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 the initiation of the event through organization of tropical convection in the Indian Ocean by a subtropical Rossby wave train. This wave train also triggers a modal meridional dipole response in the west Pacific. The eastern Asia and western Pacific portion of this wave pattern is further reinforced by downstream propagation from the Indian Ocean convection region. The wave train creates the conditions in which synoptic cold surge events can occur over China. The propagation of these surges into the Indonesian region leads to markedly increased convection there. This process may be aided by the conditions created by a tongue of high potential vorticity that is advected equatorward and westward towards the Indonesian region by the flow associated with the dipole. The Indonesian convection gives rise to a North Pacific wave pattern and increased upper tropospheric, equatorial westerlies in the eastern Pacific.

Aspects of this scenario are supported with previous theoretical studies and new numerical model experiments. It describes a mixture of eastward propagation and the flaring of stationary features of tropical convection. However, it does not describe an oscillation. It is possible that equatorial Kelvin waves of very small magnitude do play a role in making such an oscillation possible and that the variable magnitude and period of the oscillation depend on the match of the extratropical structures with the Kelvin wave.

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 subtropical and polar jet streams, with these acting as waveguides. In the Northern Hemisphere a weak waveguide belt is found near 40°N around the whole hemisphere. The patterns simulated by the model are generally in good agreement with the teleconnectivity study described above. Both the observations and the model support the existence of the Pacific–South American pattern.

Abstract

An objective front detection method is applied to ERA5, CMIP5 historical, and RCP8.5 simulations to evaluate climate model performance in simulating front frequency and to understand future projections of seasonal front activities. The study area is East Asia for two natural seasons, defined as winter (2 December–14 February) and spring (15 February–15 May), in accordance with regional circulation and precipitation patterns. Seasonal means of atmospheric circulation and thermal structures are analyzed to understand possible factors responsible for future front changes. The front location and frequency in CMIP5 historical simulations are captured reasonably. Frontal precipitation accounts for more than 30% of total precipitation over subtropical regions. Projections suggest that winter fronts will decrease over East Asia, especially over southern China. Frontal precipitation is projected to decrease for 10%–30%. Front frequency increases in the South China Sea and tropical western Pacific because of more tropical moisture supply, which enhances local moisture contrasts. During spring, southern China and Taiwan will experience fewer fronts and less frontal precipitation while central China, the Korean Peninsula, and Japan may experience more fronts and more frontal precipitation due to moisture flux from the south that enhances wet-bulb potential temperature θ _w gradients. Consensus among CMIP5 models in front frequency tendency is evaluated. The models exhibit relatively high consensus in the decreasing trend over polar and subtropical frontal zone in winter and over southern China and Taiwan in spring that may prolong the dry season. Spring front activities are crucial for water resource and risk management in the southern China and Taiwan.

Abstract

Change in extreme events in climate projections is a major concern. If the frequency of dry events is expected to increase in a warmer climate (thus, the overall number of wet days will decrease), heavy and extreme precipitation are also expected to increase because of a shift of the precipitation spectrum. However, the forecasts exhibit numerous uncertainties.

This study focuses on the Asian region, separated into the following three subregions: the East Asian region, the Indian region, and western North Pacific region, where the summer monsoon can bring heavy rainfall. Particularly emphasized herein is the reliability of the projection, using data from a large ensemble of 30 models from phase 5 of the Coupled Model Intercomparison Project. The scattering of the ensemble enables obtaining an optimal estimate of the uncertainties, and it is used to compute the correlation between projected changes of extreme events and circulation changes.

The results show clear spatial and temporal variations in the confidence of changes, with results being more reliable during the wet season (i.e., the summer monsoon). The ensemble predicts changes in atmospheric circulation with favorable confidence, especially in the low-level moisture flux convergence (MFC). However, the correlation between this mean change and the modification of extreme events is nonsignificant. Also analyzed herein are the correlation and change of MFC exclusively during these events. The horizontal MFC exerts a nonnegligible influence on the change in the intensity of extremes. However, it is mostly the change in vertical circulation and moisture advection that is correlated with the change in frequency and intensity of extreme events.

Abstract

This paper reports a new finding and related mechanism: the forcing effect of the tropical Atlantic (TA) sea surface temperature (SST) on the atmosphere–ocean coupling in the western North Pacific (WNP) and northern Indian Ocean (NIO). Since the early 1980s, the TA SST has increased and, notably, exhibited an enhanced interannual statistical relationship with the WNP subtropical high and NIO SST in boreal summer. Empirical diagnostics reveal the following spatial pattern linking the TA SST and the atmosphere–ocean coupling in the Pacific and Indian Ocean: 1) a cyclonic (anticyclonic) circulation pair straddling the equator over the eastern Pacific, 2) an anticyclonic (cyclonic) circulation pair straddling the equator in the WNP and Indian Ocean, 3) overturning circulation with ascending (descending) and descending (ascending) anomalies over the TA and tropical western Pacific, respectively, and 4) positive (negative) SST anomaly in the TA and NIO.

