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Abstract
The impact of air–sea coupling on the dynamics of the tropical Madden–Julian oscillation (MJO) is investigated with an atmospheric general circulation model (GCM) coupled to an ocean mixed layer model. In the uncoupled GCM, where climatological sea surface temperature (SST) is specified, realistic space–time spectra of near-equatorial zonal wind and precipitation are produced, with power concentrated at eastward wavenumbers 1–3 with periods of 35–90 days. However, the simulated MJO is roughly 50% stronger than observed, largely resulting from enormous activity during northern summer. Furthermore, during southern summer, when the observed MJO is most dominant across the Indian and western Pacific Oceans, intraseasonal variance in the uncoupled model is overly concentrated to the north and east of Australia with little activity extending into the equatorial Indian Ocean. Contrary to other recent modeling studies, coupling did not alleviate either of these problems nor did it have any other appreciable impact on the model’s MJO.
Feedback of the SST anomalies onto the MJO, both observed and diagnosed in other coupled models, appears to result from correlation of positive equatorial SST anomalies across the warm pool with surface low pressure to the east of the convective anomaly. This feedback is insignificant in the present coupled model because the SST anomalies, besides being too weak and not spatially coherent, do not systematically exhibit the requisite phasing with the surface pressure. The observed SST anomalies result from a combination of shortwave radiation and latent heat flux, whereby reduced shortwave radiation associated with enhanced convection slightly leads enhanced latent heat flux associated with increased surface westerlies. The model does produce realistic shortwave radiation anomalies, but its latent heat flux anomalies are too weak and do not constructively add with the shortwave radiation anomalies. It is concluded that coupling is not a panacea for problems of simulating the MJO in uncoupled GCMs and that coupling, if it is important, depends critically on the structure of the surface fluxes produced by the MJO.
Abstract
The impact of air–sea coupling on the dynamics of the tropical Madden–Julian oscillation (MJO) is investigated with an atmospheric general circulation model (GCM) coupled to an ocean mixed layer model. In the uncoupled GCM, where climatological sea surface temperature (SST) is specified, realistic space–time spectra of near-equatorial zonal wind and precipitation are produced, with power concentrated at eastward wavenumbers 1–3 with periods of 35–90 days. However, the simulated MJO is roughly 50% stronger than observed, largely resulting from enormous activity during northern summer. Furthermore, during southern summer, when the observed MJO is most dominant across the Indian and western Pacific Oceans, intraseasonal variance in the uncoupled model is overly concentrated to the north and east of Australia with little activity extending into the equatorial Indian Ocean. Contrary to other recent modeling studies, coupling did not alleviate either of these problems nor did it have any other appreciable impact on the model’s MJO.
Feedback of the SST anomalies onto the MJO, both observed and diagnosed in other coupled models, appears to result from correlation of positive equatorial SST anomalies across the warm pool with surface low pressure to the east of the convective anomaly. This feedback is insignificant in the present coupled model because the SST anomalies, besides being too weak and not spatially coherent, do not systematically exhibit the requisite phasing with the surface pressure. The observed SST anomalies result from a combination of shortwave radiation and latent heat flux, whereby reduced shortwave radiation associated with enhanced convection slightly leads enhanced latent heat flux associated with increased surface westerlies. The model does produce realistic shortwave radiation anomalies, but its latent heat flux anomalies are too weak and do not constructively add with the shortwave radiation anomalies. It is concluded that coupling is not a panacea for problems of simulating the MJO in uncoupled GCMs and that coupling, if it is important, depends critically on the structure of the surface fluxes produced by the MJO.
Abstract
The effects of a positive-only cumulus heating parameterization on equatorially trapped waves are investigated using both nonlinear and linear single vertical mode models. In the linear model the cumulus heating is shown to slow down the equatorial waves. In particular the eastward moving Kelvin wave is readily slowed from the free phase speed (45 m s−1) to the intrinsic speed associated with the observed 40–50 day oscillation (<10 m s−1). Slow eastward propagating Kelvin waves only occur for stable cumulus heating (stable meaning that adiabatic cooling is able to compensate for the cumulus heating). However these linear moist waves decay rapidly and are thus an unlikely explanation of the observed oscillation. The observed meridional wind perturbation is also unexplained by this mechanism. For unstable cumulus heating, growing linear CISK waves develop but remain stationary. Thus no satisfactory explanation of the 40–50 day oscillation is possible with this single vertical mode linear model.
For unstable cumulus heating in the nonlinear model, the growing CISK modes rapidly stabilize the atmosphere. The stability increases greatest to the west of the CISK heating which thus leads to eastward propagation. Upon equilibration, the CISK mode propagates eastward ≲10 m s−1 for a wide range of parameters. The mode has a substantial meridional velocity perturbation and appears to be a horizontally coupled Rossby-Kelvin wave. These propagating modes are quite similar to the observed 40–50 day oscillation. Various experiments are conducted to elucidate the mode of propagation. Experiments relevant to the observed atmosphere (i.e., moisture parameters that are a function of space and time) are also discussed.
