Abstract

The variability in extratropical atmospheric anomalies from one El Niño winter to another is examined. This study offers an interpretation for such observed inter–El Niño variations and discusses implications for seasonal atmospheric predictability.

The seven strongest El Niño events of the 1950–94 period are selected in order to form a composite 500-mb circulation anomaly over the Pacific–North American region. Individual events are shown to deviate significantly from such a composite. Using a large ensemble of atmospheric general circulation model simulations forced with the observed sea surface temperatures of 1950–94, the authors argue that the observed inter–El Niño atmospheric variations are primarily due to internal atmospheric variability. The observed inter–El Niño variability in spatial patterns of the extratropical circulation anomalies appears not to be a deterministic feature of the SSTs and may thus be inherently unpredictable.

Atmospheric general circulation model results further suggest that the spatial pattern of the extratropical response to El Niño consists largely of a single deterministic structure. Some variability in the spatial pattern of the simulated extratropical signal exists, but this is appreciably smaller than the internal atmospheric variability. On the other hand, the amplitude of the signal in the extratropics is shown to be a sensitive function of the particular El Niño, and the model response increases almost linearly with the strength of the SST warming. The practical implications for dynamic seasonal climate prediction in the extratropics are discussed, including an assessment of accuracy requirements for the SST predictions themselves.

Abstract

The potential for seasonal mean predictability over the Pacific and North American regions is evaluated as a function of the amplitude of equatorial Pacific sea surface temperature forcing, the phase of that forcing, and the phase of the annual cycle. The potential predictability is measured as the ratio of the seasonal mean SST-forced signal and the internally generated seasonal mean noise. The authors’ assessments are derived from the output of ensemble atmospheric general circulation model experiments forced with observed monthly SSTs for 1950–94. Using a perfect prognostic approach, results are presented on the predictability of upper-tropospheric circulation, North American land temperature, and precipitation.

Seasonal predictability is shown to depend on the amplitude of the SST-forced signal, whereas the background noise is largely independent of SSTs. To zero order, that signal grows linearly with the amplitude of anomalous SSTs. An important departure from this is with respect to the phase of tropical Pacific SST anomalies, and the simulated atmospheric signals were stronger for ENSO’s extreme warm phases compared to ENSO’s extreme cold phases. This asymmetry can be traced throughout the teleconnection chain that links the ENSO forcing region with North American climate.

With regard to the annual cycle’s role, the North American climate is shown to be most predictable during the late winter and early spring season of warm events. This stems from the fact that the SST-forced signal during warm events at that time of year is only slightly weaker than in midwinter, whereas the background noise is substantially reduced. Predictability during spring is significantly greater than that occurring in fall, due to a much weaker fall signal. Observational analyses are presented that corroborate these key model results, in particular enhanced skill during ENSO’s warm phase and a springtime predictability peak.

Finally, a comparison is made between the classic ratio of variance measure of predictability that commingles all warm, cold, and non-ENSO years to yield a single estimate, against such a ratio calculated for individual events. North American seasonal predictability for specific events can greatly exceed this single gross measure, and it is shown that the latter is a poor yardstick of the prospects for skillful predictions during extreme ENSO states.

Abstract

Using a general circulation model (GCM) dataset, the impact of postprocessing interpolation from model sigma to pressure coordinates on a diagnostic analysis of the atmospheric energy balance is examined. Various isobaric resolutions are chosen that correspond to those provided in existing analysis archives generated by the National Meteorological Center and the European Centre for Medium-Range Weather Forecasts.

Large differences are found between diabatic heating computed residually in isobaric coordinates versus sigma coordinates. Vertically averaged heating reconstructed from pressure level circulation data is found to be in error locally by 10%–50%, with poorer results occurring at coarser vertical resolution. For the zonally averaged isobaric beat balance, the error exceeds the total zonally averaged heating near the tropopause and near the surface. In the former region, the beating is quite small, whereas in the latter region it exceeds 1 K day⁻¹

In contrast, reconstructing the beat balance directly from the GCM sigma-level circulation data introduces errors generally less than 5% of the model's explicit heating. Although uncertainties exist in our results stemming from the use of a single GCM dataset and a particular vertical interpolation method, they suggest the importance of performing analyses on an assimilating model's coordinate surfaces for the purpose of quantitative climate monitoring.

