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Pao-Shin Chu and Richard W. Katz

Abstract

A relative measure of actual, rather than potential, predictability of a meteorological variable on the basis of its past history alone is proposed. This measure is predicated on the existence of a parametric time series model to represent the meteorological variable. Among other things, it provides an explicit representation of forecasting capability in terms of the individual parameters of such time series models.

As an application, the extent to which the Southern Oscillation (S0), a major component of climate, can be predicted on a monthly as well as a seasonal time scale on the basis of its past history alone is determined. In particular, on a monthly time scale up to about 44% of the variation in SO can be predicted one month ahead (zero months lead time) and about 35% two months ahead (one month lead time), or on a seasonal time scale about 53% one season ahead (zero seasons lead time) and about 31% two masons ahead (one season lead time). In general, the degree of predictability naturally decays as the lead time increases with essentially no predictability on a monthly time scale beyond ten months (nine months lead time) or on a seasonal time scale beyond seasons (two seasons lead time).

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Pao-Shin Chu and Soon-Ung Park

Abstract

A case study is presented of the cold surge over East Asia during 9–13 December 1978; using nine vertical levels of Winter MONEX data. The surge event is manifested by a rapid meridional mass flow in the lower troposphere from the midlatitudes (∼30°N) to the equator within three days. During the period studied, the lower tropospheric circulation dramatically changed in extent and intensity. With the onset of the surge event, the main divergent maximum began shifting from the South China Sea to southeast China. At the same time, the upper tropospheric circulation correspondingly changed in a reversed order from that of the lower troposphere, and a direct vertical coupling between flows in the low and high troposphere was observed.

The time-averaged meridional mass circulation between 100 and 126°E reveals a two-cell structure; the southern cell is located between the northern South China Sea and the equator, and the northern cell between midlatitudes and the northern South China Sea. Analysis of sensible heat transport indicates that the southern cell is associated with warm air in the south and cold air in the north; thus it resembles a thermally-direct local meridional circulation. Moisture transport analysis shows that the moisture source is found in the southern branch of this cell, and the sink in the northern branch. Conversely, the northern cell is thermally indirect.

The time-mean zonal mass circulation between 32°N and 4°S is marked by two cells, linked by subsidence near the longitudes of the South China Sea. The eastern cell is accompanied by heat and moisture sources while the western cell is associated with heat and moisture sinks.

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Pao-Shin Chu and James D. Clark

Tropical cyclone activity (tropical depressions, tropical storms, and hurricanes combined) in the central North Pacific has been found to be on the rise and this increase amounts to about 3.2 cyclones over the last 32 years (1966–97). An examination of time series of tropical storms and hurricanes and hurricane records alone also reveals an increasing trend in both series since 1966.

The upward trend is characterized by decadal-scale variability as manifested by fewer cyclones during the first half of the record (1966–81) and more during the second half of the record (1982–97). The maximum hurricane intensity has also increased in the central North Pacific, as well as the number of intense hurricanes from the first to the second half of the record. Relative to 1966–81, sea surface temperatures in the tropical Pacific and relative vorticity near the surface to the south of Hawaii have increased dramatically during 1982–97. The changes in the frequency and intensity of tropical cyclones in the central North Pacific appear to be modulated by decadal-scale variability of the basic state of SST, which transitioned from a cold to a warm phase in the late 1970s; this warm phase may have resulted in stronger and more frequent El Niño events as seen during the second half of the record, leading to more cyclones in the central North Pacific.

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Pao-Shin Chu and Jian-Bo Wang

Abstract

Recent climate change in tropical convection in the western Pacific and Indian Ocean regions is inferred from the outgoing longwave radiation (OLR) records. The systematic bias in the OLR series is first corrected and results of the rotated empirical orthogonal function analysis indicate that the bias, to a first approximation, has been corrected.

Linear regression analysis and nonparametric Mann–Kendall rank statistics are employed to detect trends. From 1974 to 1992, trend analyses based on the entire consecutive monthly records suggest a significant decrease in OLR over the tropical central–western Pacific and a large portion of the Indian Ocean. In contrast, northern Australia shows the largest increase in OLR over time. The significance of the local linear trend pattern has been determined via a Monte Carlo simulation technique that scrambles OLR time series at each grid point “simultaneously” and results show the field significance.

An increase in convection shows a preference to occur in the summer hemisphere. During the boreal summer half-year, this is seen in a region extending from the Arabian Sea across southeast Asia eastward to the northwest Pacific, with the largest value over the Bay of Bengal. More summer monsoon rainfall is likely to have occurred in these regions. For the austral summer half-year, enhanced convection is found over the equatorial south-central Pacific and the south-central Indian Ocean. Time series of tropical cyclone counts in the northwest Pacific, the Bay of Bengal, and the south-central Indian Ocean also reveal a general level of increase. Regardless of seasonality, a positive trend in OLR is always observed over a large portion of tropical Australia.

A sensitivity test is conducted to investigate the change in linear trend patterns by removing the years during which the El Niño–Southern Oscillation phenomenon occurred. Although the enhanced convection over the Bay of Bengal, the south Indian Ocean, and the northwest Pacific are still noticeable, it is much weaker over the equatorial south-central Pacific than when the complete duration series were used. Other sensitivity tests are also conducted to examine the change in linear trend patterns by varying data lengths and by skipping the missing 10-month observation in the OLR time series; results are basically similar to those when complete data are used. The authors speculate that monsoon convection over the tropical western Pacific and the Indian Ocean has undergone a change in the climate mean state, probably on a decadal timescale.