The characteristics of this pattern are consistent with those of a WNP–NIO coupling pattern identified in a previous study. Empirical diagnostics and numerical simulations indicate that the TA SST serves as a forcing to induce low-level divergence and streamfunction anomalies in the Indian Ocean and the western Pacific. The latter in turn induces anomalous heat storage in the NIO and enhances the WNP–NIO coupling system, which is an intrinsic pattern engendered by the atmosphere–ocean interaction in the region. Without the remote influence of the TA SST forcing, the WNP–NIO coupling pattern and its impacts on the summer monsoon and TC variability in South Asia, East Asia, and the WNP would be considerably less significant than observed.

Abstract

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.

Abstract

This study examined the impact of northward- and westward-propagating summertime intraseasonal oscillations (ISOs) on submonthly wave patterns and tropical cyclones (TCs) in the subtropical western North Pacific. In the ISO westerly phase, submonthly wave patterns associated with the northward-propagating ISO appeared to be more energetic and most of the corresponding TCs maintained their wind speed for a relatively long period. Perturbation kinetic energy exhibited a stronger maximum in the ISO northward mode than in the westward mode. The analysis of barotropic conversion in the ISO northward mode revealed that an increase in barotropic conversion can be attributed to a strong association between the perturbation zonal wind component and the background flow. Therefore, submonthly wave patterns moving in a direction similar to that of the northward-propagating ISO continuously extracted energy from the background flow to the south of the submonthly base region. However, in the westward mode, the ISO propagating in a direction almost perpendicular to the submonthly wave pattern tracks not only altered the direction of the wave pattern but also created a background environment that was detached from submonthly perturbations. Thus, the background flow transferred less energy to submonthly wave patterns, resulting in shorter TC durations in the ISO westward mode than in the northward mode.

Abstract

The development of tropical cloud clusters (TCCs) to tropical cyclones (TCs) is the process of TC formation. This study identifies five main environmental transitions for the development of TCCs to TCs in the western North Pacific by using a cluster analysis method. Of these, three transitions indicate TCCs that develop in monsoon environments and two in easterly environments. Their numbers, distributions, and interannual variability differ. On average, the development time, defined as the period from the TCC forming to it developing into a TC, for TCCs that develop in easterly environments is shorter than that in monsoon environments. For the development of TCC to TC in easterly environments, TCCs have fewer embedded mesoscale convective systems (MCSs), which are located closer to the TCC center. Moreover, there is a stronger inward short-term (less than 10 days) angular momentum flux (AMF) at middle levels (800–500 hPa) before TCC formation. Conversely, in monsoon environments, TCCs have more MCSs, which are located farther from the TCC center. A stronger inward short-term AMF at low levels (1000–850 hPa) is observed before TCC formation and develops upward during the development of TCC to TC. The characteristics of MCS and AMF are significantly correlated with the development time of TCC to TC. In summary, large-scale easterly and monsoon environments cause TCCs to have different MCS and AMF characteristics, leading to higher efficiency for TCCs developing into TCs in easterly environments compared to monsoon environments.

Abstract

This study uses a nonhierarchical cluster analysis to identify the major environmental circulation patterns associated with tropical cloud cluster (TCC) formation in the western North Pacific. All TCCs that formed in July–October 1981–2009 are examined based on their 850-hPa wind field around TCC centers. Eight types of environmental circulation patterns are identified. Of these, four are related to monsoon systems (trough, confluence, north of trough, and south of trough), three are related to easterly systems (low-latitude zone, west of subtropical high, and southwest of subtropical high), and one is associated with low-latitude cross-equatorial flow. The genesis potential index (GPI) is analyzed to compare how favorable the environmental conditions are for tropical cyclone (TC) formation when TCCs form. Excluding three cluster types with the GPI lower than the climatology of all samples, TCCs formed in monsoon environments have larger sizes, lower brightness temperatures, longer lifetimes, and higher GPIs than those of TCCs formed in easterly environments. However, for TCCs formed in easterly environments, the average GPI for those TCCs that later develop into TCs (developing TCCs) is higher than that for other TCCs (nondeveloping TCCs). This difference is nonsignificant for TCCs formed in monsoon environments. Conversely, the average magnitudes of GPI are similar for developing TCCs, regardless of whether TCCs form in easterly or monsoon environments. In summary, the probability of a TCC to develop into a TC is more sensitive to the environmental conditions for TCCs formed in easterly environments than those formed in monsoon environments.

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.