Abstract

An empirical model that predicts the evolution of the Madden–Julian oscillation (MJO) in outgoing longwave radiation (OLR) and 200-mb streamfunction is developed. The model is based on the assumption that the MJO can be well represented by a pair of empirical orthogonal functions (EOFs) of OLR and three EOFs of streamfunction. With an eye toward using this model in real time, these EOFs are determined with data only subjected to filtering that can be applied in near–real time. Stepwise lag regression is used to develop the model on 11 winters of dependent data. The predictands are the leading two principal components (PCs) of OLR and the leading three PCs of streamfunction. The model is validated with five winters of independent data and is also compared to dynamic extended range forecasts (DERFs) made with the National Centers for Environmental Prediction’s Medium Range Forecast (MRF) model.

Skillful forecasts of the MJO in OLR and streamfunction with the empirical model are achieved out to about 15 days. Initial skill arises from autocorrelation of the PCs. Subsequent skill beyond about 1 week arises primarily from the cross correlation with the other PCs that define the MJO. Inclusion of PCs not associated with the MJO as predictors appears not to reliably improve skill. Skill is found to be substantially better when the MJO is active at the initial condition than when it is inactive. The empirical forecasts are also found to be more skillful than DERF from the MRF for lead times longer than about 1 week. Furthermore, skill of DERF from the MRF is found to be better when the MJO is quiescent than when it is active at the initial condition. It is suggested that significant improvement of tropical DERF could be achieved by improvement of the representation of the MJO in the dynamic forecast model.

Abstract

The radiative transfer model from NCAR’s general circulation model CCM3 is modified to calculate monthly radiative fluxes and heating rates from monthly observations of cloud properties from the International Satellite Cloud Climatology Project and temperature and humidity from ECMWF analysis. The calculation resolves the three-dimensional structure of monthly to interannual variations of radiative heating and is efficient enough to allow a wide range of sensitivity tests.

Two modifications to the radiative transfer model improve the calculation of shortwave (SW) fluxes in a cloudy atmosphere. The first replaces an existing nonphysical parameterization of partially cloudy skies with a physically motivated one that increases substantially the accuracy of calculated SW fluxes while increasing the computational time of the calculation by only 10%. The second modification allows the specification of generalized cloud overlap properties. With these modifications, radiative fluxes are calculated from observed atmospheric properties without any tuning to observed fluxes.

Based on a comparison with top-of-the-atmosphere (TOA) fluxes observed in the Earth Radiation Budget Experiment, calculated SW and longwave (LW) fluxes at TOA have errors of less than 10 W m⁻² at 2.5° horizontal resolution, with smaller errors over ocean than over land. Errors in calculated surface fluxes are 10–20 W m⁻² based on sensitivity tests and comparisons to surface fluxes from the GEWEX Surface Radiation Budget. In contrast, TOA and surface fluxes from the NCEP/NCAR reanalysis data, which rely on cloud properties from a general circulation model, have errors larger than 30 W m⁻². Errors in the calculated fluxes result primarily from uncertainties in the observed cloud properties and specified surface albedo, with somewhat smaller errors resulting from unobserved aspects of the vertical distribution of clouds. Errors introduced into the calculation by using monthly observations and neglecting high-frequency variations are small relative to other sources of error.

Substantial uncertainty is found in many details of the vertical structure of cloud radiative forcing, which underscores the importance of performing a wide variety of sensitivity calculations in order to understand the impact of clouds on radiative heating. However, certain general features of the calculated vertical structure of cloud radiative forcing in the atmosphere are robust. Deep vertical cloud distributions at locations of active tropical convection result in deep cloud radiative heating, whereas shallow cloud distributions in the subtropics result in low-level cloud radiative cooling there. Under all conditions, SW cloud radiative forcing is systematically of opposite sign to LW cloud radiative forcing, which reduces the impact of LW cloud radiative forcing.

Abstract

The role of clouds for low-latitude atmospheric circulations is examined in a linearized calculation forced by diabatic heating rates. A comparison of the circulation calculated from total diabatic heating, obtained from reanalysis data, with observed fields determines which aspects of the calculation are realistic and which are not. The role of clouds is quantified by the circulation calculated from atmospheric cloud radiative forcing, which, in turn, has been calculated with the National Center for Atmospheric Research radiative transfer model using cloud properties observed in the International Satellite Cloud Climatology Project.

