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Brant Liebmann

Abstract

The work presented is a correlative study of the interaction between large-scale tropical convection and midlatitude wind anomalies, and the tropical wind field on subseasonal time scales. Outgoing longwave radiation (OLR) is used as a proxy for convection. Correlations are calculated from six years of 5-day averaged data for the December–February (DJF) season. The seasonal cycle and interannual variability are removed before computing the correlations.

The results show that it is appropriate to classify the tropics into two regimes based on the direction of the time-mean 200-mb zonal wind at the equator. In the easterly regime there is abundant convective activity, a large standard deviation of OLR, and a small standard deviation of zonal wind. At longitudes of upper-level westerlies there is little convection, a small standard deviation of OLR, and a large standard deviation of wind. These results imply that convection is an important determinant of time-mean flow asymmetries along the equator. The small standard deviation of wind at longitudes of easterlies is interpreted as resulting from a shielding by the easterly winds from disturbances originating in midlatitudes.

It is then shown that anomalies of OLR are better correlated with the anomalous wind field at longitudes of easterly winds than at longitudes of westerlies. This is interpreted to result from the larger forcing due to convection at longitudes of easterlies than westerlies, and from a lack of contamination of the signal by disturbances originating in midlatitudes.

Finally, one-point correlation maps are presented for OLR anomalies at the equator at 130°E correlated with the near global field of nondivergent and divergent wind at 200 and 850 mb. These results are compared with the output from a linear model which was forced with an idealized heat source. At 200 mb the nondivergent anomalies do not resemble the linear model output except perhaps near the region of heating. It is suggested that the poor correspondence results from either nonlinearities in the real atmosphere or from the slow adjustment of the atmosphere to forcing. An interesting feature is the anomalously strong Asian jet associated with heating to its south. At 850 mb the correspondence with the model is better than at 200 mb. The divergent anomalies show an apparent westward tilt of the large-scale circulation pattern with height. Also interesting is the apparent convergence into the Asian jet entrance region during periods of convection, and the fact that divergent anomalies which are of opposite sign and precede those over Indonesia appear east of the dateline. There is some evidence that wavetrains originating in midlatitudes propagate to the equator at longitudes of westerlies over the Atlantic Ocean, but, surprisingly, not over the Pacific.

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Brant Liebmann
and
Dennis L. Hartmann

Abstract

Eight Northern Hemisphere winters of five- and ten-day average midlatitude 500 mb heights and tropical outgoing IR are used in a correlative study of tropical-midlatitude interaction. The seasonal cycle and interannual variability are removed so that only intraseasonal variability remains. Results indicate that energy predominantly propagates from midlatitudes to the tropics for both five- and ten-day averaged data, although the propagation is more apparent in five-day averaged data. This is evidenced by the fact that the largest tropical IR patterns are southeastward of the 500 mb point with which the IR field is correlated. The result is interpreted in terms of a quasi-stationary Rossby wave which has an eastward component of group velocity. The southwest-northeast tilt of the 500 mb height correlation patterns, indicating poleward momentum transport or equatorward wave propagation, also supports the hypothesis that midlatitude flow drives the tropics. Lead and lag correlations show that when 500 mb heights lead IR, an upstream development appears in the 500 mb correlation pattern. The field is nearly featureless, however, when 500 mb heights lag IR. Well-defined time evolution is more evident over the eastern Pacific than over the western Pacific.

The only indication of possible forcing of the midlatitude flow by the tropics is from IR anomalies in the region of winter monsoon rainfall over the far western Pacific, which are associated with a pattern of correlations in the 500 mb field of nearly global extent. The pattern may be related to that produced by Simmons et al. with a barotropic model, when steady forcing is used to perturb a zonally-varying basic state. They hypothesize that the large global anomalies are the result of the barotropic instability of the basic state. Although the global correlation pattern is statistically significant, it explains only a small fraction of the total variance.

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Brant Liebmann
and
Harry H. Hendon

Abstract

A spectral analysis of winds analyzed and initialized at the European Centre for Medium-Range Weather Forecasts reveals an abundance of power in the 850 mb meridional wind along the equator with periods near four days. The power is mostly in the westward propagating component.

