Statistical features of the relationship among westerly wind bursts (WWBs), the El Niño-Southern Oscillation (ENSO), and intraseasonal variations (ISVs) were examined using 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis data (ERA-40) for the period of January 1979-August 2002. WWBs were detected over the Indian Ocean and the Pacific Ocean, but not over the Atlantic Ocean. WWB frequencies for each region were lag correlated with a sea surface temperature anomaly over the Niño-3 region. WWBs tended to occur in sequence, from the western to eastern Pacific, leading the El Niño peak by 9 months to 1 month, respectively, and after around 11 months, over the Indian Ocean. These results suggest that WWB occurrences are not random, but interactive with ENSO. Composite analysis revealed that most WWBs were associated with slowdowns of eastward-propagating convective regions like the Madden-Julian oscillation (MJO), with the intensified Rossby wave response. However, seasonal and interannual variations in MJO amplitude were not correlated with WWB frequency, while a strong MJO event tended to bear WWBs. It is suggested that the strong MJO amplitude promotes favorable conditions, but it is not the only factor influencing WWB frequency. An environment common to WWB generation in all regions was the existence of background westerlies around the WWB center near the equator. It is inferred that ENSO prepares a favorable environment for the structural transformation of an MJO, that is, the intensified Rossby wave response, that results in WWB generations. The role of the background wind fields on WWB generations will be discussed in a companion paper from the perspective of energetics.

No abstract

The mechanism of synoptic-scale eddy development in the generation of westerly wind bursts (WWBs) over the western–central Pacific, and their relationship with the El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO), were examined. In the WWB occurrences, barotropic structures of equatorial eddy westerlies with cyclonic disturbances were found from the surface to the upper troposphere. The dominant contributions to substantial eddy kinetic energy (EKE) were the barotropic energy conversion (KmKe) in the lower and middle tropospheres and the conversion from eddy available potential energy (PeKe) in the upper troposphere. Low-frequency environmental westerlies centered near the equator preceded strong zonal convergence and meridional shear, resulting in the substantial KmKe. The activation of synoptic convection also contributed to an increase in EKE through PeKe. These energies were redistributed to the lower-equatorial troposphere through energy flux convergence (GKe). These results showed that environmental fields contribute to the EKE increase near the equator and are important factors in WWB occurrences. Next, eddy growth was compared under different phases of MJO and ENSO. The MJO westerly phases of strong MJO events were classified into two groups, in terms of ENSO phases. Higher EKE values were found over the equatorial central Pacific in the WWB–ENSO correlated (pre–El Niño) periods. The energetics during these periods comported with those of the WWB generations. In the uncorrelated periods, the enhancement of eddy disturbances occurred far from the equator near the Philippines, where the activities of the easterly wave disturbances are well known. It is noteworthy that the enhanced region of the disturbances in the pre–El Niño periods coincided with the vicinity of large-scale MJO convection. It is suggested that coincidence corresponds with an enhancement of the internal disturbances embedded in the MJO, which is found only when the environmental conditions are favorable in association with ENSO.

The future changes in the Madden-Julian Oscillation (MJO) and its extratropical teleconnection in East Asia during the boreal winter are examined by analyzing 12 climate models with good simulation skills of the MJO convective signal in the Coupled Model Intercomparison Project phase 3 (CMIP3). The MJO convection increases over the western to central Indian Ocean (IO) under the warming climate in seven models (MJO-plus models). However, it decreases in the other five models (MJOminus models). The wintertime sea surface temperature (SST) exhibits more El Niño-like warming in the MJO-plus models. Furthermore, the predicted increase of the MJO amplitude over the IO significantly correlates with that of the amplitude in the El Niño-Southern Oscillation. In East Asia, the convective activity increases (decreases) in the MJO-plus (MJO-minus) models associated with the enhancement (weakening) of the MJO over the western to central IO. The MJO-plus models predict the intensified extratropical circulations as a wave train along the subtropical jet, and increased low-level warm-moist air transport into East Asia related to the intensified Kelvin-Rossby response. Such changes are not detected in the MJO-minus models. It is suggested that proper simulations of the change in the MJO and ENSO are required in order to predict the wintertime climate change in East Asia.