In general, cloud radiative forcing contributes about 20% to the magnitude of low-latitude circulations. It typically reinforces the circulation that is driven by convective latent heating. Cloud radiative forcing tends to have a stronger influence in the lower troposphere than at upper levels. It influences local circulations more than remote ones. In particular, cloud radiative forcing from local low cloud cover is the dominant source of diabatic heating influencing subtropical circulations over the eastern oceans. Cloud radiative forcing from low clouds is also found to be important for seasonal variations of meridional winds over the cold tongue in the eastern Pacific. This indicates that atmospheric cloud radiative forcing, and not just surface forcing, is important for ocean–atmospheric coupling there.

Additional calculations are performed that test the sensitivity of the atmospheric circulation to different sources of diabatic heating rates. These sources include radiative heating rates that have been calculated from different cloud data, different cloud overlap assumptions, and enhanced cloud short-wave absorptivity. The principal conclusions of this investigation are unchanged by these calculations. However, enhanced short-wave absorption by clouds systematically reduces the impact of clouds on atmospheric circulations.

Abstract

Three aspects of space–time spectral analysis are explored for diagnosis of the organization of tropical convection by the Madden–Julian oscillation (MJO) and other equatorial wave modes: 1) definition of the background spectrum upon which spectral peaks are assessed, 2) alternate variance preserving display of the spectra, and 3) the space–time coherence spectrum. Here the background spectrum at each zonal wavenumber is assumed to result from a red noise process. The associated decorrelation time for the red noise process for tropical convection is found to be half as long as for zonal wind, reflecting the different physical processes controlling each field. The significance of spectral peaks associated with equatorial wave modes for outgoing longwave radiation (OLR), which is a proxy for precipitating deep convection, and zonal winds that stand out above the red background spectrum is similar to that identified using a background spectrum resulting from ad hoc smoothing of the original spectrum. A variance-preserving display of the space–time power spectrum with a logarithmic frequency axis is useful for directly detecting Kelvin waves (periods 5–15 days for eastward zonal wavenumbers 1–5) and for highlighting their distinction from the MJO. The space–time coherence of OLR and zonal wind is predominantly associated with the MJO and other equatorial waves. The space–time coherence is independent of estimating the background spectrum and is quantifiable; thus, it is suggested as a useful metric for the MJO and other equatorial waves in observations and simulations. The space–time coherence is also used to quantify the association of Kelvin waves in the stratosphere with convective variability in the troposphere and for detection of barotropic Rossby–Haurwitz waves.

Abstract

Seasonal variations of cloud radiative forcing (CRF) are calculated from observed cloud properties in the International Satellite Cloud Climatology Project over the Pacific between 30°S and 30°N. Using 7 yr of data, the first annual harmonic of CRF is statistically significant with respect to the background red noise spectrum at better than a 0.99 confidence level at most locations. It is significant with respect to calculation error at better than a 0.90 confidence level at those same locations. Calculated annual variations are strongest in the subtropics and equatorial east Pacific.

In a linear analysis, annual variations of CRF are attributed to individual annual variations of cloud properties, insolation, or surface temperature. At higher latitudes, the seasonal cycle of CRF at the top of the atmosphere (TOA) and at the surface is dominated by the shortwave (SW) component and results primarily from the seasonal cycle of insolation interacting with the time mean cloud field. Annual variations of cloud fraction and of cloud optical depth are both important in the Tropics, particularly in the east Pacific. Longwave (LW) CRF at TOA is strongest at locations where the seasonal cycle of convection is strong. At those locations, annual variations of CRF result primarily from annual variations of cloud height and not from annual variations of cloud fraction. At the surface, annual variations of LW CRF are small throughout. The annual variations of atmospheric CRF are dominated by the LW component, with the SW component contributing about 20%. As with LW CRF at TOA, annual variations of atmospheric CRF are strongest over convective locations and result from annual variations of cloud height.

The impact of cloud radiative forcing on zonal circulations in the equatorial Pacific and on SST in the east Pacific was analyzed. CRF represents approximately 20% of the annual variations of diabatic heating rates over convective locations and 50% or better at nonconvective locations. Annual variations of atmospheric CRF, when strong, tend to be in phase with those of total diabatic heating rates, indicating that clouds reinforce tropical circulations driven by latent heating.

The role of clouds is particularly important in the east Pacific between 85° and 105°W. Atmospheric CRF is a major component of total diabatic heating over the cold tongue, where seasonal variations of SST are strongest. If seasonal variations of SST in the cold tongue result from seasonal variations of upwelling driven by meridional wind variability, then CRF may play an important role. In contrast, CRF at the surface has only a weak seasonal cycle, with a phase that is not consistent as a forcing for seasonal variations of SST.

