Zonal Wind Anomalies Research Articles

Summer mesospheric warmings and the quasi 2 day wave

AbstractHigh‐altitude meteorological analyses are used to study the interannual variability of mesospheric weather in the Southern extratropics over five recent January's (2005, 2006, 2008, 2009, and 2010). Two features are apparent. First, there is significant variability in the quasi 2 day wave (Q2DW) with the largest amplitudes in January 2006 and also the last half of January 2005. Second, these periods coincide with high‐latitude temperature enhancements of 8–12 K. Previous studies have linked summer mesospheric warmings to interhemispheric coupling (IHC); however, the observed temperature and zonal wind anomalies do not agree with the predictions of IHC. Rather it appears that the westward momentum forcing from these Q2DW enhancements counteracts the gravity wave drag forcing which normally produces the cold summer mesopause. Since these temperature increases have been linked to Polar Mesospheric Cloud (PMC) disappearance, the present study supports the suggestion that the Q2DW may be an important factor governing PMC variability.

Representation of the Arctic Oscillation in the CMIP5 Models

The temporal variability and spatial pattern of the Arctic Oscillation (AO) simulated in the historical experiment of 26 coupled climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are evaluated. Spectral analysis of the monthly AO index indicates that 23 out of the 26 CMIP5 models exhibit no statistically significant spectral peak in the historical experiment, as seen in the observations. These models are able to reproduce the AO pattern in the sea level pressure anomaly field during boreal winter, but the intensity of the AO pattern tends to be overestimated in all the models. The zonal-mean zonal wind anomalies associated with the AO is dominated by a meridional dipole in the mid-high latitudes of the Northern Hemisphere during boreal winter, which is well reproduced by only a few models. Most models show significant biases in both strength and location of the dipole compared to the observation. In considering the temporal variability as well as spatial structures in both horizontal and vertical directions, the MPI-ESM-P model reproduces an AO pattern that resembles the observation the best. CitationZuo, J.-Q., W.-J. Li, and H.-L. Ren, 2013: Representation of the Arctic Oscillation in the CMIP5 models.Adv. Clim. Change Res.,4(4), doi: 10.3724/SP.J.1248.2013.242.

A theoretical investigation of the tropical Indo-Pacific tripole mode

The El Nino-Southern Oscillation (ENSO) phenomenon in the tropical Pacific has been a focus of ocean and climate studies in the last few decades. Recently, the short-term climate variability in the tropical Indian Ocean has attracted increasingly more attention, especially with the proposition of the Indian Ocean Dipole (IOD) mode. However, these phenomena are often studied separately without much consideration of their interaction. Observations reveal a striking out-of-phase relationship between zonal gradients of sea surface height anomaly (SSHA) and sea surface temperature anomaly (SSTA) in the tropical Indian and Pacific Oceans. Since the two oceans share the ascending branch of the Walker cells over the warm pool, the variation within one of them will affect the other. The accompanied zonal surface wind anomalies are always opposite over the two basins, thus producing a tripole structure with opposite zonal gradients of SSHA/SSTA in the two oceans. This mode of variability has been referred to as Indo-Pacific Tripole (IPT). Based on observational data analyses and a simple ocean-atmosphere coupled model, this study tries to identify the characteristics and physical mechanism of IPT with a particular emphasis on the relationships among ENSO, IOD, and IPT. The model includes the basic oceanic and atmospheric variables and the feedbacks between them, and takes into account the inter-basin connection through an atmospheric bridge, thus providing a valuable framework for further research on the short-term tropical climate variability.

