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Klaus M. Weickmann

Abstract

An eigenvector analysis is used to characterize global-scale, intraseasonal variability in satellite-derived outgoing longwave radiation fields and NMC analyzed circulation fields. Five-day average covering November-March of 1974–75 through 1979–80 comprise the data sample. The results show that circulation eigenvector 2 and 3 (CEV2 and CEV3) describe wavenumber 1 and wavenumber 2 index cycles of the extratropical windfield, while radiation eigenvectors 2 and 4 describe large-scale fluctuations in tropical cloudiness In most winters studied, the tropical cloudiness fluctuations display a tendency for eastward propagation in the region from Africa to the central equatorial Pacific. In some winters, the intraseasonal fluctuations in the extratropical windfield are closely coupled to the propagating feature in tropical cloudiness. The “period” displayed by the fluctuations in different winters (November-March) ranges from 35 to 80 days.

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Matthew Wheeler and Klaus M. Weickmann

Abstract

A technique of near-real-time monitoring and prediction of various modes of coherent synoptic to intraseasonal zonally propagating tropical variability is developed. It involves Fourier filtering of a daily updated global dataset for the specific zonal wavenumbers and frequencies of each of the phenomena of interest. The filtered fields obtained for times before the end of the dataset may be used for monitoring, while the filtered fields obtained for times after the end point may be used as a forecast. Tests of the technique, using satellite-observed outgoing longwave radiation (OLR) data, reveal its skill for monitoring. For prediction, it demonstrates good skill for the Madden–Julian oscillation (MJO), and detectable skill for other convectively coupled equatorial modes, although the decaying amplitude of the predictions with time is a characteristic that users need to be aware of. The skill for the MJO OLR field appears to be equally as good as that obtained by the recent empirical MJO forecast methods developed by Waliser et al., and Lo and Hendon, with a useful forecast out to about 15–20 days. Unlike the previously developed methods, however, the current monitoring and prediction technique is extended to other defined modes of large-scale coherent zonally propagating tropical variability. These other modes are those that appear as equatorial wavelike oscillations in the OLR. For them, the skill shown by this empirical technique, although considerably less than that obtained for the MJO, is still deemed to be high enough for the technique to be sometimes useful, especially when compared to that of a medium-range global numerical weather prediction (NWP) model.

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Klaus M. Weickmann and Robert M. Chervin

Abstract

The seasonal cycle of the global wind field is documented for both a decadal set of analyses from the National Meteorological Center (NMC) and an extended term integration of a research version of the Community Climate Model developed at the National Center for Atmospheric Research. Composite eigenvector analysis is used to establish the dominant three dimensional coherent structures characteristic of the datasets while gridpoint harmonic analysis provides evidence of the extent to which these structures describe conventional seasonal modes. These quantitative indicators of spatial and temporal variance form a stringent measure of model performance with respect to seasonal variation. The model appears to be far more successful at capturing the annual harmonic contained in the NMC analyses than the semiannual harmonic. This discrepancy may be related to the absence of the requisite tropical forcings due to either inadequate parameterizations of certain physical processes or the lack of interannual variability in the model's boundary forcings, or both. Further numerical experimentation is likely to help resolve this issue.

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George N. Kiladis and Klaus M. Weickmann

Abstract

Statistical evidence is presented to support the notion that tropical convection in the eastern Pacific and Atlantic intertropical convergence zone (ITCZ) during northern winter can be forced by disturbances originating in the extratropics. The synoptic-scale transients in these regions are characterized at upper levels by strong positive tilts in the horizontal and appear to induce vertical motions ahead of troughs as in midlatitude baroclinic systems. Two case studies of such interactions are examined, one for the eastern North Pacific ITCZ and another somewhat different type of interaction for the South Pacific convergence zone (SPCZ) over the western South Pacific.

Both cases are associated with upper-level troughs, strong cold advection deep into the tropics, and the formation of a frontal boundary at low levels. The ITCZ case is characterized by the advection of anomalously high isentropic potential vorticity air southward, a strong poleward flux of heat and westerly momentum, and the development of a subtropical jet downstream of the disturbance. The SPCZ disturbance is not strongly tilted, but is still accompanied by a strong poleward flux of heat and momentum. Evidence for the occurrence of cross-equatorial wave dispersion in the eastern Pacific during northern winter is also presented. These observations are consistent with theory and modeling of Rossby waves in a westerly basic state extending from the midlatitudes into the tropics.

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George N. Kiladis and Klaus M. Weickmann

Abstract

The relationship between deep tropical convection and large-scale atmospheric circulation in the 6–30-day period range is examined. Regression relationships between filtered outgoing longwave radiation at various locations in the Tropics and 200- and 850-mb circulation are mapped for the standard seasons, and the spatial structure and seasonal dependence of the results are interpreted in view of the basic-state circulation.

In regions where the convection is embedded in upper-level easterlies, anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal, along with patterns of meridional outflow into subtropical anticyclonic perturbations. Lagged relationships suggest that the convection is forcing the circulation in many of these cases. The outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons, with more symmetric signals about the equator seen in the equinoctial seasons. The longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection. There tends to be a first baroclinic mode vertical structure to these circulations, such that equatorial westerlies are present at 850 mb within the convection, with closed circulations on either side of the equator resembling equatorial Rossby modes especially common over the Atlantic and Pacific sectors.

