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Michael Goss and Steven B. Feldstein

Abstract

Tropical precipitation anomalies associated with El Niño and Madden–Julian oscillation (MJO) phase 1 (La Niña and MJO phase 5) are characterized by a tripole, with positive (negative) centers over the Indian Ocean and central Pacific and a negative (positive) center over the warm pool region. However, their midlatitude circulation responses over the North Pacific and North America tend to be of opposite sign. To investigate these differences in the extratropical response to tropical convection, the dynamical core of a climate model is used, with boreal winter climatology as the initial flow. The model is run using the full heating field for the above four cases, and with heating restricted to each of seven small domains located near or over the equator, to investigate which convective anomalies may be responsible for the different extratropical responses. An analogous observational study is also performed. For both studies, it is found that, despite having a similar tropical convective anomaly spatial pattern, the extratropical response to El Niño and MJO phase 1 (La Niña and MJO phase 5) is quite different. Most notably, responses with opposite-signed upper-tropospheric geopotential height anomalies are found over the eastern North Pacific, northwestern North America, and the southeastern United States. The extratropical response for each convective case most closely resembles that for the domain associated with the largest-amplitude precipitation anomaly: the central equatorial Pacific for El Niño and La Niña and the warm pool region for MJO phases 1 and 5.

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Steven B. Feldstein and Christian Franzke

Abstract

This study addresses the question of whether persistent events of the North Atlantic Oscillation (NAO) and the Northern Annular Mode (NAM) teleconnection patterns are distinguishable from each other. Standard daily index time series are used to specify the amplitude of the NAO and NAM patterns. The above question is examined with composites of sea level pressure, and 300- and 40-hPa streamfunction, along with tests of field significance.

A null hypothesis is specified that the NAO and NAM persistent events are indistinguishable. This null hypothesis is evaluated by calculating the difference between time-averaged NAO and NAM composites. It is found that the null hypothesis cannot be rejected even at the 80% confidence level. The wave-breaking characteristics during the NAM life cycle are also examined. Both the positive and negative NAM phases yield the same wave-breaking properties as those for the NAO.

The results suggest that not only are the NAO and NAM persistent events indistinguishable, but that the NAO/NAM events are neither confined to the North Atlantic, nor are they annular.

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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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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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Dehai Luo, Yina Diao, and Steven B. Feldstein

Abstract

The winter-mean North Atlantic Oscillation (NAO) index has been mostly positive since the 1980s, with a linear upward trend during the period from 1978 to 1990 (P1) and a linear downward trend during the period from 1991 to 2009 (P2). Further calculations show that the Atlantic storm-track eddy activity is more intense during P2 than during P1, which is statistically significant at the 90% confidence level for a t test. This study proposes a hypothesis that the change in the trend of the positive NAO index from P1 to P2 may be associated with the marked intensification of the Atlantic storm track during P2.

A generalized nonlinear NAO model is used to explain the observed trend of the positive NAO index within P2. It is found that even when the Atlantic storm-track eddies are less intense, a positive-phase NAO event can form under the eddy forcing if the planetary-scale wave has an initial value with a low-over-high dipole structure during P1 and P2. A blocking flow can occur in the downstream side (over Europe) of the Atlantic basin as a result of the energy dispersion of Rossby waves during the decay of the positive-phase NAO event. This blocking flow does not strictly correspond to a negative-phase NAO event because the blocking stays mainly over the European continent. However, when the Atlantic storm-track eddies are rather strong, the blocking flow occurring over the European continent is enhanced and can retrograde into the Atlantic region and finally become a long-lived negative-phase NAO event. In this case, the NAO event can transit from the positive phase to the negative phase. Thus, the winter-mean NAO index during P2 will inevitably decline because of the increase in days of negative-phase NAO events in winter because the Atlantic storm track exhibits a marked intensification in the time interval. The transition of the NAO event from the positive phase to the negative phase can also be observed only when the downstream development of the Atlantic storm-track eddy activity is rather prominent.

Thus, it appears that there is a physical link between intraseasonal and interannual time scales of the NAO when the Atlantic storm track exhibits an interannual variability.

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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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Changhyun Yoo, Sukyoung Lee, and Steven B. Feldstein

Abstract

Using lagged composites and projections with the thermodynamic energy equation, in this study the mechanisms that drive the boreal winter Arctic surface air temperature (SAT) change associated with the Madden–Julian oscillation (MJO) are investigated. The Wheeler and Hendon MJO index, which divides the MJO into 8 phases, where phase 1 (phase 5) corresponds to reduced (enhanced) convection over the Maritime Continent and western Pacific Ocean, is used. It is shown that the more zonally localized (uniform) tropical convective heating associated with MJO phase 5 (phase 1) leads to enhanced (reduced) excitation of poleward-propagating Rossby waves, which contribute to Arctic warming (cooling). Adiabatic warming/cooling, eddy heat flux, and the subsequent change in downward infrared radiation (IR) flux are found to be important for the Arctic SAT change. The adiabatic warming/cooling initiates the Arctic SAT change, however, subsequent eddy heat flux makes a greater contribution. The resulting SAT change is further amplified by alteration in downward IR. It is shown that changes in surface sensible and latent heat fluxes oppose the contribution by the above processes.

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Tingting Gong, Steven B. Feldstein, and Sukyoung Lee

Abstract

The relationship between latent heating over the Greenland, Barents, and Kara Seas (GBKS hereafter) and Rossby wave propagation between the Arctic and midlatitudes is investigated using global reanalysis data. Latent heating is the focus because it is the most likely source of Rossby wave activity over the Arctic Ocean. Given that the Rossby wave time scale is on the order of several days, the analysis is carried out using a daily latent heating index that resembles the interdecadal latent heating trend during the winter season. The results from regression calculations find a trans-Arctic Rossby wave train that propagates from the subtropics, through the midlatitudes, into the Arctic, and then back into midlatitudes over a period of about 10 days. Upon entering the GBKS, this wave train transports moisture into the region, resulting in anomalous latent heat release. At high latitudes, the overlapping of a negative latent heating anomaly with an anomalous high is consistent with anomalous latent heat release fueling the Rossby wave train before it propagates back into the midlatitudes. This implies that the Rossby wave propagation from the Arctic into the midlatitudes arises from trans-Arctic wave propagation rather than from in situ generation. The method used indicates the variance of the trans-Arctic wave train, but not in situ generation, and implies that the variance of the former is greater than that of latter. Furthermore, GBKS sea ice concentration regression against the latent heating index shows the largest negative value six days afterward, indicating that sea ice loss contributes little to the latent heating.

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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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