Pang-chi Hsu

Assessments of surface latent heat flux associated with the Madden–Julian Oscillation in reanalyses

To understand the accuracy and uncertainty of surface latent heat flux (LHF) associated with the Madden–Julian Oscillation (MJO), the LHF from each of the six global reanalysis datasets is compared with LHF based on in situ data and the objectively analyzed air–sea flux (OAFlux), in terms of tropical intraseasonal variability. The reanalysis products used in this study include the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-I), the Modern-Era Retrospective Analysis for Research and Applications (MERRA), three generations of reanalysis from the National Center for Environmental Prediction (NCEP R1, R2 and CFSR), and the twentieth century reanalysis (20CR). We find that the intraseasonal LHF of the reanalysis products agrees well with the OAFlux over the tropical oceans in terms of patterns, but there is a significant spread in amplitude among the reanalysis products. Both ERA-I and MERRA show smaller biases in the power spectral analysis, while the other reanalysis products (NCEP R1, NCEP R2, CFSR, and 20CR) tend to overestimate the intraseasonal LHF when compared with the TAO buoy products and OAFlux. The role of anomalous LHF in supporting the MJO convection identified by previous TAO buoy data studies is confirmed by the long-term global reanalyses. The feature of increasing LHF accompanied by growing MJO observed in the recent MJO field campaign in the central Indian Ocean (DYNAMO/CINDY2011) is also well captured by the reanalysis products. Among the reanalysis datasets, MERRA has the smallest bias in temporal variability of LHF during the DYNAMO/CINDY2011 period.

Influences of Boreal Summer Intraseasonal Oscillation on Heat Waves in Monsoon Asia

AbstractBy analyzing observation-based high-resolution surface air temperature (SAT) data over the Asian monsoon region (here called “monsoon Asia”) for 1981–2007, the modulation by boreal summer intraseasonal oscillation (BSISO) of heat wave (HW) occurrence is examined. Strong SAT variability and a high probability of HW occurrence on intraseasonal time scales are found consistently in the densely populated regions over central India (CI), the Yangtze River valley in China (YR), Japan (JP), and the Korean Peninsula (KP). The two distinct BSISO modes (30–60-day BSISO1 and 10–30-day BSISO2) show different contributions to HW occurrence in monsoon Asia. A significant increase in HW occurrence over CI (YR) is observed during phases 2–3 (8–1) of BSISO2 when the 10–30-day anticyclonic and descending anomaly induce enhanced upward thermal heating and sensible heat flux (warm advection) around the areas. On the other hand, the northeastward propagating BSISO1 is closely connected to the increased HW probability ...

Fundamentals of Tropical Climate Dynamics

In this chapter, free waves in a shallow water model system at the equatorial plane are first described, followed by a discussion of vertical mode separation. Then a number of simple or intermediate tropical atmospheric and oceanic models including the Gill model, the Lindzen–Nigam model, and the Cane–Zebiak model are introduced. These dynamic frameworks will be used in the subsequent chapters for theoretical and physical investigations. 1.1 Free Waves in an Equatorial Beta-Plane In this section we discuss what free waves exist in a linear shallow water model at an equatorial beta-plane [for details, readers are referred to Matsuno (1966)]. The linear shallow water model governing equations may be written as ∂u ∂t βyv 1⁄4 ∂φ ∂x ð1:1aÞ ∂v ∂t þ βyu 1⁄4 ∂φ ∂y ð1:1bÞ ∂φ ∂t þ C20 ∂u ∂x þ ∂v ∂y 1⁄4 0 ð1:1cÞ where C0 denotes gravity wave speed. The linear shallow water model equations fit for any vertical modes in a resting environment (see detailed derivation in Sect. 1.2), with only a difference in the value of C0. For the first baroclinic mode atmosphere, C0 1⁄4 50 m/s. For the first baroclinic mode ocean, C0 1⁄4 2.5 m/s. There are two parameters in Eq. (1.1), β and C0, which form two intrinsic scales: a length scale L 1⁄4 ffiffiffiffiffiffiffiffiffiffi C0=β p (equatorial Rossby radius of deformation) and a timescale τ 1⁄4 1= ffiffiffiffiffiffiffiffi βC0 p . Let C0 as velocity scale and C20 as a scale for φ. Using these characteristic scales, Eq. (1.1) can be transferred into a set of nondimensional equations:

Role of scale interactions in the abrupt change of tropical cyclone in autumn over the Western North Pacific

