Huang-Hsiung Hsu

Rossby Wave Propagation and Teleconnection Patterns in the Austral Winter

Abstract Observational evidence of and theoretical support for the Northern and Southern Hemisphere teleconnection patterns in the austral (Southern Hemisphere) winter are examined through an upper troposphere streamfunction teleconnectivity map and time-lag cross-correlation analysis using ECMWF initialized analysis 2OO-hPa winds for the 11 June–August periods from 1979 to 1989. As was previously found for the Northern Hemisphere winter, the regions of strong teleconnectivity, particularly in the winter hemisphere, tend to he oriented in the zonal direction and coincide with the location of the major jet streams. Although equatorward propagation from the Northern and Southern Hemispheres is observed, little evidence of cross-equatorial propagation has been found. For comparison, the response of a barotropic model, linearized about a climatological 300-hPa June–August time-mean flow to localized forcing is determined. It is found that the activity tends to be trapped inside each of the Southern Hemisphere...

The 1985/86 Intraseasonal Oscillation and the Role of the Extratropics

Abstract An intraseasonal oscillation that occurred in the 1985/86 northern winter is documented in this study. The tropical convection of this event is dominated by the mixture of a standing oscillation over the maritime continent and an eastward moving feature from the Indian Ocean into the central Pacific. The time evolution of the upper tropospheric circulation patterns, instead of propagating eastward along the equator as suggested in the existing composites of the intraseasonal oscillation, is characterized by a series of wave patterns in the Northern Hemisphere and does not complete the cycle around the globe. The familiar moist Kelvin wave explanation for the intraseasonal oscillation receives little support from diagnosis of this event using zonal wind, height field, streamfunction, and potential vorticity. Only in the lower troposphere near the date line is the convincing evidence for its existence found. A scenario for the intraseasonal oscillation, which is suggested by the analysis, includes ...

Time Variation of 500 mb Height Fluctuations with Long, Intermediate and Short Time Scales as Deduced from Lag-Correlation Statistics

Abstract The time variation of Northern Hemisphere wintertime 500 mb height fluctuations with short, intermediate and long time scales is investigated, using lag-correlation patterns derived from time-filtered data. Fluctuations with short (2.5–6 day periods) time scales propagate eastward at a rate consistent with the notion of a steering level around 700 mb, which supports an interpretation in terms of baroclinic waves. The mobile teleconnection patterns associated with the intermediate (10–30 day periods) time scales exhibit a pattern of time variation suggestive a Rossby-wave dispersion, with a predominance of southward dispersion from middle latitudes into the tropics. The geographically fixed teleconnection patterns characteristic of the longer time scales do not show a well-defined pattern of time variation, but their horizontal structure resembles that of the fastest growing normal mode associated with barotropic instability of the climatological mean wintertime flow.

Evolution of Large-Scale Circulation and Heating during the First Transition of Asian Summer Monsoon

This study investigates the characteristics of large-scale circulation and heating during the first transition of the Asian summer monsoon by a compositing technique. The first transition is characterized by a sudden change in large-scale atmospheric circulation and convective activity in South and Southeast Asia. The most notable features include 1) the development of the low-level cyclonic circulation and the upper-level anticyclone in South Asia, 2) the strong convection in the Bay of Bengal, the Indochina peninsula, and the South China Sea, and 3) the warming and the subsequent cooling of the SST in the Bay of Bengal. Results show the close relationship between the fluctuations of atmospheric circulation, heating, and surface condition. It is suggested that the atmospheric circulation abruptly changes during the transition owing to the interaction between convection, large-scale circulation, and lower-boundary forcing that includes topographically lifting ocean and land surface heating.

Global Teleconnections in the 250-mb Streamfunction Field during the Northern Hemisphere Winter

Abstract Teleconnections of the streamfunction in the global domain based on ECMWF 250-mb winds for the 11 northern winters from 1978/79 through 1988/89 are documented in this study. A zonal structure with a node near the equator, indicating an out-of-phase relationship between the streamfunctions in the Northern and Southern hemisphere, appears to mask the fluctuations of the asymmetric components of streamfunction. After removing zonal means, a global pattern emerges as the dominant structure in the low-frequency band. This pattern consists of several dipoles straddling either the exit region of midlatitude jets or the equator, indicating the existence of teleconnections not only between the midlatitudes and the tropics but also between the Northern and Southern hemispheres. Teleconnection patterns in the intermediate-frequency band are predominantly wavelike. Seven waveguides are identified based on the one-point lag-correlation maps for base points near the maximum teleconnectivity. Among them are thr...

