ENSO related variation of equatorial MRG and Rossby waves and forcing from higher latitudes

The contrasting behaviour of westward-moving mixed Rossby-gravity (WMRG) and the first Rossby (R1) waves in El Niño (EN) and La Niña (LN) seasons is documented with a focus on the Northern Hemisphere winter. The eastward-moving variance in the upper troposphere is dominated by WMRG and R1 structures that appear to be Doppler-shifted by the flow and are referred to as WMRG-E and R1-E. In the East Pacific and Atlantic the years with stronger equatorial westerly winds have the stronger WMRG and WMRG- E. In the East Pacific, R1 is also a maximum in LN. However, R1-E exhibits an eastward-shift between LN and EN. The changes with ENSO phase provide a test-bed for the understanding of these waves. In the East Pacific and Atlantic, the stronger WMRG-E and WMRG with stronger westerlies are in accord with the dispersion relation with simple Doppler-shifting by the zonal flow. The possible existence of free waves can also explain stronger R1 in EN in the Eastern Hemisphere. 1-D free wave propagation theory based on wave activity conservation is also important for R1. However, this theory is unable to explain the amplitude maxima for other waves observed in the strong equatorial westerly regions in the Western Hemisphere, and certainly not their ENSO-related variation. The forcing of equatorial waves by higher latitude wave activity and its variation with ENSO phase is therefore examined. Propagation of extratropical eastward-moving Rossby wave activity through the westerly ducts into the equatorial region where it triggers WMRG-E is favoured in the stronger westerlies, in LN in the East Pacific and EN in the Atlantic. It is also found that WMRG is forced by Southern Hemisphere westward-moving wavetrains arching into the equatorial region where they are reflected. The most significant mechanism for both R1 and R1-E appear to be lateral forcing by subtropical wavetrains

The influence of the QBO on the propagation of equatorial waves into the stratosphere

The variation of stratospheric equatorial wave characteristics with the phase of the quasi-biennial oscillation (QBO) is investigated using ECMWF Re-Analysis and NOAA outgoing longwave radiation (OLR) data. The impact of the QBO phases on the upward propagation of equatorial waves is found to be consistent and significant. In the easterly phase, there is larger Kelvin wave amplitude but smaller westward-moving mixed Rossby–gravity (WMRG) and n = 1 Rossby (R1) wave amplitude due to reduced propagation from the upper troposphere into the lower stratosphere, compared with the westerly phase. Differences in the wave amplitude exist in a deeper layer in summer than in winter, consistent with the seasonality of ambient zonal winds. There is a strong evidence of Kelvin wave amplitude peaking just below the descending westerly phase, suggesting that Kelvin waves act to bring the westerly phase downward. However, the corresponding evidence for WMRG and R1 waves is less clear. In the lower stratosphere there is zonal variation in equatorial waves. This reflects the zonal asymmetry of wave amplitudes in the upper troposphere, the source for the lower-stratospheric waves. In easterly winters the upper-tropospheric WMRG and R1 waves over the eastern Pacific region appear to be somewhat stronger compared to climatology, perhaps because of the accumulation of waves that are unable to propagate upward into the lower stratosphere. Vertical propagation features of these waves are generally consistent with theory and suggest a mixture of Doppler shifting by ambient flows and filtering. Some lower-stratosphere equatorial waves have a connection with preceding tropical convection, especially for Kelvin and R1 waves in winter

Tropical moist convection an important driver of Atlantic Hadley circulation variability

The exact role of moist deep convection and associated latent heating in the tropical Hadley circulation has been debated for many years. This study investigates the connection between moist convection and the strength of the upper-level meridional circulation over the tropical Atlantic, focusing mainly on one particular boreal winter season. There is a close relationship between events of strong organised deep convection and enhanced meridional upper-level wind on many occasions. A process-based analysis of specific events suggests that moist convection impacts Hadley circulation variability on time-scales of days to months through equatorial wave dynamics. Equatorial waves play an important role, both directly by contributing to the Hadley circulation via their meridional wind component and also indirectly by triggering moist convection through low-level convergence. Specific Hadley circulation surge events, short-term, regionally confined intensifications of the upper-level meridional circulation, can be attributed to enhanced organised moist convection and equatorial wave activity in many cases, with implications for trade wind cloudiness. The findings thus elucidate how the mean Hadley circulation is shaped by and composed of temporally and spatially varying convection–circulation interactions

