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Jeffrey H. Yin
and
Grant W. Branstator

Abstract

A conceptual framework is developed for quantifying the relationship between low-frequency variability and extreme events. In this framework, variability is decomposed into low-frequency and synoptic components using complementary 10-day low-pass and high-pass filters, and a distinction is made between two ways that low-frequency variability influences extremes: the additive effect, which neglects the dependence of synoptic variability on the low-frequency state, and the multiplicative effect, which is due to the dependence of synoptic variability on the low-frequency state. The influence of various factors on the relationship between low-frequency variability and extreme events is decomposed and quantified by generating a series of simple synthetic datasets based on different assumptions about low-frequency and synoptic variability and their relationship.

These techniques are used to study the relationship between low-frequency variability and extreme westerly wind events in three datasets, an 1158-yr GCM simulation and two reanalysis datasets, with similar results for all three. Geographical variations in the low-frequency–extreme relationship are only partially explained by geographical variations in the low-frequency–synoptic variance ratio; the non-Gaussianity of low-frequency and synoptic variability and the relationship between synoptic variance and the low-frequency state are also found to be important. The simple synthetic datasets that include these factors provide good estimates of the magnitude and probability of extremes. Implications for predictability and applications to more complex low-frequency–extreme relationships are discussed.

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Kevin E. Trenberth
and
Grant W. Branstator

Abstract

Progress toward understanding the causes of and physical mechanisms involved in the 1988 North American drought is reported. An earlier study demonstrated that major sea surface temperature (SST) anomalies in the tropical Pacific Ocean, in association with the 1988 La Niña, may have disrupted atmospheric heating patterns by changing the location and intensity of the intertropical convergence zone and that such heating anomalies could have initiated the circulation anomalies across North America responsible for the drought. A key issue of when the drought circulation anomalies developed and their relation to changes in tropical Pacific SSTs is examined. Although unusually dry soil moisture and heat waves persisted into August, the anomalous atmospheric conditions that brought on the drought occurred in April, May, and June of 1988. The evolution of the Pacific SSTs and tropical convection, as revealed by outgoing longwave radiation, is shown to be consistent with the development of the conditions favorable for initiating the drought circulation pattern in April through June of 1988. On the equator at 110°W, SST anomalies exceeded −2.75°C in only April, May, and June and were largest (−4.1°C) in May 1988. The issues of how the 1988 La Niña differed from those in the past and the importance of the whole SST field in determining the anomalous diabatic heating are also discussed. Diagnostic calculations of atmospheric diabatic heating confirm that atmospheric heating anomalies existed in the tropical Pacific in association with the major SST anomalies during this time. The link between the anomalous heating and the tropical SSTs supports the view that influences external to the atmosphere were important and that the drought was not generated solely by mechanisms internal to the atmosphere. The distribution of diagnosed heating anomalies over North America, together with a planetary wave model response to idealized forcing, is described to clarify the possible role of soil moisture anomalies in perpetuating the drought. It is argued that feedback-caused soil moisture anomalies may have been secondary sources for the drought circulation but could not have been the primary instigator. For the most part, other diagnosed heating anomalies during the drought are found to have little influence on the North American region. Criteria to help judge the ability of general circulation models to simulate the drought are discussed.

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Thomas W. Schlatter
and
Grant W. Branstator

Abstract

Using an 8-day series (18–26 August 1975) of multivariate statistical analyses of European radiosonde data together with a measure of analysis error, we have estimated error statistics from 959 Nimbus 6 temperature profiles for 10 isobaric layers in the troposphere and lower stratosphere. The mean error or bias is largest near the tropopause (+0.9°C) but changes sign several times in the vertical so that the integrated mean error for the atmospheric column 1000–70 mb is small (−0.1°C). The root-mean-square error peaks at the tropopause (2.9°C) with a minimum in the midtroposphere (1.0°C). In all layers, the horizontal correlation of retrieval error shows little systematic dependence on direction but strong dependence on distance. The correlation is greater than 0.50 at distances less than 400 km and less than 0.10 at 800 km and beyond, and it can be approximated by a Gaussian curve. The vertical correlations are greatest between adjacent layers (∼0.50); negative correlations exist between layers on opposite sides of the tropopause. This information is useful in any statistical objective analysis which accounts for observational error.

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Christian Franzke
,
Andrew J. Majda
, and
Grant Branstator

Abstract

Mean phase space tendencies are investigated to systematically identify the origin of nonlinear signatures and the dynamical significance of small deviations from Gaussianity of planetary low-frequency waves. A general framework for the systematic investigation of mean phase space tendencies in complex geophysical systems is derived. In the special case of purely Gaussian statistics, this theory predicts that the interactions among the planetary waves themselves are the source of the nonlinear signatures in phase space, whereas the unresolved waves contribute only an amplitude-independent forcing, and cannot contribute to any nonlinear signature.