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Md. Rashed Chowdhury and Pao-Shin Chu

Abstract

Because of the need for information related to the variability and predictability of sea level on season-to-longer time scales, the Pacific ENSO Applications Climate (PEAC) Center runs the El Niño-Southern Oscillation (ENSO)-based canonical correlation analysis (CCA) statistical model to generate sea level forecasts for the U.S.-affiliated Pacific Islands (USAPI) region with lead times of 3–6 months in advance. However, in order to meet the increasing demand for longer lead-time (e.g., 6–12 months) forecasts, the PEAC Center, as part of the advances in operational sea level forecasts, recently incorporated both SST and zonal wind components of trade winds (U) for modulating sea level variability on longer time scales. The combined SST and U-based forecasts are found to be more skillful on longer time scales. This improvement has enabled the capability of our clients in the USAPI region to develop a more efficient long-term response plan for hazard management.

In a recent “Regional Integrated Water Level Service” meeting, it was revealed that the development and distribution of “seasonal water level outlooks” in the Pacific basin region is an area of mutual interest. We therefore synthesize the current operational forecasting, warning, and response activities of the PEAC Center and discuss the manner in which our experience in the USAPI region can contribute to the development of adaptation strategies for longer time-scale climate variability and change for the non-USAPI region in the south Pacific.

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Pao-Shin Chu and Richard W. Katz

Abstract

The theoretical spectra of certain parametric time series models with relevance to the Southern Oscillation (SO) are determined and compared with those based on a frequency-domain approach. Consistent spectral estimates are found for the two models selected in our earlier studies of the SO. All these results yield larger power at low frequencies and a dominant peak around 3–4 yr. Some reasons are offered for the slightly different behavior of the spectra as derived from the time-domain and frequency-domain approaches.

For the sake of comparison, the spectra of other simpler time series models are also calculated. While larger power is found at low frequencies, no spectral peak exists in these simpler models. Some implications of the quasi-periodic behavior found in the more complex models (i.e., an intermediate peak in the spectrum) are discussed in the context of the persistence and forecasting of the SO.

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Pao-Shin Chu and Richard W. Katz

Abstract

An index consisting of the difference of normalized sea level pressure departures between Tahiti and Darwin is used to represent the Southern Oscillation (SO) fluctuations. Using a time-domain approach, autoregressive-moving average (ARMA) progress are applied to model and predict this Southern Oscillation Index (SOI) on a monthly and seasonal basis. The ARMA process which is chosen to fit the monthly SOI expresses the index for the current month as a function of both the SOI one month and seven (or nine) mouths ago, as well as the current and previous month's random error. A purely automotive (AR) process is identified as representative of the seasonal SO fluctuations, with the SOI for the current season being derived from the index for the immediate past three seasons and a single random disturbance term for the current season. To allow for the phase locking of the SOI with the annual cycle, ARMA processes with seasonally varying coefficients are also considered.

As one example of how these models could be used, seasonal SO variations have been forecast. When SOI observations from 1935 through the summer of 1983 are employed, the seasonal model indicates forecast of positive SOI from fall 1983 through fall 1984. Forecasts based only on SOI observations from 1935 through spring 1982 show a low predictive skill for the SOI values from summer 1982 through winter 1984, whereas one-season-ahead forecasts starting with summer 1982 agree reasonably well with the actual SOI observations. These examples help illustrate the degree to which the future behavior of the SOI is predictable on the basis of its past history alone.

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Mong-Ming Lu, Pao-Shin Chu, and Yun-Ching Lin

Abstract

A Poisson generalized linear regression model cast within a Bayesian framework is applied to forecast the seasonal tropical cyclone (TC) counts in the vicinity of Taiwan. The TC season considered is June–November and the data period used for model development is 1979–2007. A stepwise regression procedure is applied for predictor selection. Three large-scale climate variables, namely, relative vorticity at 850 hPa (Vor850), vertical wind shear, and sea level pressure over the western and central North Pacific from the antecedent May, are selected as predictors. Leave-one-out cross validation is performed and forecast skill is thoroughly evaluated. The skill level of the Bayesian regression model is better than what can be achieved by climatology and persistence methods. Most importantly, the Bayesian probabilistic inference can provide an uncertainty expression in the parameter estimation. Among the three predictors, Vor850 is found to be the most important because it reflects the variation of the ridge position of the westward extension of the western Pacific subtropical high. The model shows negative bias during the years with successive TCs, which are generated by easterly waves before approaching Taiwan. Recommendations for real-time operational forecast and future development are discussed.

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Pao-Shin Chu, James Frederick, and Andrew J. Nash

Abstract

Exploratory data analysis is used to examine several key characteristics of surface wind along a ship track in the equatorial western Pacific. A month-by-month examination is undertaken, based on daily ship data from a recent 30-year record (1958–87). The characteristics considered here include the expected frequency of occurrence, the intensity, the range of variability, and the extreme value of the wind from the eight-point compass directions. A hodograph and constancy of monthly mean wind vectors are also presented.

Results from the 30-year climatology suggest that the equatorial western Pacific is affected by monsoonal and trade flows from each winter hemisphere and by the eastern Pacific subtropical highs during the transition season (i.e., May). Equatorial westerlies peak in November (20%) and December (18%).

Composite analysis reveals the further influence of the trade flows from the northwest Pacific from January to May, and the Southern Hemisphere influence from June to September during a year when ENSO has occurred. The reliability of the ENSO composite has been tested using a Monte Carlo simulation technique. Westerlies indeed increase their frequency of occurrence from November of the antecedent year to November of the ENSO year. This increase, however, is small relative to the decrease in easterlies in the same period.

Westerly wind events are examined in terms of their duration and timing of occurrence. Westerly wind events with a period of 5–7 days do occur more often than events with a longer duration, but their frequency of occurrence has reduced substantially during El Niño years.

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Pang-Chi Hsu, Pao-Shin Chu, Hiroyuki Murakami, and Xin Zhao

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.

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