Dehai Luo

Seasonal response of Asian monsoonal climate to the Atlantic Multidecadal Oscillation

[1] The influence of the Atlantic Multidecadal Oscillation (AMO) on Asian monsoonal climate in all four seasons is investigated by comprehensive observational analyses and ensemble experiments with atmospheric general circulation models (AGCMs). Three AGCMs are forced by prescribed climatological seasonal cycle of sea surface temperature (SST) or with additional SST anomalies representing the warmth phase of the AMO. The results in both the observations and the models consistently suggest that the warm AMO phase gives rise to elevated air temperatures in East Asia and northern India but decreased air temperatures in much of central-southern India in all four seasons. This positive AMO anomaly also causes more rainfall in central and southern India in every season, particularly in summer and fall. In contrast, the sign of AMO influences on East Asian rainfall is season-dependent: in southeastern China, it induces increased rainfall in summer but suppressed rainfall in autumn. It is suggested that these AMO influences are realized by warming Eurasian middle and upper troposphere in all four seasons, resulting in weakened Asian winter monsoons but enhanced summer monsoons. Furthermore, the formation of the troposphere heating anomaly may be related to the wave guidance mechanism associated with the Asian upper jet.

Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part I: A Simple Model of North Atlantic Oscillations

Abstract A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of...

A Barotropic Envelope Rossby Soliton Model for Block–Eddy Interaction. Part I: Effect of Topography

Abstract A new forced envelope Rossby soliton model in an equivalent barotropic beta-plane channel is proposed to describe the interaction between an incipient block (planetary scale) and short synoptic-scale eddies. This model is based on two assumptions, motivated by observations that (i) there exists a zonal scale separation between the planetary-scale and synoptic-scale waves and (ii) that the range of synoptic-scale zonal wavenumber is comparable to the planetary-scale zonal wavenumber. These assumptions allow an analytical treatment. The evolution of the planetary-scale block under the influence of synoptic-scale eddies is described by a forced nonlinear Schrodinger equation that is solved numerically, while the feedback of block development on the preexisting synoptic-scale eddies is derived analytically. It is shown that the planetary-scale projection of the nonlinear interaction between synoptic-scale eddies is the most important contributor to the amplification and decay of the planetary-scale b...

Impact of Ural Blocking on Winter Warm Arctic–Cold Eurasian Anomalies. Part I: Blocking-Induced Amplification

AbstractIn Part I of this study, the impact of Ural blocking (UB) on the warm Arctic–cold Eurasian (WACE) pattern associated with the winter (DJF) arctic sea ice loss during 1979–2013 is examined by dividing the arctic sea ice reduction region into two dominant subregions: the Barents and Kara Seas (BKS) and the North American high-latitude (NAH) region (Baffin and Hudson Bay, Davis Strait, and Labrador Sea). It is found that atmospheric response to arctic sea ice loss resembles a negative Arctic response oscillation with a dominant positive height anomaly over the Eurasian subarctic region. Regression analyses of the two subregions further show that the sea ice loss over the BKS corresponds to the UB pattern together with a positive North Atlantic Oscillation (NAO+) and is followed by a WACE anomaly, while the sea ice reduction in the NAH region corresponds to a negative NAO (NAO−) pattern with a cold anomaly over northern Eurasia.Further analyses reveal that the UB pattern is more persistent during the ...

A new blocking index and its application: Blocking action in the Northern Hemisphere

Abstract In this paper, a new two-dimensional blocking index is proposed by defining a difference between the daily 500-hPa geopotenial heights at the reference latitude and its north side. The reference latitude is determined by a composite latitude-dependent 500-hPa geopotenial height of blocking events in different seasons and sectors. The new index can take account of the duration, intensity, propagation, and spatial structure of a blocking event. Using this index, the characteristics (frequency, duration, intensity, and preferred occurrence region) of the blocking action in the North Hemisphere (NH) are investigated using a 42-yr sample of blocking events from the NCEP–NCAR reanalyses. It is found that blocking events in the NH are more frequent in the Atlantic–Europe sector than in the Pacific sector in winter and spring and autumn, but more persistent in the Atlantic–Europe sector than in the Pacific sector for all seasons. Blocking events in the Pacific sector tend to have larger amplitudes than t...

