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Sumant Nigam

Abstract

The minimum meridional resolution needed for an adequate numerical simulation of the linear and “quasi-linear” baroscopic vorticity dynamics in the vicinity of a critical latitude is determined by using a semi-spectral nondivergent barotropic model on a sphere. The high resolution barotropic calculations of Nigam and Held in which the stationary waves are forced by the earth's orography are repeated with several lower meridional resolutions. Comparison of the lower resolution simulations with the higher resolution ones (the “true solutions”) shows the quality of both the linear and the quasi-linear simulations to deteriorate with decreasing meridional resolution.

An unresolved critical latitude results in spurious sensitivity of the steady linear response to the tropical zonal wind structure, whereas a critical latitude resolved using a strong damping coefficient rather than a fine latitudinal grid may result in the attenuation of any genuinely reflected wave at the critical latitude. For a Rayleigh damping coefficient of (13.5 days)−1, a latitudinal resolution of Δθ <3° is found to be sufficient for an adequate simulation of planetary waves in the quasi-linear model; the linear model, for a commensurate quality of simulation, needs a Δθ< 2°. While this choice of the damping coefficient is arbitrary to some extent, the obtained solutions do have structure similar to that seen in the observed wintertime stationary planetary waves.

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Sumant Nigam

Abstract

The latitude-height structure of variability of the monthly-mean zonally-averaged zonal wind (Ū) is objectively documented for the 9-year period (1980–88) during which both ECMWF and NMC global analyses are available. Modes, resulting from a rotated principal component analysis of the wintertime variability in each dataset, are compared not only with each other but also with those present in a longer dataset (1963–77) of NMC's geostrophically analyzed extratropical winds.

In the northern extratropics, there is considerable agreement between the two modern datasets on the structure of wintertime variability: the first two modes, which together account for over 58% of the integrated variance, have largest amplitudes (∼3 m s−1) at the tropopause level and little, if any, phase variation with height. The first mode, which explains over 40% of the variance (in the ECMWF, and over 32% in the NMC data), has meridionally a dipole structure centered approximately at the latitude of the subtropical jet—suggestive of small latitudinal shifts of the jet core. The dominant mode of fluctuation in the 14-year NMC's geostrophic wind record, however, has a node at ∼40°N, which is suggestive more of “in place” fluctuations in the jet speed rather than in the “jet-location.”

In the tropics and subtropics, the variability in both 9-year datasets is dominated by a mode that represents fluctuations in the intensity of tropical convection. The time series associated with this mode is rather intriguing.

An examination of variability in the winter troposphere/stratosphere in an 8-year (1978/79–1985/86) record of zonal-mean zonal winds, derived from “NMC/CAC-analyzed” geopotential heights, reveals interesting baroclinic-type modes of variability.

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Sumant Nigam

Abstract

The dynamical basis for the Asian summer monsoon rainfall-El Niño linkage is explored through diagnostic calculations with a linear steady-state multilayer primitive equation model. The contrasting monsoon circulation during recent El Niño (1987) and La Niña (1988) years is first simulated using orography and the residually diagnosed heating (from the thermodynamic equation and the uninitialized, but mass-balanced, ECMWF analysts) as forcings, and then analyzed to provide insight into the importance of various regional forcings, such as the El Niño–related heating anomalies over the tropical Indian and Pacific Oceans.

The striking simulation of the June–August (1987–1988) near-surface and upper-air tropical circulation anomalies indicates that tropical anomaly dynamics during northern summer is essentially linear even at the 150-mb level. The vertical structure of the residually diagnosed heating anomaly that contributes to this striking simulation differs significantly from the specified canonical vertical structure (used in generating 3D heating from OLR/precipitation distributions) near the tropical tropopause.

The dynamical diagnostic analysis of the anomalous circulation during 1987 and 1988 March–May and June–August periods shows the orographically forced circulation anomaly (due to changes in the zonally averaged basic-state flow) to be quite dominant in modulating the low-level moisture-flux convergence and hence monsoon rainfall over Indochina. The El Niño–related persistent (spring-to-summer) heating anomalies over the tropical Pacific and Indian Ocean basins, on the other hand, mostly regulate the low-level westerly monsoon flow intensity over equatorial Africa and the northern Indian Ocean and, thereby, the large-scale moisture flux into Sahel and Indochina.

The anomalous summer monsoon rainfall over Asian/African longitudes in turn, forces modest surface westerlies over the equatorial western and south tropical Pacific, which contribute positively to the ongoing El Niño's development.

