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K.-M. Lau and Hengyi Weng

In this paper, the application of the wavelet transform (WT) to climate time series analyses is introduced. A tutorial description of the basic concept of WT, compared with similar concepts used in music, is also provided. Using an analogy between WT representation of a time series and a music score, the authors illustrate the importance of local versus global information in the time–frequency localization of climate signals. Examples of WT applied to climate data analysis are demonstrated using analytic signals as well as real climate time series. Results of WT applied to two climate time series—that is, a proxy paleoclimate time series with a 2.5-Myr deep-sea sediment record of δ18 O and a 140-yr monthly record of Northern Hemisphere surface temperature—are presented. The former shows the presence of a 40-kyr and a 100-kyr oscillation and an abrupt transition in the oscillation regime at 0.7 Myr before the present, consistent with previous studies. The latter possesses a myriad of oscillatory modes from interannual (2–5 yr), interdecadal (10–12 yr, 20–25 yr, and 40–60 yr), and century (~180 yr) scales at different periods of the data record. In spite of the large difference in timescales, common features in time–frequency characteristics of these two time series have been identified. These features suggest that the variations of the earth's climate are consistent with those exhibited by a nonlinear dynamical system under external forcings.

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K-M. Lau and Hengyi Weng

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In this paper, the authors have identified three coherent modes of summertime rainfall variability over China and global sea surface temperature (SST) for the period of 1955–98 by Singular Value Decomposition. Based on these modes, the impacts of the 1997–98 El Niño on major drought and flood occurrences over China have been assessed. The first mode can be identified with the growing phase of El Niño superimposed on a warming trend since the mid-1950s. This mode strongly influences rainfall over northern China. The second mode comprises a quasi-biennial (QB) variability manifested in alternate wet and dry years over the Yangtze River Valley (YRV) of central China. The third mode is dominated by a quasi-decadal oscillation in eastern China between the Yangtze River and the Yellow River, with an opposite tendency in southern China.

Using a mode-by-mode reconstruction, the contributions of these leading modes to the 1997 and 1998 observed rainfall anomalies are evaluated. It is found that the severe drought in northern China, and to a lesser degree the flood in southern China, in 1997 is likely a result of the influence of anomalous SST forcing during the growing phase of the 1997–98 El Niño. The severe flood over YRV in 1998 is associated with the biennial tendency of basin-scale SST anomaly during the transition from El Niño to La Niña in 1997–98. In addition, the prolonged dry tendency over northern China and wet tendency over YRV since the 1970s may be associated with a long-term warming trend in the tropical Indian Ocean and western Pacific. The long-term dry background exacerbated the drought situation over northern China in 1997, and the wet background exacerbated the flood situation over YRV in 1998, under the impacts of the 1997–98 El Niño. In contrast, the rainfall variability in southern China is most chaotic, with no clear dominance of either El Niño or QB signals. The significance, reliability, and stability of these results are also discussed.

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K-M. Lau and Hengyi Weng

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Sea surface temperature (SST) data from the NOAA analysis for the period of 1955–97 are used to identify dominant spatial and temporal patterns associated with interannual, decadal–interdecadal variabilities, and global warming. For the global annual mean SST variation, a linear trend plus pronounced decadal–interdecadal and interannual variabilities are found. The linear trend reflects a slow warming of about 0.1°C in the past four decades. Superimposed on the linear trend is a relatively fast warming in the past two decades of approximately 0.2°–0.3°C, which is associated with the advent of the warm phase of a decadal–interdecadal oscillation. Also noted is the more frequent occurrence of warm events and the dearth of cold events on interannual timescales in the early and mid-1990s. For the last four decades, the tropical oceans have been warming at the faster rate while the extratropical North Pacific and the northwestern North Atlantic has been cooling at the slower rate. The exceptional intensity of the 1997 warm event stems from the superposition of the warm phases of a moderate interannual signal, a decadal–interdecadal oscillation, and a steady warming trend since 1955.

