[1] The water-vapor profiles obtained from the Atmospheric Infrared Sounder (AIRS) on Satellite Aqua are used to document the 3-D moisture structure of the boreal-summer Tropical Intraseasonal Oscillation (TISO) over the Indo-western Pacific region. With the support of several other Satellite data sets, the interactions between the TISO and underlying ocean are also investigated. The AIRS data reveal much larger tropospheric moisture perturbations than those depicted in previous reanalysis and analysis data sets. It also reveals a drying Atmospheric Boundary Layer (ABL) below the TISO convection probably due to the associated downdrafts. Before the occurrence of TISO convection, both positive Sea Surface Temperature (SST) anomaly and ABL moistening are observed. Further analysis suggests that the positive SST anomaly is the major contributor to the ABL moistening through enhancing surface evaporation. Therefore, the intraseasonal SST anomaly could positively feed back to the atmosphere through moistening the boundary layer, destabilizing the troposphere, and contributing to the northeastward propagation of the TISO. Citation: Fu, X., B. Wang, and L. Tao (2006), Satellite data reveal the 3-D moisture structure of Tropical Intraseasonal Oscillation and its coupling with underlying ocean, Geophys. Res. Lett., 33, L03705,
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°S-30°N, 60°-160°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.
In this study, we have investigated contributions of tropical Indian Ocean (IO) sea surface temperature (SST) warming and El Niño-Southern Oscillation (ENSO) to the interannual variability of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) between 1948 and 2010 and the involved physical mechanisms. Both ENSO and tropical IO Basin Mode (IOBM) warming are found to play important roles in modulating the WNP TC genesis frequency, but their effects are significantly different.The time series of seasonal empirical orthogonal function of tropical IO and tropical Pacific SST with trend and multidecadal variability removed are defined as the IOBM index and ENSO index, respectively. The results show that the IO warming year is usually the El Niño decaying year. The number of total TCs, especially weak TCs, decreases during the tropical IO warming year when an anomalous anticyclonic circulation is observed over the tropical Northwest Pacific off the equator. On the other hand, the number of intense TCs increases in the El Niño developing year because of the eastward shifts of both the western Pacific monsoon trough and the cyclonic shear of the equatorial westerlies, and thus the eastward shift of the main TC genesis region. It is also found that the relationship between ENSO and the frequency of intense TCs has a decadal variation. During 1968During -1987, the number of intense TCs was not related to ENSO with a correlation coefficient of only 0.19, while the correlation coefficient is 0. 63 and 0.73 during 1948-1967 and 1988-2007, respectively. The extent to which the WNP anomalous anticyclone is forced by IO warming is investigated by using the global climate model (European Centre Hamburg Model, ECHAM) with imposed SST anomalies over the tropical IO. The results suggest that the WNP anomalous anticyclone that develops from June to September results mainly from the northward shift of the tropical IO warming from boreal spring to boreal summer.
A recent finding is the significant impact of the sea surface temperature anomaly (SSTA) over the east Indian Ocean (EIO) on the genesis frequency of tropical cyclones (TCs) over the western North Pacific (WNP). In this study it is shown that such an impact is significant only after the late 1970s. The results based on both data analysis and numerical model experiments demonstrate that prior to the late 1970s the EIO SSTA is positively correlated with the equatorial central Pacific SSTA and the latter produces an opposite atmospheric circulation response over the WNP to the former. As a result, the impact of the EIO SSTA on the TC genesis over the WNP is largely suppressed by the latter. After the late 1970s, the area coverage of the EIO SSTA is expanding. This considerably enhances the large-scale circulation response over the WNP to the EIO SSTA and significantly intensifies the impact of the EIO SSTA on TC genesis frequency over the WNP. The results from this study have great implications for seasonal prediction of TC activity over the WNP.
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