Connections between the stratospheric quasi‐biennial oscillation and tropospheric circulation over Asia in northern autumn

We investigate the variations in tropospheric circulation over Asia associated with the stratospheric quasi‐biennial oscillation (QBO) during Northern Hemisphere (NH) autumn (September–November) using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis and NCEP‐Department of Energy (DOE) Reanalysis II data sets for a 25 year period (1980–2004). The anomaly fields in this study are expressed as the easterly years minus the westerly years of the QBO. The zonal wind distribution at 200 hPa indicates easterly anomalies at low latitudes from India through Taiwan, statistically significant westerly anomalies at midlatitudes including the Tibetan Plateau to northern China, and significant easterly anomalies in the high‐latitude Eurasia north of 50°N. A momentum balance analysis indicates that these anomalies over Asia are attributable to atmospheric waves and residual mean meridional flow. Significant northward and descending anomalies are observed in the lower stratosphere over Asia, with detectable signals in the troposphere around Tibet. The adiabatic heating due to descending motions in the region centering on India and Bangladesh is consistent with the distribution of rainfall anomalies. The formation of zonal wind anomalies over South Asia seems to be associated with not only wave activities but also adiabatic heating and thermal advection. We suggest that the QBO may be linked to tropospheric circulation over Asia through wave activities and convective activities in autumn, and the wave activities in the low latitudes associated with the QBO seem to be connected with the rainfall distribution over the Asian monsoon region.

Distribution of the tropical Pacific surface zonal wind anomaly and its relation with two types of El Niño

El Nino events with an eastern Pacific pattern (EP) and central Pacific pattern (CP) were first separated using rotated empirical orthogonal functions (REOF). Lead/lag regression and rotated singular value decomposition (RSVD) analyses were then carried out to study the relation between the surface zonal wind (SZW) anomalies and sea surface temperature (SST) anomalies in the tropical Pacific. A possible physical process for the CP El Nino was proposed. For the EP El Nino, strong westerly anomalies that spread eastward continuously produce an anomalous ocean zonal convergence zone (ZCZ) centered on about 165A degrees W. This SZW anomaly pattern favors poleward and eastward Sverdrup transport at the equator. For the CP El Nino, westerly anomalies and the ZCZ are mainly confined to the western Pacific, and easterly anomalies blow in the eastern Pacific. This SZW anomaly pattern restrains poleward and eastward Sverdrup transport at the equator; however, there is an eastward Sverdrup transport at about 5A degrees N, which favors the warming of the north-eastern tropical Pacific. It is found that the slowness of eastward propagation of subsurface warm water (partly from the downwelling caused by Ekman convergence and the ZCZ) is due to the slowdown of the undercurrent in the central basin, and vertical advection in the central Pacific may be important in the formation and disappearance of the CP El Nino.

Role of thermocline–SST coupling in the evolution of IOD events and their regional impacts

The evolution of sea surface temperature (SST) and thermocline (represented by 20 °C isotherm depth, D20) in the east equatorial Indian Ocean (EIO) associated with the Indian Ocean Dipole (IOD) years is studied for the period of 50 years from 1958 to 2007. A new IOD index based on combined anomalies of surface winds, D20 and SST over the equatorial Indian Ocean is defined to identify strong and weak IOD events. It is found that the evolution of strong IOD events is driven by ocean dynamics in the form of thermocline–SST coupling and is strongly interactive with the atmosphere, whereas the weak IOD events are mere response to surface winds without such dynamical coupling. The easterly wind anomalies extend up to the western equatorial Indian Ocean (WIO) during strong IOD years and support enhanced EIO air–sea interactions. On the other hand, the evolution of zonal wind anomalies is weak during the weak IOD years. Thermocline–SST coupling is robust in both EIO and WIO during strong IOD years, which is primarily responsible for the enhanced SST gradient, strong enough to establish anomalous Walker circulation within the Indian Ocean. The strong convection over the WIO associated with the Indian Ocean Walker cell triggers a secondary cell with subsidence over the African landmass. This double cell structure over the equatorial Indian Ocean is not reported before. Such double cell structure is not evident in weak IOD years and instead the convection over WIO extends up to African landmass. These are well supported by the spatial pattern of anomalous precipitable water during strong and weak IOD years. Strengthening of monsoon flow and local Hadley cell associated with strong IOD events enhances precipitation over the Indian subcontinent, whereas weak IOD years have less impact on the Indian summer monsoon circulation and rainfall. Analysis reveals that the EIO thermocline index and combined index could be potential predictors for the central Indian rainfall during summer.

