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  • Author or Editor: M. C. Wu x
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M. C. Wu
,
W. L. Chang
, and
W. M. Leung

Abstract

The impact of El Niño–Southern Oscillation (ENSO) episodes on the variability in the landfalling pattern of tropical cyclones in the western North Pacific is studied using the bootstrap technique.

It is found that, relative to neutral years, in the months September, October, and November or the late season of El Niño years the number of tropical cyclones landfalling in the landmasses rimming the western North Pacific is significantly reduced. The exception is Japan and the Korean Peninsula. On the other hand, in the late season of La Niña years, China can expect significantly more landfalls. The predictability of the number of landfalling tropical cyclones in the western North Pacific is found to be the highest for China in the late season of La Niña years.

The reduction in the number of landfalls during the late season of El Niño years seems to be related to an eastward shift in the mean tropical cyclone genesis position and a break in the 500-hPa subtropical ridge near 130°E. In contrast, the increase in the number of landfalls during the late season of La Niña years appears to be related to a westward shift in the mean genesis position together with a contiguous 500-hPa subtropical ridge.

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K. M. Lau
,
H. T. Wu
,
Y. C. Sud
, and
G. K. Walker

Abstract

The sensitivity of tropical atmospheric hydrologic processes to cloud microphysics is investigated using the NASA Goddard Earth Observing System (GEOS) general circulation model (GCM). Results show that a faster autoconversion rate leads to (a) enhanced deep convection in the climatological convective zones anchored to tropical land regions; (b) more warm rain, but less cloud over oceanic regions; and (c) an increased convective-to-stratiform rain ratio over the entire Tropics. Fewer clouds enhance longwave cooling and reduce shortwave heating in the upper troposphere, while more warm rain produces more condensation heating in the lower troposphere. This vertical differential heating destabilizes the tropical atmosphere, producing a positive feedback resulting in more rain and an enhanced atmospheric water cycle over the Tropics. The feedback is maintained via secondary circulations between convective tower and anvil regions (cold rain), and adjacent middle-to-low cloud (warm rain) regions. The lower cell is capped by horizontal divergence and maximum cloud detrainment near the freezing–melting (0°C) level, with rising motion (relative to the vertical mean) in the warm rain region connected to sinking motion in the cold rain region. The upper cell is found above the 0°C level, with induced subsidence in the warm rain and dry regions, coupled to forced ascent in the deep convection region.

It is that warm rain plays an important role in regulating the time scales of convective cycles, and in altering the tropical large-scale circulation through radiative–dynamic interactions. Reduced cloud–radiation feedback due to a faster autoconversion rate results in intermittent but more energetic eastward propagating Madden–Julian oscillations (MJOs). Conversely, a slower autoconversion rate, with increased cloud radiation produces MJOs with more realistic westward-propagating transients embedded in eastward-propagating supercloud clusters. The implications of the present results on climate change and water cycle dynamics research are discussed.

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J. A. Maslanik
,
A. H. Lynch
,
M. C. Serreze
, and
W. Wu

Abstract

Simulations of Arctic climate require treatment of land, ocean, ice, and atmospheric processes, and are further complicated by the dynamic nature of the sea ice cover. Here, the ability of a climate system model to simulate conditions over the Arctic Ocean during April–September 1990, a period of anomalous atmospheric circulation and sea ice conditions, is investigated. Differences between observations and model results are used to gain insight into the mechanisms that contributed to the observed record reduction in ice extent in late summer. The coupled model reproduces the general patterns seen in comparison sea level pressure fields in most months, but the discrepancies significantly affect the model’s ability to simulate details of sea ice transport and warm air advection linked to the unusual ice conditions. The use of prescribed sea ice fraction in the climate model yields relatively small changes in the surface energy balance compared to the fully-coupled simulation with dynamic ice cover, but significantly affects atmospheric circulation in spring and late summer. Analyses of observations, coupled model experiments, and stand-alone ice model output suggest a positive feedback between ice dynamics and ice melt that contributed to the ice extent anomaly. The results highlight the importance of regional atmospheric circulation in driving interannual variations in Arctic ice extent, and illustrate the level of model performance needed to simulate such variations.

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