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Dehai Luo, Tingting Gong, and Linhao Zhong

Abstract

In this paper, it is shown from an analytical solution that in the presence of a preexisting jet the interaction between the zonal jet and the topography of the land–sea contrast (LSC) in the Northern Hemisphere (NH) tends to induce a dipole component that depends crucially upon whether this zonal jet exhibits a north–south excursion. This phenomenon cannot be observed if the zonal jet has no north–south shift. When the preexisting jet is located more northward (southward), the induced dipole can have a low-over-high (high-over-low) structure and thus can make the center of the stationary wave anomaly shift southward (northward), which can be regarded as an initial state or embryo of a positive (negative) phase North Atlantic Oscillation (NAO). This dipole component can be amplified into a typical NAO event under the forcing of synoptic-scale eddies. To some extent, this result provides an explanation for why the positive (negative) phase of the NAO can be controlled by the northward (southward) shift of the zonal jet prior to the NAO.

In addition, the impact of the jet shift on the occurrence of NAO is examined in a weakly nonlinear NAO model if the initial state of an NAO is prespecified. It is found that the northward (southward) shift of a zonal jet favors the occurrence of the subsequent positive (negative) phase NAO event and then results in a northward (southward)-intensified jet relative to the preexisting jet. In addition, during the decaying of the positive phase NAO, a strong blocking activity is easily observed over Europe as the jet is moved to the north.

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Linhao Zhong, Lijuan Hua, and Dehai Luo

Abstract

In this paper, an ideal model of the role of mesoscale eddies in the Kuroshio intruding into the South China Sea (SCS) is developed, which represents the northwestern Pacific and the SCS as two rectangular basins connected by a gap. In the case of considering only intrinsic ocean variability, a time-dependent western boundary current (WBC) driven by steady wind is modeled under both eddy-resolving and noneddy-resolving resolutions. Almost all simulated WBCs intrude into the adjacent sea in the form of the Loop Current with multiple-state transitions and eddy-shedding processes, which have aperiodic variations on intraseasonal or interannual scales, determined by the eddy-induced WBC variation. For the parameters considered in this paper, the WBC intrusion exhibits a 30–90-day cycle in the presence of the subgrid-scale eddy forcing (SSEF) but a 300–500-day cycle in the absence of SSEF. Moreover, the roles of the grid-scale and subgrid-scale eddies in the WBC intrusion are studied by using the dynamically consistent decomposition developed by Berloff. Based on the large-sample composite analysis of the intrusion events, it is found that the Loop Current intensity is mainly determined by baroclinic processes through grid-scale, eddy–eddy interaction and subgrid-scale, eddy–flow interaction. The intrusion position and period are mainly regulated by the SSEF to the west of gap, where the balance between relative vorticity and isopycnal thickness SSEFs determines eddy detachment. Generally, the relative vorticity SSEF therein tends to induce WBC intrusion. However, the isopycnal thickness SSEF tends to induce eddy shedding, and WBC retreat thus determines the intrusion cycle through counteracting relative vorticity SSEF.

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Linhao Zhong, Lijuan Hua, and Dehai Luo

Abstract

Water vapor is critical to Arctic sea ice loss and surface air warming, particularly in winter. Whether the local process or poleward transport from lower latitudes can explain the Arctic warming is still a controversial issue. In this work, a hydrological tool, a dynamical recycling model (DRM) based on time-backward Lagrangian moisture tracking, is applied to quantitatively evaluate the relative contributions of local evaporation and external sources to Barents–Kara Seas (BKS) moisture in winter during 1979–2015. On average, the local and external moistures explain 35.4% and 57.3% of BKS moisture, respectively. The BKS, Norwegian Sea, and midlatitude North Atlantic are the three major sources and show significant increasing trends of moisture contribution. The local moisture contribution correlates weakly to downward infrared radiation (IR) but significantly to sea ice variation, which suggests that the recent-decade increase of local moisture contribution is only a manifestation of sea ice melting. In contrast, the external moisture contribution significantly correlates to both downward IR and sea ice variation, thus suggesting that meridional moisture transport mainly explains the recent BKS warming.

The moisture contributions due to different sources are governed by distinct circulation patterns. The negative Arctic Oscillation–like pattern suppresses external moisture but favors local evaporation. In the case of dominant external moisture, a well-organized wave train spanning from across the midlatitude Atlantic to mid–high-latitude Eurasia has the mid–high-latitude components similar to a positive-phase North Atlantic Oscillation with a Ural blocking to the east. Moreover, the meridional shift of the wave train pathway and the spatial scale of the wave train anomalies determine the transport passage and strength of the major external moisture sources.

