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Michael Goss and Steven B. Feldstein

Abstract

The response to the Madden–Julian oscillation (MJO) over the Pacific–North American (PNA) region is investigated. In addition, the sensitivity of this response to the interaction between Madden–Julian oscillation forcing and the extratropical initial flow is explored. First, a simple dynamical model is run with an ensemble of 100 randomly selected initial conditions from ERA-Interim data, with no heating, MJO phase-1-like heating, and MJO phase-5-like heating. The 300-hPa geopotential height field is separated into a part that would evolve without an active MJO present, and a part that is a consequence of the MJO heating. A negative 300-hPa geopotential height anomaly centered over northeastern China bounded by positive anomalies on its equatorward and poleward flanks is found to be followed by a large amplitude negative PNA-like response for MJO phase 1 and a positive PNA-like response for phase 5.

A similar study is carried out using observational data. An analog approach—using projections to determine the analogs—is used to approximate the part of the flow that results from the MJO heating. The composite initial flow that corresponds to a large MJO response in observational data somewhat matches that in the model, although there is more variability between phases. Finally, the analog method is used to examine questions related to predictability and the MJO. It is found that predictability is improved by taking into account the presence of the MJO and by choosing analogs with high projections. The presence of an active MJO also increases predictability.

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Michael Goss and Steven B. Feldstein

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The dynamical core of a dry global model is used to investigate the role of central Pacific versus warm pool tropical convection on the extratropical response over the North Pacific and North America. A series of model runs is performed in which the amplitude of the warm pool (WP) and central Pacific (CP) heating anomalies associated with the MJO and El Niño–Southern Oscillation (ENSO) is systematically varied. In addition, model calculations based on each of the eight MJO phases are performed, first using stationary heating, and then with heating corresponding to a 48-day MJO cycle and to a 32-day MJO cycle.

In all model runs, the extratropical response to tropical convection occurs within 7–10 days of the convective heating. The response is very sensitive to the relative amplitude of the heating anomalies. For example, when heating anomalies in the WP and CP have similar amplitude but opposite sign, the amplitude of the extratropical response is much weaker than is typical for the MJO and ENSO. For the MJO, when the WP heating anomaly is much stronger than the CP heating anomaly (vice versa for ENSO), the extratropical response is amplified. For the MJO heating, it is found that the extratropical responses to phases 4 and 8 are most distinct. A likely factor contributing to this distinctiveness involves the relative amplitude of the two heating anomalies. The stationary and moving (48- and 32-day) heating responses are very similar, revealing a lack of sensitivity to the MJO phase speed.

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Dehai Luo, Jing Cha, and Steven B. Feldstein

Abstract

In this study, the relationship between weather regime transitions and the interannual variability of the North Atlantic Oscillation (NAO) in winter during 1978–2008 is examined by using a statistical approach. Four classical weather regimes—the two phases of the NAO (NAO+, NAO) and the Scandinavian blocking and Atlantic ridge patterns—are obtained with k-means cluster analysis. Observations show that the transition between the NAO+ and NAO regimes is markedly different between 1978–90 (P1) and 1991–2008 (P2). Within P1 (P2), the frequency of the NAO to NAO+ (NAO+ to NAO) transition events is almost twice that of the NAO+ to NAO (NAO to NAO+) transition events. On this basis, further cluster analysis performed for two cases with and without NAO transition events indicates that within P1 (P2) the NAO+ (NAO) anomaly is markedly enhanced as the NAO to NAO+ (NAO+ to NAO) transitions take place. Furthermore, the NAO regime transition is found to be more likely to enhance the eastward shift of the NAO+ (NAO) anomaly. Thus, it is hypothesized that the interannual change in the winter-mean NAO index from P1 to P2 is related to the intraseasonal NAO to NAO+ (NAO+ to NAO) transition events during P1 (P2) because of the variation of the NAO pattern in intensity, location, and frequency (number of days). This finding is also seen from calculations of the winter monthly mean NAO index with and without NAO regime transitions.

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Joseph P. Clark and Steven B. Feldstein

Abstract

Radiative transfer calculations are conducted to determine the contribution of temperature and water vapor anomalies toward the surface clear-sky downward longwave radiation (DLR) anomalies of the NAO. These calculations are motivated by the finding that the NAO’s skin temperature anomalies are driven primarily by changes in surface DLR. The clear-sky radiative transfer calculations follow the result that the clear-sky surface DLR anomalies can account for most of the all-sky surface DLR anomalies of the NAO. The results of the radiative transfer calculations prompt an analysis of the thermodynamic energy and total column water (TCW) budget equations, as water vapor and temperature anomalies are found to be equally important drivers of the surface DLR anomalies of the NAO. Composite analysis of the thermodynamic energy equation reveals that the temperature anomalies of the NAO are wind driven: the advection of climatological temperature by the anomalous wind drives the NAO’s temperature anomalies at all levels except for those in the upper troposphere–lower stratosphere where the advection of anomalous temperature by the climatological wind becomes dominant. A similar analysis of the TCW budget reveals that changes in TCW are driven by water flux convergence. In addition to determining the drivers of the temperature and TCW anomalies, the thermodynamic energy and water budget analyses reveal that the decay of the temperature anomalies occurs primarily through vertical mixing, and that of the water anomalies mostly by evaporation minus precipitation.

