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The impact of air--sea coupling on the predictability of monsoon intraseasonal oscillations (MISO) has been investigated with an atmosphere--ocean coupled model and its atmospheric component. From a 15-yr coupled control run, 20 MISO events are selected. A series of twin perturbation experiments have been conducted for all the selected events using both the coupled model and the atmosphere-only model. Two complementary measures are used to quantify the MISO predictability: (i) the ratio of signal-to-forecast error and (ii) the spatial anomaly correlation coefficient (ACC). In the coupled model, the MISO predictability is generally higher over the Indian sector than that over the western Pacific with a maximum of 35 days in the eastern equatorial Indian Ocean. Air--sea coupling significantly improves the predictability in almost the entire Asian-western Pacific region. The mean predictability of the MISO-related rainfall over its active area (10[degrees]S-30[degrees]N, 60[degrees]-160[degrees]E) reaches about 24 days in the coupled model and is about 17 days in the atmosphere-only model. This result suggests that including an interactive ocean allows the MISO predictability of an atmosphere-only model to be extended by about a week. The extended predictability is primarily due to the coupled model capturing the two-way interactions between the MISO and underlying sea surface. The MISO forces a coherent intraseasonal SST response in underlying ocean that in return exerts an external control on the future evolutions of the MISO. The break phase of the MISO is more predictable than the active phase in both the atmosphere-only model and the coupled model as revealed in the observations. Air--sea coupling appears to extend the MISO predictability uniformly regardless of the active or break phases.

The atmospheric moisture and temperature profiles from the Atmospheric Infrared Sounder (AIRS)/Advanced Microwave Sounding Unit on the NASA Aqua mission, in combination with the precipitation from the Tropical Rainfall Measuring Mission (TRMM), are employed to study the vertical moist thermodynamic structure and spatial--temporal evolution of the Madden--Julian oscillation (MJO). The AIRS data indicate that, in the Indian Ocean and western Pacific, the temperature anomaly exhibits a trimodal vertical structure: a warm (cold) anomaly in the free troposphere (800-250 hPa) and a cold (warm) anomaly near the tropopause (above 250 hPa) and in the lower troposphere (below 800 hPa) associated with enhanced (suppressed) convection. The AIRS moisture anomaly also shows markedly different vertical structures as a function of longitude and the strength of convection anomaly. Most significantly, the AIRS data demonstrate that, over the Indian Ocean and western Pacific, the enhanced (suppressed) convection is generally preceded in both time and space by a low-level warm and moist (cold and dry) anomaly and followed by a low-level cold and dry (warm and moist) anomaly. The MJO vertical moist thermodynamic structure from the AIRS data is in general agreement, particularly in the free troposphere, with previous studies based on global reanalysis and limited radiosonde data. However, major differences in the lower-troposphere moisture and temperature structure between the AIRS observations and the NCEP reanalysis are found over the Indian and Pacific Oceans, where there are very few conventional data to constrain the reanalysis. Specifically, the anomalous lower-troposphere temperature structure is much less well defined in NCEP than in AIRS for the western Pacific, and even has the opposite sign anomalies compared to AIRS relative to the wet/dry phase of the MJO in the Indian Ocean. Moreover, there are well-defined eastward-tilting variations of moisture with height in AIRS over the central and eastern Pacific that are less well defined, and in some cases absent, in NCEP. In addition, the correlation between MJO-related midtropospheric water vapor anomalies and TRMM precipitation anomalies is considerably more robust in AIRS than in NCEP, especially over the Indian Ocean. Overall, the AIRS results are quite consistent with those predicted by the frictional Kelvin--Rossby wave/conditional instability of the second kind (CISK) theory for the MJO.

Our ability to predict active and break periods of the Asian summer monsoon is intimately tied to our ability to predict the intraseasonal oscillation (ISO). The present study analyzes the upper limit of potential predictability of the northern summer ISO, as it is simulated by the ECHAM5 atmospheric general circulation model forced with climatological SSTs. The leading extended empirical orthogonal functions of precipitation, computed from a 10-yr control simulation, are used to define four different phases of the ISO. Fourteen-member ensembles of 90-day hindcasts are run for each phase of the three strongest ISO events identified in the 10-yr control run. Initial conditions for each ensemble are created from the control simulation using a breeding method. The signal-to-noise ratio is analyzed over a region that covers the core of the Asian summer monsoon activity. Over Southeast Asia, the upper limit for predictability of precipitation and 200-hPa zonal wind is about 27 and 33 days, respectively. Over India, values of more than 15 days occur for both variables. A spatial analysis of the different phases of the ISO reveals that the predictability follows the eastward- and northward-propagating ISO during the active and break phases of the monsoon. Precipitation reveals increased predictability at the end of the convective phase. Analogous, 200-hPa zonal wind shows strongest predictability during low and easterly anomalies. This potential predictability is considerably higher than for numerical forecasts of typical weather variations, particularly for the Tropics, indicating that useful forecasts of monsoon active and break events may be possible with lead times of more than two weeks for precipitation and the dynamics. A closer look at the breeding method used here to initialize the hindcasts shows the importance of appropriate ensemble experiment designs.

