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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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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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Yongjing Zhao
and
Sumant Nigam

Abstract

The claim for a zonal-dipole structure in interannual variations of the tropical Indian Ocean (IO) SSTs—the Indian Ocean dipole (IOD)—is reexamined after accounting for El Niño–Southern Oscillation’s (ENSO) influence. The authors seek an a priori accounting of ENSO’s seasonally stratified influence on IO SSTs and evaluate the basis of the related dipole mode index, instead of seeking a posteriori adjustments to this index, as common.

Scant observational evidence is found for zonal-dipole SST variations after removal of ENSO’s influence from IO SSTs: The IOD poles are essentially uncorrelated in the ENSO-filtered SSTs in both recent (1958–98) and century-long (1900–2007) periods, leading to the breakdown of zonal-dipole structure in surface temperature variability; this finding does not depend on the subtleties in estimation of ENSO’s influence. Deconstruction of the fall 1994 and 1997 SST anomalies led to their reclassification, with a weak IOD in 1994 and none in 1997.

Regressions of the eastern IOD pole on upper-ocean heat content, however, do exhibit a zonal-dipole structure but with the western pole in the central-equatorial IO, suggesting that internally generated basin variability can have zonal-dipole structure at the subsurface.

The IO SST variability was analyzed using the extended-EOF technique, after removing the influence of Pacific SSTs; the technique targets spatial and temporal recurrence and extracts modes (rather than patterns) of variability. This spatiotemporal analysis also does not support the existence of zonal-dipole variability at the surface. However, the analysis did yield a dipole-like structure in the meridional direction in boreal fall/winter, when it resembles the subtropical IOD pattern (but not the evolution time scale).

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Stephen Baxter
and
Sumant Nigam

Abstract

The 2013/14 boreal winter (December 2013–February 2014) brought extended periods of anomalously cold weather to central and eastern North America. The authors show that a leading pattern of extratropical variability, whose sea level pressure footprint is the North Pacific Oscillation (NPO) and circulation footprint the West Pacific (WP) teleconnection—together, the NPO–WP—exhibited extreme and persistent amplitude in this winter. Reconstruction of the 850-hPa temperature, 200-hPa geopotential height, and precipitation reveals that the NPO–WP was the leading contributor to the winter climate anomaly over large swaths of North America. This analysis, furthermore, indicates that NPO–WP variability explains the most variance of monthly winter temperature over central-eastern North America since, at least, 1979. Analysis of the NPO–WP related thermal advection provides physical insight on the generation of the cold temperature anomalies over North America. Although NPO–WP’s origin and development remain to be elucidated, its concurrent links to tropical SSTs are tenuous. These findings suggest that notable winter climate anomalies in the Pacific–North American sector need not originate, directly, from the tropics. More broadly, the attribution of the severe 2013/14 winter to the flexing of an extratropical variability pattern is cautionary given the propensity to implicate the tropics, following several decades of focus on El Niño–Southern Oscillation and its regional and far-field impacts.

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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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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

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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Bin Guan
and
Sumant Nigam

Abstract

During the week of Christmas 2021, winter storms pummeled the Pacific Northwest and broke daily temperature and snowfall records in scores, especially west of the Cascades and notably in Oregon. With La Niña ruling the tropical Pacific, the record-setting, disruptive snowfall during Christmas week raised questions about its origin, especially as the seasonal outlook was for below-normal precipitation. We show that Pacific–North American (PNA) teleconnection—a well-documented subseasonal variability pattern during winter—reigned over the region in its negative phase; it was the strongest 7-day PNA episode in December in more than 50 years. It led to robust northwesterly onshore flow, whose interaction with the Coastal, Cascade, and Sierra ranges led to blockbuster snowfall and precipitation. Note that one seldom encounters circulation anomalies consisting of just one winter teleconnection pattern. Also worth noting is the tremendous power of subseasonal variability in recharging Western water resources in the context of the seasonal gloom from a La Niña–intensified West Coast drought.

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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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