Abstract

Four observed El Niño-Southern Oscillation (ENSO) events are studied to determine the mechanisms responsible for the anomalous extratropical atmospheric circulation during northern winter. A parallel analysis of a GCM's response to El Niño is performed in order to assess if similar mechanisms are operative in the model atmosphere. The observed stationary wave anomalies over the Pacific/North American (PNA) region are found to he similar during the four winters despite appreciable differences in sea surface temperatures. The anomalous transient vorticity fluxes are remarkably robust over the North Pacific during each event, with an eastward extension of the climatological storm track leading to strong cyclonic forcing near 40°N, 150°W. This forcing is in phase with the seasonal mean Aleutian trough anomaly suggesting the importance of eddy-mean flow interactions. By comparison, the intersample variability of the GCM response over the PNA region is found to exceed the observed inter-El Niño variability. This stems primarily from a large variability in the model's anomalous transients over the North Pacific.

Further analysis using a linear stationary wave model reveals that the extratropical vorticity transients are the primary mechanism maintaining the stationary wave anomalies over the PNA region during all four observed ENSO winters. In the case of the GCM, the organization of transient eddies is ill defined over the North Pacific, a behavior that appears more indicative of model error than the unpredictable component of seasonal mean storm track anomalies. A physical model is proposed to explain the robustness of the tropical controlling influence of the extratropical transients in nature. A simple equatorial Pacific heat source directly forces a tropical anticyclone whose phase relative to the climatological tropical anticyclone leads to an eastward extension of the subtropical jet stream. This mechanism appears to be equally effective for a beat source located either in the central or eastern Pacific basin.

Abstract

Remarkable among the atmospheric phenomena associated with El Niño–Southern Oscillation (ENSO) is the lag in the zonal mean tropical thermal anomalies relative to equatorial east Pacific sea surface temperatures (SSTs). For the period 1950–99, the maximum correlation between observed zonal mean tropical 200-mb heights and a Niño-3.4 (5°N–5°S, 120°–170°W) SST index occurs when the atmosphere lags by 1–3 months, consistent with numerous previous studies. Results from atmospheric general circulation model (GCM) simulations forced by the monthly SST variations of the last half-century confirm and establish the robustness of this observed lag.

An additional feature of the delay in atmospheric response that involves an apparent memory or lingering of the tropical thermal anomalies several seasons beyond the Niño-3.4 SST index peak is documented in this study. It is characterized by a strong asymmetry in the strength of the zonal mean tropical 200-mb height response relative to that peak, being threefold stronger in the summer following the peak compared to the preceding summer. This occurs despite weaker Niño-3.4 SST forcing in the following summer compared to the preceding summer.

The 1–3-month lag in maximum correlation is reconciled by the fact that the rainfall evolution in the tropical Pacific associated with the ENSO SST anomalies itself lags one season, with the latter acting as the immediate forcing for the 200-mb heights. This aspect of the lagged behavior in the tropical atmospheric response occurs independent of any changes in SSTs outside of the tropical east Pacific core region of SST variability related to ENSO. The lingering of the tropical atmospheric thermal signal cannot, however, be reconciled with the ENSO-related SST variability in the tropical eastern Pacific. This part of the tropical atmospheric response is instead intimately tied to the tropical ocean's lagged response to the equatorial east Pacific SST variability, including a warming of the tropical Indian and Atlantic SSTs that peak several seasons after the Niño-3.4 warming peak.

Abstract

Atmospheric response patterns associated with tropical forcing are examined with general circulation models driven by global sea surface temperature (SST) variations during 1950–99. Specifically the sensitivity of midlatitude responses to the magnitude and position of tropical SST anomalies is explored. This controversial problem, spanning more than a quarter century now, centers on whether response patterns over the Pacific–North American region are affected or changed by inter–El Niño variability in tropical forcing. Ensemble methods are used in this study to reliably identify the signals related to various tropical SST forcings, and the sensitivity is determined from analysis of four different climate models.