Tropical cyclone (TC) activity in autumn (September–November) over the western North Pacific experienced an abrupt change in 1998, which can be detected by the Bayesian change-point analysis. During the decade before the regime shift (1988–1997), the occurrence frequency of TC genesis increased significantly over the tropical western Pacific, where the seasonal cyclonic flow, intraseasonal oscillation (ISO) and synoptic-scale eddy (SSE) were all strengthened, compared to those observed in the decade after 1998 (1998–2007). The TC trajectories also exhibited spatial differences. During the active decade, the TCs had a higher probability to move westward into the Philippine Sea and the South China Sea, and recurved northeastward toward the east of Japan. Meanwhile, the northwestward propagating TCs approaching Taiwan and southeastern coast of China were reduced. To understand the role of mean flow–ISO–SSE interaction in the decadal changes of SSE and associated TC activity, we diagnosed a newly proposed SSE kinetic energy (KE) equation that separates the contributions of seasonal-mean circulation and ISO to the SSE. The results show that, during the active TC decade, the SSE obtained higher KE from both mean flow and ISO through eddy barotropic energy conversion when the enhanced SSE momentum flux interacted with the strengthened monsoon trough and vigorous ISO cyclonic anomaly over the western tropical Pacific. The increased SSE KE contributed positively to the increased TC genesis over the main genesis region (7.5°–20°N, 130°–170°E). It also benefited the growth of TCs over the Philippine Sea and the South China Sea during the active decade. The decadal change in TC frequency over the extratropics was related to the eddy baroclinic energy conversion instead of the barotropic conversion associated with scale interaction. During the active TC decade, SSE gained more (less) KE from the SSE available potential energy over the east of Japan (the East China Sea), favoring (disfavoring) the succeeding development of TCs in this region.

Extended-range forecast of spring rainfall in southern China based on the Madden–Julian Oscillation

Spring (March–May) rainfall after a dry period in winter has a substantial impact on agriculture and water management in populous southern China. The occurrence of low-frequency spring rainfall anomalies has been linked with the tropical Madden–Julian Oscillation (MJO) through its modulation of large-scale circulation and moisture supply over southern China. Using the spatial and temporal information of the MJO as a predictor, an empirical model for extended-range forecasting of spring rainfall in southern China was constructed. We first obtained the coupled patterns between the preceding MJO evolutions (real-time multivariate MJO index) and the succeeding rainfall variability in southern China based on singular value decomposition analysis. Then, a prediction was carried out by projecting the predictor onto the spatiotemporal coupled patterns. Useful skill, in terms of the temporal correlation coefficient (TCC) between the predicted and observed rainfall over southern China, persisted up to a forecast lead-time of six pentads. The forecast amplitude bias in terms of root-mean-square error was around 1.0 standard deviation. Also, the forecast skill was highly dependent on the strength of the MJO signal. During active MJO periods, the TCC skill was around twofold larger than that during weak MJO periods. The current statistical model shows encouraging ability, but additional work is required to improve its forecasting skill.

Tropical Cyclone Formation

Introduction: The formation (genesis) of tropical cyclones (TCs) is a complicated process that involves interactions among multi-scale circulations. While tropical disturbances exist all the time, only a few of them may develop into TCs. In this study, the daily global analysis from the Navy Operational Global Atmospheric Prediction System (NOGAPS) is examined to understand why some disturbances develop into TCs while others do not. Significantly different characteristics of atmospheric variables associated with developing and nondeveloping disturbances are found. The most important parameter controlling TC formation is vertical wind shear in the North Atlantic (NA) and low-level, large-scale convergence in the western North Pacific (WNP). The result suggests that different cyclogenesis mechanisms may operate for TC formation in the two oceanic basins.

Monsoon Dynamics and Its Interactions with Ocean

In this chapter, the basic concept of the monsoon is introduced, followed by the discussion of monsoon variability on the quasi-biennial and lower-frequency timescales. The mechanisms responsible for the tropospheric biennial oscillation (TBO) and the monsoon – warm ocean interaction – are discussed. Next, how the ENSO affects the monsoon variability over East Asia and South Asia and how the remote and local SST anomalies affect western Pacific circulation anomalies are discussed. Finally, the physical mechanisms responsible for the in-phase and out-of-phase relationships among the Indian monsoon, western North Pacific monsoon, and Australian monsoon are discussed.

Madden-Julian Oscillation: Observations and Mechanisms

In this chapter, the observed features of the Madden–Julian Oscillation (MJO) are first introduced, followed by the discussions of physical mechanisms responsible for the eastward propagation and initiation of MJO. Next the boreal summer intraseasonal oscillation (BSISO) is described, with a special emphasis on the mechanisms responsible for its northward propagation over the monsoon regions. Finally the interaction of MJO or BSISO with higher-frequency motions is discussed.

Roles of Air–Sea Interaction in Shaping Tropical Mean Climate

In this chapter, the observed characteristics of the mean climate such as the equatorial asymmetry of the intertropical convergence zone (ITCZ) and the equatorial annual cycle in the tropical Pacific are first described. Next the physical mechanisms responsible for the ITCZ asymmetry and the annual cycle are discussed, from theoretical analysis and idealized modeling perspectives.

Dynamics of El Niño–Southern Oscillation

In this chapter, the observed characteristics of the El Nino – Southern Oscillation (ENSO) is described, followed by the discussion of the ENSO instability and oscillatory theories. Next, the mechanisms of its phase locking to the annual cycle are discussed. Through the oceanic mixed-layer budget analysis, physical mechanisms responsible for El Nino and La Nina amplitude asymmetry and El Nino and La Nina evolution asymmetry are investigated. How the interdecadal mean state modulates the El Nino behavior is further discussed. Finally, an air–sea interaction mode with a characterized zonal-dipole structure in the tropical Indian Ocean is described.