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Harry H. Hendon
and
John Glick

Abstract

The relationships between intraseasonal (periods <100 days) variations of convection, sea surface temperature (SST), surface wind stress, and surface fluxes of latent heat and radiation in the warm pool of the equatorial Indian and western Pacific Oceans are examined using 7 yr of gridded outgoing longwave radiation (OLR), SST, and surface stress and latent heat flux based on European Centre for Medium-Range Weather Forecasts analyses. In the warm pool region enhanced evaporation, which results from enhanced surface westerlies, lags enhanced convection by ∼1 week. Intraseasonal SST fluctuations lag decreased evaporation by ∼1 week and decreased convection (which implies increased insolation) by ∼2 weeks, suggesting that anomalous latent heat flux and surface insolation drive SST changes on intraseasonal timescale.

The relationship between anomalous SST, surface wind stress and surface fluxes of latent heat and shortwave radiation for the Madden–Julian oscillation (MJO), which dominates the intraseasonal variability of convection and surface winds over the warm pool, is developed. Spatially coherent SST anomalies, with amplitude of ∼1/3°C, develop in the Indian Ocean and propagate eastward along with the large-scale convective anomaly, but with 1/4 cycle lag. The SST anomalies in the Indian Ocean are postulated to be driven predominantly by surface insolation anomalies associated with the anomalous large-scale convection. The SST anomalies in the western Pacific are postulated to be driven by a combination of anomalous latent heat flux and insolation. The differing behavior in each ocean reflects structural changes of the MJO as it evolves through its life cycle. Data collected during TOGA COARE are used to quantify the role of surface heat flux anomalies for driving the SST changes in the western Pacific.

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Toshiaki Shinoda
,
Harry H. Hendon
, and
John Glick

Abstract

Composites of sea surface temperature (SST), surface heat, momentum, and freshwater flux anomalies associated with intraseasonal oscillations of convection are developed for the warm pool of the western Pacific and Indian Oceans during 1986–93. The composites are based on empirical orthogonal function analysis of intraseasonally filtered outgoing longwave radiation (OLR), which efficiently extracts the Madden–Julian oscillation (MJO) in convection. Surface fluxes are estimated using gridded analyses from the European Centre for Medium-Range Weather Forecasts, weekly SST, OLR, microwave sounding unit precipitation, and the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm. At intraseasonal timescales, these surface flux estimates agree reasonably well with estimates based on mooring observations collected during TOGA COARE.

The amplitude of the composite SST variation produced by the MJO is about 0.25°C in the western Pacific, 0.35°C in the Indonesian region, and 0.15°C in the Indian Ocean. The intraseasonal anomalies of SST and net surface heat flux propagate eastward at about 4 m s−1 along with the convective anomaly. The amplitude of the net surface heat flux variation is 50–70 W m−2 in the western Pacific, with anomalous insolation and latent heat flux making similar contributions. Across the Indian Ocean, the net surface heat flux anomaly is weaker (30–40 W m−2), and anomalous insolation appears to make a greater contribution than anomalous latent heat flux. Across the entire warm pool, the net surface heat flux leads the SST variation by about one-quarter cycle, which is consistent with the notion that surface heat flux variations are driving the SST variations at these intraseasonal timescales. The intraseasonal SST variation, however, is estimated to significantly reduce the amplitude of the latent and sensible heat fluxes produced by the MJO.

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Toshiaki Shinoda
,
Harry H. Hendon
, and
John Glick