Using high-pass filtered data it is shown that the waves have westward phase and eastward group propagation relative to the mean wind. The longest wavelengths are found over the Pacific Ocean, while the shortest are found over the convectively variable regions of Indonesia, South America, and Africa. Mean phase speeds at 850 mb are positively correlated with the mean wind on the equator at 500 mb and below, and negatively correlated with the mean wind above that level. The effective advecting zonal wind of the disturbances seems to be the density weighted average of the lower troposphere.

The structure of the disturbances bears resemblance to the expected structure of an equatorially trapped mixed Rossby-gravity wave over the central Pacific and Atlantic oceans, although the anomalies, while statistically significant, are extremely small. The outgoing longwave radiation (OLR) pattern is consistent with the flow field, suggesting that the waves are not merely a model artifact. Over the Atlantic there is a mode well defined by the zonal wind at the equator, but the OLR pattern is not consistent. Over the far western Pacific, there is evidence of meridional propagation from Northern Hemisphere midlatitudes. North of the equator there is meridional propagation at every longitude.

The strongest disturbances are primarily confined to the lower half of the troposphere, but at many longitudes there is evidence of a weak first baroclinic-mode structure within the troposphere. North of the equator the structures are barotropic.

Effective equivalent depths are estimated by comparing dispersion characteristics with mixed Rossby-gravity dispersion curves. Where the assumption of a mixed Rossby-gravity mode is believed to be valid, the equivalent depths are found empirically to lie between 1–60 m.

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Harry H. Hendon
and
Brant Liebmann

Abstract

Onset of the Australian summer monsoon is identified each year (1957–87) using the wind and rainfall record at Darwin. Onset is defined as the first occurrence of wet, 850 mb westerly winds. Composites of atmospheric fields at stations in and about the Australian tropics are constructed relative to the onset date at Darwin.

The composite onset is accompanied by the development of a convectively driven, baroclinic circulation over northern Australia. Upper tropospheric easterlies expand about the equator and the subtropical jet shifts poleward at onset. This behavior is interpreted as a transient southerly shift of the local Hadley circulation concurrent with the development of an upper level anticyclone over northern Australia.

The composite onset coincides with the arrival of an eastward propagating convective anomaly. The anomaly originates in the southern Indian Ocean, propagates eastward at 5 m s−1 and is detectable as far east as the date line. An eastward propagating zonal wind anomaly also is detectable at tropical stations east and west of Darwin. These features are indicative of the 40–50 day oscillation and thus the composite onset is concluded to coincide with the traversal of the oscillation across northern Australia. The composite onset is further shown to coincide with the first occurrence of the convectively active 40–50 day oscillation during each southern summer.

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Harry H. Hendon
and
Brant Liebmann

Abstract

The tropical intraseasonal (30–50 day) oscillation manifests itself in the Australian summer monsoon by a pronounced modulation of the monsoonal westerlies. These 30-50 day fluctuations of the monsoonal westerlies are coherent with rainfall and OLR across northern Australia. The OLR fluctuation originates in the Indian Ocean and systematically propagates eastward at 5 m s−1, consistent with previous studies of the intraseasonal oscillation.

The detailed evolution of the intraseasonal oscillation of the monsoon is studied via composites of upper air data in and about the Australian tropics. During the summer periods 1957-87, 91 events were identified at Darwin, Australia. The composite oscillation at Darwin has a very deep baroclinic structure with westerlies extending up to 300 mb. The westerly phase lasts about ten days and lags a similar duration rainfall event by about four days. During the westerly phase, the upper troposphere is warm and the extreme lower troposphere is cool. This structure is consistent with midtropospheric latent heating and lower tropospheric cooling due to evaporation of falling rain. The magnitude of the composite oscillation at Darwin is about 5 m s−1 in zonal wind, 1 m s−1 in meridional wind, 0.5°K in temperature, 5 mm rainfall per day, and 10% in relative humidity. The oscillation at Darwin is readily traced as far west as Cocos Island and as far east as Pago Pago.

Above northern Australia, enhanced synoptic scale variability develops during the wet-westerly phase of the oscillation. Analysis of a single station record precludes documentation of the structure of these synoptic fluctuations. In the Northern Hemisphere midlatitudes, a wave train in 500 mb heights appears to emanate from the longitude of the Australian tropics during the wet-westerly phase. The magnitude of this wave train is only about 50 m while the wave train undergoes a systematic evolution as the tropical convective anomaly moves west to east, no sense of dispersion from a localized low-latitude heat source is evident.