The predictions of the general framework are studied for a simple stochastic climate model. This toy model has statistics that are very close to being Gaussian and a strong nonlinear signature in the form of a double swirl in the mean phase space tendencies of its low-frequency variables, much like recently identified signatures of nonlinear planetary wave dynamics in prototype and comprehensive atmospheric general circulation models (GCMs). As predicted by the general framework for the Gaussian case, the double swirl results from nonlinear interactions of the low-frequency variables. Mean phase space tendencies in a reduced space of a prototype atmospheric GCM are also investigated. Analysis of the dynamics producing nonlinear signatures in these mean tendencies shows a complex interplay between waves resolved in the subspace and unresolved waves. The interactions among the resolved planetary waves themselves do not produce the nonlinear signature. It is the interaction with the unresolved waves that is responsible for the nonlinear dynamics. Comparing this result with the predictions of the general framework for the Gaussian case shows that the impact of the unresolved waves is due to their small deviations from Gaussianity. This suggests that the observed deviations from Gaussianity, even though small, are dynamically relevant.

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Haiyan Teng
,
Grant Branstator
, and
Gerald A. Meehl

Abstract

Predictability of the Atlantic meridional overturning circulation (AMOC) and associated oceanic and atmospheric fields on decadal time scales in the Community Climate System Model, version 3 (CCSM3) at T42 resolution is quantified with a 700-yr control run and two 40-member “perfect model” climate change experiments. After taking into account both the mean and spread about the mean of the forecast distributions and allowing for the possibility of time-evolving modes, the natural variability of the AMOC is found to be predictable for about a decade; beyond that range the forced predictability resulting from greenhouse gas forcing becomes dominant. The upper 500-m temperature in the North Atlantic is even more predictable than the AMOC by several years. This predictability is associated with subsurface and sea surface temperature (SST) anomalies that propagate in an anticlockwise direction along the subpolar gyre and tend to be prominent during the 10 yr following peaks in the amplitude of AMOC anomalies. Predictability in the North Atlantic SST mainly resides in the ensemble mean signals after three to four forecast years. Analysis suggests that in the CCSM3 the subpolar gyre SST anomalies associated with the AMOC variability can influence the atmosphere and produce surface climate predictability that goes beyond the ENSO time scale. However, the resulting initial-value predictability in the atmosphere is very weak.

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Gerald A. Meehl
,
Julie M. Arblaster
, and
Grant Branstator

Abstract

A linear trend calculated for observed annual mean surface air temperatures over the United States for the second-half of the twentieth century shows a slight cooling over the southeastern part of the country, the so-called warming hole, while temperatures over the rest of the country rose significantly. This east–west gradient of average temperature change has contributed to the observed pattern of changes of record temperatures as given by the ratio of daily record high temperatures to record low temperatures with a comparable east–west gradient. Ensemble averages of twentieth-century climate simulations in the Community Climate System Model, version 3 (CCSM3), show a slight west–east warming gradient but no warming hole. A warming hole appears in only several ensemble members in the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset and in one ensemble member of simulated twentieth-century climate in CCSM3. In this model the warming hole is produced mostly from internal decadal time-scale variability originating mainly from the equatorial central Pacific associated with the Interdecadal Pacific Oscillation (IPO). Analyses of a long control run of the coupled model, and specified convective heating anomaly experiments in the atmosphere-only version of the model, trace the forcing of the warming hole to positive convective heating anomalies in the central equatorial Pacific Ocean near the date line. Cold-air advection into the southeastern United States in winter, and low-level moisture convergence in that region in summer, contribute most to the warming hole in those seasons. Projections show a disappearance of the warming hole, but ongoing greater surface temperature increases in the western United States compared to the eastern United States.

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Qinghua Ding
,
Bin Wang
,
John M. Wallace
, and
Grant Branstator

Abstract

Maximum covariance analysis is performed on the fields of boreal summer, tropical rainfall, and Northern Hemisphere (NH) 200-hPa height for the 62-yr period of record of 1948–2009. The leading mode, which appears preferentially in summers preceding the peak phases of the El Niño–Southern Oscillation (ENSO) cycle, involves a circumglobal teleconnection (CGT) pattern in the NH extratropical 200-hPa height field observed in association with Indian monsoon rainfall anomalies. The second mode, which tends to occur in summers following ENSO peak phases, involves a western Pacific–North America (WPNA) teleconnection pattern in the height field observed in association with western North Pacific summer monsoon rainfall anomalies. The CGT pattern is primarily a zonally oriented wave train along the westerly waveguide, while the WPNA pattern is a wave train emanating from the western Pacific monsoon trough and following a great circle. The CGT is accompanied by a pronounced tropical–extratropical seesaw in the zonally symmetric geopotential height and temperature fields, and the WPNA is observed in association with hemispherically uniform anomalies. These ENSO-related features modulate surface air temperature in both the tropics and extratropics. ENSO also affects the wave structure of the CGT and WPNA indirectly, by modulating the strengths of the Indian and western North Pacific monsoons. Linear barotropic mechanisms, including energy propagation and barotropic instability of the basic-state flow, also act to shape and maintain the CGT. The implications of these findings for seasonal prediction of the NH extratropical circulation are discussed.