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Tingting Gong, Steven B. Feldstein, and Dehai Luo

Abstract

This study investigates the relationship between El Niño–Southern Oscillation (ENSO) and southern annular mode (SAM) events with an idealized general circulation model. A series of model calculations are performed to examine why positive (negative) intraseasonal SAM events are observed to occur much more frequently during La Niña (El Niño). Seven different model runs are performed: a control run, three El Niño runs (the first with a zonally symmetric heating field, the second with a zonally asymmetric heating/cooling field, and the third that combines both fields), and three La Niña runs (with heating fields of opposite sign).

The model runs with the zonally symmetric and combined heating fields are found to yield the same relationship between the phase of ENSO and the preferred phase for SAM events as is observed in the atmosphere. In contrast, the zonally asymmetric model runs are found to have the opposite SAM–ENSO phase preference characteristics. Since a reduced midlatitude meridional potential vorticity gradient has been linked to a greater frequency of positive-phase SAM events, and vice versa for negative SAM events, the meridional potential vorticity gradient in the various model runs was compared. The results suggest that the phase preference of SAM events during ENSO arises from the impact of the zonal-mean heating on the midlatitude meridional potential vorticity gradient.

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Michael Goss, Steven B. Feldstein, and Sukyoung Lee

Abstract

The interference between transient eddies and climatological stationary eddies in the Northern Hemisphere is investigated. The amplitude and sign of the interference is represented by the stationary wave index (SWI), which is calculated by projecting the daily 300-hPa streamfunction anomaly field onto the 300-hPa climatological stationary wave. ERA-Interim data for the years 1979 to 2013 are used. The amplitude of the interference peaks during boreal winter. The evolution of outgoing longwave radiation, Arctic temperature, 300-hPa streamfunction, 10-hPa zonal wind, Arctic sea ice concentration, and the Arctic Oscillation (AO) index are examined for days of large SWI values during the winter.

Constructive interference during winter tends to occur about one week after enhanced warm pool convection and is followed by an increase in Arctic surface air temperature along with a reduction of sea ice in the Barents and Kara Seas. The warming of the Arctic does occur without prior warm pool convection, but it is enhanced and prolonged when constructive interference occurs in concert with enhanced warm pool convection. This is followed two weeks later by a weakening of the stratospheric polar vortex and a decline of the AO. All of these associations are reversed in the case of destructive interference. Potential climate change implications are briefly discussed.

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Tingting Gong, Steven B. Feldstein, and Dehai Luo

Abstract

This study examines the relationship between intraseasonal southern annular mode (SAM) events and the El Niño–Southern Oscillation (ENSO) using daily 40-yr ECMWF Re-Analysis (ERA-40) data. The data coverage spans the years 1979–2002, for the austral spring and summer seasons. The focus of this study is on the question of why positive SAM events dominate during La Niña and negative SAM events during El Niño. A composite analysis is performed on the zonal-mean zonal wind, Eliassen–Palm fluxes, and two diagnostic variables: the meridional potential vorticity gradient, a zonal-mean quantity that is used to estimate the likelihood of wave breaking, and the wave breaking index (WBI), which is used to evaluate the strength of the wave breaking. The results of this investigation suggest that the background zonal-mean flow associated with La Niña (El Niño) is preconditioned for strong (weak) anticyclonic wave breaking on the equatorward side of the eddy-driven jet, the type of wave breaking that is found to drive positive (negative) SAM events. A probability density function analysis of the WBI, for both La Niña and El Niño, indicates that strong anticyclonic wave breaking takes place much more frequently during La Niña and weak anticyclonic wave breaking during El Niño. It is suggested that these wave breaking characteristics, and their dependency on the background flow, can explain the strong preference for SAM events of one phase during ENSO. The analysis also shows that austral spring SAM events that coincide with ENSO are preceded by strong stratospheric SAM anomalies and then are followed by a prolonged period of wave breaking that lasts for approximately 30 days. These findings suggest that the ENSO background flow also plays a role in the excitation of stratospheric SAM anomalies and that the presence of these stratospheric SAM anomalies in turn excites and then maintains the tropospheric SAM anomalies via a positive eddy feedback.