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Harry H. Hendon
and
Brant Liebmann

Abstract

The signature of 4–5-day period Rossby–gravity waves is searched for in the tropical convection field across the Indian-Pacific oceans. The convergence/divergence field of these waves in the lower troposphere is anticipated to produce an antisymmetric fluctuation in tropical convection. Antisymmetric fluctuations of tropical convection are shown to exhibit a pronounced spectral peak at a 4–5-day period only during boreal fall and only within about 30° longitude of the date line. The peak amplitude occurs around 7.5° latitude. These fluctuations propagate westward at 15–20 m s−1 with zonal wavelength of about 7000&–9000 km. The fluctuations of convection are coherent and out of phase with the equatorial meridional wind, which also exhibits a pronounced spectral peak at a 4–5-day period in the lower troposphere near the date line. The antisymmetric zonal wind also is strongly coherent with the antisymmetric convective fluctuations in this region. The horizontal distributions of the 4–5-day power and coherence of the winds and convection are consistent with that produced by a convectively coupled Rossby–gravity wave that is confined near the date line.

The localization of the convectively coupled Rossby–gravity wave activity near the date line during boreal fall is postulated to be due to the unique meridional distribution of sea surface temperature at this location. The equatorial minimum flanked by maxima at about 5°–10° latitude is thought to encourage antisymmetric convection, which interacts efficiently with Rossby–gravity waves. The fall maximum in convectively coupled Rossby–gravity wave activity is consistent with these unique sea surface temperatures occurring only during fall.

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Brant Liebmann
and
Dennis L. Hartmann

Abstract

Interannual variability of outgoing IR in the tropical Pacific Ocean is studied using measurements derived from the NOAA scanning radiometer. In addition to the usual mean maps, seasonal anomaly maps are constructed from June, July, August 1974-December 1977, January, February 1978. These IR anomalies are closely related to changes in convective cloudiness patterns. Time series representing the equatorial eastern Pacific sea-surface temperature (SST) anomalies and monthly anomalies at various locations are also plotted. During this period a “warming event” occurs, in which SSTs in the eastern Pacific rapidly become anomalously warm. Dramatic changes in outgoing IR occur simultaneously with this SST increase. The region of convergence over Indonesia shifts eastward and connects to a well-developed intertropical convergence zone (ITCZ). The South Pacific convergence zone (SPCZ) is also connected to the Indonesian convergence zone, but develops more slowly and does not reach its maximum strength until more than a year after the SST increases occur. By this time the ITCZ has returned to its pro-warming state. Eastward movement of the SPCZ is also apparent.

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Harry H. Hendon
,
Brant Liebmann
, and
John D. Glick

Abstract

The relationship between the Madden–Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical troposphere, and the Kelvin waves that dominate the variability of the equatorial thermocline in the central and eastern Pacific Oceans is explored. The Kelvin waves have period near 70 days, which is distinctly longer than the dominant period of the MJO (40–50 days). Their zonal wavelength is roughly the width of the Pacific basin, which is about twice the zonal scale of the zonal stress anomalies produced by the MJO across the western Pacific. Their eastward phase speed is about 2.3 m s−1, which is indistinguishable from the gravest baroclinic mode using the observed stratification in the Pacific.

The stress anomalies that force the Kelvin waves are shown to be associated with the lower-frequency components of the MJO (i.e., periods greater than about 60 days). These stress anomalies move eastward at less than 5 m s−1 from the Indian Ocean to the date line, where their local wavelength is about 15000 km. East of the date line, where the convective component of the MJO weakens, the phase speed of the stress anomalies increases to greater than 10 m s−1. The similarity of the phase speeds of the MJO west of the date line and of the gravest baroclinic Kelvin wave is shown to result in near-resonant forcing by the relatively weak, but zonally broad, stress anomalies induced by the MJO. Despite the large increase in phase speed east of the date line, the MJO-induced stress anomalies are shown to continue to positively project onto the Kelvin waves to about 130°W, which is where the observed thermocline perturbations are the largest. East of this longitude, the MJO-induced stress anomalies detract from the amplitude of the Kelvin waves. The large spatial scale of the zonal stress anomalies produced by the MJO and the near-resonant forcing west of the date line helps explain the observed spectral peak near 70 days for the Kelvin waves despite the higher central frequency of the MJO.

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