First, the fraction of Pacific–North American (PNA) wintertime 500-hPa height variability that is potentially predictable and is linked to interannual variations in the global SSTs is identified. This SST-forced component accounts for as much as 20%–30% of the total seasonal mean height variability over portions of the PNA region, and the most important boundary forcing originates from the tropical Pacific Ocean. The spatial expression of the teleconnections that are linked to this potentially predictable SST-forced fraction of height variability is next identified. The leading model pattern is similar to the classic observed teleconnection associated with the linear ENSO signal, and explains 80% of the SST-forced height variance over parts of the North Pacific and North America. Two additional wavelike patterns are identified that are also associated with tropical forcing. One is related to the pattern of tropical SST variations often seen during the transition of the tropical ocean that marks the interlude between ENSO extremes, and the pattern of forcing related to it is distinctly non-ENSO in character. The other is related to the nonlinear component of the atmospheric response to ENSO's extreme opposite phases. Response patterns having annular-like structures over the Northern Hemisphere that are related to multidecadal variations in tropical Indo-Pacific and Atlantic SSTs are also highlighted.

Subtle modifications in upper-level responses to different tropical SST forcings are shown to yield disproportionate sensitivity in North American surface climate. Particularly pronounced is the reversal in sign of the precipitation anomalies over the region spanning the Canadian border to southern California in response to equatorial Pacific convection anomalies shifting from 170°E to 140°W. The behavior is reproduced in experiments using both realistic and idealized SST anomalies, and this behavior is found to emerge particularly when the far eastern equatorial Pacific Ocean is strongly warmed as occurred during the 1982/83 and 1997/98 El Niños.

Despite the existence of different response patterns to tropical SST forcings, it is shown that the seasonal hindcast skill of PNA 500-hPa heights for 1950–99 originates mainly from the single, leading teleconnection structure. The conclusion drawn from this result is that the atmospheric sensitivity to different tropical SST forcings, though real, is weak and easily masked by the year-to-year climate variations due to internal atmospheric processes.

Abstract

The origin of extreme climate states during the exceptional 1982–83 El Niño event has continued to be a source of controversy and debate. On the one hand, empirical analyses of extratropical climate patterns during past El Niño events suggests the observed anomalies during 1982–83 were consistent with tropical forcing. On the other hand, the large amplitude of those anomalies have not been replicated in atmospheric general circulation model (AGCM) simulations for that period performed as part of the Atmospheric Model Intercomparison Project (AMIP).

It has recently become apparent, however, that the sea surface boundary conditions used to drive the multitude of AMIP simulations were deficient, in that at least 30% of available tropical Pacific SST observations were discarded in the analysis cycle due to excessive trimming constraints. It is shown from a reanalysis of the sea surface temperatures that the observed east equatorial Pacific waters were 1.5°C warmer than original estimates.

In order to address the extent to which simulations of the extratropical climate of 1982–83 are sensitive to different SST analyses of that period, a parallel suite of AGCM simulations using two SST prescriptions is performed. One set is based on the blended satellite–in situ data used also in the AMIP runs, whereas the other is based on the optimum interpolation (OI) reanalysis. A nine-member ensemble of such simulations is performed, and this is compared with observations. The model response using the original blended SSTs is shown to severely underestimate the tropical rainfall anomalies, and this contributes to the simulation of a weak extratropical response as reported earlier in the AMIP experiments. A larger, more realistic response during 1982–83 is shown to occur in an identical set of runs based on the OI SST boundary conditions, and most aspects of the observed pattern and strength of the Pacific–North American climate anomalies during that winter are reproduced in the model’s ensemble mean response.

Further analysis of the models’ intersample variability are performed to ascertain the extent to which the observed anomalies may have been influenced by atmospheric initial conditions. It is shown from the OI runs that the observed tropical Pacific rainfall anomalies and the Southern Oscillation index were phenomena causally determined by the El Niño. Even over the Pacific–North American region, the spatial pattern of the anomalies in individual runs was highly reproducible, and several members of the OI runs achieved climate anomalies exceeding in amplitude those observed. The findings strongly indicate the important role of El Niño in determining the climate state over the Pacific–North American region during 1982–83, and various competing hypotheses are critiqued in light of these new model results.

Abstract

The semiarid U.S. Great Plains is prone to severe droughts having major consequences for agricultural production, livestock health, and river navigation. The recent 2012 event was accompanied by record deficits in precipitation and high temperatures during the May–August growing season. Here the physics of Great Plains drought are explored by addressing how meteorological drivers induce soil moisture deficits during the growing season. Land surface model (LSM) simulations driven by daily observed meteorological forcing from 1950 to 2013 compare favorably with satellite-derived terrestrial water anomalies and reproduce key features found in the U.S. Drought Monitor. Results from simulations by two LSMs reveal that precipitation was directly responsible for between 72% and 80% of the soil moisture depletion during 2012, and likewise has accounted for the majority of Great Plains soil moisture variability since 1950. Energy balance considerations indicate that a large fraction of the growing season temperature variability is itself driven by precipitation, pointing toward an even larger net contribution of precipitation to soil moisture variability.

