Search Results

You are looking at 1 - 6 of 6 items for :

  • Author or Editor: Zhe-Min Tan x
  • Journal of Climate x
  • Refine by Access: All Content x
Clear All Modify Search
Yi-Peng Guo and Zhe-Min Tan

Abstract

This study investigated the impacts of the interannual variability in the boreal spring regional Hadley circulation over the Indo-Pacific warm pool (IPWP) on the tropical cyclone (TC) activity over the western North Pacific (WNP). The principal modes of the interannual variability in the IPWP Hadley circulation were calculated using empirical orthogonal function (EOF) analysis. The leading mode (EOF-1) features cross-equatorial southerly wind anomalies over the Indian Ocean and Maritime Continent and has an evident impact on WNP TC activity during summer. In the summer following a positive phase of the EOF-1, a cyclonic circulation anomaly, with upward motion, positive relative vorticity anomalies, and weak sea level pressure, dominates the WNP, and this favors increased TC genesis. However, large positive vertical wind shear anomalies over the South China Sea and Philippine Sea inhibit the TC intensification. A positive wind–sea surface temperature (SST)–precipitation feedback was found to facilitate the ability of the signal of the EOF-1 to persist until the summer. The westerly wind anomalies converge around 10°N over the WNP, thus increasing precipitation, and this increased precipitation enhances the westerly wind anomalies via a Gill-type response. The strengthened westerly wind anomalies increase total wind speeds, which in turn cool the SST in the Bay of Bengal and the South China Sea, and warm the SST in the eastern WNP, increasing the zonal SST gradient. Consequently, this increased zonal SST gradient further enhances the westerly wind anomalies, strengthens the monsoon trough, and increases the WNP precipitation further. Therefore, the WNP precipitation anomalies are sustained into the summer.

Open access
Yan Liu, Zhe-Min Tan, and Zhaohua Wu

Abstract

Recent study indicates that the non-instantaneous interaction of convection and circulation is essential for evolution of large-scale convective systems. It is incorporated into cumulus parameterization (CP) by relating cloud-base mass flux of shallow convection to a composite of subcloud moisture convergence in the past 6 h. Three pairs of 19-yr simulations with original and modified CP schemes are conducted in a tropical channel model to verify their ability to reproduce the Madden–Julian oscillation (MJO). More coherent tropical precipitation and improved eastward propagation signal are observed in the simulations with the modified CP schemes based on the non-instantaneous interaction. It is found that enhanced feedback between shallow convection and low-level moisture convergence results in amplified shallow convective heating, and then generates reinforced moisture convergence, which transports more moisture upward. The improved simulations of eastward propagation of the MJO are largely attributed to higher specific humidity below 600 hPa in the free troposphere to the east of maximum rainfall center, which is related to stronger boundary layer moisture convergence forced by shallow convection. Large-scale horizontal advection causes asymmetric moisture tendencies relative to rainfall center (positive to the east and negative to the west) and also gives rise to eastward propagation. The zonal advection, especially the advection of anomalous specific humidity by mean zonal wind, is found to dominate the difference of horizontal advection between each pair of simulations. The results indicate the vital importance of non-instantaneous feedback between shallow convection and moisture convergence for convection organization and the eastward MJO propagation.

Restricted access
Yi-Peng Guo and Zhe-Min Tan

Abstract

The variation in the interannual relationship between the boreal winter Hadley circulation (HC) and El Niño–Southern Oscillation (ENSO) during 1948–2014 is investigated. The interannual variability of the HC is dominated by two principal modes: the equatorial asymmetric mode (AM) and the equatorial symmetric mode (SM). The AM of the HC during ENSO events mainly results from a combined effect of the ENSO sea surface temperature (SST) anomalies and the climatological background SST over the South Pacific convergence zone. Comparatively, the SM shows a steady and statistically significant relationship with ENSO; however, the interannual relationship between the AM and ENSO is strengthened during the mid-1970s, which leads to a HC regime change—that is, the interannual pulse of the HC intensity and its response to ENSO are stronger after the mid-1970s than before. The long-term warming trend of the tropical western Pacific since the 1950s and the increased ENSO amplitude play vital roles in the HC regime change. Although the tropical eastern Pacific also experienced a long-term warming trend, it has little influence on the HC regime change due to the climatologically cold background SST over the cold tongue region.