An assessment of Indo-Pacific oceanic channel dynamics in the FGOALS-g2 coupled climate system model

Lag correlations of sea surface temperature anomalies (SSTAs), sea surface height anomalies (SSHAs), subsurface temperature anomalies, and surface zonal wind anomalies (SZWAs) produced by the Flexible Global Ocean-Atmosphere-Land System model: Grid-point Version 2 (FGOALS-g2) are analyzed and compared with observations. The insignificant, albeit positive, lag correlations between the SSTAs in the southeastern tropical Indian Ocean (STIO) in fall and the SSTAs in the central-eastern Pacific cold tongue in the following summer through fall are found to be not in agreement with the observational analysis. The model, however, does reproduce the significant lag correlations between the SSHAs in the STIO in fall and those in the cold tongue at the one-year time lag in the observations. These, along with the significant lag correlations between the SSTAs in the STIO in fall and the subsurface temperature anomalies in the equatorial Pacific vertical section in the following year, suggest that the Indonesian Throughflow plays an important role in propagating the Indian Ocean anomalies into the equatorial Pacific Ocean. Analyses of the interannual anomalies of the Indonesian Throughflow transport suggest that the FGOALS-g2 climate system simulates, but underestimates, the oceanic channel dynamics between the Indian and Pacific Oceans. FGOALS-g2 is shown to produce lag correlations between the SZWAs over the western equatorial Pacific in fall and the cold tongue SSTAs at the one-year time lag that are too strong to be realistic in comparison with observations. The analyses suggest that the atmospheric bridge over the Indo-Pacific Ocean is overestimated in the FGOALS-g2 coupled climate model.

Interannual variability in the North Pacific meridional overturning circulation

We analyzed the temporal and spatial variation, and interannual variability of the North Pacific meridional overturning circulation using an empirical orthogonal function method, and calculated mass transport using Simple Ocean Data Assimilation Data from 1958-2008. The meridional streamfunction field in the North Pacific tilts N-S; the Tropical Cell (TC), Subtropical Cell (STC), and Deep Tropical Cell (DTC) may be in phase on an annual time scale; the TC and the STC are out of phase on an interannual time scale, but the interannual variability of the DTC is complex. The TC and STC interannual variability is associated with ENSO (El Nio-Southern Oscillation). The TC northward, southward, upward, and downward transports all weaken in El Nios and strengthen in La Nias. The STC northward and southward transports are out of phase, while the STC northward and downward transports are in phase. Sea-surface water that reaches the middle latitude and is subducted may not completely return to the tropics. The zonal wind anomalies over the central North Pacific, which control Ekman transport, and the east-west slope of the sea level may be major factors causing the TC northward and southward transport interannual variability and the STC northward and southward transports on the interannual time scale. The DTC northward and southward transports decrease during strong El Nios and increase during strong La Nias. DTC upward and downward transports are not strongly correlated with the Nio-3 index and may not be completely controlled by ENSO.

Meridional movement of wind anomalies during ENSO events and their role in event termination

Observational analysis has shown that when El Niño‐Southern Oscillation (ENSO) events typically reach their peak amplitude in boreal winter, the associated zonal wind anomalies abruptly shift southward so that the maximum anomalous zonal wind is located around 5°–7°S. Here, an analysis utilizing multiple wind products identifies a clear ENSO phase nonlinearity in the extent of this meridional wind movement and its dynamically linked changes in equatorial heat content. It is shown that the meridional wind movement and its discharging effect increase with increasing El Niño amplitude, while both remain relatively small regardless of La Niña amplitude. This result implies that asymmetries in the extent of the meridional wind shift may contribute to the observed asymmetry in the duration of El Niño and La Niña events. We also evaluate the result sensitivities to wind product selection and discuss Eastern Pacific (EP) and Central Pacific (CP) El Niño event differences.