The Impact of ENSO on Wave Breaking and Southern Annular Mode Events

Abstract This study examines the relationship between intraseasonal southern annular mode (SAM) events and the El Nino–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 Nina and negative SAM events during El Nino. 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 Nina (El Nino) 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 ...

Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part III: Meridional Displacement of Westerly Jet Anomalies during Two Phases of NAO

Abstract In this paper, the north–south variability of westerly jet anomalies during the two phases of the North Atlantic Oscillation (NAO) is examined in a theoretical model. It is found that the north–south variability of the zonal mean westerly anomaly results from the interaction between the eddy-driven anomalous stationary waves with a dipole meridional structure (NAO anomalies) and topographically induced climatological stationary waves with a monopole structure, which is dependent upon the phase of the NAO. The westerly jet anomaly tends to shift northward during the positive NAO phase but southward during the negative phase. Synoptic-scale eddies tend to maintain westerly jet anomalies through the excitation of NAO anomalies, but the climatological stationary wave and its position relative to the eddy-driven anomalous stationary wave appear to dominate the north–south shift of westerly jet anomalies. On the other hand, it is shown that when the climatological stationary wave ridge is located downs...

Impact of Ural Blocking on Winter Warm Arctic–Cold Eurasian Anomalies. Part II: The Link to the North Atlantic Oscillation

AbstractIn Part I of this study, the Ural blocking (UB)-induced amplification role of winter warm Arctic–cold Eurasian (WACE) anomalies has been examined. It was found that the long-lived UB together with the positive North Atlantic Oscillation (NAO+) significantly contributes to the amplification of the WACE pattern. The present study examines how the UB variability affects quasi-biweekly WACE (QB-WACE) anomalies and depends on the NAO+ and North Atlantic conditions by classifying the UB based on a case study of a cold event that occurred over southern China in January 2008. A composite analysis during 1979–2013 shows that the QB-WACE anomalies associated with the UB that often occur with the NAO+ are strong and influenced by the North Atlantic jet (NAJ) and zonal wind strengths over Eurasia. For NAO+-related UB, the QB-WACE anomaly depends strongly on the location of UB, and the UB anomalies lag the NAO+ by approximately 4–7 days.The strength of the NAJ determines whether the combined NAO+ and UB anomal...

The Variability of the Atlantic Storm Track and the North Atlantic Oscillation: A Link between Intraseasonal and Interannual Variability

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 si...

Interaction between antecedent planetary‐scale envelope soliton blocking anticyclone and synoptic‐scale eddies: Observations and theory

The conditions of 30 blocking events were examined at the time of onset over the 1993/1994–1996/1997 cold season (November to March). The antecedent synoptic-scale eddies in 83.3% of the total events are shown to induce a large-scale low/high eddy forcing pattern upstream of the incipient block, and the mean basic westerly flow is weak, 76.7% of which possesses cyclonic shear. It appears that both the antecedent synoptic-scale eddies that induce a low/high eddy forcing pattern and the weak background westerly flow (zonal wave number O) that allows quasi-stationary preblock ridge are two necessary preconditions for the onset of blocking anticyclone, but the cyclonic shear of the background westerly flow is a favorable preblock environment. In addition, a theoretical model, represented by the superposition of dipole and monopole envelope Rossby solitons with weak orographic forcing for zonal wave number 2, is proposed to confirm these observations. It is found that the dipole envelope soliton is dominant and exhibits a quasi-2-week oscillation during the interaction between an antecedent blocking anticyclone and upstream synoptic-scale eddies. The monopole soliton tends to break up, indicating that blocking anticyclone circulation is dominated by the amplification of dipole component associated with synoptic-scale eddies. Finally, the evolution of time-dependent eddy feedback is shown to control the direction of planetary-scale potential vorticity transports induced by transient eddies, which is an indicator of the onset, maintenance, and decay of blocking. Moreover, the changes of amplified blocking anticyclone and synoptic-scale eddies obtained theoretically are also in agreement with those of observed blocking and synoptic eddies.