The first annual empirical orthogonal function (EOF) mode shows an El Niño signal, modified by a warming trend. In this mode, the tropical eastern Pacific and Indian Oceans are warmed relative to the western Pacific, leading to a relaxation of the climatological east–west SST gradient. The second annual EOF mode shows that there is a secondary cooling trend in the equatorial eastern Pacific, centered near the Niño-3 (5°S–5°N, 150°–90°W) area. This cooling is most pronounced in the boreal fall and summer. It was strongest in the 1980s, but considerably weakened in the 1990s. The cooling is well correlated with the observed surface wind divergence in the equatorial eastern Pacific cold tongue region and with anomalous surface easterly wind in the equatorial central Pacific, signaling an enhancement of the Walker circulation. The net result of the warming and cooling tendencies in the tropical eastern Pacific produces slightly less warming in the Niño-3 area compared with the equatorial central and eastern Pacific, but with little change in large-scale SST gradient across the Pacific. The implications of the present results on the mechanisms and detection of global warming are discussed.

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Song Yang and K-M. Lau

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The authors have conducted a series of experiments with a general circulation model to understand the influences of sea surface temperature (SST) and ground wetness (GW) (measured by snow amount and soil moisture content) on the Asian summer monsoon. The experiments are designed to illustrate the dominant features of monsoon response to SST and GW forcings and to delineate the relative importance of each forcing function in contributing to the variability of the monsoon.

Results indicate that ocean basin-scale SST anomalies exert a stronger control on the interannual variability of the monsoon compared to GW anomalies. The impact of SST anomalies on the monsoon appears nonlinear with respect to warm and cold events. The monsoon is weakened during the warm events but changes less noticeably during the cold events. The diminution of monsoon circulation associated with the warm SST anomalies is accompanied by a broad-scale reduction in water vapor convergence and monsoon rainfall.

Results also indicate that, following wet land surface conditions (enhanced snow and soil moisture) in the Asian continent during previous cold seasons, the summer monsoon becomes moderately weaker. Antecedent land surface processes mainly influence the early part of the monsoon. Wetter and colder conditions occur in the Asian continent during warm SST events. This results in reduced land–sea thermal contrast, which reinforces the weak monsoon anomalies produced initially by warm SST forcing. These interactive forcings are also responsible for the changes in the winter–spring westerlies over subtropical Asia, which are key precursory signals for the subsequent summer monsoon.

It should be pointed out that this study is conducted for the climate decade of 1979–88 only. The general robustness of the results needs to be explored by further investigations. In addition, chaotic features may have affected the results because of sampling errors.

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Hengyi Weng and K-M. Lau

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In this paper, preliminary results in using orthogonal and continuous wavelet transform (WT) to identify period doubling and time-frequency localization in both synthetic and real data are presented. First, the Haar WT is applied to synthetic time series derived from a simple nonlinear dynamical system-a first-order quadratic difference equation. Second, the complex Morlet WT is used to study the time-frequency localization of tropical convection based on a high-resolution Japanese Geostationary Meteorological Satellite infrared (IR) radiance dataset.

The Haar WT of the synthetic time series indicates the presence and distinct separation of multiple frequencies in a period-doubling sequence. The period-doubling process generates a multiplicity of intermediate frequencies, which are manifested in the nonuniformity in time with respect to the phase of oscillations in the lower frequencies. Wavelet transform also enables the detection of extremely weak signals in higher-order subharmonics resulting from the period-doubling bifurcations. These signals are either undetected or considered statistically insignificant by traditional Fourier analysis.

The Morlet WT of the IR radiance dataset indicates the presence of multiple timescales, which are localized in both frequency and time. There are two regimes in the variation of IR radiance, corresponding to the wet and dry periods. Multiple timescales, ranging from semidiurnal, diurnal, synoptic, to intraseasonal with embedding structures, are active in the wet regime. In particular, synoptic variability is more prominent during the wet phase of an intensive intraseasonal cycle. These are not only consistent with, but also show more details than, previous findings by using other techniques. The phase-locking relationships among the oscillations with different time-scales suggest that both synoptic and intraseasonal variations may be mixed oscillations due to the interaction of self-excited oscillations in the tropical atmosphere and external forcings such as annual and diurnal solar radiation variations.

Both examples show that WT is a powerful tool for analysis of phenomena involving multiscale interactions that exhibit localization in both frequency and time. A discussion on the caveats in the use of WT in geophysical data analysis is also presented.