The Relationship of Rainfall Variability in Western Equatorial Africa to the Tropical Oceans and Atmospheric Circulation. Part I: The Boreal Spring

Abstract This paper examines the factors governing rainfall variability in western equatorial Africa (WEA) during the April–June rainy season. In three of the five regions examined some degree of large-scale forcing is indicated, particularly in the region along the Atlantic coast. Interannual variability in this coastal sector also demonstrates a strong link to changes in local sea surface temperatures (SSTs) and the South Atlantic subtropical high. To examine potential causal mechanisms, various atmospheric parameters are evaluated for wet and dry composites. The results suggest that the intensity of the zonal circulation in the global tropics is a crucial control on rainfall variability over WEA. A La Niña (El Niño)–like signal in both SSTs and zonal circulation over the Pacific is apparent in association with the wet (dry) conditions in the western sector. However, remote forcing from the Pacific modulates the circulation over Africa indirectly by way of synchronous changes in the entire Indian or Atlantic Ocean. Anomalies in the local zonal winds are similar in all three regions: the wet (dry) composite is associated with an intensification (weakening) of the upper-tropospheric easterlies and low-level westerlies, but a weakening (intensification) of the midlevel easterlies. This work also suggests that, in most cases, the relationship between local SSTs and rainfall reflects a common remote forcing by the large-scale atmosphere–ocean system. This forcing is manifested via changes in the zonal circulation. Thus, the statistical associations between rainfall and SSTs do not indicate direct forcing by local SSTs. One point of evidence for this conclusion is the stronger association with atmospheric parameters than with SSTs.

Observational Zonal Mean Flow Anomalies: Vacillation or Poleward Propagation?

North-south displacements and meridional vacillations of the eddy-driven jet are widely accepted as the dominant cause of variability of the observational zonal-mean zonal wind anomalies (denoted [...

Multi‐Model Projection of the Interannual Relationship Between Spring Hadley Circulation and East Asian Summer Circulation under Global Warming

AbstractObservations indicate that spring Hadley circulation is tightly linked to East Asian summer circulations and precipitation on the interannual timescale. Whether does such a relationship tend to change under the background of global warming in the future? To answer this question, this work chose five climate models (GFDL_CM2_0, GFDL_CM2_1, IAP_FGOALS_1.0g, NCARCCSM, UKMO_HADCM3), which can well reproduce the observed interannual variability of spring Hadley circulation in the North Hemisphere and its linkage to East Asian summer atmospheric circulations and precipitation, based on overall assessment of models' simulation ability. Then, the potential change of the interannual variability of spring Hadley circulation and its interannual connection with East Asian summer circulations and precipitation under the A1B scenario are projected by using the outputs of these five climate models. To facilitate quantitative estimation of their changes, five indices are exploited in this study. The Northern Hemispheric Hadley circulation index (HCI) is defined as the maximum value of the zonal mean mass stream function occurring within the latitudinal zone of 0°–30°N, and the western Pacific subtropical high index (WPSHI) as the 850 hPa geopotential height anomalies averaged over the region (10°N–30°N, 110°E–150°E). The difference of zonal wind anomalies at 850 hPa between the region (10°N–20°N, 100°E–150°E) and (25°N–35°N, 100°E–150°E) and that at 200 hPa between the region (25°N–35°N, 80°E–150°E) and (40°N–50°N, 80°E–150°E) are used to define the East Asian summer monsoon index (EASMI) and the East Asian Jet index (EAJI), respectively. The East Asian rainfall index is represented by mean precipitation over the region (27°N–32°N, 110°E–130°E). The multi‐model ensemble (MME) results show that the interannual variability of the Northern Hemispheric Hadley circulation in spring tends to decrease by 23% in the late 21st century (2070–2099) as compared to the late 20th century (1970–1999). Concurrent with the decreasing interannual variability of spring Hadley circulation, its linkage to summer western Pacific subtropical high and East Asian summer monsoon is also projected to be weakened. The correlation coefficients between spring HCI and summer WPSHI (EASMI) decrease from 0.44 (–0.46) in the late 20th century to 0.19 (–0.15) in the late 21st century. Such a change can be simulated consistently by individual models. In addition, the MME results suggest that the association of spring Hadley circulation with summer East Asian jet and precipitation will be weakened as well. However, there are discrepancies in the changes of their relationships among individual models.