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.

Abstract

Sea surface temperature (SST) variations associated with the atmospheric intraseasonal oscillation in the tropical Indian and western Pacific Oceans, are examined using a one-dimensional mixed layer model. Surface fluxes associated with 10 well-defined intraseasonal events from the period 1986–93 are used to force the model. Surface winds from the European Centre for Medium-Range Weather Forecasts daily analyses and SST from the mixed layer model are used to compute latent and sensible heat fluxes and wind stress with the TOGA COARE bulk flux algorithm. Surface freshwater flux is estimated from the Microwave Sounding Unit precipitation data. Net shortwave radiation is estimated, via regression analysis, from outgoing longwave radiation. An idealized diurnal cycle of shortwave radiation is also imposed. The intraseasonal SST variation from the model, when forced by the surface fluxes estimated from gridded analyses, agrees well with the SST observed at a mooring during the COARE. The model was then integrated for the 10 well-defined intraseasonal events at grid points from 75° to 175°E at 5°S, which spans the warm pool of the equatorial Indian and western Pacific Oceans. The one-dimensional model is able to simulate the amplitude of the observed intraseasonal SST variation throughout this domain. Variations of shortwave radiation and latent heat flux are equally important for driving the SST variations in the western Pacific, while latent heat flux variations are less important in the Indian Ocean. The phasing of the intraseasonal variation of precipitation relative to wind stress results in little impact of the freshwater flux variation on the intraseasonally varying mixed layer. The diurnal cycle of shortwave radiation is found to significantly increase the intraseasonal amplitude of SST over that produced by daily mean insolation.

Abstract

Australia typically experiences drought during El Niño, especially across the eastern two-thirds of the continent during austral spring (September–November). There have, however, been some interesting departures from this paradigm. For instance, the near-record-strength El Niño of 1997 was associated with near-normal rainfall. In contrast, eastern Australia experienced near-record drought during the modest El Niño of 2002. This stark contrast raises the issue of how the magnitude of the drought is related to the character and magnitude of El Niño, for instance as measured by the broadscale sea surface temperature (SST) anomaly in the equatorial eastern Pacific. Internal (unpredictable) atmospheric noise is one plausible explanation for this contrasting behavior during these El Niño events. Here, the authors suggest that Australian rainfall is sensitive to the zonal distribution of SST anomalies during El Niño and, in particular, the greatest sensitivity is to the SST variations on the eastern edge of the Pacific warm pool rather than in the eastern Pacific where El Niño variations are typically largest. Positive SST anomalies maximized near the date line in 2002, but in 1997 maximum anomalies were shifted well into the eastern Pacific, where their influence on Australian rainfall appears to be less. These findings provide a plausible physical basis for the view that forecasting the strength of El Niño is not sufficient to accurately predict rainfall variations across Australia during El Niño.

Abstract

Rectification of (Madden–Julian oscillation) MJO-induced wind speed and latent heat flux variations across the tropical Indian and western Pacific Oceans is estimated using 51 yr of NCEP–NCAR reanalysis. The rectified wind speed anomaly is calculated from the difference in wind speed based on 30- and 90-day low-pass-filtered winds. During periods when the MJO is active, the wind speed is typically enhanced by about 1 m s⁻¹ south of the equator in the western Pacific. The largest rectified latent heat flux occurred during the large MJO event of March 1997 in the western Pacific warm pool. The magnitude of the rectification is found to depend strongly on the mean wind speed, and this affects the temporal and spatial variations of the rectification.

Abstract

The upper-ocean heat budget in response to the atmospheric Madden–Julian oscillation (MJO) in the western equatorial Pacific is examined using a tropical Pacific basin general circulation model. The model is forced with surface fluxes associated with 10 well-defined MJO events from the period 1986–93. Surface fluxes were estimated from gridded operational analyses from the European Centre for Medium-Range Weather Forecasts and independent satellite data.

A 10-event composite of the model results was formed. The simulated composite SST agrees well with the observed composite from weekly SST analyses. Also, the simulated intraseasonal SST variation for the large MJO event during TOGA COARE (December 1992) agrees reasonably well with SST observed at a mooring. The strong equatorial jet associated with this MJO event is also well simulated.

The heat budget of the warm pool is calculated from the model output in order to investigate the role of three-dimensional processes in driving the intraseasonal SST variability. Although horizontal advection of heat is locally large, it is incoherent on the scale of MJO. It is confirmed that the intraseasonal SST variation in the western Pacific warm pool is primarily controlled by the surface heat flux variation and vertical processes.