As a contrast, in regions located within upper-level westerlies or along the margin of influence of upper westerly disturbances, convection appears to be forced by upper-level wave energy propagating into the deep Tropics, with the heating located in the upward motion region ahead of upper-level troughs. This occurs over the Atlantic and eastern Pacific sectors during northern winter and spring, and over Australia, the South Pacific, and South America during southern summer, when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection. The results confirm theoretical and modeling ideas that suggest that Rossby wave energy is able to propagate into the deep Tropics in regions where upper-level westerlies exist in the Tropics.

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George N. Kiladis and Klaus M. Weickmann

Abstract

Lagged cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses are utilized to isolate the preferred upper-level and surface circulation anomalies associated with tropical convection during northern winter. Three intraseasonal time scales are studied: 30–70, 14–30, and 6–14 days. In the 30–70-day band, the upper-level circulation signals are zonally elongated, with zonal wavenumbers 0–2 dominant. Higher-frequency signals are dominated by zonal wavenumbers 5 and 6. In the 14–30-day band, convection over the eastern hemisphere is associated with upper-level anticyclones in the subtropics and appears to be linked in some cases to midlatitude wave trains. The strongest signals are for convection over Africa, Australia, and the eastern Indian Ocean. Only weak signals are seen for convection over Indonesia. In these regions of upper-level easterlies, OLR anomalies peak prior to the maximum anomalies in wind, suggesting forcing of the circulation by tropical heating.

In contrast, 14–30-day and 6–14-day convection over the eastern tropical Pacific, eastern South America, and central South Pacific is primarily associated with the intrusion of troughs in the westerlies originating in the extratropics. These are regions of mean upper level westerly flow, or where upper-westerlies lie adjacent to tropical convergence zones overlain by only weak easterly flow aloft. The large amplitude of these troughs prior to the OLR anomaly is indicative of the forcing of the convection by these disturbances.

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Jeffrey S. Whitaker and Klaus M. Weickmann

Abstract

A statistical prediction model for weekly rainfall during winter over western North America is developed that uses tropical outgoing longwave radiation (OLR) anomalies as a predictor. The effects of El Niño–Southern Oscillation (ENSO) are linearly removed from the OLR to isolate the predictive utility of subseasonal variations in tropical convection. A single canonical correlation (CCA) mode accounts for most of the predictable signal. The rank correlation between this mode and observed rainfall anomalies over southern California is 0.2 for a 2-week lag, which is comparable to correlation between a weekly ENSO index and weekly rainfall in this region. This corresponds to a doubling of the risk of extreme rainfall in southern California when the projection of tropical OLR on the leading CCA mode two weeks prior is extremely large, as compared with times when it is extremely small. “Extreme” is defined as being in the upper or lower quintile of the probability distribution.

The leading CCA mode represents suppressed convection in the equatorial Indian Ocean and enhanced convection just south of the equator east of the date line. OLR regressed on the time series of this mode shows an eastward progression of the suppressed region to just south of the Philippines at the time of maximum California rainfall enhancement. The region of enhanced convection east of the date line remains quasi stationary. Associated with this tropical OLR evolution is the development of upper-tropospheric westerly wind anomalies near 30°N in the eastern Pacific. Synoptic-scale weather systems are steered farther east toward California by these enhanced westerlies.

Because most operational weather prediction models do not accurately simulate subseasonal variations in tropical convection, statistical prediction models such as the one presented here may prove useful in augmenting numerical predictions. An analysis of 4 yr of operational week-2 ensemble predictions indicates that the level of skill provided by the statistical model is comparable to that of the operational ensemble mean. Given that by week 2 the operational forecast model has lost its ability to represent convectively coupled circulation associated with the subseasonal tropical convective variability, the statistical model provides essentially independent information for the forecaster.

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Klaus M. Weickmann and Prashant D. Sardeshmukh

Abstract

The period 1 December 1984 to 3 February 1985 was associated with strong intraseasonal fluctuations in both the global atmospheric angular momentum (AAM) and tropical convection. Consistent changes were observed in the length of day. The AAM budget for the 65-day period is examined here using circulation data from the National Meteorological Center. Surprisingly well-balanced global and zonal budgets are obtained for the vertically integrated AAM. This enables a closer examination of regional changes, to assess how they might be responsible for the changes in the global AAM.

Both friction and mountain torques are important in the global AAM budget. The increase of AAM is associated first with a positive friction torque, then with a positive mountain torque. The subsequent decrease of AAM results from a negative friction torque. The accompanying regional changes are mostly confined to the Northern Hemisphere, with high global AAM associated with a stronger and southward-displaced subtropical jet. In the zonal budget, meridional AAM fluxes by the zonally asymmetric eddies are important and appear to lead the torques by a few days.