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Sumant Nigam

Abstract

The forcing of the March to May southerly surface-wind tendency along the equatorial South American coast, which leads to the annual transition of the eastern tropical Pacific basin’s climate from its peak warm phase in April, is explored through diagnostic modeling.

Modeling experiments with a high-resolution (18 σ-levels, Δθ = 2.5°, 30 zonal waves) steady-state global linear primitive equation model that produces a striking simulation of most aspects of the March to May change in the lower tropospheric circulation over the eastern tropical Pacific, including the notable southerly surface-wind tendency, have provided unique insight into the role of various physical processes. The model is forced by the 3D distribution of the residually diagnosed diabatic heating and the submonthly momentum and thermal transients, all obtained from the twice-daily 2.5° × 2.5° European Centre for Medium-Range Weather Forecasts uninitialized analyses for 1985–95. The principal findings are the following:

  • The initial southerly surface-wind tendency along the equatorial South American coast in April is forced by the March to May abatement in deep heating (p ≲ 900 mb) over the Amazon due to the northward migration of continental convection, and by the elevated Andean cooling.

  • The increased Northern Hemisphere deep heating due to the developing Central American monsoons and the eastern Pacific ITCZ also contributes to the generation of the initial coastal southerly wind tendency, but not more strongly than the March to May cooling over South America.

  • The March to May cooling of the lower troposphere (600–900 mb) over the southeastern tropical Pacific, which likely results from the longwave radiative cooling from the developing stratocumulus cloud tops, generates relatively strong southerly surface-wind tendencies over the eastern Pacific, particularly at the equatorial South American coast.

Based on the last finding, a new feedback mechanism can be envisioned for the rapid development of the coastal southerly surface-wind tendency and stratocumulus clouds—in which the lower tropospheric cooling over the southeastern tropical Pacific, due to longwave radiative cooling from the stratocumulus cloud tops, generates southerly surface winds, which in turn foster stratocumulus growth from the increased meridional cold advection and latent heat flux.

With respect to the role of stratus clouds in the coupled annual cycle evolution, the new feedback, based on the dynamic response of cloud-top longwave cooling, should proceed more rapidly than the feedback based on the thermodynamic impact of stratus shading on SST.

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Eric DeWeaver
and
Sumant Nigam

Abstract

Seasonally averaged 200-mb circulations for recent winters (1987/88 and 1988/89) that represent opposite phases of El Niño and a zonal-mean zonal flow index cycle are diagnosed using data assimilated by the Goddard Earth Observing System (GEOS) and operational analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). The comparison is undertaken to determine whether there are significant differences in the 200-mb vorticity dynamics implied by the mean meridional circulations in the two datasets and whether these differences can be related to the Incremental Analysis Update (IAU) method used in the GEOS assimilation.

The two datasets show a high degree of similarity in their depictions of the large-scale rotational flow, but there are substantial differences in the associated divergent circulations. For the zonal-mean flow, the zonal winds are substantially the same, but the meridional wind in the Tropics and subtropics is considerably weaker in the GEOS assimilation than its counterparts in both the ECMWF data and the GEOS analyses used to produce the assimilation.

The authors examine the assimilation of the Hadley circulation using a zonally symmetric f-plane model. For this model, the IAU method easily assimilates the rotational flow but fails to assimilate the divergent circulation. This deficiency of the IAU method may explain the weakness of the Hadley cell in the GEOS assimilation, at least in relation to the GEOS analysis.

For this simple model, an alternative assimilation method, based on constraints imposed by the analyzed potential vorticity and mean meridional circulation fields, is proposed that simultaneously assimilates both rotational and divergent flow components.

Barotropic modeling suggests that an accurate representation of mean meridional flow anomalies can be important for the diagnosis of both zonal-mean and eddy rotational flow perturbations, particularly during extreme phases of the zonal-mean zonal flow fluctuation.

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Bin Guan
and
Sumant Nigam

Abstract

A consistent analysis of natural variability and secular trend in Pacific SSTs in the twentieth century is presented. By focusing on spatial and temporal recurrence, but without imposition of periodicity constraints, this single analysis discriminates between biennial, ENSO, and decadal variabilities, leading to refined evolutionary descriptions, and between these natural variability modes and secular trend, all without advance filtering (and potential aliasing) of the SST record. SST anomalies of all four seasons are analyzed together using the extended-EOF technique.

