Jingliang Huangfu

Influences of sea surface temperature in the tropical Pacific and Indian Oceans on tropical cyclone genesis over the western North Pacific in May

This study investigated the effects of sea surface temperature (SST) in different tropical regions on tropical cyclone (TC) genesis over the western North Pacific (WNP) in May. The results revealed that positive SST anomalies over the tropical Pacific (Indian) Ocean in March may lead to increased (decreased) numbers of TCs over the WNP in May by affecting the atmospheric circulation anomalies over the WNP. Warmer SSTs over the tropical Pacific Ocean (TPO) may lead to stronger low-level southwesterly winds and enhanced convection. However, warmer SSTs over the tropical Indian Ocean (TIO) may enhance the western part of the western Pacific subtropical high (WPSH) and suppress convection. Further analysis suggests that changes in the atmospheric circulation anomalies over the WNP are related to changes in environmental factors. Concomitantly with the positive SST anomalies over the TPO, a favorable environment for TC genesis in May is present, with stronger low-level relative vorticity and upper-level divergence, smaller vertical wind shear and abundant water vapor. In contrast, the positive SST anomalies over the TIO might lead to an unfavorable environment for TC genesis over the WNP. This study also investigated the joint contributions of the TPO and TIO, and the results indicate that positive SST anomalies over the TPO and negative SST anomalies over the TIO may lead to an increased number of TC geneses. The analysis of the energy budget suggests that the joint activity of the TPO and TIO influences the barotropic eddy kinetic energy conversions and is mainly attributed to the contributions from the meridional shear of the mean zonal winds and the zonal wind convergence.

Statistical analysis and a case study of tropical cyclones that trigger the onset of the South China Sea summer monsoon

This paper addresses whether a tropical cyclone can trigger the onset of the South China Sea (SCS) summer monsoon (SM). We conducted a statistical analysis of tropical cyclones (TCs) generated over the western North Pacific (WNP) between late-April and May. The results showed that there were cases in which TCs were generated before the onset of the SCSSM, accounting for 43.2% of the TCs generated during this season. This study examined a representative case, Super Typhoon Chanchu (0601), which was determined to be influential in the onset of the SCSSM. With a northwestward track, Chanchu brought strong convection and westerly winds to the SCS on 12 May, which triggered the intrusion of the southwesterly winds from the Bay of Bengal and the eastward retreat of the western Pacific subtropical high. Super Typhoon Chanchu provides an example in which a TC triggered the onset of the SCSSM. The negative correlation between the onset date of the SCSSM and the number of TCs generated over the WNP used to be interpreted as the influence of the monsoon trough on TC genesis. This work provides a supplementary illustration that this relationship also includes the impact of TCs on the onset of the SCSSM.

East Asian Winter Monsoon Impacts the ENSO-related Teleconnections and North American Seasonal Air Temperature Prediction

El Niño-Southern Oscillation (ENSO) is a key feature for seasonal weather and climate prediction in the extra-tropics since related sea surface temperature anomalies induce precipitation anomalies that generate poleward propagating Rossby waves and teleconnections. The East Asian winter monsoon (EAWM) is driven by processes originating over the Asian continent and, to a lesser degree, by ENSO-related tropical convection. EAWM also strongly affects convection and precipitation patterns over the western tropical Pacific by cold air outbreaks reaching equatorial latitudes. Hence, one can expect a modulating effect of EAWM on the generation of Rossby wave trains related to ENSO. By increasing the convective heating over the western Pacific, strong EAWM strengthens the Pacific Walker circulation, and weakens (strengthens) the El Niño (La Niña) related effects on the extra-tropics via a modulation of the Pacific North America teleconnection pattern. Our results indicate that, for seasonal prediction over North America, along with ENSO the variability of EAWM should also be taken into account. The climate anomalies over the North America for the same phase of ENSO are significantly different for strong and weak EAWM.

Interdecadal variation of tropical cyclone genesis and its relationship to the convective activities over the central Pacific

Using the outgoing longwave radiation (OLR) data from the National Oceanic and Atmospheric Administration archives, this paper analyzes the interdecadal variation of convective activities over the central Pacific (CP) from July to October of 1979–2013 and its impact on tropical cyclone (TC) genesis in the western North Pacific (WNP). Concurrent with the interdecadal decrease of TC genesis, the tropical convection underwent a significant interdecadal change in the late 1990s. Overall, the first leading empirical orthogonal function mode of the tropical OLR during July–October turned from a zonal dipole pattern during 1979–1997 to a tripole pattern during 1998–2013. Concomitant to this change, the boreal part of the Walker circulation shrank westward, with its downdraft branch located over the CP. The downward motion anomalies over the CP increased after the late 1990s, as did the trade easterlies. Consistent with the CP convective activity anomalies, the negative low-level relative vorticity anomalies and upper-level divergence anomalies, positive vertical wind shear anomalies and anomalous abundant water vapor can be observed over the southeastern part of the WNP. Additionally, the tropical depression (TD)-type waves associated with the CP convective activities are significantly different before and after the late 1990s. Before the late 1990s, the off-equatorial TD-type waves could be distinctly observed, with clear transitions located along the WNP monsoon trough. However, these transitions were vague after the late 1990s. Therefore, the convective activities over the CP may have played an important role in affecting the interdecadal change of TC genesis by affecting the genesis of TD-type waves.

Impact of the cross-tropopause wind shear on tropical cyclone genesis over the Western North Pacific in May

The role of the 50- to 200-hPa zonal wind shear in modulating tropical cyclone (TC) genesis over the western North Pacific (WNP) in May is investigated in this study. Concurrent with the strong cross-tropopause shear over the key region (0°–5°N, 160°–180°E), suppressed convection was observed over the tropical WNP, especially over the South China Sea and the Philippines. The monsoon trough (MT) was confined westward. However, enhanced convection occurred in the weak shear years and the MT extended eastward. This cross-tropopause wind shear is negatively correlated with TC genesis in May, with a decreased (increased) number of TCs corresponding to strong (weak) cross-tropopause wind shear.This cross-tropopause wind shear can be treated as the combined impacts of the El Niño-Southern Oscillation (ENSO) events and the stratospheric quasi-biennial oscillation (QBO). When decaying El Niño events coupled with the easterly phase of the QBO were noted, the cross-tropopause wind shear was stronger with weakened convection, and an enhanced western Pacific subtropical high was observed. TCs are rarely generated during these years. In contrast, the modulation of the QBO westerly phase on decaying La Niña events is limited. Affected by the QBO westerly phase, TC genesis in the May following La Niña events is only slightly enhanced. The energy analysis indicates that the combined impacts of the decaying El Niño events and the QBO easterly phase might suppress the barotropic eddy kinetic energy conversion in May, whereas the decaying La Niña events and the QBO west phase act in an opposite manner.