The ocean–atmosphere coupling in the northeastern subtropical Pacific is dominated by a Pacific meridional mode (PMM), which spans between the extratropical and tropical Pacific and plays an important role in connecting extratropical climate variability to the occurrence of El Niño. Analyses of observational data and numerical model experiments were conducted to demonstrate that the PMM (and the subtropical Pacific coupling) experienced a rapid strengthening in the early 1990s and that this strengthening is related to an intensification of the subtropical Pacific high caused by a phase change of the Atlantic multidecadal oscillation (AMO). This PMM strengthening favored the development of more central Pacific (CP)-type El Niño events. The recent shift from more conventional eastern Pacific (EP) to more CP-type El Niño events can thus be at least partly understood as a Pacific Ocean response to a phase change in the AMO.

SUMMARYUsing the reanalyses from the European Centre for Medium-Range Weather Forecasts, outgoing longwave radiation (OLR) and precipitation data for 1979-93, this paper examines major factors contributing to the onset of the Australian summer monsoon. The low-level (850 hPa) westerly wind and convective activity (OLR, precipitation) over a region in northern Australia (NAU) are used to determine the onset dates. Composite results are then derived, based on these dates. Daily apparent heat source and apparent moisture sink values are obtained as residuals of the large-scale heat and moisture budgets to help clarify the roles of the various heating processes in the onset. Four major factors contributing to the onset are identi ed: (1) land-sea thermal contrast, (2) barotropic instability, (3) arrival of the Madden-Julian oscillation (MJO) and (4) intrusion of midlatitude troughs. The thermal contrast due to differential heating between land and sea acts as a seasonal preconditioning for the onset. The sensible heating over the continent leads to a reversal of meridional temperature gradient between the Australian continent and the Arafura Sea in a layer below 800 hPa in September-March, and sets up a thermally induced meridional-vertical circulation which helps to transport low-level moist air inland. The barotropic instability criterion is often met at 850 hPa in NAU several days prior to the onset. The sudden onset is then triggered by the arriving MJO and at times by the intrusion of a midlatitude trough.

By estimating the differences between the original and tropical cyclone (TC)-removed fields derived from the 40-yr (ECMWF) Re-Analysis (ERA-40) and NCEP–NCAR 40-Year Reanalysis, this study reveals that TCs contribute significantly (exceeding 50% in certain regions) to the seasonal mean and the intraseasonal and interannual variance of the 850-hPa vorticity along the TC tracks in the tropical western North Pacific. Similar effects on the precipitation are also seen, as presented by the examples located in Taiwan. While the low-frequency, large-scale circulation produces a clustering effect on TCs, the latter, which has a large positive vorticity and tends to occur in the positive vorticity background flow, significantly enhances the strength of the positive vorticity. The contribution from TCs, which is not offset by the synoptic systems with weak negative vorticity, can therefore leave marked footprints in the climate signal and variability. This effect is not removed by long-term averaging and low-pass filtering, which are often used to retrieve the climate perturbations. This study reveals that the climate variability, as it is defined, is not contributed to merely by the low-frequency large-scale fluctuations. Instead, the TC effect has to be taken into account to understand the climate variability in the tropical western North Pacific. Subsequently, the ensemble effect of TCs, at least in the statistical sense, has to be resolved in the climate model to obtain a better simulation of the climate variability in the TC-prone region, such as the tropical western North Pacific.

[1] This study found that the spring (February -April) rainfall in northern Taiwan has fluctuated concurrently with the Pacific Decadal Oscillation (PDO) since the early 20th century. It is proposed that this fluctuation in spring rain is induced by the tropical sea surface temperature (SST) anomaly embedded in the PDO, which has been known to oscillate in opposite phase to the SST in the extra-tropical North Pacific. When the PDO index is positive (negative), the SST over the tropical Central-Eastern Pacific is warmer (colder) than normal, and a low-level anti-cyclonic (cyclonic) anomalous flow is induced over the Philippine Sea. This anomalous anti-cyclonic (cyclonic) flow results in southwesterly (northeasterly) anomalous winds to the east of Taiwan and enhances (reduces) the trough extending southwestward from southern Japan to northern Taiwan. As a consequence, more (less) spring rain occurs in northern Taiwan during the positive (negative) PDO phases.

The rainfalls associated with the Asian summer monsoon have significant correlation with succeeding Australian summer monsoon rainfalls. This is partly due to the typical life cycle of the El Niño-Southern Oscillation (ENSO) phase locked with the annual cycle within an Asian monsoon year (May-April). This feature is referred to as the symmetric behavior of the Asian-Australian monsoon system. However, preceding Australian summer monsoon rainfalls have no significant impact on succeeding Asian summer monsoon rainfalls. Thus, there is a one-way (asymmetric) relationship between the two monsoons and a communication gap exists in boreal spring prior to the onset of the Asian summer monsoon. The annual cycles of precipitation associated with the intertropical convergence zone (ITCZ) and the sea surface temperature (SST) anomaly in the Indonesia sector (90Њ-120ЊE) that links the two monsoon regions are compared. The continuous propagation of the ITCZ from the Asian to the Australian summer monsoon regions in Northern Hemisphere (NH) fall provides a one-way link between them. However, the SST anomaly shows a sudden change of sign in November, suggesting that the anomaly is mainly a response to the ITCZ movement rather than its cause. From January to May, the ITCZ is confined at the latitudes south of 5ЊN due to the subsidence in oceanic regions adjacent to the southern coast of Asia. This subsidence prevents the ITCZ from propagating northward until the abrupt onset of the Asian summer monsoon with the reversal of meridional temperature gradient. The discontinuity of the ITCZ movement and the seasonal heating over the Asian continent result in the communication gap in NH spring.