Vertical Structure of Wintertime Teleconnection Patterns

Abstract Orthogonal rotated principal component analysis of the wintertime, Northern Hemisphere, 5-day mean sea level pressure field yielded five modes which are of some dynamical interest. One can be identified with the well-known North Atlantic Oscillation and another with the Pacific/North American pattern. Three of the other modes are highly baroclinic in the sense that their sea level pressure patterns and their associated 500 mb height patterns are different in shape and opposite in polarity over substantial areas. These more baroclinic patterns attain their largest amplitudes in the vicinity of the Himalayas and Rockies. Their spatial patterns evolve very differently in the lower and middle troposphere: the sea level pressure patterns exhibit a distinctive eastward and/or equatorward phase propagation, parallel to contours of surface elevation, along the northern and/or eastern side of the mountain ranges, while the corresponding 500 mb patterns evolve in a manner consistent with the concept of Ros...

Compounding effects of warm sea surface temperature and reduced sea ice on the extreme circulation over the extratropical North Pacific and North America during the 2013–2014 boreal winter

Unprecedented atmospheric circulations with extreme weather were observed in the extratropical Northern Hemisphere during the winter of 2013–2014. The anomalous circulations were the manifestation of the Pacific pattern or the North Pacific Oscillation/Western Pacific pattern but with extremely large amplitude. Simulation results suggest that the anomalous atmospheric circulations were constructively induced by anomalous sea surface temperature in the tropical Pacific and extratropical North Pacific, as well as the low sea ice concentration in the Arctic. Natural variability played a major role in inducing the anomaly pattern, whereas the anomalously warm sea surface temperature and low Arctic sea ice concentration in the Bering Sea contributed to the intensity. If the anthropogenic warming has a significant impact on causing the synchronization of the aforementioned anomalies in sea surface temperature and sea ice concentration and this trend continues, severe winters similar to that in 2013–2014 may occur more frequently in the future.

Contrasting Characteristics between the Northward and Eastward Propagation of the Intraseasonal Oscillation during the Boreal Summer

Abstract This study investigates the structural and evolutionary characteristics of the eastward- and northward-propagating intraseasonal oscillation (ISO) in the Indian Ocean and western Pacific during the boreal summer. Along the equator, the near-surface moisture convergence located to the east of the deep convection region appears to result in the eastward propagation of the ISO, consistent with the frictional wave–CISK (conditional instability of the second kind) mechanism proposed in previous studies. The eastward propagation is characterized by sequentially downstream development of deep convection occuring mainly in certain regions such as 60°, 95°, 120°, and 145°E, and the date line. The northward propagation of deep convection can be attributed to the low-level moisture convergence located to the north. This convergence is a deep structure extending from the surface to the middle troposphere. Near-surface convergence appears only after the systems approach the landmass in the north. It is sugges...

Global impacts of the 1980s regime shift

Abstract Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change‐point analysis and a sequential t‐test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earths biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur.

Topographic Effects on the Eastward Propagation and Initiation of the Madden-Julian Oscillation

Abstract This study investigates the relationship between deep convection (and heating anomaly) in the Madden–Julian oscillation (MJO) and the tropical topography. The eastward propagation of the deep heating anomalies is confined to two regions: the Indian Ocean and the western Pacific warm pool. Superimposed on the eastward propagation is a series of quasi-stationary deep heating anomalies that occur sequentially and discretely downstream in a leapfrog manner in the central Indian Ocean, the Maritime Continent, tropical South America, and tropical Africa. The deep heating anomaly, usually preceded by near-surface moisture convergence and shallow heating anomalies, tends to occur on the windward side of the tropical topography in these regions (except the central Indian Ocean) under the prevailing surface easterly anomaly of the MJO. It is suggested that the lifting and frictional effects of the tropical topography and landmass induce the near-surface moisture convergence anomaly, which in turn triggers ...