Observational evidence for the mechanism of the poleward propagation of zonal wind anomalies over the North Atlantic

AbstractThis study examines the physical mechanisms that drive the poleward propagation of zonal‐mean flow anomalies over the North Atlantic during the boreal winter with ECMWF ERA40 reanalysis data that spans the years 1958–2001. An empirical orthogonal function (EOF) analysis is performed on the zonal wind anomalies for the North Atlantic domain. Based on time‐lag correlations between the first two EOFs, 14 poleward propagation events are identified.A series of lagged composite calculations of the poleward propagation events reveals the following processes: midlatitude synoptic‐scale Rossby waves propagate equatorward and break when they encounter their critical latitudes in the Tropics and Subtropics; a new critical latitude is generated slightly poleward of the previous critical latitude, and the subsequent wave breaking occurs at this new critical latitude. This interaction between the wave breaking and the critical latitude drives the poleward propagation of zonal‐mean zonal wind anomaly. A comparison between the poleward propagation events and zonal index events indicates that the poleward propagation is associated with a relatively weak potential vorticity gradient over the Subtropics. These North Atlantic poleward propagation events are found to be preceded by anomalously strong transient eddy activity over the subtropical North Pacific, and by anomalously weak tropical convection over the tropical Atlantic Ocean and the Amazon. Copyright © 2012 Royal Meteorological Society

The Effect of the South Pacific Convergence Zone on the Termination of El Niño Events and the Meridional Asymmetry of ENSO*

AbstractDuring large El Niño events the westerly wind response to the eastern equatorial Pacific sea surface temperature anomalies (SSTAs) shifts southward during boreal winter and early spring, reaching latitudes of 5°–7°S. The resulting meridional asymmetry, along with a related seasonal weakening of wind anomalies on the equator are key elements in the termination of strong El Niño events. Using an intermediate complexity atmosphere model it is demonstrated that these features result from a weakening of the climatological wind speeds south of the equator toward the end of the calendar year. The reduced climatological wind speeds, which are associated with the seasonal intensification of the South Pacific convergence zone (SPCZ), lead to anomalous boundary layer Ekman pumping and a reduced surface momentum damping of the combined boundary layer/lower-troposphere surface wind response to El Niño. This allows the associated zonal wind anomalies to shift south of the equator. Furthermore, using a linear shallow-water ocean model it is demonstrated that this southward wind shift plays a prominent role in changing zonal mean equatorial heat content and is solely responsible for establishing the meridional asymmetry of thermocline depth in the turnaround (recharge/discharge) phase of ENSO. This result calls into question the sole role of oceanic Rossby waves in the phase synchronized termination of El Niño events and suggests that the development of a realistic climatological SPCZ in December–February/March–May (DJF/MAM) is one of the key factors in the seasonal termination of strong El Niño events.

Optimal Initial Perturbations for Ensemble Prediction of the Madden–Julian Oscillation during Boreal Winter

AbstractAn initialization strategy, tailored to the prediction of the Madden–Julian oscillation (MJO), is evaluated using the Goddard Earth Observing System Model, version 5 (GEOS-5), coupled general circulation model (CGCM). The approach is based on the empirical singular vectors (ESVs) of a reduced-space statistically determined linear approximation of the full nonlinear CGCM. The initial ESV, extracted using 10 years (1990–99) of boreal winter hindcast data, has zonal wind anomalies over the western Indian Ocean, while the final ESV (at a forecast lead time of 10 days) reflects a propagation of the zonal wind anomalies to the east over the Maritime Continent—an evolution that is characteristic of the MJO.A new set of ensemble hindcasts are produced for the boreal winter season from 1990 to 1999 in which the leading ESV provides the initial perturbations. The results are compared with those from a set of control hindcasts generated using random perturbations. It is shown that the ESV-based predictions have a systematically higher bivariate correlation skill in predicting the MJO compared to those using the random perturbations. Furthermore, the improvement in the skill depends on the phase of the MJO. The ESV is particularly effective in increasing the forecast skill during those phases of the MJO in which the control has low skill (with correlations increasing by as much as 0.2 at 20–25-day lead times), as well as during those times in which the MJO is weak.