LIP: Learning Instance Propagation for Video Object Segmentation

In recent years, the task of segmenting foreground objects from background in a video, i.e. video object segmentation (VOS), has received considerable attention. In this paper, we propose a single end-to-end trainable deep neural network, convolutional gated recurrent Mask-RCNN, for tackling the semi-supervised VOS task. We take advantage of both the instance segmentation network (Mask-RCNN) and the visual memory module (Conv-GRU) to tackle the VOS task. The instance segmentation network predicts masks for instances, while the visual memory module learns to selectively propagate information for multiple instances simultaneously, which handles the appearance change, the variation of scale and pose and the occlusions between objects. After offline and online training under purely instance segmentation losses, our approach is able to achieve satisfactory results without any post-processing or synthetic video data augmentation. Experimental results on DAVIS 2016 dataset and DAVIS 2017 dataset have demonstrated the effectiveness of our method for video object segmentation task.Comment: ICCVW1

LIP:Learning instance propagation for video object segmentation

In recent years, the task of segmenting foreground objects from background in a video, i.e. video object segmentation (VOS), has received considerable attention. In this paper, we propose a single end-to-end trainable deep neural network, convolutional gated recurrent Mask-RCNN, for tackling the semi-supervised VOS task. We take advantage of both the instance segmentation network (Mask-RCNN) and the visual memory module (Conv-GRU) to tackle the VOS task. The instance segmentation network predicts masks for instances, while the visual memory module learns to selectively propagate information for multiple instances simultaneously, which handles the appearance change, the variation of scale and pose and the occlusions between objects. After offline and online training under purely instance segmentation losses, our approach is able to achieve satisfactory results without any post-processing or synthetic video data augmentation. Experimental results on DAVIS 2016 dataset and DAVIS 2017 dataset have demonstrated the effectiveness of our method for video object segmentation task.</p

Relationship between extensive and persistent extreme cold events in China and stratospheric circulation anomalies

This study examines the relationship between the extensive and persistent extreme cold events (EPECEs) in China and geopotential height anomalies in the stratosphere using daily mean fields of outgoing long wave radiation (OLR) produced by the NCAR and daily atmospheric circulations produced by the NCEP/NCAR. The OLR composite analysis for the EPECE in China demonstrates that the negative OLR height anomalies (cold air) originated from Siberia influence China progressively from north to south. The largest negative OLR height anomaly (cooling event) occurs in the region to the north of the Nanling Mountains. This suggests that the OLR height anomalies can be used to represent the temporal and spatial characteristics of extreme low temperatures and cold air activities in winter in China. The composite analysis of large-scale atmospheric circulations during the EPECE reveals characteristic evolutions of stratospheric and tropospheric circulations during the extreme cold event. We demonstrate the important role of atmospheric circulation anomalies in the outbreak and dissipation of the EPECE in China. We also demonstrate that significant perturbations in the stratospheric circulation occur more than 10 days prior to the outbreak of the EPECE, with positive height anomalies in the Arctic stratosphere. These positive anomalies propagate downward from the stratosphere and affect the formation and development of the high pressure ridge in the middle troposphere over the Ural Mountains. Significant changes also occur in the atmospheric circulation in the mid-latitude stratosphere. These changes propagate downward from the stratosphere and strengthen the low pressure trough in the troposphere in the region to the east of Lake Balkhash and Lake Baikal. Therefore, the changes in the stratospheric circulation during the EPECE in China occur prior to changes in the tropospheric circulation and are very useful for predicting extreme wintertime cold temperatures in China

Correlation between meridional migration of the East Asian jet stream and tropical convection over Indonesia in winter

