Southeastern Western North Pacific Research Articles

An Abrupt Decrease in the Late-Season Typhoon Activity over the Western North Pacific*

Abstract In 1995 an abrupt shift in the late-season (October–December) typhoon activity over the western North Pacific (WNP) is detected by a Bayesian changepoint analysis. Interestingly, a similar change also occurs in the late-season sea surface temperature series over the western Pacific, eastern North Pacific, and portions of the Indian Ocean. All of the counts, lifespans, and accumulated cyclone energy of the late-season typhoons during the 1995–2011 epoch decreased significantly, compared with typhoons that occurred during the 1979–94 epoch. The negative vorticity anomaly is found to be the leading contributor to the genesis potential index (GPI) decrease over the southeastern sector of the WNP during 1995–2011. To elucidate the origin of the epochal change in the dynamic environmental conditions, a suite of sensitivity experiments is conducted based on the latest version of the Japan Meteorological Research Institute atmospheric general circulation model (MRI AGCM). The ensemble simulations suggest that the recent change to a La Niña–like state induces an unfavorable dynamic condition for typhoon genesis over the southeastern WNP. Warming in the Indian Ocean, however, contributes insignificantly to the circulation anomaly related to typhoon genesis over the southeastern WNP. The frequency of typhoon occurrence reveals a basinwide decrease over the WNP in the recent epoch, except for a small increase near Taiwan. An empirical statistical analysis shows that the basinwide decrease in the frequency of the typhoon occurrence is primarily attributed to a decrease in typhoon genesis, while the change in track is of less importance.

Contribution of East Indian Ocean SSTA to Western North Pacific tropical cyclone activity under El Niño/La Niña conditions

ABSTRACTThis study investigates, under El Niño/La Niña conditions, the effects of sea surface temperature anomaly (SSTA) in the East Indian Ocean (EIO) on tropical cyclone (TC) genesis over the western North Pacific (WNP), and a possible mechanism is proposed. It is found that the EIO SSTA exerts a secondary modulation to cyclogenesis relative to the El Niño‐Southern Oscillation (ENSO), and the negative correlation between the EIO SSTA and WNP TC frequency is significantly weakened by the ENSO events. Under El Niño (La Niña) conditions, no matter what phases of EIO SSTA, TC genesis generally shows similar features with low (high) frequency in the western‐central WNP (15°–25°N, 120°–150°E) and high (low) frequency in the southeastern WNP (5°–15°N, 150°–170°E). In particular, the effects of warm EIO SSTA on the WNP TC activity represent an asymmetry between El Niño and La Niña. That is, the warm EIO SSTA and El Niño share combined effects to decrease TC genesis over the western‐central WNP. By contrast, under La Niña conditions, the influence of warm EIO SSTA on TC activity over the western‐central WNP is partially drown out by La Niña. Meanwhile, weaker monsoon trough with intense anomaly of easterly jointly induced by the two factors suppresses TC genesis over the southeastern WNP.

Revealing the effects of the El Niño-southern oscillation on tropical cyclone intensity over the western north pacific from a model sensitivity study

Five sets of model sensitivity experiments are conducted to investigate the influence of tropical cyclone (TC) genesis location and atmospheric circulation on interannual variability of TC intensity in the western North Pacific (WNP). In each experiment, bogus TCs are placed at different initial locations, and simulations are conducted with identical initial and boundary conditions. In the first three experiments, the specified atmospheric and SST conditions represent the mean conditions of El Nio, La Nia, and neutral years. The other two experiments are conducted with the specified atmospheric conditions of El Nio and La Nia years but with SSTs exchanged. The model results suggest that TCs generated in the southeastern WNP incurred more favorable environmental conditions for development than TCs generated elsewhere. The different TC intensities between El Nio and La Nia years are caused by difference in TC genesis location and low-level vorticity (VOR). VOR plays a significant role in the intensities of TCs with the same genesis locations between El Nio and La Nia years.

Future Change of Western North Pacific Typhoons: Projections by a 20-km-Mesh Global Atmospheric Model*

Abstract Projected future changes in tropical cyclone (TC) activity over the western North Pacific (WNP) under the Special Report on Emissions Scenarios (SRES) A1B emission scenario were investigated using a 20-km-mesh, very-high-resolution Meteorological Research Institute (MRI)–Japan Meteorological Agency (JMA) atmospheric general circulation model. The present-day (1979–2003) simulation yielded reasonably realistic climatology and interannual variability for TC genesis frequency and tracks. The future (2075–99) projection indicates (i) a significant reduction (by about 23%) in both TC genesis number and frequency of occurrence primarily during the late part of the year (September–December), (ii) an eastward shift in the positions of the two prevailing northward-recurving TC tracks during the peak TC season (July–October), and (iii) a significant reduction (by 44%) in TC frequency approaching coastal regions of Southeast Asia. The changes in occurrence frequency are due in part to changes in large-scale steering flows, but they are due mainly to changes in the locations of TC genesis; fewer TCs will form in the western portion of the WNP (west of 145°E), whereas more storms will form in the southeastern quadrant of the WNP (10°–20°N, 145°–160°E). Analysis of the genesis potential index reveals that the reduced TC genesis in the western WNP is due mainly to in situ weakening of large-scale ascent and decreasing midtropospheric relative humidity, which are associated with the enhanced descent of the tropical overturning circulation. The analysis also indicates that enhanced TC genesis in the southeastern WNP is due to increased low-level cyclonic vorticity and reduced vertical wind shear. These changes appear to be critically dependent on the spatial pattern of future sea surface temperature; therefore, it is necessary to conduct ensemble projections with a range of SST spatial patterns to understand the degree and distribution of uncertainty in future projections.