Canonical ENSO variability is encapsulated in two modes that depict the growth (east-to-west along the equator) and decay (near-simultaneous amplitude loss across the basin) phases. Another interannual mode, energetic in recent decades, is shown linked to the west-to-east SST development seen in post–climate shift ENSOs: the noncanonical ENSO mode. The mode is closely related to Chiang and Vimont’s meridional mode, and leads to some reduction in canonical ENSO’s oscillatory tendency.

Pacific decadal variability is characterized by two modes: the Pan-Pacific mode has a horseshoe structure with the closed end skirting the North American coast, and a quiescent eastern equatorial Pacific. The mode exhibits surprising connections to the tropical/subtropical Atlantic, with correlations there resembling the Atlantic multidecadal oscillation. The second decadal mode—the North Pacific mode—captures the 1976/77 climate shift and is closer to Mantua’s Pacific decadal oscillation. This analysis shows, perhaps for the first time, the striking links of the North Pacific mode to the western tropical Pacific and Indian Ocean SSTs. The physicality of both modes is assessed from correlations with the Pacific biological time series.

Finally, the secular trend is characterized: implicit accommodation of natural variability leads to a nonstationary SST trend, including midcentury cooling. The SST trend is remarkably similar to the global surface air temperature trend. Geographically, a sliver of cooling is found in the central equatorial Pacific in the midst of widespread but nonuniform warming in all basins.

An extensive suite of sensitivity tests, including counts of the number of observational analogs of the modes in test analyses, supports the robustness of this analysis.

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Eric DeWeaver
and
Sumant Nigam

Abstract

This research is an attempt to understand the dynamical mechanisms that drive the wintertime North Atlantic oscillation (NAO) on monthly and longer timescales. In an earlier work by DeWeaver and Nigam, the authors showed that momentum fluxes from stationary waves play a large role in maintaining the zonal-mean zonal wind ( u ) perturbations associated with the NAO. In this paper, a linear stationary wave model is used to show that zonal-mean flow anomalies in turn play a large role in maintaining the NAO stationary waves. A strong two-way coupling thus exists between u and the stationary waves, in which each is both a source of and a response to the other.

When forced by zonal-eddy coupling terms—terms that represent the interaction between NAO-covariant zonal-mean zonal wind anomalies and the climatological eddy flow—together with heating and transient fluxes, the model produces a realistic simulation of the observed stationary wave pattern. Zonal-eddy coupling terms make the largest contribution to the simulated stationary waves. Every feature of the stationary wave pattern is forced to some extent by zonal-eddy coupling, and the upper-level trough over Greenland is forced almost entirely by the coupling terms. The stationary waves generated by zonal-eddy coupling are well positioned to provide additional momentum to the u anomalies, demonstrating the strong positive feedback between zonal-mean and eddy flow components.

The NAO is known for its effect on tropospheric temperatures over northern Eurasia, and the model produces a realistic simulation of these temperature changes at midtropospheric levels. Zonal-eddy coupling, including the zonal advection of land–sea thermal contrasts, is partly responsible for the temperature changes. However, diabatic heating anomalies associated with the displacement of the Atlantic storm track are also influential, causing more than half of the warming over Scandinavia and most of cooling from North Africa to the Caspian Sea.

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Sumant Nigam
and
Chul Chung

Abstract

The structure of surface-wind anomalies associated with ENSO variability is extracted from ComprehensiveOcean–Atmosphere Dataset observations and European Centre for Medium-Range Forecasts (ECMWF) and National Centers for Environmental Prediction (NCEP) reanalyses, along with estimates of uncertainty. The targets are used to evaluate ENSO surface winds produced by the National Center for Atmospheric Research’s atmospheric GCM known as the Community Climate Model, version 3 (CCM3), when integrated in the climate-simulation mode. Simulated anomalies have stronger easterlies in the off-equatorial Tropics and stronger equatorward flow in the Pacific than any of the observational estimates do. CCM3’s wind departures are found to be large when compared with the difference of the reanalysis anomalies and should thus be considered to be errors.

In a companion paper, the authors make a compelling case for the presence of robust errors in CCM3’s ENSO heating distribution, based on comparisons with the residually diagnosed heating anomalies from ECMWF and NCEP reanalyses.

