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Ka-Ming Lau
and
Paul H. Chan

Abstract

Various aspects of the intraseasonal variability (ISV) of tropical convection are studied using outgoing longwave radiation (OLR) data. From empirical orthogonal function analysis, it was found that the most dominant mode of the ISV of tropical convection consists of an east-west oriented dipole that propagates eastward at a speed of approximately 4–5 m s−1 over the equatorial Indian/Western Pacific Ocean but stalls and intensifies as it reaches cast of Indonesia and the equatorial central Pacific. When one center of the dipole is established over Indonesia, the associated anomalous convection extends from the equator southeastward over Northern Australia to the Southern Hemisphere midlatitudes. The oscillation covers a wide range of periods ranging from 30–60 days and is likely to affect strongly the transient fluctuation of the Pacific Walker circulation and the South Pacific Convergence Zone. The foregoing features are identified with the so-called “40–50” day oscillation in the tropics.

Distinct patterns of extratropical OLR anomalies are found to evolve coherently with this “40–50” day oscillation. When the dipole centers are situated over the Indian Ocean and the equatorial western Pacific, major anomalous convection centers are found over the Asian sector. When the centers are shifted about 3000–4000 km (about one-quarter wavelength) eastward to Indonesia and the equatorial central Pacific, the most pronounced extratropical feature is found over the eastern North Pacific. These features reflect possible connection between tropical and extratropical convection/circulation systems. Because of the seesaw nature in the variation of a tropical heat source/sink, we suggest that extratropical anomalies arising from tropical forcing are associated with changes in the overall tropical beating distribution rather than just from a local source.

We also present some evidence of a plausible connection between the “40–50” day mode and the monthly and interannual variability in tropical convection. It is shown that large fluctuations in the monthly averages of OLR over the western and central Pacific are mainly due to the modulation by the “40–50” day oscillation over these regions. A possible relationship between this oscillation and the El Niñ/Southern Oscillation is discussed.

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Ernest C. Kung
and
Paul H. Chan

Abstract

Energetics characteristics of the Asian winter monsoon are studied with twice-daily upper air data during a 20-year period over its source region. It is found that the energetics features over Siberia, North-eastern Asia, China Main and the Japan Sea, are distinctly different reflecting the different roles and characteristic flow patterns of these regions in the system of the winter monsoon.

Siberia, under the dominance of the anticyclonic flow, shows a general adiabatic destruction of kinetic energy through cross-isobaric motion. Northeastern Asia, under the influence of a major cyclonic system, is dominated by typical patterns of energy transformations as observed in most areas of transient synoptic disturbances. The strong westerlies dominate over the general area of the China Main, East China Sea and Japan Sea and the kinetic energy is intensely generated in this portion of the flow. Most of the kinetic energy generated over China Main is exported. Over the Japan Sea area where the series of cyclonic disturbances develops, a strong dissipation takes place.

During the cold air outbreaks, which are recognized with the intrusion of a strong cyclonic upper level flow into China Main, the kinetic energy generation and dissipation are drastically increased in the general area of the China Main, East China Sea and Japan Sea. However, basic energetics features of different regions in the prevailing systems of the Asian winter monsoon remain unchanged. A case study of a cold air outbreak is also presented.

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Ka-Ming Lau
and
Paul H. Chan

Abstract

Satellite-inferred short-term climate variability and atmospheric teleconnections are studied using seven years (1974–81) of Outgoing Longwave Radiation (OLR) data from NOAA polar orbiters. This study utilizes composite, partition-of-variance and multiple correlation techniques to investigate the simultaneous relationship in OLR fluctuation between remotely separated regions of the globe.

Results show that two dominant modes of variation are present in the monthly anomalous OLR time series fluctuation in the tropics. They are 1) variations of 2–3 month time scale associated with quasi-stationary fluctuation in the Walker Circulation and 2) an abrupt shift in the mean level of fluctuation related to the Southern Oscillation. The distribution of diabatic heat sources and sinks appears to exhibit a dipole-like oscillation that alternates between dry and wet periods over the equatorial central Pacific region and the maritime continent of Indonesia. The wet period is characterized by strong convection over the equatorial central Pacific, an eastward migration of the South Pacific convergence zone, and an equatorial migration of the ITCZ over the central and eastern Pacific, resulting in considerable shrinkage of the eastern Pacific dry zone. The dry period corresponds to intense convection over the maritime continent and an extensive eastern Pacific dry zone. While one major transition from dry to wet conditions is believed to occur from March 1976 to December 1977 associated with the 1975–76 El Niño, minor transitions between the two periods also occur frequently in the time scale of 2–3 months.