The increase of AAM begins with a shift of the tropical convection from the east Indian to the west Pacific Ocean. The consequent enhancement of the trades east of the Philippines gives a positive friction torque. The friction torque also has a contribution from enhanced trades over Central America and the tropical Atlantic Ocean, which appear to be linked to an equatorward propagating upper-tropospheric wave over the region. A persistent high pressure anomaly subsequently develops to the east of the Himalayas, giving a positive mountain torque. The global AAM rises in response to these torques, but as the circumpolar vortex expands the trades are weakened, causing a negative friction torque and the final reduction of the AAM.

Interestingly, no coherent signals are seen in the weak zonal-mean convection anomalies accompanying these AAM changes. Rather, the AAM budget suggests that the tropical Madden–Julian oscillation and the global AAM are linked through the interaction of Rossby waves generated by the tropical heating with a zonally varying ambient flow and with mountains. The surface stresses have both a local component related to the convection and a remote component induced by upper-tropospheric AAM fluxes.

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Thomas R. Knutson and Klaus M. Weickmann

Abstract

Life cycles of the 30–60 day atmospheric oscillation were examined by compositing 30–60 day filtered NMC global wind analyses (250 mb and 850 mb) and outgoing longwave radiation (OLR) for the years 1979–84. Separate composite life cycles were constructed for the May–October and November–April seasons using empirical orthogonal function analysis of the large-scale divergent wind field (250 mb velocity potential) to define the oscillation's phase. Monte Carlo simulations were used to assess the statistical significance of the composite OLR and vector wind fields.

Large-scale (wavenumber one) tropical divergent wind features propagate eastward around the globe throughout the seasonal cycle. The spatial relationships between these propagating circulation features and OLR are shown using sequences of composite maps. Good agreement exists between areas of upper-air divergence and areas of convection inferred from the OLR satellite data. Convection anomalies are smaller over tropical Africa and South America than over the Indian and western Pacific oceans. Anomalies of OLR are nearly negligible over cooler tropical sea surfaces. Fluctuations in summer monsoon region convection are influenced by the global-scale eastward-moving wave.

The oscillation's vertical structure varies with latitude. In the tropics, upper-level and lower-level tropospheric wind anomalies are about 180° out of phase. Poleward of about 20°, there is no pronounced phase shift between levels. In tropical and subtropical latitudes, analysis of the nondivergent circulation composites at 250 mb (ψ250) reveals cyclones to the east of the convection and anticyclones alongside or west of the convection. While convection anomalies are most pronounced in the summer hemisphere tropics, the tropical and subtropical ψ250 features are most prominent in the winter hemisphere. There is some evidence of symmetry of cyclonic and anticyclonic circulations about the equator.

A subset of the composite extratropical vector wind fields were statistically significant (95% level) at 850 and 250 mb in the winter hemisphere (25°–85° latitude), based upon a Monte Carlo simulation. During the November-April season, the East Asian jet is retracted toward Asia when positive 30–60 day convection anomalies are occurring over the equatorial Indian Ocean. The eastward shift of convection into the western and central Pacific is accompanied by a series of circulation features over northern Asia and an eastward extension of the East Asian jet. During the May-October season, the shift of large-scale tropical convection anomalies from the Indian Ocean and Indian monsoon regions to the tropical western Pacific is followed (10–15 days later) by the occurrence of strengthened westerlies over southern Australia. In contrast, the extratropical “response” in the summer hemisphere for both the May–October and November–April seasons was not statistically significant.

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Michael A. Alexander and Klaus M. Weickmann

Abstract

Recent observational analyses have indicated that tropospheric quasi-biennial oscillations (QBs) may play a fundamental role in regulating the timing and strength of El Niño and the Southern Oscillation. The biennial variability is examined in the tropical troposphere of a 35-year general circulation model (GCM) simulation forced by observed sea surface temperatures (SSTs). The results of spectral analyses and temporal filtering applied to the SST boundary conditions and the simulated lower- and upper-tropospheric zonal winds, precipitation, and sea level pressure anomalies are compared with observations and used to investigate the relationship between variables.

The GCM obtains regions of coherent biennial variability over the tropical Indian and Pacific Oceans in close correspondence with observations. In addition, the evolution of the stronger QBs and the physical relationship between variables are fairly well simulated. Zonal wind anomalies, with a simple baroclinic structure, tend to propagate eastward from the Indonesian region to the central Pacific where they increase in strength. The amplitude of the zonal wind and SST anomalies in the central Pacific vary together, with the largest anomalies occurring during the mid-1960s, mid-1970s, and early 1980s. During the time of the warmest SSTs, low pressure is found in the east Pacific with high pressure over Indonesia, and precipitation is enhanced between the date line and 120°W. However, the model underestimates the low-frequency variability in general and has approximately one-half to two-thirds of the observed variability in the biennial range. In addition, the observed phasing of the biennial and annual cycles in the zonal winds over the eastern Indian Ocean is not reproduced by the model.

The authors have also compared the amount of biennial variability of the near-surface zonal winds in the 35-year run with observed SSTs to two 35-year periods in a 100-year control run with climatological SSTs that repeat the seasonal cycle. Only the simulation with observed SSTs has an organized region of enhanced biennial variability near the equator, suggesting a strong oceanic component to the forcing of the QB.

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