Xi Cao

Effects of Monsoon Trough Intraseasonal Oscillation on Tropical Cyclogenesis over the Western North Pacific

AbstractThe effects of intraseasonal oscillation (ISO) of the western North Pacific (WNP) monsoon trough on tropical cyclone (TC) formation were investigated using the Advanced Research Weather Research and Forecasting (ARW) Model. A weak vortex was specified initially and inserted into the background fields containing climatological-mean anomalies associated with active and inactive phases of monsoon trough ISOs.The diagnosis of simulations showed that monsoon trough ISO can modulate TC development through both dynamic and thermodynamic processes. The dynamic impact is attributed to the lower–midtropospheric large-scale vorticity associated with monsoon trough ISO. Interactions between cyclonic vorticity in the lower middle troposphere during the active ISO phase and a vortex lead to the generation of vortex-scale outflow at the midlevel, which promotes the upward penetration of friction-induced ascending motion and thus upward moisture transport. In addition, the low-level convergence associated with ac...

Modulation of Western North Pacific Tropical Cyclone Genesis by Intraseasonal Oscillation of the ITCZ: A Statistical Analysis

The present study investigates modulation of western North Pacific (WNP) tropical cyclone (TC) genesis in relation to different phases of the intraseasonal oscillation (ISO) of ITCZ convection during May to October in the period 1979–2008. The phases of the ITCZ ISO were determined based on 30–80-day filtered OLR anomalies averaged over the region (5°–20°N, 120°–150°E). The number of TCs during the active phases was nearly three times more than during the inactive phases. The active (inactive) phases of ISO were characterized by low-level cyclonic (anticyclonic) circulation anomalies, higher (lower) midlevel relative humidity anomalies, and larger (smaller) vertical gradient anomalies of relative vorticity associated with enhanced (weakened) ITCZ convection anomalies. During the active phases, TCs tended to form in the center of the ITCZ region.Barotropic conversion from the low-level mean flow is suggested to be the major energy source for TC formation. The energy conversion mainly depended on the zonal and meridional gradients of the zonal flow during the active phases. However, barotropic conversion weakened greatly during the inactive phases. The relationship between the meridional gradient of absolute vorticity and low-level zonal flow indicates that the sign of the absolute vorticity gradient tends to be reversed during the two phases, whereas the same sign between zonal flow and the absolute vorticity gradient is more easily satisfied in the active phases. Thus, the barotropic instability of low-level zonal flow might be an important mechanism for TC formation over the WNP during the active phases of ISO.

Intensified impact of northern tropical Atlantic SST on tropical cyclogenesis frequency over the western North Pacific after the late 1980s

Previous studies suggest that spring SST anomalies over the northern tropical Atlantic (NTA) affect the tropical cyclone (TC) activity over the western North Pacific (WNP) in the following summer and fall. The present study reveals that the connection between spring NTA SST and following summer–fall WNP TC genesis frequency is not stationary. The influence of spring NTA SST on following summer–fall WNP TC genesis frequency is weak and insignificant before, but strong and significant after, the late 1980s. Before the late 1980s, the NTA SST anomaly-induced SST anomalies in the tropical central Pacific are weak, and the response of atmospheric circulation over the WNP is not strong. As a result, the connection between spring NTA SST and following summer–fall WNP TC genesis frequency is insignificant in the former period. In contrast, after the late 1980s, NTA SST anomalies induce pronounced tropical central Pacific SST anomalies through an Atlantic–Pacific teleconnection. Tropical central Pacific SST anomalies further induce favorable conditions for WNP TC genesis, including vertical motion, mid-level relative humidity, and vertical zonal wind shear. Hence, the connection between NTA SST and WNP TC genesis frequency is significant in the recent period. Further analysis shows that the interdecadal change in the connection between spring NTA SST and following summer–fall WNP TC genesis frequency may be related to the climatological SST change over the NTA region.