Abstract

Reliability of the surface fluxes from National Centers for Environmental Prediction (NCEP) reanalyses is assessed across the warm pool of the western Pacific and Indian Oceans. Emphasis is given to the spatial distribution and coherence of the fluxes on intraseasonal (25–100 day) periods, as intraseasonal variability predominates the subseasonal variability across the warm pool. Comparison is made with surface fluxes estimated from data collected at a mooring during the Coupled Ocean–Atmosphere Response Experiment and with independent gridded estimates based on operational wind and surface pressure analyses and satellite observations of rainfall, shortwave radiation, and outgoing longwave radiation. In general, fluxes that depend primarily on surface wind variations (e.g., stress and latent heat flux) agree more favorably than fluxes that are largely dependent on fluctuations of convection (e.g., surface shortwave radiation and freshwater or precipitation). In particular, the intraseasonal variance of shortwave radiation and precipitation in the NCEP reanalyses is about half of that estimated from in situ observations and from satellite observations. Composite surface flux variations for the Madden–Julian oscillation, which is the dominant mode of intraseasonal variability in the warm pool, are also constructed. Again, the composite variations of wind stress and latent heat flux from the NCEP reanalyses agree reasonably well, both in magnitude and phasing, with the composite fluxes from the independent gridded data. However, the composite intraseasonal shortwave radiation and precipitation from the NCEP reanalyses, while agreeing in phase, exhibit less than half the amplitude of the satellite-based estimates.

The impact of the underestimation of these surface flux variations in the NCEP reanalyses on the intraseasonal evolution of sea surface temperature (SST) in the warm pool is investigated in the context of a one-dimensional mixed layer model. When forced with the intraseasonal surface fluxes from the NCEP reanalyses, the amplitude of the intraseasonal SST variation is some 30%–40% smaller than observed or than that from forcing with the independent gridded fluxes. This reduced amplitude is primarily caused by the underestimation of the intraseasonal shortwave radiation variations in the NCEP reanalyses.

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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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Harry H. Hendon
,
Chidong Zhang
, and
John D. Glick

Abstract

Interannual variability of the Madden–Julian oscillation (MJO), the dominant mode of intraseasonal variability in the Tropics, is investigated during the extended austral summer season November–March, which is when the MJO is most prominent. Indexes of the level of MJO activity are developed using outgoing longwave radiation and zonal wind analyses at 850 mb for 1974–98. Based on these indexes, interannual variations in the level of MJO activity are found to be primarily associated with changes in the number of discrete MJO events each year and with changes in the intensity of intraseasonal convection across the Indian and western Pacific Oceans, where the MJO is normally prominent. An eastward shift of MJO activity east of the date line does occur during El Niño events. However, the overall level of MJO activity is found to be uncorrelated with El Niño, except during exceptional warm events when MJO activity is diminished. The level of MJO activity is shown to be weakly related to sea surface temperature anomalies in the equatorial Indian and western Pacific Oceans, but the weak correlations imply that much of the year-to-year variability of the MJO is internally generated, independent of any slowly varying boundary forcing. Such year-to-year variations of the intensity of the MJO are, however, associated with changes in the distribution of seasonal mean convection across the tropical Indian and western Pacific Oceans. This interannual variation of convection unrelated to SST variability may thus act as a limit to seasonal predictions that rely heavily on equatorial Pacific SST anomalies.

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Harry H. Hendon
,
Brant Liebmann
,
Matthew Newman
,
John D. Glick
, and
J. E. Schemm

Abstract

Systematic forecast errors associated with active episodes of the tropical Madden–Julian oscillation (MJO) are examined using five winters of dynamical extended range forecasts from the National Centers for Environmental Prediction reanalysis model. Active episodes of the MJO are identified as those periods when the amplitude of either of the first two empirical orthogonal functions of intraseasonally filtered outgoing longwave radiation, which efficiently capture the MJO, is large. Forecasts initialized during active episodes of the MJO are found not to capture the eastward propagation of the tropical precipitation and circulation anomalies associated with the MJO. Rather, the MJO-induced anomalies of precipitation and winds are systematically forecast to weaken and even retrograde. By about day 7 of the forecast the convectively coupled, tropical circulation anomalies produced by the MJO are largely gone. Systematic errors in the extratropical 200-mb streamfunction also fully develop by day 10. The initial development of these errors is argued to result from the collapse of the tropical divergence forcing produced by the MJO and, thus, the lack of correct Rossby wave source. Forecast skill in the Tropics and Northern Hemisphere extratropics is found to be systematically reduced during active periods of the MJO as compared to quiescent times. This reduced skill is suggested to result because the MJO is the dominant mode of convective variability and not because the model is better able to forecast intraseasonal convection unrelated to the MJO.