Abstract
The effects of a positive-only cumulus heating parameterization on equatorially trapped waves are investigated using both nonlinear and linear single vertical mode models. In the linear model the cumulus heating is shown to slow down the equatorial waves. In particular the eastward moving Kelvin wave is readily slowed from the free phase speed (45 m s−1) to the intrinsic speed associated with the observed 40–50 day oscillation (<10 m s−1). Slow eastward propagating Kelvin waves only occur for stable cumulus heating (stable meaning that adiabatic cooling is able to compensate for the cumulus heating). However these linear moist waves decay rapidly and are thus an unlikely explanation of the observed oscillation. The observed meridional wind perturbation is also unexplained by this mechanism. For unstable cumulus heating, growing linear CISK waves develop but remain stationary. Thus no satisfactory explanation of the 40–50 day oscillation is possible with this single vertical mode linear model.
For unstable cumulus heating in the nonlinear model, the growing CISK modes rapidly stabilize the atmosphere. The stability increases greatest to the west of the CISK heating which thus leads to eastward propagation. Upon equilibration, the CISK mode propagates eastward ≲10 m s−1 for a wide range of parameters. The mode has a substantial meridional velocity perturbation and appears to be a horizontally coupled Rossby-Kelvin wave. These propagating modes are quite similar to the observed 40–50 day oscillation. Various experiments are conducted to elucidate the mode of propagation. Experiments relevant to the observed atmosphere (i.e., moisture parameters that are a function of space and time) are also discussed.
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Abstract
The variability and time-mean response to orographic forcing are examined in a nonlinear atmospheric model. Distinct signatures from both high-frequency (synoptic-scale) and low-frequency (periods greater than 10 days) transients are seen in the temporal variance and eddy fluxes. Downstream of the orography, in the region of the time-mean jet stream, high-frequency transients are organized into a storm track and exhibit baroclinic energy conversions. The low-frequency transients, while producing, greater variability in the same region as the storm track, exhibit significantly less baroclinic energy generation. The structure of the low-frequency transients downstream of the orography is similar to the observed PNA pattern. The time scale of the eddies in this region appears to be longer than typical time scales associated with stationary Rossby wave dispersion. These eddies exhibit large local barotropic conversion of mean kinetic energy due to the large zonal gradient of the mean zonal wind. These barotropic processes downstream of the mountain give the appearance of low-frequency waves propagating out of the tropics even though there is no low latitude forcing in this model.
Midlatitude orography is shown to influence the tropical time-mean circulation; a weak easterly jet along the equator develops due south of the orography. The influence on the tropical variability is restricted to increased high-frequency variance with limited effects on the low-frequency transients.
Abstract
The variability and time-mean response to orographic forcing are examined in a nonlinear atmospheric model. Distinct signatures from both high-frequency (synoptic-scale) and low-frequency (periods greater than 10 days) transients are seen in the temporal variance and eddy fluxes. Downstream of the orography, in the region of the time-mean jet stream, high-frequency transients are organized into a storm track and exhibit baroclinic energy conversions. The low-frequency transients, while producing, greater variability in the same region as the storm track, exhibit significantly less baroclinic energy generation. The structure of the low-frequency transients downstream of the orography is similar to the observed PNA pattern. The time scale of the eddies in this region appears to be longer than typical time scales associated with stationary Rossby wave dispersion. These eddies exhibit large local barotropic conversion of mean kinetic energy due to the large zonal gradient of the mean zonal wind. These barotropic processes downstream of the mountain give the appearance of low-frequency waves propagating out of the tropics even though there is no low latitude forcing in this model.
Midlatitude orography is shown to influence the tropical time-mean circulation; a weak easterly jet along the equator develops due south of the orography. The influence on the tropical variability is restricted to increased high-frequency variance with limited effects on the low-frequency transients.
Abstract
The response to steady tropical diabatic forcing in a nonlinear model of the atmosphere is analyzed. For sufficiently small diabatic heating, upper tropospheric anticyclones develop along the equator to the west of the heating, as predicted by linear theory. For sufficiently large diabatic heating, the anticyclones shift eastward to the same longitude as the heating (nonlinear response). The structure of the nonlinear response agrees more favorably with the observations than does the linear response. Inclusion of strong tropospheric dissipation causes the weaker diabatic forcing to produce the structural characteristics of the nonlinear response. Analysis of the time-mean vorticity budget reveals that the relative magnitude of the steady nonlinear flux divergence is substantially larger for the strong forcing as compared to the weak forcing. This appears to be the mechanism responsible for the marked difference between the two responses. Significant differences in the extratropical response exist for the two cases.