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Dehai Luo, Xiaodan Chen, and Steven B. Feldstein

Abstract

Observations reveal that the North Atlantic Oscillation (NAO) exhibits a strong asymmetry: large amplitude, long persistence, and westward movement in its negative phase (NAO) and conversely in its positive phase (NAO+). Further calculations show that blocking days occur frequently over the North Atlantic (Eurasia) after the NAO (NAO+) peaks, thus indicating that North Atlantic blocking occurs because of the retrogression of the NAO, whereas blocking occurs over Eurasia because of enhanced downstream energy dispersion of the NAO+.

Motivated by a unified nonlinear multiscale interaction (UNMI) model, the authors define dispersion, nonlinearity, and movement indices to describe the basic characteristics of the NAO. On this basis, the physical cause of the strong asymmetry or symmetry breaking of the NAO is examined. It is revealed that the strong asymmetry between the NAO+ and NAO may be associated with the large difference of the North Atlantic jet in intensity and latitude between both phases. When the NAO+ grows, the North Atlantic jet is intensified and shifts northward and corresponds to reduced nonlinearity and enhanced energy dispersion because of an increased difference between its group velocity and phase speed related to enhanced meridional potential vorticity gradient. Thus, the NAO+ has smaller amplitude, eastward movement, and less persistence. Opposite behavior is seen for the NAO because of the opposite variation of the North Atlantic jet during its life cycle. Thus, the above results suggest that the NAO+ (NAO) tends to be a linear (nonlinear) process as a natural consequence of the NAO evolution because of different changes in the North Atlantic jet between both phases.

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Changhyun Yoo, Sukyoung Lee, and Steven B. Feldstein

Abstract

Using an initial-value approach with an idealized general circulation model, the mechanisms by which the Madden–Julian oscillation (MJO) influences the Arctic surface air temperature (SAT) are investigated. Model calculations corresponding to MJO phases 1 and 5 are performed, as previous studies have shown that these two phases are associated with a cooling and warming of the Arctic surface, respectively. Observed MJO-like tropical heating profiles are specified, with the phase 5 (phase 1) heating taking on a more zonally localized (uniform) spatial structure. A large ensemble of model runs is performed, where the initial flow of each ensemble member consists of the winter climatology together with an initial perturbation that is selected randomly from observational data. The model calculations show that MJO phase 5 (phase 1) is followed by a strengthening (weakening) in the poleward wave activity propagation out of the tropics, which leads to an increase (decrease) in Arctic SAT. Examination of the corresponding eddy momentum flux convergence and mass streamfunction fields shows that an eddy-induced mean meridional circulation warms (cools) the Arctic for phase 5 (phase 1). Further Arctic warming (cooling) takes place through changes in the planetary-scale, poleward eddy heat flux. In addition, calculations with a passive tracer added to the model show an increase (decrease) in the high-latitude tracer concentration for MJO phase 5 (phase 1). These results suggest that the observed changes in Arctic downward infrared radiation associated with the MJO may be associated with changes in poleward moisture transport.

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Nathaniel C. Johnson and Steven B. Feldstein

Abstract

This study combines k-means cluster analysis with linear unidimensional scaling to illustrate the spatial and temporal variability of the wintertime North Pacific sea level pressure (SLP) field. Daily wintertime SLP data derived from the NCEP–NCAR reanalysis are used to produce 16 SLP anomaly patterns that represent a discretized approximation of the continuum of North Pacific SLP patterns. This study adopts the continuum perspective for teleconnection patterns, which provides a much simpler framework for understanding North Pacific variability than the more commonly used discrete modal approach.

The primary focus of this research is to show that variability in the North Pacific—on intraseasonal, interannual, and interdecadal time scales—can be understood in terms of changes in the frequency distribution of the cluster patterns that compose the continuum, each of which has a time scale of about 10 days. This analysis reveals 5–6 Pacific–North American–like (PNA-like) patterns for each phase, as well as dipoles and wave trains. A self-organizing map (SOM) analysis of coupled SLP and outgoing longwave radiation data shows that many of these patterns are associated with convection in the tropical Indo-Pacific region. On intraseasonal time scales, the frequency distribution of these patterns, in particular the PNA-like patterns, is strongly influenced by the Madden–Julian oscillation (MJO). On interannual time scales, the El Niño–Southern Oscillation (ENSO) impacts the North Pacific continuum, with warm ENSO episodes resulting in the increased frequency of easterly displaced Aleutian low pressure anomaly patterns and cold ENSO episodes resulting in the increased frequency of southerly displaced Aleutian high pressure anomaly patterns. In addition, the results of this analysis suggest that the interdecadal variability of the North Pacific SLP field, including the well-known “regime shift” of 1976/77, also results from changes in the frequency distribution within the continuum of SLP patterns.

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