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Brant Liebmann
,
JoséA. Marengo
,
John D. Glick
,
Vernon E. Kousky
,
Ilana C. Wainer
, and
Oswaldo Massambani

Abstract

Observed rainfall, outgoing longwave radiation (OLR), divergence, and precipitation from the reanalysis project of the National Centers for Environmental Prediction and the National Center for Atmospheric Research are compared over the Amazon Basin. The spatial pattern of the mean and the phase of the annual cycle generally compare well, except that the amplitude of the annual cycle of model precipitation is much smaller than observed. On 10–30-day timescales, it is shown that averaging stations within a 5° radius is approximately equivalent to total wavenumber 20 (T20) spatial scale, although it is more important to have a high density of stations than an exact match of spatial scales. Ideally, there should be one station per 20 000 km2. On 10–30-day scales, observed rainfall is best correlated with OLR. Correlations between OLR and 150-mb divergence are larger than between observed rainfall and divergence or between rainfall and model precipitation. For example, if 10–30-day filtered OLR and divergence are truncated at T20 and rainfall is averaged to include stations within a 5° radius, OLR is correlated with rainfall at about −0.6, OLR is correlated with divergence at about −0.35, and rainfall is correlated with divergence at about 0.2. At least part of the lack of correlation is due to inadequate spatial sampling of rainfall. Correlations improve with larger spatial scale. The major seasonal transitions from dry to rainy regimes are captured well by OLR but not by the model quantities. The mean diurnal cycle is represented reasonably by 150-mb divergence.

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Brant Liebmann
,
George N. Kiladis
,
JoséA. Marengo
,
Tércio Ambrizzi
, and
John D. Glick

Abstract

Relationships between deep convection over South America and the atmospheric circulation are examined, with emphasis on submonthly variations of the South Atlantic convergence zone (SACZ) during austral summer. Outgoing longwave radiation (OLR) is used as a proxy for convection, while the associated circulation patterns are depicted by the National Centers for Environmental Prediction Reanalysis.

Over South America and the adjacent oceans, OLR fluctuations with periods less than 90 days show maximum variance in the SACZ and over central South America during December–February. There is a local minimum in variance over the southern Amazon Basin, where mean convection is at a maximum. OLR spectra display several statistically relevant peaks corresponding to periods of less than 30 days over tropical South America, with the relative proportion of higher-frequency power increasing as the base grid point is moved to the southeast within the SACZ.

Correlations between submonthly (2–30-day) OLR in the vicinity of the SACZ and 200-mb streamfunction reveal the preferred path of Rossby wave energy impinging on the SACZ from the midlatitudes of the Southern Hemisphere. Episodes of enhanced convection within the SACZ, indicated by negative OLR anomalies, occur at the leading edge of upper-level troughs propagating into the region. The corresponding pattern at 850 mb reveals that the disturbances are nearly equivalent barotropic west of South America but tilt westward with height in the region of the SACZ. Negative low-level temperature anomalies lie to the southwest of the convection. The results are consistent with baroclinic development along an associated cold front.

Convection over the southwestern Amazon Basin on submonthly timescales is seen to progress into the region from the south. Upper-level anomalies, which at times may play a role in the initiation of the convection, move eastward and rapidly become decoupled from the convection. Low-level cold air along the eastern flank of the Andes appears linked to the convection as it moves northward. In contrast, convection over the southeastern Amazon is accompanied by disturbances moving into the area from the Atlantic, but there is little sign of a low-level temperature anomaly. In this case convection seems to result in cross-equatorial outflow into the North Atlantic, rather than be the result of forcing from the extratropics.

The authors speculate that the relatively stable position of the SACZ is associated with a Rossby wave guide, which ultimately is related to the large-scale circulation driven by sources and sinks of diabatic heating. It also appears that the SACZ forms when the northwesterly flow associated with a low-level trough is able to tap moisture from the Amazon.

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