Effects of monsoon trough interannual variation on tropical cyclogenesis over the western North Pacific

The western North Pacific monsoon trough (MT) exhibits marked interannual variation (IAV) associated with El Nino–Southern Oscillation forcing. The role of MT IAV in tropical cyclone (TC) development was investigated using the Advanced Research Weather Research and Forecasting model placed on a beta plane. It was found that MT IAV has a great influence on vortex development. In strong years, the MT provides more favorable environmental conditions—primarily through enhanced low-level vorticity, convergence and midlevel moisture—for TC formation and vice versa in weak years. Sensitivity experiments that separated the dynamic and thermodynamic (moisture) factors from strong MT IAV showed that the thermodynamic impact associated with MT IAV is comparable to the dynamic impact.

On the weakened relationship between spring Arctic Oscillation and following summer tropical cyclone frequency over the western North Pacific: A comparison between 1968–1986 and 1989–2007

This study documents a weakening of the relationship between the spring Arctic Oscillation (AO) and the following summer tropical cyclone (TC) formation frequency over the eastern part (150°–180°E) of the western North Pacific (WNP). The relationship is strong and statistically significant during 1968–1986, but becomes weak during 1989–2007. The spring AOrelated SST, atmospheric dynamic, and thermodynamic conditions are compared between the two epochs to understand the possible reasons for the change in the relationship. Results indicate that the spring AO leads to an El Ni˜no-like SST anomaly, lower-level anomalous cyclonic circulation, upper-level anomalous anticyclonic circulation, enhanced ascending motion, and a positive midlevel relative humidity anomaly in the tropical western–central Pacific during 1968–1986, whereas the AOrelated anomalies in the above quantities are weak during 1989–2007. Hence, the large-scale dynamic and thermodynamic anomalies are more favorable for TC formation over the eastern WNP during 1968–1986 than during 1989–2007.

Relationship of boreal summer 10–20-day and 30–60-day intraseasonal oscillation intensity over the tropical western North Pacific to tropical Indo-Pacific SST

The present study contrasts interannual variations in the intensity of boreal summer 10–20-day and 30–60-day intraseasonal oscillations (ISOs) over the tropical western North Pacific and their factors. A pronounced difference is found in the relationship of the two ISOs to El Niño-Southern Oscillation. The 10–20-day ISO intensity is enhanced during El Niño developing summer, whereas the 30–60-day ISO intensity is enhanced during La Niña decaying summer. The above different relationship is interpreted as follows. The equatorial central and eastern Pacific SST anomalies modify vertical wind shear, lower-level moisture, and vertical motion in a southeast-northwest oriented band from the equatorial western Pacific to the tropical western North Pacific where the 10–20-day ISOs originate and propagate. These background field changes modulate the amplitude of 10–20-day ISOs. Preceding equatorial central and eastern Pacific SST anomalies induce SST anomalies in the North Indian Ocean in summer, which in turn modify vertical wind shear and vertical motion over the tropical western North Pacific. The modified background fields influence the amplitude of the 30–60-day ISOs when they reach the tropical western North Pacific from the equatorial region. A feedback of ISO intensity on local SST change is identified in the tropical western North Pacific likely due to a net effect of ISOs on surface heat flux anomalies. This feedback is more prominent from the 10–20-day than the 30–60-day ISO intensity change.

Contrast of 10–20-day and 30–60-day intraseasonal SST propagation during summer and winter over the South China Sea and western North Pacific

