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Cheng Sun and Jianping Li

Abstract

In this paper the authors use the NCEP–Department of Energy (DOE) Reanalysis 2 (NCEP2) data from 1979 to 2004 to expand the daily 500-hPa geopotential height in the Southern Hemisphere (SH, 90°–20°S) into a double Fourier series, and analyze the temporal frequency characteristics of the expansion coefficients over various spatial scales. For the daily series over the whole year, the coefficient series of the extratropical-mean height is characterized by a significant low-frequency (10–30 day) variation. For zonal waves with (k, l) = (1–5, 1), where k and l are the zonal and meridional wavenumbers, respectively, the low-frequency variability is most pronounced for zonal wavenumbers 3 and 4; while the short wave with zonal wavenumber 5 has significant high-frequency (4–8 day) variability. For meridional waves with (k, l) = (0, 2–6), the meridional dipole (l = 2) makes a major contribution to the low-frequency variability, consistent with the intraseasonal space–time features of the southern annular mode (SAM). The meridional tripole (l = 3) also exhibits low-frequency variability. For two-dimensional waves (k, l) = (1–5, 2–6), the dipole is a preferred meridional structure for intraseasonal modes with large zonal scales, indicating an out-of-phase relationship between low-frequency planetary-scale waves at mid- and high latitudes. The diagnostic results outlined above can be explained, to a certain extent, by the dispersion relation for Rossby waves. Theoretical analysis indicates that zonal wavenumber 3, zonally symmetric flow such as SAM, and planetary-scale waves with meridional dipole structures may be interpreted as low-frequency eigenmodes of the atmosphere.

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Ruiqiang Ding and Jianping Li

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This study investigates the persistence characteristics of the sea surface temperature anomaly (SSTA) in the northern tropical Atlantic (NTA). It is found that a persistence barrier exists around December and January. This winter persistence barrier (WPB) is prominent during the mature phase of strong ENSO events but becomes indistinct during weak ENSO and normal (non-ENSO) events. During strong El Niño events, the NTA SSTA shows a reversal in sign and a rapid warming during December and January. It is possible that this SSTA sign reversal reduces the persistence, leading to the occurrence of the NTA WPB. The present analyses indicate a dynamic relationship among the Pacific ENSO, the NTA SSTA, and the NTA WPB on a quasi-biennial time scale: a strong El Niño event is usually preceded by a strong La Niña event, which leads to a sign reversal of the NTA SSTA in winter as a delayed response to ENSO, finally resulting in the NTA WPB. Analyses also suggest that the NTA WPB is affected by the North Atlantic Oscillation (NAO). The NAO enhances the persistence of the NTA SSTA during winter, tending to weaken the NTA WPB.

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Junlu Li and Jianping Gan

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Based on a physics-oriented modeling study, we investigate the underlying forcing processes of the North Equatorial Undercurrent (NEUC). Made up of large-scale (~90%) and mesoscale (~10%) components, the NEUC weakens eastward with a longitude-independent seasonality. The large-scale component reflects the effect of the meridional baroclinic pressure gradient force (PGF_BC). The vertical velocity shear forms the eastward NEUC, when the PGF_BC exceeds the meridional barotropic pressure gradient force (PGF_BT). The mesoscale variability with alternating jets is linked to the wind stress curl in different regions of the tropical North Pacific. Spatially, the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S), which are mainly attributed to the transports from Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC), respectively. The LUC of ~3 Sv (1 Sv ≡ 106 m3 s−1) feeds the NEUC_N in summer, while the MUC of ~4 Sv fuels the NEUC_S in autumn and the two branches do not coexist. The total NEUC transport peaks in August/September, and there exist three distinct periods in a 1-yr cycle: the non-NEUC period in winter, the LUC-driven period in summer, and the MUC-driven period in autumn. Based on the layer-integrated vorticity equation, we diagnose quantitatively that the variation of the NEUC is dominated by the lateral planetary vorticity influx from the LUC and the MUC. These external influxes interact with the internal dynamics of pressure torques and stress curls in the NEUC layer, to jointly govern the NEUC and its variability. Meanwhile, the nonlinearity due to relative vorticity advection near the coast modulates the strength of the NEUC.

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Ruiqiang Ding and Jianping Li

Abstract

This study confirms a weak spring persistence barrier (SPB) of sea surface temperature anomalies (SSTAs) in the western tropical Indian Ocean (WIO), a strong fall persistence barrier (FPB) in the South China Sea (SCS), and the strongest winter persistence barrier (WPB) in the southeastern tropical Indian Ocean (SEIO). During El Niño events, a less abrupt sign reversal of SSTAs occurs in the WIO during spring, an abrupt reversal occurs in the SCS during fall, and the most abrupt reversal occurs in the SEIO during winter. The sign reversal of SSTA implies a rapid decrease in SSTA persistence, which is favorable for the occurrence of a persistence barrier. The present results indicate that a more abrupt reversal of SSTA sign generally corresponds to a more prominent persistence barrier. El Niño–induced changes in atmospheric circulation result in reduced evaporation and suppressed convection. This in turn leads to the warming over much of the TIO basin, which is an important mechanism for the abrupt switch in SSTA, from negative to positive, in the northern SCS and SEIO. The seasonal cycle of the prevailing surface winds has a strong influence on the timing of the persistence barriers in the TIO.

