer-to-fall precipitation regressions of the warm Trend, cold Pacific, and warm Atlantic modes of annual mean SST variability (used in forcing the DWG idealized model experiments) tend to dry the southwestern, midwestern, and southeastern regions of the United States in the observations and, to a lesser extent, in the simulations.

The similarity of the idealized SST-forced droughts in DWG modeling experiments with AMIP precipitation regressions of the corresponding SST principal components, evident especially in the case of the cold Pacific pattern, suggests that the routinely conducted AMIP simulations could have served as an effective proxy for the more elaborated suite of DWG modeling experiments.

Full access
Sumant Nigam
and
Horng-Syi Shen

Abstract

The recurrent modes of combined oceanic and atmospheric low-frequency variability over the tropical Pacific and Indian oceans are calculated to provide quantitatively and structurally well-defined targets for simulation/prediction studies of ocean-atmosphere variability. Within the atmosphere, this study focuses on the structure of the large-scale near-surface flow variability.

The structure of combined variability of outgoing longwave radiation (OLR) and the Comprehensive Ocean-Atmosphere Data Set (COADS) SSTs and surface winds (1974–87) is analyzed using the covariance-based rotated principal component analysis technique during three nonoverlapping periods of the calendar year. The leading combined variability mode in each of these periods is found to be EJ Niño related, and the structural relationships among its four components quite revealing. For example, the surface divergence is found to be largely determined by the meridional winds, and while there is broad correspondence between the similarly signed OLR and surface divergence anomalies across the Pacific, pronounced oppositely signed anomalies are also present, notably over the Maritime Continent during the December–February (DJF) months.

The second leading mode of combined variability during the DJF months is robust, has nondescript equatorial SST or OLR anomalies, and represents coherent Jarge-scale expansion/contraction of the Pacific trade-wind system-north-south in case of the northeast trades and cast-west in case of the southeast trades.

In a related modeling study, the leading combined variability mode ~ on COADS observations was used to provide both the input (its OLR component) and the target (its surface wind components) for a simplified “Gill-type” model of the tropics (the atmospheric component of the Cane-Zebiak model) in order to ascertain the quality of its surface wind simulation.

Full access
Natalie Thomas
and
Sumant Nigam

Abstract

Twentieth-century trends in seasonal temperature and precipitation over the African continent are analyzed from observational datasets and historical climate simulations. Given the agricultural economy of the continent, a seasonal perspective is adopted as it is more pertinent than an annual-average one, which can mask offsetting but agriculturally sensitive seasonal hydroclimate variations. Examination of linear trends in seasonal surface air temperature (SAT) shows that heat stress has increased in several regions, including Sudan and northern Africa where the largest SAT trends occur in the warm season. Broadly speaking, the northern continent has warmed more than the southern one in all seasons. Precipitation trends are varied but notable declining trends are found in the countries along the Gulf of Guinea, especially in the source region of the Niger River in West Africa, and in the Congo River basin. Rainfall over the African Great Lakes—one of the largest freshwater repositories—has, however, increased. It is shown that the Sahara Desert has expanded significantly over the twentieth century, by 11%–18% depending on the season, and by 10% when defined using annual rainfall. The expansion rate is sensitively dependent on the analysis period in view of the multidecadal periods of desert expansion (including from the drying of the Sahel in the 1950s–80s) and contraction in the 1902–2013 record, and the stability of the rain gauge network. The desert expanded southward in summer, reflecting retreat of the northern edge of the Sahel rainfall belt, and to the north in winter, indicating potential impact of the widening of the tropics. Specific mechanisms for the expansion are investigated. Finally, this observational analysis is used to evaluate the state-of-the-art climate simulations from a comparison of the twentieth-century hydroclimate trends. The evaluation shows that modeling regional hydroclimate change over the African continent remains challenging, warranting caution in the development of adaptation and mitigation strategies.

Full access
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.

Full access
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.

Full access