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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 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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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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Massimo Bollasina and Sumant Nigam

Abstract

The Thar Desert between northwestern India and Pakistan is the most densely populated desert region in the world, and the vast surrounding areas are affected by rapid soil degradation and vegetation loss. The impact of an expanded desert (implemented by changing vegetation type and related greenness fraction, albedo, surface roughness length, emissivity, among others) on the South Asian summer monsoon hydroclimate is investigated by means of 7-month, 4-member ensemble sensitivity experiments with the Weather Research and Forecasting model.

It is found that extended desertification significantly affects the monsoon at local and large scales. Locally, the atmospheric water cycle weakens because precipitation, evaporation, and atmospheric moisture convergence all decrease; soil moisture and runoff reduce too. Air temperature cools because of an increase in albedo (the desert makes the area brighter) and a reduction of surface turbulent fluxes; the cooling is partially offset by adiabatic descent, generated to maintain thermodynamic balance and originating at the northern flank of the low-level anticyclone forced by desert subsidence. Regionally, an anomalous northwesterly flow over the Indo-Gangetic Plain weakens the monsoon circulation over northeastern India, causing precipitation to decrease and the formation of an anomalous anticyclone in the region. As a result, the middle troposphere cools because of a decrease in latent heat release, but the ground heats up because of a reduction in cloudiness. At larger scale, the interaction between the anomalous circulation and the mountains leads to an increase in precipitation over the eastern Himalayas and Indochina.

The findings of this study reveal that the expansion of the Thar Desert can lead to a pronounced and large-scale impact on summer monsoon hydroclimate, with a potential to redistribute precious water over South Asia.

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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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Eric DeWeaver and Sumant Nigam

Abstract

The linearity, or extent of antisymmetry, of El Niño and La Niña heating and circulation anomalies is examined for the period 1950–2000. Characteristic structures are obtained by compositing winter season anomalies for positive and negative values of the Niño-3.4 sea surface temperature (SST) index in excess of one standard deviation. Eight winters meet this condition in each ENSO phase, and the warm and cold years are equitably distributed relative to the 1976/77 climate transition.

ENSO SSTs have a direct effect on the large-scale atmospheric circulation through their impact on diabatic heating and subsequent upper-level divergence over the equatorial Pacific. These fields show a significant westward displacement for the La Niña composite compared to the El Niño composite, as expected from the SST threshold condition for convection. But despite the westward shift in convection, the 200-mb height composites are almost antisymmetric over the Pacific, with only a small (∼10°) westward shift for the extratropical La Niña pattern. The upper-level height response in the Tropics, including the position of the El Niño anticyclones, is found to be even more antisymmetric than the extratropical response. The responses are less antisymmetric over eastern North America and the Atlantic.

These results are broadly consistent with idealized experiments in which the midlatitude circulation response to equatorial heating is insensitive to shifts in the longitude of the heating. However, the finding of antisymmetry in the upper-level Pacific height responses to warm and cold ENSO events is in disagreement with the observational composites of Hoerling et al., which show a large shift between El Niño and La Niña height patterns over the North Pacific. In their composites, the La Niña response resembles the Pacific–North American (PNA) pattern, a result not in evidence here. This difference can be understood as a consequence of decadal variability, particularly the 1976/77 climate transition.

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Chul Chung and Sumant Nigam

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The Asian summer monsoon heating anomalies are parameterized in terms of the concurrent ENSO SST anomalies and used as additional forcing in the Cane–Zebiak (CZ) Pacific ocean–atmosphere anomaly model. The Asian heating parameterization is developed from the rotated principal component analysis of combined interannual variability of the tropical Pacific SSTs, residually diagnosed tropical diabatic heating at 400 mb (from ECMWF’s analyses), and the 1000-mb tropical winds during the 1979–97 summer months of June, July, and August.

Analysis of the 95 000-yr-long model integrations conducted with and without the interactive Asian sector heating anomalies reveals that their influence on the Pacific surface winds leads to increased ENSO occurrence—an extra ENSO event every 20 yr or so. An examination of the ENSO distribution w.r.t. the peak SST anomaly in the eastern equatorial Pacific shows increased El Niño occurrence in the 2.2–3.6 K range (and −1.0 to −1.6 K range in case of cold events) along with a modest reduction in the 0.6–1.2 K range, that is, a population shift due to the strengthening of weak El Niños in the monsoon run. The interaction of ENSO-related Asian summer monsoon heating with the CZ model’s ocean–atmosphere also results in a wider period distribution of ENSO variability, but with the El Niño peak phase remaining seasonally locked with the northern winter months.

The above modeling results confirm the positive feedback between Asian summer monsoon and ENSO suggested by previous empirical and diagnostic modeling studies; the feedback is generated primarily by the diabatic heating changes in the Asian Tropics.

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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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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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