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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 JoséA. Marengo

Abstract

Interannual variability of seasonal rainfall in the Brazilian Amazon basin is examined in context of its relationship to sea surface temperatures in the tropical Pacific and Atlantic Oceans. Linear correlations reveal strong relationships, but rainfall patterns are of regional scale. Areas of rainfall exhibiting strong relationships with SST are confined to the equatorial region of the Brazilian Amazon. The best relationships are found either during the season of transition between wet and dry regimes, or entirely within the dry season. It is hypothesized, and results are shown in support, that during the transition seasons, an important contributor to the SST control on seasonal totals is its influence on the timing on the rainy season onset or end. That influence appears to be stronger than the SST influence on the rainy season rain rate.

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Brant Liebmann and Dave Allured
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Brant Liebmann and Arnold Gruber

Abstract

The annual variation of the diurnal cycle of outgoing longwave radiation (OLR) is examined. Our results are based on the climatological amplitude and phase of the first diurnal harmonic for each month. The diurnal harmonic was extracted from a composite daily cycle from several polar orbiting satellites that flew in different years with ten different equator crossing times. We compute a “diurnal vector standard deviation” which is the square root of the sum of the variances of both components of the 12 climatological monthly diurnal vectors. This allows contributions from both phase and amplitude changes of the diurnal vector.

A map of the diurnal vector standard deviation is presented. The values over land are an order of magnitude larger than over the ocean. The maxima are located over the seasonally migrating monsoons and over the midlatitude semi-arid zones. In midlatitudes the large standard deviation results from an increased daily cycle of insolation during summer and from clouds associated with midlatitude storms which reduce the diurnal cycle during winter. In the tropical monsoon regions a large variability of the diurnal cycle results from a larger daily cycle of cloudiness during the wet season than in the dry season. At some locations over the monsoons, however, the diurnal amplitude is actually a minimum during the wet summer season. We believe the minimum is caused by the pervasive cloudiness in the most convective regions. In the midlatitudes and during the dry season in the tropics, the maximum emission generally occurs between 1200 and 1400 local time. During the rainy season it occurs between 0600 and 0900.

We hypothesize that there should be a spatial relationship between the diurnal cycle variability and the standard deviation of the 12 climatological monthly means of OLR, and we compare maps of the two quantities The large-scale features are in broad agreement and the correlation between the two maps is marginally statistically significant. A detailed comparison, however, reveals that the diurnal vector standard deviation is of much smaller scale than the standard deviation of OLR. We attribute the regional structure of the diurnal cycle variability to varying geography, vegetation, and available moisture. Some of the small-scale structure, however, undoubtedly arises because the diurnal cycle involves day-night differences which are inherently more noisy than the OLR field itself.

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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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Prashant D. Sardeshmukh and Brant Liebmann

Abstract

Any discussion of intraseasonal and interannual variability in the atmosphere must presume a reliable assessment of the observed variability. In spite of continued improvements in observing systems, quality control techniques, and data analysis schemes, however, and also because of them, this assessment remains difficult in the tropics.

In this paper the authors examine the mean tropical circulation during two Januarys, 1988 and 1989, as described by the circulation analyses produced at two weather prediction centers, the National Meteorological Center (NMC) in Washington, D.C., and the European Center for Medium-Range Weather Forecast (ECMWF) in Reading, England. In particular, the authors’ focus is on the change in the circulation between 1988 and 1989 as estimated by these two sets of analyses, especially the change in the 200-mb wind divergence associated with organized deep convection. The authors find that in many regions the discrepancy between thew estimates is of the order of the change itself. A comparison with maps of the outgoing longwave radiation (OLR) is not quantitatively useful in this regard.

One way out of this dilemma is to derive divergence fields that are consistent with the 200-mb vorticity balance. The authors do so by solving the “chi problem” of Sardeshmukh and Hoskins. Because the large-scale vorticity fields generated by NMC and ECMWF are highly correlated (∼98%), the divergence fields derived in this manner are also better correlated than the analyzed fields and enable a more reliable assessment of the observed change between these two periods.

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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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Harry H. Hendon and Brant Liebmann

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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

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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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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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