The tropical variability in the presence of the time-mean asymmetries in the basic state is examined. Strong maxima and minima in the transient kinetic energy are observed in the regions of equatorial westerlies and easterlies, respectively. The penetration into the regions of reduced easterlies by equatorward propagating extratropical waves is shown to be the major cause of the asymmetry in the variance. Most incident midlatitude waves are seen to be absorbed at their low-latitude critical line. Because the spectrum of atmospheric waves produced in midlatitudes includes some westward moving waves, the existence of equatorial westerlies is not required for the asymmetry to occur.
Abstract
The response to steady tropical diabatic forcing in a nonlinear model of the atmosphere is analyzed. For sufficiently small diabatic heating, upper tropospheric anticyclones develop along the equator to the west of the heating, as predicted by linear theory. For sufficiently large diabatic heating, the anticyclones shift eastward to the same longitude as the heating (nonlinear response). The structure of the nonlinear response agrees more favorably with the observations than does the linear response. Inclusion of strong tropospheric dissipation causes the weaker diabatic forcing to produce the structural characteristics of the nonlinear response. Analysis of the time-mean vorticity budget reveals that the relative magnitude of the steady nonlinear flux divergence is substantially larger for the strong forcing as compared to the weak forcing. This appears to be the mechanism responsible for the marked difference between the two responses. Significant differences in the extratropical response exist for the two cases.
The tropical variability in the presence of the time-mean asymmetries in the basic state is examined. Strong maxima and minima in the transient kinetic energy are observed in the regions of equatorial westerlies and easterlies, respectively. The penetration into the regions of reduced easterlies by equatorward propagating extratropical waves is shown to be the major cause of the asymmetry in the variance. Most incident midlatitude waves are seen to be absorbed at their low-latitude critical line. Because the spectrum of atmospheric waves produced in midlatitudes includes some westward moving waves, the existence of equatorial westerlies is not required for the asymmetry to occur.
Abstract
The previously reported spectral peak near 50 days in time series of length of day (LOD) is shown to occur in conjunction with episodes of tropical convective activity associated with the Madden-Julian oscillation (MJO). When the convective signal of the MJO is absent, LOD exhibits a red spectrum at intraseasonal time-scales. LOD is shown to be in phase with the convective anomaly due to the MJO over the date line and out of phase with the convective anomaly over the Indian Ocean. A composite angular momentum budget, made relative to the convective signal of the MJO, reveals that the zonal surface stress only partially accounts for the observed tendency of LOD. Not only is the amplitude some 50% too weak, the phase is shifted ahead of the LOD tendency by about 1/8 cycle. Hence, in order to balance the angular momentum budget, an additional mountain torque is postulated to occur. This additional torque is required to lag the frictional torque by about 1/4 of a cycle, but be of similar amplitude.
The composite surface stress anomalies appear to result predominantly from zonal mean zonal wind anomalies. An important role for the zonally symmetric convective anomaly due to the MJO is suggested. The surface zonal wind anomalies at low latitudes, which exhibit a high degree of equatorial symmetry with zero amplitude on the equator, appear to be accounted for as the linear response to zonal mean convective heating in the presence of strong dissipation. The upper-tropospheric zonal wind anomalies, which mimic the angular momentum anomalies, are not accounted for by simple linear momentum balance. In particular, maximum zonal wind anomaly occurs on the equator, which suggests an important role for eddy fluxes of momentum during the life cycle of the MJO.
Abstract
The previously reported spectral peak near 50 days in time series of length of day (LOD) is shown to occur in conjunction with episodes of tropical convective activity associated with the Madden-Julian oscillation (MJO). When the convective signal of the MJO is absent, LOD exhibits a red spectrum at intraseasonal time-scales. LOD is shown to be in phase with the convective anomaly due to the MJO over the date line and out of phase with the convective anomaly over the Indian Ocean. A composite angular momentum budget, made relative to the convective signal of the MJO, reveals that the zonal surface stress only partially accounts for the observed tendency of LOD. Not only is the amplitude some 50% too weak, the phase is shifted ahead of the LOD tendency by about 1/8 cycle. Hence, in order to balance the angular momentum budget, an additional mountain torque is postulated to occur. This additional torque is required to lag the frictional torque by about 1/4 of a cycle, but be of similar amplitude.
The composite surface stress anomalies appear to result predominantly from zonal mean zonal wind anomalies. An important role for the zonally symmetric convective anomaly due to the MJO is suggested. The surface zonal wind anomalies at low latitudes, which exhibit a high degree of equatorial symmetry with zero amplitude on the equator, appear to be accounted for as the linear response to zonal mean convective heating in the presence of strong dissipation. The upper-tropospheric zonal wind anomalies, which mimic the angular momentum anomalies, are not accounted for by simple linear momentum balance. In particular, maximum zonal wind anomaly occurs on the equator, which suggests an important role for eddy fluxes of momentum during the life cycle of the MJO.
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 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
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
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
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.