The latitude preference of the intertropical convergence zone (ITCZ) is examined on the basis of observations, theory, and a modeling analysis. Observations show that convection is enhanced at latitudes of about 4 degrees to 10 degrees relative to the equator, even in regions where the sea surface temperature (SST) is maximum on the equator. Both linear shallow-water theory and a moist primitive equation model suggest a new explanation for the off-equatorial latitude preference of the ITCZ that requires neither the existence of zonally propagating disturbances nor an off-equatorial maximum in SST. The shallow-water theory indicates that a finite-width, zonally oriented, midtropospheric heat source (i.e., an ITCZ) produces the greatest local low-level convergence when placed a finite distance away from the equator. This result suggests that an ITCZ is most likely to be supported via low-level convergence of moist energy when located at these 'preferred' latitudes away from the equator. For a plausible range of heating widths and damping parameters, the theoretically predicted latitude is approximately equal to the observed position(s) of the ITCZ(s). Analysis with an axially symmetric, moist, primitive equation model indicates that when the latent heating field is allowed to be determined internally, a positive feedback develops between the midtropospheric latent heating and the low-level convergence, with the effect of enhancing the organization of convection at latitudes of about 4 degrees to 12 degrees. Numerical experiments show that 1) two peaks in convective precipitation develop straddling the equator when the SST maximum is located on the equator; 2) steady ITCZ-like structures form only when the SST maximum is located away from the equator; and 3) peaks in convection can develop away from the maximum in SST, with a particular preference for latitudes of about 4 degrees to 12 degrees, even in the ('cold') hemisphere without the SST maximum. The relationship between this mechanism and earlier theories is discussed, as are implications for the coupled ocean-atmosphere system and the roles played by midlevel latent heating and SST gradients in forcing the low-level atmospheric circulation in the tropics.

The objective of this study is to examine the relationship between interannual SST variability and the activity and predictability of tropical intraseasonal variability (TISV). A 10-yr simulation forced by climatological SSTs and a 10-member 10-yr (1979-88) ensemble of simulations forced by observed SSTs from the NASA Goddard Laboratory for the Atmospheres GCM coupled to a weakly interacting slab ocean mixed layer are analyzed. The climatological simulation provides a measure of the natural variability associated with TISV, while the observed SST simulations provide an indication of how externally imposed SST anomalies modify this variability. Analysis is conducted on both the eastward-propagating (winter mode) and northeastward-propagating (summer mode) forms of TISV. Indices of TISV are constructed from the amplitude time series of the leading EOFs of intraseasonally bandpassed model precipitation and 850-hPa zonal wind. Analysis of the TISV activity indices from the climatological SST simulation shows that considerable natural variability exists for both the winter and summer TISV modes. Based on the TISV indices constructed, the internal variability of TISV is about 50% of the mean signal. Moreover, the interannual standard deviation in TISV activity from the climatological SST simulation ranges from about the same size to about twice the standard deviation associated with the seasonal evolution of TISV activity. A ratio between an estimate of the externally forced TISV variability in the observed SST simulations to an estimate of natural TISV variability from the climatological SST simulations is constructed to give a measure of TISV predictability. While these predictability values exhibit some seasonal dependence, on average they suggest little to no predictability associated with interannual TISV variations for either the winter or summer TISV mode. However, the ensemble does demonstrate significantly enhanced predictability of the summer TISV mode during the 1982/83 winter. This same characteristic was found during the 1997/98 winter in a second 10-member ensemble that was conducted for the period September 1996 to August 1998. In each of these cases, the ensemble means exhibited a decrease in TISV activity, a feature that is also echoed in the observations. Thus, under very specific circumstances, anomalous SST may have some predictable influence over the level of TISV activity, at least for the summer mode. In addition, the simulations show evidence that generalized intraseasonal variance (i.e., no constraint on spatial structure) displays some predictable characteristics over the central and eastern equatorial Pacific in association with ENSO-related SST anomalies. Considerations of the use of the ocean mixed layer coupling in the context of the above study are discussed as well as the agreement between the observed levels of TISV activity for the periods modeled and the levels simulated by the GCM ensemble.

In this study, the authors compare the Madden-Julian oscillation (MJO) variability in the Goddard Laboratory for Atmospheres atmospheric general circulation model for two different sea surface temperature (SST) boundary conditions. In the 'control' simulation, the model employs specified annual cycle SSTs. In the 'coupled' simulation, the model employs the same annual cycle SSTs but in addition is coupled to a slab ocean mixed layer that provides prognostic SST anomalies equatorward of 24 [degrees]. The results show that the simplified interactive SST facilitates a better simulation with respect to a number of general model shortcomings associated with the MJO that were recently documented by Slingo et al. in an Atmospheric Model Intercomparison Project study. These improvements include 1) increased variability associated with the MJO, 2) a tendency for the timescales of the modeled intraseasonal variability to more closely match and consolidate around the timescales found in the observations, 3) a reduced eastward phase speed in the Eastern Hemisphere, and 4) an increased seasonal signature in the MJO with relatively more events occurring in the December-May period. The above changes are associated with a systematic change in SST, with warming of about 0.10 [degree]-0.15 [degree] C before the passage of the MJO convection center and cooling of the same magnitude after its passage, both of which appear to be roughly consistent with observations. These changes in SST are primarily due to decreased latent heat flux ([approximately]25 W [m.sup.-2]) and slightly enhanced surface shortwave flux ([approximately]10 W [m.sup.-2]) to the east of the convection, enhanced latent heat flux and diminished shortwave flux ([approximately]25 W [m.sup.-2]) coincident with the convection, and enhanced latent heat flux ([approximately]25 W [m.sup.-2]) just west of the convection. The results indicate that the enhanced SST to the east of the convection reinforces the meridional convergence associated with the frictional wave-CISK (conditional instability of the second kind) mechanism that appears to be at work within the model. This enhanced meridional convergence transports more low-level moisture into the region lying just east of convection. The resulting increase in moist static energy helps destabilize the disturbance and/or maintain it against dissipation more effectively relative to the case without SST coupling. The above results are discussed in terms of their relation to current MJO theory, as well as to their implications for medium- to extended-range weather forecasting.

Examines the relationship between interannual sea surface temperature (SST) variability and the activity of tropical intraseasonal variability. Effects of the imposed SST anomalies on the variability; Role of the observed SST simulations in the anomalies.