The major significant teleconnections using the equatorial central Pacific OLR fluctuation as reference are (identified by key geographical areas, with the sense of the correlation denoted by the signs in parentheses):
i1520-0469-40-12-2735-e1

It is suggested that the dry period over the equatorial central Pacific corresponds dynamically to stronger zonal teleconnections by equatorially trapped Kelvin and low-order Rossby-type response, and the wet periods, to stronger downstream meridional teleconnections characteristic of external higher-order Rossby-wave response to tropical diabatic forcing.

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Ka-Ming Lau
and
Paul H. Chan

Abstract

As a sequel to Part I of this study, lagged relationships in atmospheric teleconnections associated with outgoing longwave radiation (OLR) are investigated using Lagged Cross Correlations (LCC). The feasibility of extratropical seasonal-to-interannual predictions using satellite-derived observation is also quantitatively assessed. It is found that the global influence of teleconnectivity of the atmosphere is strongest for diabatic forcing located near the equatorial central Pacific, but much reduced for forcings over the maritime continent and to the east of the dateline. The LCC patterns show that at zero-lag, the OLR fluctuation over the equatorial central Pacific is associated with simultaneous excitation of quasi-stationary waves in the tropics. These tropics–tropics teleconnections eventually (in about 5 months) transform into tropics–midlatitude and midlatitude–midlatitude teleconnections associated with possible excitation of extratropical quasi-stationary waves in both hemispheres.

Analysis of the LCC pattern with the Southern Oscillation (SO) signal removed shows that during 1974–81, both the SO signal and the variability in the 2–3 month time scale contribute substantially to the observed LCC patterns. The presence of a convective heat source in the equatorial central Pacific is found to be important in forcing the tropics–midlatitude and the midlatitude–midlatitude teleconnections, which appear also to be phase-locked with the normal seasonal cycle. A mechanism is proposed to explain the observed lagged relationships. This mechanism is consistent with both internal atmospheric dynamics related to the seasonal cycle and with external influences such as sea surface temperature anomalies associated with the El Niño/Southern Oscillation.

Initial assessment of the predictability of regional climate using satellite-derived atmospheric teleconnection shows that about 30–40% of the wintertime OLR variance over the southeastern United States is accounted for by a 5-month antecedent OLR variation over the equatorial central Pacific. Because of the close relation between OLR variation and synoptic disturbances, the satellite-derived teleconnections described in Part I and Part II of this study can be used to identified regions with potentially higher seasonal-to-interannual predictability.

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Ka-Ming Lau
and
Paul H. Chan

Abstract

Intraseasonal and interannual variations of tropical convection are studied using 12 years of NOAA outgoing longwave radiation (OLR) data. The spatial patterns of OLR variance in the 1–5 day, 20–70 day and longer-than-180 day bands am examined separately for the northern summer (May–October) and winter (November&ndash April). In the 1–5 day band, the most prominent features are the Pacific and Atlantic ITCZs during the northern summer and the large variability over the western Pacific and the tropical continents during the northern winter. For the time scales beyond 10 days, it was found that the maximum variance shifts north and south of the equator from summer to winter but is generally located between 20°S and 20°N in the longitudinal sector between the Indian and Pacific oceans. Over the Indian Ocean and the western Pacific, variations with 30–60 day period are most pronounced. In the central and eastern Pacific variations with periods over one year are more prevalent.

The natural variability of tropical convection is estimated. Results show that the climate signal (interannual variability) in convection is 1argest and most easily detectable over the equatorial central Pacific. Results of empirical orthogonal function analyses show that the most dominant mode of variation in tropical convection exhibits multiple time scales that can be identified respectively with the 40–50 day oscillation, the annual cycle and an aperiodic variation with several year' time scale. These variations are associated with fluctuations of the equatorial Walker circulation. It is suggested that an interaction of these time scales may be instrumental in leading to the onset of ENSO. The possibility of the 40–50 day oscillation as a trigger to ENSO is further supported by the seemingly systematic frequency and amplitude modulation of these oscillations before and after the 1982–83 ENSO. Discussion of the results in light of recent development in the dynamics of the coupled ocean-atmosphere system is also presented.

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