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K-M. Lau and L. Peno

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In this paper, the mechanisms of northern summertime teleconnections are investigated using a barotropic model. In a series of numerical experiments we study the atmospheric response over the eastern Pacific-North America to an idealized local divergence source corresponding to the northward displacement of the ITCZ in the eastern Pacific. It is found that the response is much stronger in June than in May and is strongest when the forcing is located north of about 10°N. This can be explained in terms of the refractive properties of the climatoloigical summertime subtropical jet stream over North America. In another series of experiments we examine the global response as a function of the longitudinal location of the tropical forcing. A wave train emanating from the subtropics of the western Pacific near the Philippines, arching across the Aleutians and the Gulf of Alaska, and terminating with a high anomaly over the continental United States appears over a wide longitudinal range of local forcing, suggesting the existence of a normal mode for the northern summertime climatological flow. The normal-mode concept is supported by further experiments using extratropical forcings as well as free-mode integrations. The upstream anomalous low over the Gulf of Alaska is found to be essential for the development of the anomalous high over the continental United States. These results indicate that an above-normal high over the continent may occur when the anomalous forcing (both tropical and extratropical) acts to amplify the normal-mode structure. The caveats and implications of the present results to the possible linkage between tropical forcing and United States droughts are also discussed.

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K-M. Lau and Song Yang

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In this paper, a description of the global and regional features of the Asian summer monsoon in the Goddard Laboratory Atmospheres (GLA) general circulation model (GCM) is presented based on a 10-yr (1979–1988) integration of the model under the Atmospheric Model Intercomparison Project. It is found that the earliest signal of onset of the Asian monsoon is represented by the emergence of convection over the Indo-China “land bridge” in early May, followed by the sudden jump of the equatorial ITCZ (intertropical convergence zone) to 10°N over the South China Sea and Indian Ocean in mid-May. This sudden jump is linked to an abrupt northward shift of the ascending branch of the local Hadley circulation, possibly stemming from symmetric instability of the basic flow in May. In agreement with observations, the model shows a pronounced intraseasonal oscillation along the equator and enhanced synoptic-scale activities identified as easterly waves in the tropical and subtropical western Pacific. These intraseasonal and synoptic activities strongly regulate rainfall variability over East Asia, especially during the premonsoon period.

While the model simulates a similar range of variability in the broad-scale structure and evolution of the Asian monsoon as observed, there is considerable scope for improvement in the model's ability to simulate regional features of the monsoon. Compared to observed climatology, the model produces deficient rainfall over northern India but excessive rainfall over the South China Sea and the western Pacific region due to an overactive ITCZ at 10°N. The intraseasonal transition between the equatorial ITCZ and the monsoon (land) convection to the north is weaker than in nature. Moreover, the East Asian monsoon trough and Mei-yu rainband are under-developed in the model. These discrepancies are related to the limited northward extension of the model monsoon circulation and the confinement of the upper-level easterly flow near the equator.

Overall, the GLA GCM provides a reasonably realistic description of the seasonal and subseasonal climatology of the Asian monsoon and yields important information that sheds new light on the dynamical underpinnings of the multiscale variabilities associated with the Asian summer monsoon.

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K-M. Lau and L. Peng

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A theory of the origin of intraseasonal oscillations of the tropical atmosphere is presented and tested by simple model experiments. This study forces on the validation of the basic theory against key features of the observed 40–50 day oscillation. It is shown that the observed eastward propagation of intraseasonal oscillation in the tropical atmosphere arises as an intrinsic mode of oscillation resulting from an interaction of convection and dynamics via the so-called “mobile” wave-CISK mechanism. Through this mechanism, the heat source feeds on the east-west asymmetry of forced equatorial waves. As a result, Kelvin waves are selectively amplified, which in turn causes the heat source to propagate eastward. This mechanism also prevents small-scale waves from immediate destabilization, contrary to the results of traditional wave-CISK theory. The “mobile” wave-CISK establishes a new dynamics equilibrium state between convection and the wind field to form a wave packet or collective motion with relatively fixed horizontal and vertical structure. Relative to the steady state solutions with stationary heat source the new equilibrium state has suppressed Rossby-wave resonance to the west and enhanced Kelvin-wave response to the propagating heat source.