The Indian Ocean dipole (IOD) alone can cause a weak WPB in the SEIO. El Niño events co-occurring with positive IOD further strengthen the SEIO WPB. The SEIO WPB appears to be more strongly influenced by ENSO than by the IOD. In contrast, the WIO SPB and the SCS FPB are relatively independent of the IOD.

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Xia Zhao and Jianping Li

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The spatiotemporal characteristics of the winter-to-winter recurrence (WWR) of sea surface temperature anomalies (SSTA) in the Northern Hemisphere (NH) are comprehensively studied through lag correlation analysis. On this basis the relationships between the SSTA WWR and the WWR of the atmospheric circulation anomalies, El Niño–Southern Oscillation (ENSO), and SSTA interdecadal variability are also investigated.

Results show that the SSTA WWR occurs over most parts of the North Pacific and Atlantic Oceans, but the spatiotemporal distributions of the SSTA WWR are distinctly different in these two oceans. Analyses indicate that the spatiotemporal distribution of the SSTA WWR in the North Atlantic Ocean is consistent with the spatial distribution of the seasonal cycle of its mixed layer depth (MLD), whereas that in the North Pacific Ocean, particularly the recurrence timing, cannot be fully explained by the change in the MLD between winter and summer in some regions. In addition, the atmospheric circulation anomalies also exhibit the WWR at the mid–high latitude of the NH, which is mainly located in eastern Asia, the central North Pacific, and the North Atlantic. The sea level pressure anomalies (SLPA) in the central North Pacific are essential for the occurrence of the SSTA WWR in this region. Moreover, the strongest positive correlation occurs when the SLPA lead SSTA in the central North Pacific by 1 month, which suggests that the atmospheric forcing on the ocean may play a dominant role in this region. Therefore, the “reemergence mechanism” is not the only process influencing the SSTA WWR, and the WWR of the atmospheric circulation anomalies may be one of the causes of the SSTA WWR in the central North Pacific. Finally, the occurrence of the SSTA WWR in the NH is closely related to SSTA interdecadal variability in the NH, but it is linearly independent of ENSO.

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Ruiqiang Ding and Jianping Li

Abstract

An analysis has been made of the trend and decadal variability of persistence of daily 500-mb (hPa) geopotential height anomalies for the winter season. The persistence is measured based on autocorrelations at 1- and 5-day lags (denoted r 1 and r 5, respectively) and the effective time between independent samples T 0. The results from linear trend analysis show that there exist significant trends of persistence of daily 500-mb geopotential height anomalies in some regions of the world. The regions with a significant decreasing trend are found to be mainly located at mid–high latitudes of the Northern and Southern Hemispheres, while the regions with a significant increasing trend are mainly located in the tropical Pacific Ocean. For other variables including sea level pressure (SLP), 1000-mb height, and 200-mb height, the persistence of daily anomalies also exhibits similar trends in these regions. It is speculated that the enhanced baroclinicity and advection are possibly responsible for the significant downward trend of persistence mainly occurring in the southern and northern mid–high latitudes, while the increased coupling between the atmospheric circulation and sea surface temperature (SST) could contribute to the increase of persistence in the tropical Pacific. An empirical orthogonal function (EOF) analysis based on the 7-yr Gaussian low-pass-filtered series of winter season r 1 and r 5 of 500-mb height (linear trend removed before the low-pass filtering) is presented. The results suggest that there is prominent decadal variability of persistence in some regions of the Northern and Southern Hemispheres and tropics. When compared with r 1, r 5 has decadal variations with larger magnitude and larger spatial scale. It is found that the decadal variability of persistence is closely related to decadal fluctuations of large-scale atmospheric circulation patterns.

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Jianping Li and Ruiqiang Ding

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To quantify the predictability limit of a chaotic system, the authors recently developed a method using the nonlinear local Lyapunov exponent (NLLE). The NLLE method provides a measure of local predictability limit of chaotic systems and is intended to supplement existing predictability methods. To apply the NLLE in studies of actual atmospheric predictability, an algorithm based on local dynamical analogs is devised to enable the estimation of the NLLE and its derivatives using experimental or observational data. Two examples are given to illustrate the effectiveness of the algorithm, involving the Lorenz63 three-variable model and the Lorenz96 forty-variable model; they reveal that the algorithm is applicable in estimating the NLLE of a chaotic system from its experimental time series. On this basis, the NLLE method is used to investigate temporal–spatial distributions of predictability limits of the daily geopotential height and wind fields. The limit of atmospheric predictability varies widely with region, altitude, and season. The predictability limits of the daily geopotential height and wind fields are generally less than 3 weeks in the troposphere, whereas they are approximately 1 month in the lower stratosphere, revealing a potential predictability source for forecasting weather from the stratosphere. Further work is required to examine broader applications of the NLLE method in predictability studies of the atmosphere, ocean, and other systems.