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Dehai Luo, Linhao Zhong, and Christian L. E. Franzke
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Dehai Luo, Linhao Zhong, Rongcai Ren, and Chunzai Wang

Abstract

In this part, the spatial evolution of an initial dipole anomaly in a prescribed jet is at first investigated by numerically solving linear and nonlinear models without forcing in order to examine how the spatial pattern of a dipole anomaly depends on the meridional distribution of a specified jet. It is shown that in a linear experiment an initial symmetric dipole anomaly in the meridional direction can evolve into a northeast–southwest (NE–SW) or northwest–southeast (NW–SE) tilted dipole structure if the core of this jet is in higher latitudes (the north) or in lower latitudes (the south). This is in agreement with the result predicted by the linear Rossby wave theory in slowly varying media. The conclusion also holds for the nonlinear and unforced experiment.

North Atlantic Oscillation (NAO) events are then reproduced in a fully nonlinear barotropic model with a wavemaker that mimics the Atlantic storm-track eddy activity. In the absence of topography the spatial tilting of the eddy-driven NAO pattern is found to be independent of the NAO phase. The eddy-driven NAO pattern for the positive (negative) phase can exhibit a NE–SW (NW–SE) tilting only when the core of a prescribed jet prior to the NAO is confined in the higher latitude (lower latitude) region. However, in the presence of the wavenumber-2 topography (two oceans and continents) in the Northern Hemisphere the spatial tilting of the eddy-driven NAO dipole anomaly can be dependent on the NAO phase. Even when the specified basic flow prior to the NAO is uniform, the eddy-driven positive (negative) NAO phase dipole anomaly can also show a NE–SW (NW–SE) tilting because the northward (southward) shift of the excited westerly jet can occur in the presence of topography. In addition, it is found that when the wavemaker is closer to the position of the initial NAO, the eddy-driven positive (negative) NAO phase pattern can display a whole eastward shift and a more distinct NE–SW (NW–SE) tilting. This thus explains why the first empirical orthogonal function of the NAO pattern observed during 1998–2007 exhibits a more pronounced NE–SW tilting than during 1978–97. It appears that the latitudinal shift of the jet, the large-scale topography, and the zonal position of the Atlantic storm-track eddy activity are three important factors for controlling the spatial tilting and zonal shift of eddy-driven NAO dipole anomalies.

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Dehai Luo, Zhihui Zhu, Rongcai Ren, Linhao Zhong, and Chunzai Wang

Abstract

This paper presents a possible dynamical explanation for why the North Atlantic Oscillation (NAO) pattern exhibits an eastward shift from the period 1958–77 (P1) to the period 1978–97 (P2) or 1998–2007 (P3). First, the empirical orthogonal function analysis of winter mean geopotential heights during P1, P2, and P3 reveals that the NAO dipole anomaly exhibits a northwest–southeast (NW–SE) tilting during P1 but a northeast–southwest (NE–SW) tilting during P2 and P3. The NAO pattern, especially its northern center, undergoes a more pronounced eastward shift from P1 to P2. The composite calculation of NAO events during P1 and P2 also indicates that the negative (positive) NAO phase dipole anomaly can indeed exhibit such a NW–SE (NE–SW) tilting. Second, a linear Rossby wave formula derived in a slowly varying basic flow with a meridional shear is used to qualitatively show that the zonal phase speed of the NAO dipole anomaly is larger (smaller) in higher latitudes and smaller (larger) in lower latitudes during the life cycle of the positive (negative) NAO phases because the core of the Atlantic jet is shifted to the north (south). Such a phase speed distribution tends to cause the different movement speeds of the NAO dipole anomaly at different latitudes, thus resulting in the different spatial tilting of the NAO dipole anomaly depending on the phase of the NAO. The zonal displacement of the northern center of the NAO pattern appears to be more pronounced because the change of the mean flow between two phases of the NAO is more distinct in higher latitudes than in lower latitudes.

In addition, a weakly nonlinear analytical solution, based on the assumption of the scale separation between the NAO anomaly and transient synoptic-scale waves, is used to demonstrate that an eastward shift of the Atlantic storm-track eddy activity that is associated with the eastward extension of the Atlantic jet stream is a possible cause of the whole eastward shift of the center of action of the NAO pattern during P2/P3.