This study documents the structure and propagation of intraseasonal sea surface temperature (SST) variations and relative contribution of surface latent heat flux and shortwave radiation to the SST propagation in the South China Sea (SCS) and western North Pacific (WNP) regions. The emphasis is on the contrast of intraseasonal SST propagation between summer and winter and between 10–20-day and 30–60-day time scales. The dominant SST pattern during summer displays a tilted southwest–northeast band from the SCS to the subtropical WNP on both time scales, but with a larger value in the subtropical WNP on the 10–20-day time scale and in the SCS on the 30–60-day time scale. The dominant SST pattern during winter resembles that during summer, but with a larger value in the SCS. In summer, the SST anomalies show obvious northwestward and northward propagations in the SCS–WNP region on the 10–20-day and 30–60-day time scales, respectively. The cloud–radiation effect is a dominant factor for the SST propagation on both time scales in the SCS–WNP region, with a supplementary effect from the wind–evaporation effect on the 10–20-day time scale. In winter, the SST anomalies show southward propagation on both time scales in the SCS, while the southward propagation in the WNP is weak and confined to the subtropics on the 10–20-day time scale. The wind–evaporation effect makes a larger contribution to the SST propagation than the cloud–radiation effect on both time scales in the SCS–WNP region.

Surface Wind Speed-SST Relationship During the Passage of Typhoons Over the South China Sea

The present study investigates the relationship of sea surface temperature (SST) and surface wind speed changes in the South China Sea (SCS) region during the passage of typhoons for May-October of 2000-2010. Analysis shows that the SST drop in a 2° × 2° box during the passage of typhoons is about 1-3°C in two days around the time of maximum surface wind speed for most of the SCS typhoon cases. The typhoon-induced SST cooling displays a tendency of increase with surface wind speed. There is no clear statistical relationship between the SST cooling and the surface wind weakening during the analysis period. The relative location of the cold SST patch and the low surface wind region varies from case to case and from time to time. The present result does not support the previously proposed SST cooling impact on the surface wind speed based on case studies.

Origins and interrelationship of Intraseasonal rainfall variations around the Maritime Continent during boreal winter

Large intraseasonal rainfall variations are identified over the southern South China Sea (SSCS), tropical southeastern Indian Ocean (SEIO), and east coast of the Philippines (EPHI) in boreal winter. The present study contrasts origins and propagations and investigates interrelations of intraseasonal rainfall variations on the 10–20- and 30–60-day time scales in these regions. Different origins are identified for intraseasonal rainfall anomalies over the SSCS, SEIO, and EPHI on both time scales. On the 10–20-day time scale, strong northerly or northeasterly wind anomalies related to the East Asian winter monsoon (EAWM) play a major role in intraseasonal rainfall variations over the SSCS and EPHI. On the 30–60-day time scale, both the intraseasonal signal from the tropical Indian Ocean and the EAWM-related wind anomalies contribute to intraseasonal rainfall variations over the SSCS, whereas the EAWM-related wind anomalies have a major contribution to the intraseasonal rainfall variations over the EPHI. No relation is detected between the intraseasonal rainfall variations over the SEIO and the EAWM on both the 10–20-day and 30–60-day time scales. The anomalies associated with intraseasonal rainfall variations over the SSCS and EPHI propagate northwestward and northeastward, respectively, on the 10–20- and 30–60-day time scales. The intraseasonal rainfall anomalies display northwestward and northward propagation over the Bay of Bengal, respectively, on the 10–20- and 30–60-day time scales.

Simulations of tropical cyclogenesis associated with different monsoon trough patterns over the western North Pacific

The numerical simulations of tropical cyclone (TC) genesis during the strong and weak monsoon trough (MT) years, in which meteorological fields are composited, are conducted using advanced research weather research and forecasting model. The simulation results show that both tropical disturbances tend to form in the east of the western North Pacific (WNP) near 160°–170°E during the strong and weak MT years. During the strong MT years, there is a faster formation rate of TC. The eastward-extending MT gradually evolves into a closed monsoon gyre over the WNP during the early stage. The following rapid development of TC can be attributed to the enhanced lower-level southwesterly flows induced by the cross-equatorial currents, enhanced easterly winds, and weak vertical wind shear, which provide a favorable environment for TC genesis. The strengthened large-scale circulation spawns abundant convective updrafts resulting in the aggregation of cyclonic vorticity. In contrast, during the weak MT years, the westward-retreated MT gradually evolves into expansive easterly winds over the WNP. Two episodes of convective updrafts are triggered with a longer interval, and thus lead to a slower TC genesis compared with that during the strong MT years.