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Bette L. Otto-Bliesner
,
Grant W. Branstator
, and
David D. Houghton

Abstract

A global, spectral, primitive equation model is developed to study the seasonal climatology of the large-scale features of the atmosphere. The model resolution is five equally-spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. Included in the model are: orography; time-varying (but prescribed) sea-surface temperatures, snowcover, and solar declination angle; parameterizations for radiation, convection, condensation, diffusion, and surface transports; and a surface heat budget. The external seasonal forcing of the model atmosphere is composed of sinusoidal time variations in the incoming solar radiation and latitude of the snowline and more complicated variations in the albedo of the snow and the sea-surface temperatures. A five-year seasonal simulation has been analyzed. The model reasonably reproduces the general features of the observed atmospheric circulation, seasonal cycles, interannual variations and hemispheric differences. The success of this low-resolution model in simulating the large-scale features of the atmospheric seasonal cycle illustrates the usefulness of such models for climate studies in conjunction with high-resolution general circulation model simulations.

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Haiyan Teng
,
Ruby Leung
,
Grant Branstator
,
Jian Lu
, and
Qinghua Ding

Abstract

Significant surface air temperature warming during summer 1979–2020 is not uniformly distributed in the northern midlatitudes over land but rather is confined to several longitudinal sectors including Europe, central Siberia and Mongolia, and both coasts of North America. These hot spots are accompanied by a chain of high pressure ridges from an anomalous, circumglobal Rossby wave train in the upper troposphere. From reanalysis data and several baseline experiments from phase 6 of the Coupled Model Intercomparison Project (CMIP6), we find that the circulation trend pattern is associated with fluctuations of the Atlantic multidecadal variability (AMV) and the interdecadal Pacific oscillation. The phase shift of AMV in the 1990s is particularly noteworthy for accelerating warming averaged over the northern midlatitude land. The amplitude of the observed trend in both surface air temperature and the upper-level geopotential height generally falls beyond the range of multidecadal trends simulated by the CMIP6 preindustrial control runs, supporting the likelihood that anthropogenic forcing played a critical role in the observed trend. On the other hand, the fidelity of the simulated low-frequency modes of variability and their teleconnections, especially on multidecadal time scales, is difficult to assess because of the relatively short observational records. Our mechanistic modeling results indicate that synoptic eddy–mean flow interaction is a key to the formation of the anomalous wave train but how the multidecadal modes can modulate the synoptic eddies through atmosphere–ocean and atmosphere–land interactions remains poorly understood. This gap in our knowledge makes it challenging to quantify the roles of the low-frequency modes and external forcings in causing the observed multidecadal trends.

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Gerald A. Meehl
,
Julie M. Arblaster
,
Grant Branstator
, and
Harry van Loon

Abstract

The 11-yr solar cycle [decadal solar oscillation (DSO)] at its peaks strengthens the climatological precipitation maxima in the tropical Pacific during northern winter. Results from two global coupled climate model ensemble simulations of twentieth-century climate that include anthropogenic (greenhouse gases, ozone, and sulfate aerosols, as well as black carbon aerosols in one of the models) and natural (volcano and solar) forcings agree with observations in the Pacific region, though the amplitude of the response in the models is about half the magnitude of the observations. These models have poorly resolved stratospheres and no 11-yr ozone variations, so the mechanism depends almost entirely on the increased solar forcing at peaks in the DSO acting on the ocean surface in clear sky areas of the equatorial and subtropical Pacific. Mainly due to geometrical considerations and cloud feedbacks, this solar forcing can be nearly an order of magnitude greater in those regions than the globally averaged solar forcing. The mechanism involves the increased solar forcing at the surface being manifested by increased latent heat flux and evaporation. The resulting moisture is carried to the convergence zones by the trade winds, thereby strengthening the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). Once these precipitation regimes begin to intensify, an amplifying set of coupled feedbacks similar to that in cold events (or La Niña events) occurs. There is a strengthening of the trades and greater upwelling of colder water that extends the equatorial cold tongue farther west and reduces precipitation across the equatorial Pacific, while increasing precipitation even more in the ITCZ and SPCZ. Experiments with the atmosphere component from one of the coupled models are performed in which heating anomalies similar to those observed during DSO peaks are specified in the tropical Pacific. The result is an anomalous Rossby wave response in the atmosphere and consequent positive sea level pressure (SLP) anomalies in the North Pacific extending to western North America. These patterns match features that occur during DSO peak years in observations and the coupled models.

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