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Juan Feng and Jianping Li

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The possible influences of two types of ENSO [i.e., the canonical ENSO and ENSO Modoki (EM)] on Hadley circulation (HC) during the boreal spring are investigated during 1979–2010. El Niño events are featured with a symmetric pattern in equatorial zonal-mean sea surface temperature anomalies (SSTA), with a maximum around the equator. In contrast, the zonal-mean SSTA associated with El Niño Modoki events shows an asymmetric structure with a maximum around 10°N. The contrasting underlying thermal structures corresponding with ENSO and EM have opposite impacts on the simultaneous HC. In El Niño years, a symmetric anomalous meridional circulation is seen, with enhanced rising around the equator and anomalous descent at about 15°N and 20°S. In contrast, an asymmetric equatorial meridional circulation is observed for El Niño Modoki years, with anomalous ascent around 10°N and descent at about 10°S and 20°N. The contrasting meridional circulation anomalies within ENSO and EM are caused by their different meridional SSTA structure. This result is theoretically explained, indicating that anomalous meridional circulation is subject to the meridional SSTA gradient. Moreover, the observed results are reproduced in numerical experiments driven by anomalous warming in the eastern and central Pacific Ocean. Thus, the authors conclude that the anomalous HC linked to ENSO and EM is induced by the accompanying meridional gradient in zonal-mean SSTA.

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Xiao-Feng Li, Jianping Li, and Yun Li

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The middle–lower valley of the Yangtze River (MLY), located in the middle of eastern China, has been one of the largest economic centers of China since ancient times. Winter precipitation variability over the MLY is important for China because of its significant influence on the local economy. However, few studies have focused on the long-term variability of winter precipitation over the MLY. This study reports a significant wetting trend over the MLY in winter during the three decades since the late 1970s, forming a “mid-east-China winter wetting” pattern, which has become an important feature of precipitation change under the weakening East Asian winter monsoon. This wetting trend in the MLY also implies the poleward extension of the precipitation belts of southern China.

Further investigation reveals that the increasing sea surface temperature (SST) in the tropical Indian Ocean (TIO) is the dominant factor responsible for recent increases in precipitation over the MLY. The thermal forcing driven by warming of the TIO SST gives rise to an anomalous cyclonic circulation along the coast of eastern China. This transports more water vapor onto the Chinese mainland, shifts and causes anomalous convergence over the MLY, and generates the increase in precipitation there. As such, the increasing SST in the TIO induces over 80% of the observed wetting trend over the MLY. This mechanism was verified by results obtained from two sets of sensitivity experiments using a numerical spectral atmospheric general circulation model. Thus, increasing SST in the TIO has made a dominant contribution to the recent winter precipitation increase over the MLY.

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Hyacinth C. Nnamchi and Jianping Li

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This paper demonstrates a causal link between the air–sea phenomenon referred to as the South Atlantic Ocean (SAO) dipole (SAOD) and the interannual variability of precipitation over West Africa during the boreal summer monsoon rainy season in which most of the annual rainfall is recorded using an array of observational datasets. Analyses show that positive precipitation anomalies exceeding 40 mm month−1 over most locations at the Guinea Coast are associated with the positive phase of the SAOD, which is characterized by warm sea surface temperature anomalies (SSTAs) in the northeastern part of the SAO or the northeast pole (NEP)—that is, the Atlantic Niño sector—and cool SSTA in the southwestern part [southwest pole (SWP)] off the Argentina–Uruguay–Brazil coast. On the other hand, interannual variability in the Sahel is closely connected to the West African summer monsoon and the Atlantic Niño.

The results of this study reveal that the well-known influence of the Atlantic Niño on Guinea Coast precipitation in the literature represents only a component of the ocean–atmosphere interactions in the SAO causing the precipitation anomalies. Indeed, correlation and composite analyses using Guinea Coast precipitation indices consistently yield significant links to both the NEP and SWP centers of action. The hypothesized physical mechanism through which the SAOD-type SST gradients could induce Guinea Coast precipitation anomalies is the Lindzen–Nigam process. During the positive phase of the SAOD, the imprint of SST gradients gives rise to divergence over the SWP linked to convergence and vigorous upward motion over the NEP thereby leading to enhancement of precipitation over the Guinea Coast.

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