The linkage between specific features of CCM3’s surface-wind and heating errors is investigated using a steady, linear, global, primitive equation model (18 vertical σ levels, 30 zonal waves, and latitude spacing of 2.5°). Diagnostic modeling indicates that stronger equatorward flow in the Pacific results largely from excessive diabatic cooling in the off-equatorial Tropics, a key heating error linked to a more meridional redistribution of ENSO heating in CCM3. The “bottom-heavy” structure of CCM3’s equatorial heating anomalies, on the other hand, is implicated in the generation of zonal-wind errors in the central and eastern tropical Pacific.

In the diagnostic simulation of CCM3’s ENSO variability, the longwave heating anomalies, with peak values near 850 mb, contribute as much to surface zonal winds as do all other heating components together—a novel finding, needing corroboration.

This study, along with the companion paper, illustrates the dynamical diagnosis strategy—of circulation and forcing intercomparisons with observed counterparts, followed by diagnostic modeling—for analyzing errors in the GCM’s simulation of climate variability.

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Sumant Nigam
and
Yi Chao

Abstract

The structure of ocean-atmosphere annual cycle variability is extracted from the revised Comprehensive Ocean-Atmosphere Data Set SSTs, surface winds, and the latent heat (LH) and net shortwave (SW) surface fluxes using the covariance-based rotated principal component analysis method.

The coupled annual cycle variability is concisely described using two modes that are in temporal quadrature. The first, peaking in June/July (and Dec/Jan), represents monsoonal flow onto Indochina, Central America, and western Africa. The second mode peaks in September/October and March/April when it represents the extreme phases of the SST annual cycle in the eastern oceans.

Analysis of the surface momentum balance in the Pacific cold tongue core shows the equatorial flow, and in particular the zonal wind, to be dynamically consistent with the SST gradient during both the cold tongue's nascent (Jun/Jul) and mature (Sep/Oct) phases; the dynamical consistency improves when the impact of near-surface static stability variation on horizontal momentum dissipation is also considered. Evolution structure of the extracted annual cycle, moreover, shows the easterly wind tendency to lead SST cooling in the off-coastal zone. Taken together, these findings suggest that the Pacific cold tongue westward expansion results from local interaction of the zonal wind and zonal SST gradient, as encapsulated in the proposed “westward expansion hypothesis” -a simple analytic model of which is also presented.

Although positive LH flux tendency leads SST cooling in the off-coastal zone, its modest magnitude (∼5 W m−2/mo) indicates that its direct impact on SSTs, while additive, is secondary to the impact of equatorial upwelling. Comparison of the open ocean and coastal annual evolutions reveals that the northward expansion of the Pacific cold tongue likely results from the positive feedback between coastal meridional winds and the upwelled meridional SST gradient, but suggests that the reason for the nonobservance of equatorially antisymmetric SSTs is the counter LH-flux impact northward of the equator.

The comparatively modest SST annual cycle in the northern equatorial Indian Ocean is forced by the Asian-monsoon-driven (i.e., nonlocally forced) surface winds through coastal upwelling along the Somali coast and from the monsoon-cloudiness-impacted net SW surface flux and wind-speed-influenced LH flux in the off-coastal sector.

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Agniv Sengupta
and
Sumant Nigam

Abstract

The northeast monsoon (NEM) brings the bulk of annual rainfall to southeastern peninsular India, Sri Lanka, and the neighboring Southeast Asian countries. This October–December monsoon is referred to as the winter monsoon in this region. In contrast, the southwest summer monsoon brings bountiful rainfall to the Indo-Gangetic Plain. The winter monsoon region is objectively demarcated from analysis of the timing of peak monthly rainfall. Because of the region’s complex terrain, in situ precipitation datasets are assessed using high-spatiotemporal-resolution Tropical Rainfall Measuring Mission (TRMM) rainfall estimates, prior to their use in monsoon evolution, variability, and trend analyses. The Global Precipitation Climatology Center’s in situ analysis showed the least bias from TRMM.

El Niño–Southern Oscillation’s (ENSO) impact on NEM rainfall is shown to be significant, leading to stronger NEM rainfall over southeastern peninsular India and Sri Lanka but diminished rainfall over Thailand, Vietnam, and the Philippines. The impact varies subseasonally, being weak in October and strong in November. The positive anomalies over peninsular India are generated by anomalous anticyclonic flow centered over the Bay of Bengal, which is forced by an El Niño–related reduction in deep convection over the Maritime Continent.

The historical twentieth-century climate simulations informing the Intergovernmental Panel on Climate Change’s Fifth Assessment (IPCC-AR5) show varied deficiencies in the NEM rainfall distribution and a markedly weaker (and often unrealistic) ENSO–NEM rainfall relationship.

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