The simulated Madden-Julian oscillation (MJO) in the climate of the 20th Century (20C3M) experiment of 23 models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) is examined. The models having moisture-convergence-type convection schemes well simulate the MJO signal in precipitation. By analyzing the data from these models, we confirm that the MJO convection is active from the Indian Ocean to the western Pacific, consistent with the observations. However, analyses of the structure of the simulated MJO reveal that the convergence of the surface wind tends to be overestimated in the models. In addition, the peak of the MJO signal over the Indian Ocean is relatively weaker than that over the maritime continent and the western Pacific, and shifts westward in the models. These inconsistencies appear to arise from the distribution of sea-surface temperature (SST) bias. As a general tendency in all of the climate models studied, it is also demonstrated that the skill of the MJO simulation correlates positively with that of the climatological SST. Potential importance of the basic field and the air-sea coupling in the MJO simulation is suggested.

[1] Equatorial westerly wind bursts (WWBs) and their relationship with El Niño-Southern Oscillation (ENSO) in the 18 climate models presented in the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3) are examined. Some models depict a realistic eastward shift of collective occurrences of WWBs over the Pacific as the warm pool expands eastward. These models that depict the frequent western Pacific WWBs preceding El Niño peak, known to trigger or enhance El Niño, tend to reproduce westerly background states and ENSO more accurately. Thus the reproducibility of the westerly background states is suggested to be fundamental for WWB occurrences as well as the following El Niño. Although WWBs generate with active convection in most of the models as observations, various kinds of intraseasonal disturbances that cause the active convection are found. It is suggested that organized convection is essential for the WWB generation but is prepared by each model's own dominant mode in the tropics. Under global warming, WWBs tend to increase over the eastern Pacific and decrease over the Indian Ocean whereas the total number of WWBs does not change consistently. This might arise from an increase of short-period convective disturbances over the eastern Pacific due to a sea surface temperature increase. Although there is a weak relationship between changes in the ENSO amplitude and the eastern Pacific WWBs in general, good models reproducing the WWB-ENSO relationship in the current climate tend to show consistent changes, suggesting the possibility of the eastern Pacific WWBs to intensify ENSO.

The cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011) was conducted to capture atmospheric and oceanic characteristics of the Madden‐Julian Oscillation (MJO) in the central Indian Ocean from late 2011 to early 2012. During CINDY2011, the research vessel (R/V) MIRAI stayed at 8°S, 80.5°E for two months during the special observing period (SOP). Intraseasonal convection associated with the MJO was organized in the central Indian Ocean in late October and late November during the SOP. In the middle of November, both sea surface temperature (SST) and mixed layer temperature decreased suddenly when cold low salinity water intruded into the upper layer around the R/V MIRAI. This intrusion was accompanied by a surface current change from southwestward to westward/west‐northwestward associated with the passage of the annual oceanic downwelling Rossby wave. The mixed layer heat budget analysis shows that horizontal advection plays an important role in the abrupt cooling whereas the net surface heat flux cannot account for the cooling. This is an interesting result because the associated downwelling Rossby wave is usually considered to increase SST through a reduction of entrainment cooling. In addition, for the second MJO event convection was activated around 20 November over the central north and equatorial Indian Ocean but not in the south. It is suggested that the cooler surface waters (as seen at the location of the R/V MIRAI) tended to suppress the initial atmospheric convection, resulting in the lagged convective onset in the end of November over the central south Indian Ocean.

Oceanic responses to relatively strong MaddenJulian Oscillations (MJOs) and background winds controlled by El Ni no-Southern Oscillation (ENSO) are examined. The MJO's arrival excites dominant downwelling and upwelling Kelvin waves during El Ni no developing (pre-El Ni no: PEN) and other (non-PEN) phases, respectively. These opposite signals come from background wind directions under different ENSO phases and exert opposite impacts on SST. In addition, MJO convection itself develops accompanied by larger surface wind variations during PEN phases, which can be related to the interactive amplifications of synopticand planetary-scale disturbances when westerly wind bursts occur. Consequently, the strength of westerly forcing and its oceanic response during PEN phases are larger than that of the corresponding easterly forcing and its response during non-PEN phases. These results suggest that modulations of MJO amplitude and structure under the background westerly and easterly winds associated with ENSO phases exert opposite but asymmetric impacts on the ocean.