The role of high-frequency eddy forcing in the Southern Hemisphere Annular Mode (SAM) is diagnosed with the use of the 21-yr (1979-1999) NCEP/NCAR daily reanalysis dataset. The SAM is described as a meridional vacillation of the polar jet. The present study focuses on a zonally localized sector, 30°W-20°E, where both the polar jet and high-frequency eddies are dominant. In the maintenance of the extreme phases of the SAM, two positive feedback processes play important roles. The storm tracks move with the polar jet between the extreme positions of the polar jet. The displacement of the storm tracks sustains their anomalous positions. Together with this process, the alternation of the horizontal eddy structure also plays a role in the maintenance of the extreme phases. The southeast-northwest tilt of eddy is emphasized when the polar jet resides at higher latitudes, as has been mentioned in earlier studies. The southeast-northwest oriented eddies transport momentum to higher latitudes, keeping the jet in the higher latitudes. To both the poleward and equatorward jet transitions, momentum flux anomalies due to highfrequency eddies contribute. Characteristics of the eddies associated with these momentum flux anomalies are examined. For the poleward transitional phase, flux anomalies are induced by both increased eddy kinetic energy and an emphasized southeast-northwest tilt of the eddy. For the equatorward transitional phase, changes in the horizontal eddy structure are found to be more important.
Abstract The effects of low- and high-frequency eddies (time scales longer and shorter than 10 days, respectively) on the transitional processes of the Southern Hemisphere “Annular Mode” are investigated, based on NCEP–NCAR daily reanalysis data for the period 1979–2001. Special attention is focused on the zonal symmetry/asymmetry and the temporal evolution of the eddy forcing. For the poleward transitional process, the effects of low-frequency eddies precede those of high-frequency eddies in driving the jet transition. Quasi-stationary Rossby waves propagating along the polar jet with wavelengths of 7000 km play an important role. The waves, originally come from the Indian Ocean through the waveguide associated with the polar jet, dissipate equatorward over the eastern Pacific Ocean. This anomalous equatorward dissipation of wave activity induces an anomalous poleward momentum flux, which is responsible for changes in the polar jet over the Pacific Ocean during the beginning stage. Following the low-frequency eddy forcing, momentum forcing anomalies due to the high-frequency eddies rapidly appear. This forcing continues to drive the polar jet poleward over the whole of longitude, while the low-frequency eddies have completed their role of inducing the anomalous poleward momentum flux during the earlier stage. For the equatorward transitional events, the roles of the low-frequency eddy forcing differ from that in the poleward ones. Anomalous equatorward momentum fluxes due to low-frequency eddies appear simultaneously with that due to high-frequency eddies. Quasi-stationary Rossby waves with wavelengths of 7000 km propagate southeastward through the waveguide over the Pacific Ocean. The convergence of their wave activity results in the deceleration of the westerlies over the higher latitudes of the Pacific Ocean. On the other hand, the high-frequency eddy forcing contributes to the equatorward jet drift longitudinally over the whole of the hemisphere.
Properties of the quasi-stationary Rossby waves along the westerly jets are investigated with the space-time spectral analysis of 200hPa meridional wind velocity for four regions in the Northern Hemisphere summer using ECMWF data for 1980 to 1993. The observed zonal wavenumber of eastward (westward) propagating disturbances increases (decreases) as the frequency increases. The eastward propagating disturbances are stronger than the westward propagating ones. The quasi-stationary Rossby waves are seen not only in 10- to 30-day time scales but also in 30- to 90-day time scales.These observed zonal wavenumber-frequency relationships are reproduced by a β-channel model with step-like basic states. Calculated zonal wavenumber of eastward (westward) propagating solution of this model increases (decreases) as the frequency increases. The eastward propagating solutions are more strongly trapped in the westerly jet than the westward propagating ones. For these four waveguides mentioned, the results of the spectral analysis agree with the properties of the solutions deduced from the β-channel model with basic states derived from the climatological basic flow near these waveguides.
The circulation and SST anomalies during the post El Niño Asian summer monsoon season were examined through data analysis and linear equatorial β plane model calculations. Over the Philippine Sea, a negative precipitation anomaly and low‐level anti‐cyclonic anomaly were found. Over the western equatorial Pacific, a low‐level easterly anomaly prevailed. An east‐west SST anomaly contrast dominated over the tropical Indian and Pacific Oceans, with positive anomalies over the Indian and western Pacific Oceans and negative anomalies over the central to eastern Pacific. The β plane model demonstrated that the anti‐cyclonic anomaly over the Philippine Sea was a Rossby response to the negative precipitation anomaly found in this region. The easterly anomaly along the equator was part of a Kelvin response to the SST anomaly contrast. On the northern side of this anomalous easterly, a negative vorticity anomaly developed. This could induce the moist Rossby wave over the Philippine Sea.
The rain drop size distribution (DSD) at Cherrapunji, Northeast India was observed by a laser optical disdrometer Parsivel 2 from May to October 2017; this town is known for the world’s heaviest orographic rainfall recorded. The disdrometer showed a 30% underestimation of the rainfall amount, compared with a collocated rain gauge. The observed DSD had a number of drops with a mean normalized intercept log 10 N w > 4.0 for all rain rate categories, ranging from <5 to >80 mm h − 1 , comparable to tropical oceanic DSDs. These results differ from those of tropical oceanic DSDs, in that data with a larger N w were confined to the stratiform side of a stratiform/convective separation line proposed by Bringi et al. (2009). A large number of small drops is important for quantitative precipitation estimates by in-situ radar and satellites, because it tends to miss or underestimate precipitation amounts. The large number of small drops, as defined by the second principal component (>+1.5) while using the principal component analysis approach of Dolan et al. (2018), was rare for the pre-monsoon season, but was prevalent during the monsoon season, accounting for 16% (19%) of the accumulated rainfall (precipitation period); it tended to appear over weak active spells or the beginning of active spells of intraseasonal variation during the monsoon season.
After the climate shift of 1976/1977, associated with the interdecadal variability in the tropical Pacific Ocean, persistence barriers of El Niño/Southern Oscillation (ENSO) and tropical mean tropospheric temperature (TMTT) variations are detected in the boreal spring and autumn, respectively. Prior to the climate shift, however, the TMTT persistence barrier is almost non-existent, despite the prominence of the ENSO persistence barrier. Thus, the phase lag between ENSO and the TMTT variations is not fixed prior to the climate shift, while there is a fixed lag after the climate shift. This interdecadal variability is most remarkable for the TMTT anomalies in December. After the climate shift, the TMTT anomalies in December tend to persist 4 months later than prior to the climate shift. This is further examined in the comparison to the SST averaged over the strongly precipitating regions only, that is, the rainy-region SST. The SST variations in the rainy-region well correspond to those over the remote ocean basins, such as the Indian Ocean and the Western Pacific, that show a lagged response to the equatorial eastern Pacific SST anomalies. The TMTT anomalies are positively correlated with the rainy-region SST anomalies in both periods, prior to and after the climate shift. The interdecadal variability of the rainy-region SST persistence is similar to that of the TMTT persistence, although the variability is not as distinct as that of the TMTT.
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