To assess robustness across a larger sample of drought events, daily meteorological output from 1050 years of climate simulations, representative of conditions in 1979–2013, are used to drive two LSMs. Growing season droughts, and low soil moisture conditions especially, are confirmed to result principally from rainfall deficits. Antecedent meteorological and soil moisture conditions are shown to affect growing season soil moisture, but their effects are secondary to forcing by contemporaneous rainfall deficits. This understanding of the physics of growing season droughts is used to comment on plausible Great Plains soil moisture changes in a warmer world.

Abstract

An analysis of the Northern Hemispheric zonal mean flow anomalies during El Niño is performed, and the dynamical effect of such atmospheric flows on the wintertime climatological stationary waves over the Pacific/North American (PNA) region is assessed. Only in the subtropical latitudes can one identify a statistically significant zonal flow anomaly during the El Ni˜os of the historical record. Strong zonal flow anomalies in the midlatitudes are observed during individual El Ni˜o events, although these appear to be manifestations of chaotic atmospheric behavior. The observational results are confirmed by GCM climate simulations using prescribed SSTs for the 1982-93 period. The principal SST-forced zonal flow signal in these experiments is located on the equatorward flank of the subtropical jet.

Using a linear diagnostic modes, the authors find the climatological stationary waves over the PNA region to be insensitive to zonal mean flow anomalies in the subtropics. On the other hand, zonal mean anomalies in the midiatitudes are found to induce large amplitude stationary wave anomalies, and these resemble the Pacific/North American pattern. The zonal/eddy effect may thus account for an important fraction of the interannual variability of the wintertime North American climate, although this component is evidently unpredictable from boundary-forced experiments only. Further evidence is derived from GCM hindcasts for the 1986/87 and 1991/92 El Ni˜o winters. A lack of model skill in predicting the observed midlatitude zonal flow anomalies for these cases is shown to place a limit on the accuracy of boundary-forced simulations of North American seasonal climate anomalies.

Seasonal forecasts may yet benefit from the zonal-eddy relationship in view of the fact that the upper-tropospheric zonal flow anomalies in midlatitudes are frequently long-lived. It is thus possible that inclusion of initial atmospheric conditions, together with relevant boundary information. will be more skillful than just boundary-forced simulations alone.

Abstract

The paradigm of an atmospheric system varying linearly with respect to extreme phases of the El Niño–Southern Oscillation is questioned. It is argued that the global response to tropical Pacific sea surface temperature forcing will be inherently nonlinear. A physical basis for this intrinsic nonlinearity is the thermodynamic control on deep convection.

Climate statistics for warm and cold events of the tropical Pacific are analyzed separately for the northern winter periods during 1950–96. Composite analysis of 500-mb heights reveal planetary-scale teleconnection patterns, as noted in earlier studies. A new result is the evidence for an appreciable 35° longitude phase shift between the warm and cold event circulation composites, and the two wave trains appear to have different tropical origins. A large nonlinear component in North American surface climate anomalies is also found, which is consistent with such a phase shift in teleconnections. In the Tropics, rainfall anomalies also show evidence of nonlinear behavior. The maximum rain anomalies along the equator are located east of the date line during warm events, but west of the date line during cold events. The interpretation of this behavior is complicated, however, by the fact that composite warm event SST anomalies are not the exact inverse of their cold event counterparts.

Idealized atmospheric general circulation model (AGCM) experiments are performed in order to test the question of whether the observed nonlinearity is an intrinsic property of the atmospheric system. The model is forced with a composite SST anomaly that undergoes a realistic seasonally varying ENSO life cycle, as described by E. Rasmusson and T. Carpenter. Both positive and negative phases of the SST anomaly are used, and a 40-member ensemble of warm and cold event model simulations is conducted. A nonlinear climate response in the AGCM is found that closely resembles the observed composites, including a shift in the equatorial positions of the maxmium rain responses and a phase shift of teleconnection patterns in the upper troposphere. Barotropic model experiments indicate that the inherent nonlinearity in the tropical rain response may itself be responsible for the phase shift in the extratropical teleconnection patterns.