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Dehai Luo, Linhao Zhong, and Christian L. E. Franzke

Abstract

In this paper, based on a new wave–eddy interaction framework, the interaction mechanism between the North Atlantic Oscillation (NAO) and synoptic-scale eddies is revealed by using the analytical solutions of a two-scale model as a description of the inverse energy cascade from nonuniform synoptic-scale eddies to the large-scale NAO flow. It is found that the spatial shape of the eddy-induced large-scale streamfunction tendency prior to the NAO onset determines the direction of eddy energy transfer, as well as the phase and growth of the NAO. However, the feedback of the intensified NAO anomaly on synoptic eddies can affect significantly the asymmetry of the NAO between negative (NAO) and positive (NAO+) phases in amplitude and persistence through the presence or absence of the eddy straining related to cyclonic wave breaking (CWB).

For the NAO+, the stretching deformation role of the NAO+ field seems dominant in the eddy variation. Because the eddy energy generation rate (EGR) weakens and tends to be negative in the downstream side of the NAO+ region, the synoptic eddies lose their energy to the NAO+-type zonal flow, thus leading to the weakening of synoptic-scale eddies. However, for the NAO, the EGR variation shows that synoptic eddies grow over the two upstream sides of the NAO region by extracting energy from the NAO shearing deformation field, while losing energy to the mean flow over the upstream middle region through the stretching deformation. This process results in the eddy straining (splitting and strengthening) associated with the CWB.

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Dehai Luo, Yao Yao, Aiguo Dai, Ian Simmonds, and Linhao Zhong

Abstract

In Part I of this study, it was shown that the Eurasian cold anomalies related to Arctic warming depend strongly on the quasi stationarity and persistence of the Ural blocking (UB). The analysis here revealed that under weak mean westerly wind (MWW) and vertical shear (VS) (quasi barotropic) conditions with weak synoptic-scale eddies and a large planetary wave anomaly, the growth of UB is slow and its amplitude is small. For this case, a quasi-stationary and persistent UB is seen. However, under strong MWW and VS (quasi baroclinic) conditions, synoptic-scale eddies are stronger and the growth of UB is rapid; the resulting UB is less persistent and has large amplitude. In this case, a marked retrogression of the UB is observed.

The dynamical mechanism behind the dependence of the movement and persistence of UB upon the background conditions is further examined using a nonlinear multiscale model. The results show that when the blocking has large amplitude under quasi-baroclinic conditions, the blocking-induced westward displacement greatly exceeds the strong mean zonal-wind-induced eastward movement and hence generates a marked retrogression of the blocking. By contrast, under quasi-barotropic conditions because the UB amplitude is relatively small the blocking-induced westward movement is less distinct, giving rise to a quasi-stationary and persistent blocking. It is further shown that the strong mid–high-latitude North Atlantic mean zonal wind is the quasi-barotropic condition that suppresses UB’s retrogression and thus is conducive to the quasi stationarity and persistence of the UB. The model results show that the blocking duration is longer when the mean zonal wind in the blocking region or eddy strength is weaker.

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Dehai Luo, Wenqi Zhang, Linhao Zhong, and Aiguo Dai

Abstract

In this paper, an extended nonlinear multiscale interaction model of blocking events in the equivalent barotropic atmosphere is used to investigate the effect of a slowly varying zonal wind in the meridional direction on dipole blocking that is regarded as a nonlinear Rossby wave packet. It is shown that the meridional gradient of potential vorticity (PVy=PV/y) prior to the blocking onset, which is related to the background zonal wind and its nonuniform meridional shear, can significantly affect the lifetime, intensity, and north–south asymmetry of dipole blocking, while the blocking dipole itself is driven by preexisting incident synoptic-scale eddies. The magnitude of the background PVy determines the energy dispersion and nonlinearity of blocking. It is revealed that a small background PVy is a prerequisite for strong and long-lived eddy-driven blocking that behaves as a persistent meandering westerly jet stream, while the blocking establishment further reduces the PVy within the blocking region, resulting in a positive feedback between blocking and PVy. When the core of the background westerly jet shifts from higher to lower latitudes, the blocking shows a northwest–southeast-oriented dipole with a strong anticyclonic anomaly to the northwest and a weak cyclonic anomaly to the southeast as its northern pole moves westward more rapidly and has weaker energy dispersion and stronger nonlinearity than its southern pole because of the smaller PVy in higher latitudes. The opposite is true when the background jet shifts toward higher latitudes. The asymmetry of dipole blocking vanishes when the background jet shows a symmetric double-peak structure. Thus, a small prior PVy is a favorable precursor for the occurrence of long-lived and large-amplitude blocking.

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