The relationship between tropical convective activities and meridional (north-south) migration of the East Asian jet stream (EAJS) in winter (December-February) is investigated for improving our knowledge of processes affecting the meridional migration of the EAJS. The monthly mean fields of outgoing longwave radiation (OLR) produced by NCAR and monthly atmospheric circulations produced by the NCEP/NCAR are used in this study. For 31 winter seasons between 1980 and 2011, the meridional migration of the winter EAJS is found to be strongly correlated with the present and preceding conditions of tropical convection over Indonesia. The anomalies in the tropical convection over the region in the preceding autumn and even preceding summer are a very useful indicator for the abnormal meridional migration of the wintertime EAJS. When the tropical convection over Indonesia weakens (strengthens), the EAJS has an abnormal southward (northward) migration. The atmospheric circulation associated with the abnormal meridional migration of the EAJS features abnormal air temperatures over the EAJS and its south side. The center of abnormal air temperatures occurs over the region south of the Yangtze River. Abnormal air pressures generated by abnormal air temperatures lead to abnormal winds. In the case of weakened tropical convection (positive OLR anomaly) over Indonesia, ascending motion of air mass over Indonesia is reduced, and the strength of Hadley circulation is weakened over the meridional range of the western Pacific Ocean. Consequently, the high-level air mass to the south of the core of the EAJS abnormally ascends and cools and the nearly southerly divergent winds at high-altitudes weaken, leading to significant reduction of heat transport from tropics to the southern China, with negative anomalies of air temperatures in the EAJS and its south side. The above processes increase thermal winds to the south of the Yangtze River and enhance the high-level westerly winds. To the north of the Yangtze River, both thermal winds and the high-level westerly winds are reduced. As a result, the EAJS has an abnormal south migration. In the case of enhanced tropical convection (negative OLR anomaly) over Indonesia, the opposite happens, in which Hadley circulation strengthens, the air mass to the south of the core of the EAJS abnormally descends and warms, heat transport increases from tropics to the southern China with positive air temperatures anomalies over the EAJS and its south side, and the EAJS has an abnormal northward migration

Linking African easterly wave activity with equatorial waves and the influence of Rossby waves from the Southern Hemisphere

A connection is found between African easterly waves (AEWs), equatorial westward-moving mixed Rossby-gravity (WMRG) waves and equivalent barotropic Rossby waves (RWs) from the Southern Hemisphere (SH). The amplitude and phase of equatorial waves is calculated by projection of broad-band filtered ERA-Interim data onto a horizontal structure basis obtained from equatorial wave theory. Mechanisms enabling interaction between the wave types are identified. AEWs are dominated by a vorticity wave which tilts eastwards below the African Easterly Jet and westwards above: the tilt necessary for baroclinic wave growth. However, a strong relationship is identified between amplifying vorticity centres within AEWs and equatorial WMRG waves. Although the waves do not phase-lock, positive vorticity centres amplify whenever the cross-equatorial motion of the WMRG wave lies at the same longitude in the upper troposphere (southwards flow) and east of this in the lower troposphere (northwards flow). Two mechanisms could explain the vorticity amplification: vortex stretching below the upper-tropospheric divergence and ascent associated with latent heating in convection in the lower-tropospheric moist northwards flow. In years of strong AEW activity, SH and equatorial upper-tropospheric zonal winds are more easterly. Stronger easterlies have two effects: i) they Doppler shift WMRG waves so that their period varies little with wavenumber (3-4 days) and ii) they enable westward-moving RWs to propagate into the tropical wave guide from the SH. RW phase speeds can match those of WMRG waves, enabling sustained excitation of WMRG. The WMRG waves have an eastwards group velocity with wave activity accumulating over Africa and invigorating AEWs at similar frequencies through the vorticity amplification mechanism

Lattice study on ηc2\eta_{c2} and X(3872)

Properties of $2^{-+}$ charmonium $\eta_{c2}$ are investigated in quenched lattice QCD. The mass of $\eta_{c2}$ is determined to be 3.80(3) GeV, which is close to the mass of $D$-wave charmonium $\psi(3770)$ and in agreement with quark model predictions. The transition width of $\eta_{c2}\to \gamma J/\psi$ is also obtained with a value $\Gamma=3.8(9)$ keV. Since the possible $2^{-+}$ assignment to X(3872) has not been ruled out by experiments, our results help to clarify the nature of X(3872).Comment: 15 pages, 8 figures. typos, grammatical errors and some references corrected, redundant discussions deleted, conclusion does not change. published versio