Yao Ha

Decadal change of South China Sea tropical cyclone activity in mid‐1990s and its possible linkage with intraseasonal variability

This study focuses on the decadal variability of tropical cyclone (TC) activity over the South China Sea (SCS) since the 1970s and its possible cause behind. It is found that TC activity over the SCS experiences a significant decadal change around the mid-1990s. Compared to the period from the 1970s to the early 1990s, the number of TCs formed in the SCS remarkably increases from the mid-1990s through the 2000s. In particular, this change of TC genesis is closely related to a decadal shift in atmospheric intraseasonal variability (ISV) that occurred in 1994. The ISV on the 30–60 days time scale over the SCS has been increasing since the mid-1990s, and the increased TC frequency after 1994 is attributed primarily to the active convection induced by the enhancement of the SCS ISV. In addition, the TC activities before the mid-1990s are mostly confined within the SCS basin. However, more TCs form over the SCS and move northeastward since the mid-1990s and finally enter the East China Sea and the Philippine Sea. Anomalies of westerly over the northern SCS after 1994 are responsible for the northeastward moving of TCs.

Different Pacific Ocean Warming Decaying Types and Northwest Pacific Tropical Cyclone Activity

AbstractThis study focuses on statistical analysis of anomalous tropical cyclone (TC) activities and the physical mechanisms behind these anomalies. Different patterns of decaying of the warm sea surface temperature anomaly (SSTA) over the equatorial central-eastern Pacific are categorized into three types: eastern Pacific warming decaying to La Nina (EPWDL), eastern Pacific warming decaying to a neutral phase (EPWDN), and a central Pacific warming decaying year (CPWD). Differences in TC activity over the western North Pacific (WNP) corresponding to the above three types are discussed, and possible mechanisms are proposed. For EPWDL, TC genesis shows a significant positive (negative) anomaly over the northwestern (southeastern) WNP and more TCs move westward and make landfall over the southern East Asian coast. This is attributed primarily to the combined modulation of La Nina and the warm equatorial east Indian Ocean SSTA. For EPWDN, enhanced TC genesis is observed over the northeastern WNP, and suppress...

Influences of ENSO on Western North Pacific Tropical Cyclone Kinetic Energy and Its Meridional Transport

AbstractThis study investigates the influences of ENSO on tropical cyclone (TC) kinetic energy and its meridional transport in the western North Pacific (WNP) using the TC wind field obtained after a method for removing TC vortices from reanalysis data is applied. Results show that ENSO strongly modulates TC kinetic energy and its meridional transport in the WNP, but their effects and regions differ. The TC kinetic energy is positively correlated with the Nino-3.4 index in the entire WNP, and its poleward transport is positively (negatively) correlated with the Nino-3.4 index in the eastern WNP (the western WNP and the South China Sea); these correlations are statistically significant. The maximum TC kinetic energy is located around 25°N, 135°E (25°N, 125°E) in the warm (cold) year, showing an east–west pattern during different ENSO phases. The meridional transport of TC kinetic energy exhibits a dipole pattern over the WNP, with the poleward (equatorward) transport in the eastern (western) WNP. Both pole...

The opposite effects of inner and outer sea surface temperature on tropical cyclone intensity

A suite of semiidealized numerical experiments are conducted to investigate the sensitivity of tropical cyclone (TC) intensity to changes of sea surface temperature (SST) over different radial extents. It is found that the increase of inner SST within the range 1.5–2.0 times the radius of maximum wind (RMW), defined as the effective radius (ER), contributes greatly to the increase of TC intensity and the reduction of TC inner-core size, whereas the increase of outer SST (defined as SST outside the ER) reduces TC intensity and increases TC inner-core size. Further analysis suggests that the effects of SST inside and outside the ER on TC intensity rely on the factors that influence the TC development. As the SST increases inside the ER, more surface enthalpy flux enters the TC eyewall and less enters the outer spiral rainbands. This will decrease the RMW, leading to a smaller eyewall radius where strong latent heating is released. As a result, the central pressure of the TC deepens with stronger radial pressure gradient. Meanwhile, the difference between SST and upper tropospheric temperature increases. All factors above contribute to TC intensification as the inner SST increases. The opposite happens as the SST increases outside the ER. How TC intensity responds to the change of the entire SST depends on the competitive and opposite effects of inner and outer SST. Moreover, understanding the mechanisms is vital to the forecast of variations in TC intensity and inner-core size when a TC comes across an ocean cold or warm pool.