Open access
Yi-Peng Guo and Zhe-Min Tan

Abstract

El Niño–Southern Oscillation (ENSO), which features an equatorial quasi-symmetric sea surface temperature anomaly (SSTA), is related to both the symmetric and asymmetric components of the Hadley circulation (HC) variability. However, the mechanisms for such a nonlinear HC–ENSO relationship are still unclear. Using 36-yr monthly reanalysis datasets, this study shows that the month-to-month HC variability is dominated by two principal modes, the asymmetric mode (AM) and symmetric mode (SM), both of which are highly correlated with ENSO variability. Furthermore, the relationship between the HC principal modes and the ENSO SSTA is modulated by the western Pacific SST annual cycle. When the zonal mean western Pacific SST peaks off (on) the equator, the ENSO SSTA leads to the AM (SM) of HC variability. This is because the zonal mean western Pacific SST peak provides a warmer background favorable for the SSTA to stimulate convection, indicating the important role of the combined effect of the SST annual cycle and the ENSO SSTA in affecting the HC variability. Importantly, the western Pacific SST annual cycle has no such modulation effect during central Pacific El Niño or La Niña events. The results have important implications for simulating and predicting the climatic impacts of ENSO and HC variability.

Open access
Dingzhu Hu, Yi-Peng Guo, Zhe-Min Tan, and Zhaoyong Guan

Abstract

This study investigates the interannual relationship and the dynamical linkage between the boreal spring Arctic Oscillation (AO) and the Northern Hemisphere Hadley circulation extent (HCE). The spring AO is positively correlated with the HCE, with one standard positive deviation of the AO index corresponding to approximately 0.42° latitude poleward shift of the HCE. The interaction between the planetary wave and the zonal winds over the subtropics results in an anomalous eddy momentum flux divergence, which shifts the HCE poleward. The AO related transient eddy momentum flux divergence makes nearly 2 times larger contributions than those of the stationary component to the HCE change. The increased equatorward transient wave flux over the subtropics is possibly related to the larger meridional gradient of the transient wave refractive index there. The AO positive phase corresponds to an enhanced planetary wave propagation from the midlatitude Atlantic Ocean to the subtropics, which resembles the North Atlantic Oscillation pattern. The autumn and winter AO–HCE relationship is similar to that during spring, while summer has the weakest relationship, which could be mainly attributed to the far poleward extension of the climatological HCE during summer.

Open access
Xi Guo, James P. Kossin, and Zhe-Min Tan

Abstract

Tropical cyclone (TC) translation speed (TCTS) can affect the duration of TC-related disasters, which is critical to coastal and inland areas. The long-term variation of TCTS and its relationship to the variability of the midlatitude jet stream and storm migration is discussed here for storms near the North Atlantic coast during 1948–2019. Our results reveal the prominent seasonality in the long-term variation of TCTS, which can be largely explained by the seasonality in the covariations of the midlatitude jet stream and storm locations. Specifically, significant increases of TCTS occur in June and October during the past decades, which may result from the equatorward displacement of the jet stream and poleward migration of storm locations. Prominent slowdown of TCTS is found in August, which is related to the weakened jet strength and equatorward storm migration. In September, the effects of poleward displacement and weakening of the jet stream on TCTS are largely compensated by the poleward storm migration, and therefore no significant change in TCTS is observed. Meanwhile, the multidecadal variability of the Atlantic may contribute to the multidecadal variability of TCTS. Our findings emphasize the significance in taking a seasonality view in discussing the variability and trends of near-coast Atlantic TCTS under climate change.

Full access