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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 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
and
Eric DeWeaver

Abstract

The contribution of the interaction between tropically forced circulation anomalies and the extratropicalmountains in the generation of extratropical circulation anomalies during the 1987/88 and 1988/89 winter seasons is diagnosed using a divergent barotropic model that solves for both the zonal-mean and eddy components of the 200-mb rotational anomalies. Barotropic modeling shows that the orographic modulation of the rotational response to the 200-mb tropical divergence anomaly can be substantial over the Pacific–North American region.

  • The modulation consists of a large-scale wave pattern with a ridge in the central subtropical Pacific, a trough over the Gulf of Alaska, and a weak ridge extending across North America from Baja California to Greenland. These features have an amplitude of ∼40 gpm, and the orographic modulation is thus about one-third as strong as the primary wave pattern.

  • The associated 200-mb zonal wind is strongest (∼5 m s−1) in the vicinity of the eastern end of the East Asian jet, thus contributing to the southeastward jet extension during El Niño winters.

  • The Himalayan–Tibetan complex is the major locus of orographic interaction in the model, for it alone accounts for all the features and over two-thirds of the amplitude modulation.

  • The eddy and zonal-mean parts of the tropically forced flow anomalies make comparable contributions to orographic modulation. However, the midlatitude eddy anomalies themselves result, in part, from the interaction of the zonal-mean zonal wind anomaly and the climatological vorticity gradients, that is, from “zonal–eddy”interaction. The strength of this interaction depends on the arbitrarily specified distribution of the compensating zonal-mean subsidence in the model.

These findings indicate the potential importance of secondary orographic interaction in the generation of extratropical circulation anomalies in response to tropical heating anomalies. Experiments with more complete dynamical models that predict both the rotational and divergent components of the flow in response to tropical heating anomalies are clearly warranted.

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Bin Guan
and
Sumant Nigam

Abstract

Atlantic SST variability in the twentieth century is analyzed factoring the influence of natural SST variability in the Pacific basin and the secular change in global SSTs. The tropical and northern extratropical basins are analyzed together using the extended EOF technique, which permits extraction of the interannual and multidecadal modes in the pan-Atlantic basin in a single step.

The leading mode of Pacific-uninfluenced SST variability is a multidecadal oscillation focused in the extratropical basin, with a period of ∼70 yr. The mode differs from the conventional Atlantic multidecadal oscillation (AMO) in the near quiescence of the tropical–subtropical basin, highlighting the significant influence of the Pacific basin on this region in conventional analysis; as much as 45% of the regional variance resulting from the conventional AMO is due to this influence.

The second and third modes capture the growth (east-to-west development) and decay (near-simultaneous loss of amplitudes) of interannual SST variability in the eastern tropical Atlantic. A nominal 4-yr evolution cycle is identified, but phase transitions are irregular.

The fourth mode describes a north–south tripole with the mature-phase structure resembling the North Atlantic Oscillation’s (NAO’s) SST footprint in winter. The mode lags the NAO by two seasons. Modal evolution involves eastward extension of the main lobe (centered near the separation of the Gulf Stream) along with shrinkage of the oppositely signed two side lobes.

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Eric DeWeaver
and
Sumant Nigam

Abstract

The role of zonal-mean zonal flow ( u ) perturbations in generating anomalous stationary waves has been acknowledged since the 1939 study by Rossby and his collaborators. However, the dynamical mechanisms, which in turn produce the u anomalies, are still not well understood. Here, the authors examine the forcing of u anomalies in the NCEP–NCAR 40-yr Reanalysis by regressing the zonal-mean zonal momentum budget against the leading empirical orthogonal function (EOF) of monthly December–January–February u in the domain covering 30°S to 90°N. The authors find that momentum fluxes arising from the interaction of climatological and anomalous stationary waves constitute the primary source of zonal-mean zonal momentum for the leading u EOF, which resembles the zonal index fluctuations discussed by Rossby, Namias, and others. When combined with previous studies that show the generation of stationary waves by u anomalies, the results presented here indicate a cooperative dynamical relationship between the u and stationary wave anomalies associated with the zonal index—a relationship in which each is both a source of and a response to the other.

The role of stationary waves in driving u anomalies is further examined for the canonical Northern Hemisphere teleconnection patterns. The authors use a rotated principal component analysis of 200-mb geopotential height to identify the u anomalies associated with the North Atlantic oscillation (NAO), the Pacific–North American (PNA) pattern, and an El Niño–related pattern. The NAO and PNA pattern are both accompanied by midlatitude u anomalies resembling EOF1. However, the two do not contribute equally to the leading u EOF: the NAO accounts for 64% of the variance, and the PNA pattern accounts for about 10%. Furthermore, the NAO clearly shows coherent expansion and contraction of the entire polar vortex, but the PNA pattern does not. The time series of the NAO may thus be a better indicator of the expansion and contraction of the polar vortex than are indices based on the zonally averaged circulation.

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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

ENSO teleconnections were originally regarded as a single train of stationary Rossby waves generated by a compact region of enhanced (reduced for La Niña) equatorial convective heating. While more recent studies have greatly enhanced this dynamical picture, the dominant conceptual model of the teleconnections still identifies this monopolar convective heat source as the ultimate driver of the teleconnections.

This note presents evidence that the surrounding regions of diabatic cooling are just as important as equatorial heating in producing the ENSO teleconnections. In simulations with a linear diagnostic model, heating and cooling anomalies derived from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis make comparable contributions to the upper-level eddy height anomalies. In particular, remote cooling is just as important as local heating in determining the central longitude of the subtropical El Niño anticyclones.

The same diagnosis is applied to the ENSO response of an atmospheric general circulation model (AGCM) forced by observed sea surface temperatures in an integration performed by the NASA Seasonal-to-Interannual Prediction Project (NSIPP). Despite differences in the climatological basic state and diabatic heating, positive and negative heating anomalies play the same complimentary roles for the simulated ENSO response as they do for the observed ENSO pattern.

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Chul Chung
and
Sumant Nigam

Abstract

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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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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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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