Signals of El Niño Modoki in the tropical tropopause layer and stratosphere

Abstract. The effects of El Niño Modoki events on the tropical tropopause layer (TTL) and on the stratosphere were investigated using European Center for Medium Range Weather Forecasting (ECMWF) reanalysis data, oceanic El Niño indices, and general climate model outputs. El Niño Modoki events tend to depress convective activities in the western and eastern Pacific but enhance convective activities in the central and northern Pacific. Consequently, during El Niño Modoki events, negative water vapor anomalies occur in the western and eastern Pacific upper troposphere, whereas there are positive anomalies in the central and northern Pacific upper troposphere. The spatial patterns of the outgoing longwave radiation (OLR) and upper tropospheric water vapor anomalies exhibit a tripolar form. The empirical orthogonal function (EOF) analysis of the OLR and upper tropospheric water vapor anomalies reveals that canonical El Niño events are associated with the leading mode of the EOF, while El Niño Modoki events correspond to the second mode. The composite analysis based on ERA-interim data indicate that El Niño Modoki events have a reverse effect on middle-high latitudes stratosphere, as compared with the effect of typical El Niño events, i.e., the northern polar vortex is stronger and colder but the southern polar vortex is weaker and warmer during El Niño Modoki events. According to the simulation' results, we found that the reverse effect on the middle-high latitudes stratosphere is resulted from a complicated interaction between quasi-biennial oscillation (QBO) signal of east phase and El Niño Modoki signal. This interaction is not a simply linear overlay of QBO signal and El Niño Modoki signal in the stratosphere, it is El Niño Modoki that leads to different tropospheric zonal wind anomalies with QBO forcing from that caused by typical El Niño, thus, the planetary wave propagation from troposphere to the stratosphere during El Niño Modoki events is different from that during canonical El Niño events. However, when QBO is in its west phase, El Niño Modoki events have the same effect on middle-high latitudes stratosphere as the typical El Niño events. Our simulations also suggest that canonical El Niño and El Niño Modoki activities actually have the same influence on the middle-high latitudes stratosphere when in the absence of QBO forcing.

Important factors for long‐term change in ENSO transitivity

AbstractEl Niño and La Niña exhibit significant asymmetry in their duration. El Niño tends to turn rapidly into La Niña after the mature, while La Niña tends to persist for up to 2 years. Reconstructed historical sea surface temperatures (SST) show a significantly increase in the intensity of El Niño‐Southern Oscillation (ENSO) asymmetry, particularly El Niño transitivity, during the last six decades. Atmospheric observational data have shown that the relationship between El Niño and surface zonal wind anomalies over the equatorial Western Pacific (WP) has strengthened, and anomalous WP easterlies have appeared after the 1970s climate regime shift. To investigate the dependency of ENSO transitivity on its amplitude, a suite of idealized experiments using an atmospheric general circulation model (AGCM) is performed by imposing 12 different ENSO‐related SST anomalies exhibiting equal spatiotemporal distribution but different amplitude. Our AGCM experiments show strong nonlinearity in the WP zonal wind against the amplitude of the warm phase. In the strong (weak) El Niño state, the WP response tends to accelerate (prevent) the transition from El Niño to La Niña; however, this relationship is not applicable to the La Niña phase. The asymmetry in transitivity/persistency is found to differ in terms of their sensitivity to the ENSO amplitude; this difference is also detected in the long‐term control simulation of Geophysical Fluid Dynamics Laboratory‐Climate Model version 2.1. The long‐term variation in El Niño transitivity in this model correlates strongly with the decadal variation in the ENSO amplitude and Indo‐Pacific interbasin coupling. However, this result is not apparent in the opposite phase. The results of this study indicate that the decadal change in ENSO amplitude and Indian Ocean feedback could be significantly related to the decadal change in the cyclic nature of ENSO and should be discussed separately for El Niño and La Niña. Copyright © 2012 Royal Meteorological Society

Intraseasonal atmospheric variability and its interannual modulation in Central Africa