The dynamic and thermodynamic effects of relative and absolute sea surface temperature on tropical cyclone intensity

Several numerical experiments were performed to investigate the dynamic and thermodynamic effects of sea surface temperature (SST) on tropical cyclone (TC) intensity. The results reveal that the relative SST within a radius of 2–3 times the radius of maximum wind contributes positively and greatly to TC intensity, while the remote SST far away from the TC center could reduce storm intensity. The change of air-sea temperature and moisture differences may be the reason why TC intensity is more sensitive to the relative rather than the absolute SST. As the inflow air moves toward the eyewall, warmer (colder) remote SST can gradually increase (decrease) the underlying surface air temperature and moisture, and thus decrease (increase) the air-sea temperature and moisture differences, which lead to less (more) energy fluxes entering the eyewall and then decrease (increase) the TC intensity and make it less sensitive to the absolute SST change. Finally, with all the related dynamic and thermodynamic processes being taken into account, a schematic diagram for the effects of relative SST and absolute SST on TC intensity is proposed.

Performance of a New Convective Parameterization Scheme on Model Convergence in Simulations of a Tropical Cyclone at Grey-Zone Resolutions

AbstractThe latest version of the Weather Research and Forecasting model (WRFV3.5) is used to evaluate the performance of the Grell and Freitas (GF13) cumulus parameterization scheme on the model convergence in simulations of a tropical cyclone (TC) at gray-zone resolutions. The simulated TC intensity converges to a finite limit as the grid spacing varies from 7.5 to 1 km. The reasons for the model convergence are investigated from perspectives of subgrid-scale processes and thermodynamic and dynamic structures. It is found that the impacts of above factors are notably different with varying model resolutions. The convective heating and drying increase as the grid spacing decreases, which inhibits the explicit microphysical parameterization preventing the simulated TC from overly intensifying. As the grid spacing decreases from 7.5 to 5 km, the TC intensity increases because of a stronger secondary circulation, a larger magnitude and proportion of strong eyewall updraft, and a greater amount of latent hea...

Out-of-phase decadal changes in boreal summer rainfall between Yellow-Huaihe River Valley and southern China around 2002/2003

This study investigates the decadal variability of rainfall over China in boreal summer (June–August) since the early 1990s. Results show that the rainfall experiences an abrupt decadal change at around 2002/2003. The decadal change is statistically significant and characterized by an out-of-phase pattern between southern China (SC) and the Yellow-Huaihe River Valley (YHRV). The rainfall over SC decreases during the decade 2003–2012 compared to that in the preceding decade 1993–2002. A simultaneous decadal increase in rainfall has occurred over the YHRV. Meanwhile, a significant sea surface temperature warming appears over the western Pacific Ocean and the northern Indian Ocean after 2002 on the decadal time scale. Further analysis reveals that enhanced convections are activated over the tropical regions between 130°E and 160°E and west of 80°E due to the SST anomalies, which induce the dry air in an area of anomalous subsidence located over SC and the northern South China Sea (SCS) via zonal circulation. Accompanied by the anomalous descending flow over the northern SCS, tropical cyclone (TC) activities in the SCS also experience a concurrent decadal reduction. The decrease in landfall TCs contributes to the decadal decrease in SC rainfall since 2003. Corresponding to the anomalous descending motion that is dominant south of 30°N, an anomalous moist ascending flow develops over the YHRV at around 35°N. Meanwhile, the western Pacific subtropical high becomes stronger and extends further westward during 2003–2012, leading to enhanced moisture transport by the southwesterly in the northwestern flank of subtropical high. As a result, more precipitation occurs over the YHRV. The above analysis has revealed the physical–dynamical processes involved in the decadal changes in rainfall over China. The mechanisms behind the out-of-phase pattern of rainfall changes between SC and the YHRV that occurred at 2002/2003 are explored.