Results also suggest that the periodicity of the oscillation is determined by the time taken for the Kelvin wave to complete one circuit around the globe in the equatorial region. The propagation speed (∼19 m s−1) of the model disturbance, which is about twice as fast as the observed, is found to coincide with the real part of the complex phase speed of the model's unstable normal mode modified by internal heating. The speed and the growth rate are dependent on the vertical structure of the heating profile and the static stability of the basic gate. In addition to the eastward propagation, many observed features, such as pressure and wind distribution, amplitude modulation by SST, and dominance of low wavenumber response, are well simulated in the idealized experiments. The theory also predicts that the low-frequency disturbance should have a westward tilt with height. This is partially confirmed in real observation and in GCM simulations. While the basic theory appears to explain some fundamental feature of the 40–50 day oscillation, large discrepancies still exist The possibility of examining further detailed features of the oscillation in the present theoretical framework is also discussed.

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Jiayu Zhou and K-M. Lau

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The climatology and the basic state of the summertime circulation and rainfall over South America are studied using assimilation products from the data assimilation system of Goddard Earth Observing System-1 (GEOS-1) and satellite-derived rainfall. Results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical South American highland centered at the Altiplano Plateau. Sensitivity of the results to the assimilation scheme is tested by comparing that with the National Centers for Environmental Prediction (NCEP) reanalysis and with satellite rainfall estimates. Results show general agreement between the model produced rainfall anomaly and the satellite estimates, as well as consistency between the basic circulation features in the GEOS-1 and the NCEP reanalyses.

A case study of 1989–90 South American summer monsoon (SASM) reveals the following characteristics.

1) In late spring, the onset of SASM is signaled by an abrupt merging of the upper-tropospheric double westerly jets, one in the subtropics and the other in the subpolar region, into a single jet in the midlatitudes. This is followed by the establishment of a vortex to the southeast of Altiplano and occurrence of heavy precipitation over subtropical eastern Brazil.

2) During the mature phase of SASM, the heavy rainfall zone moves over the Altiplano Plateau and the southernmost Brazilian highland. The fully established SASM features are the following: (a) an enhancement of equatorial North Atlantic trade wind, which emanates from the Sahara high and crosses the equator over the South American continent; (b) a buildup of strong northwesterlies along the eastern side of the tropical Andes;and (c) development of the South Atlantic convergence zone in the southernmost position with strong convective activity. Meanwhile, the upper-tropospheric return flow emerges from an anticyclone formed over the Altiplano Plateau, crosses the equator, and sinks over northwestern Africa.

3) The withdrawal of SASM in late summer is signaled by the resplitting of the midlatitude westerly jet. At the same time, the low-level northwest monsoon flow diminishes, reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland.

Results also show that the above-mentioned characteristics of SASM are clearly linked to the tropospheric temperature changes over the central South American highland. Sensible versus latent heating over the highland are bound to play an important role in the evolution of SASM.

To provide further support of presence of a monsoon climate over South America, SASM is compared and contrasted to the “classic” east Asian summer monsoon (EASM). Many similar features, including evolution characteristics between the two systems, have been identified. Contrasting aspects of the SASM from the EASM are also discussed. It is pointed out that a number of monsoonal characteristics of the climate of South America, such as the seasonal reversal of the low-level wind, become apparent only when the strong annual mean wind is removed. Based on the characteristic features and their evolution, the authors conclude that a monsoon climate does exist over South America.

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K-M. Lau and L. Peng

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This is the third in a series of papers to study the origin of intraseasonal oscillations. In this paper, we address the issue of the interaction of the monsoon large-scale circulation and intraseasonal oscillations. We show that as a result of the interaction of the large scale monsoon flow with the near-equatorial intraseasonal oscillation, unstable baroclinic disturbances are generated over the monsoon region. These disturbances have spatial scales of approximately 3000–4000 km and periods of 5–6 days with the vertical wave axis tilting eastward with height. The rapid development of these cyclonic disturbances along 15°–20°N and the concomitant weakening of the equatorial disturbances are accompanied by the rapid northward shift of the rising branch of the local Hadley circulation. They may also be identified with the observed sudden jump of the Mei-yu rainband over East Asia and the inverse relationship between the monsoon ITCZ and the equatorial ITCZ over India and East Asia.

From a linear stability analysis of quasi-geostrophic motion in a two-level model, it is shown that the westward propagating disturbances generated over the monsoon region are the manifestation of heat-induced unstable Rossby waves. The instability is favored in the region with large vertical wind shear and reduced effective static stability. The monsoon large scale circulation over India and southeast Asia and the plentiful supply of moisture in the region appear to be favorable for the development of these unstable waves. It is argued that the prevailing easterly waves found over the subtropical western Pacific during northern summer may also be due to the above unstable Rossby wave mechanism.

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