The spatial and temporal structures of the intraseasonal atmospheric variability over central Africa is investigated using 2.5° × 2.5° daily outgoing longwave radiation (OLR) and National Centers for Environmental Prediction (NCEP) Reanalysis zonal winds for the period 1980–2010. The study begins with an overview of the Central African rainfall regime, noting in particular the contrast amongst Western and Eastern parts, with different topography and surface conditions features. The annual mean rainfall and OLR over the region revealed a zone of intense convective activity centered on the equator near 30°E, which extends southward and covers almost all the Congo forest. The annual cycle of rainfall reflects the classical bi-annual shift of Inter-Tropical Convergence Zone across the equatorial belt, between 10°S and 10°N. The result of the empirical orthogonal functions (EOFs) analysis has shown that the three leading EOF modes explain about 45 % of total intraseasonal variability. The power spectra of all the three corresponding principal components (PCs) peak around 45–50 days, indicating a Madden–Julian Oscillation (MJO) signal. The first mode exhibits high positive loadings over Northern Congo, the second over Southern Ethiopia and the third over Southwestern Tanzania. The PCs time series revealed less interannual modulation of intraseasonal oscillations for the Congo mode, while Ethiopian and Tanzanian modes exhibit strong interannual variations. Hovmoller plots of OLR, 200 and 850 hPa NCEP zonal winds found the eastward propagating features to be the dominant pattern in all the three times series, but this propagation is less pronounced in the OLR than in the 850 and 200 hpa zonal wind anomalies. An index of MJO strength was built by averaging the 30–50 day power for each day. A plot of MJO indices and El Nino Southern Oscillation (ENSO) cycle confirm a strong interannual modulation of MJO over Eastern central Africa partially linked with the ENSO events (El Nino and La Nina). Strong MJO activity is observed during La Nina years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Nino episodes.

Annular Mode–Like Variation in a Multilayer Quasigeostrophic Model

Abstract Eddy–zonal flow interactions in the annular modes are investigated in this study using a modified beta-plane multilayer quasigeostrophic (QG) channel model. This study shows the different response of high- and low-phase-speed (frequency) eddies to the zonal wind anomalies and suggests a baroclinic mechanism through which the two eddies work symbiotically maintaining the positive eddy feedback in the annular modes. Analysis also indicates that the different roles played by these two eddies in the annular modes are related to the differences in their critical line distributions. Eddies with higher phase speeds experience a low-level critical layer at the center of the jet. They drive the zonal wind anomalies associated with the annular mode but weaken the baroclinicity of the jet in the process. Lower-phase-speed eddies encounter low-level critical lines on the jet flanks. While their momentum fluxes are not as important for the jet shift, they play an important role by restoring the lower-level baroclinicity at the jet center, creating a positive feedback loop with the fast eddies that extends the persistence of the jet shift. The importance of the lower-level baroclinicity restoration by the low-phase-speed eddies in the annular modes is further demonstrated in sensitivity runs, in which surface friction on eddies is increased to selectively damp the low-phase-speed eddies. For simulations in which the low-phase-speed eddies become inactive, the leading mode of the zonal wind variability shifts from the position fluctuation to a pulsing of the jet intensity. Further studies indicate that the response of the lower-level baroclinicity to the zonal wind anomalies caused by the low-phase-speed eddies can be crucial in maintaining the annular mode–like variations.

Propagation of planetary-scale zonal mean wind anomalies and polar oscillations

Global atmospheric variables can be physically decomposed into four components: (1) the zonal time averaged climate symmetric component, (2) the time averaged climate asymmetric, (3) the zonal-mean transient symmetric anomaly, and (4) the transient asymmetric anomaly. This study analyzes the relationships between the intra-seasonal and inter-annual variability of planetary scale decomposed zonal and meridional winds in the tropopause, and oscillations such as those from the El Nino-Southern Oscillation (ENSO), the Arctic Oscillation (AO) and the Antarctic Oscillation (AAO). The tropical inter-annual zonal mean wind anomalies in the tropopause are linked with the ENSO cycle and can propagate into the subtropics, mid-latitudes, and polar front regions via abnormal meridional vertical cells. Similarly, tropical intra-seasonal (40–60-d) zonal wind anomalies can reach the subtropics and mid-latitudes. The polar intra-seasonal zonal wind anomalies in the tropopause can propagate toward high-latitude areas. Thus, the AO and the AAO are the result of the interaction and propagation of these planetary scale zonal wind anomalies.