Structure and dynamics of the summertime Pacific–Japan teleconnection pattern

Abstract

SUMMARYConvective activity over the tropical western Pacific is known to influence the extratropical circulation over East Asia in the boreal summer in the form of teleconnection, called the 'Pacific-Japan (PJ) pattern', but its structure and dynamics have not yet been studied in depth. In this study, a composite analysis is performed for 32 monthly events of enhanced convection observed to the east of the Philippines. The composited monthly mean vorticity anomalies associated with the PJ pattern are elongated zonally with a distinct poleward tilt with height. This structure differs fundamentally from a combination of the first baroclinic mode in the tropics and the barotropic structure in midlatitudes, as has widely been accepted as a conceptual model of the PJ pattern. A waveactivity flux points polewards only in the lower troposphere, indicating that Rossby wave teleconnection occurs primarily through a low-level south-westerly jet. Those tilted anomalies over the western Pacific can effectively gain kinetic energy in the exits of the mean jet streams in the upper and lower troposphere and available potential energy (APE) in the presence of the vertically sheared jets. The enhanced convection can generate APE effectively, and the associated low-level anomalous circulation acts to increase moisture supply into the convective region while enhancing evaporation from the pre-warmed ocean surface. It is thus hypothesized that the PJ pattern may be regarded as a dynamical mode that can be effectively excited in the zonally asymmetric baroclinic mean flow associated with the Asian summer monsoon with an efficient self-sustaining mechanism through moist processes.

Impacts of tropical Indian Ocean SST on the meridional displacement of East Asian jet in boreal summer

“…Kosaka and Nakamura (2006) found the PJ teleconnection pattern not only exists at lower level, but also at upper level. In addition, their study showed that the teleconnection pattern tilts northward with height.…”

“…Our analysis confirms these results ( Figure 5). Anomalous precipitation near Japan and the vertical structure of the 2078 X. QU AND G. HUANG mean flow may be responsible for the tilt (Kosaka and Nakamura, 2006;Lu and Lin, 2009). Thus, over the coast of East Asia, the mid-latitude cyclonic wind anomalies at upper level are slightly poleward in comparison to those at low level (Figure 1(a) andFigure 4).…”

Two major modes of variability of the East Asian summer monsoon

“…Looking at the first EOF (Figure 2(a)), we begin by noting that the spatial anomalies at both 850 hPa and 200 hPa are similar to those associated with the negative phase of the so-called Pacific-Japan (PJ) pattern (Nitta, 1987;Kosaka and Nakamura, 2006). At 850 hPa, the circulation anomalies are dominated, in the positive phase, by a dipole pattern with an anticyclonic anomaly in the subtropical western Pacific and a cyclonic anomaly centred near Japan.…”

“…Indeed, while it is clear that anomalous diabatic heating over the Indian Ocean/maritime continent region is important in the dynamics of both modes, the main difference between the two EOFs occurs at middle latitudes and appears to be related to the way in which these two circulation anomalies interact with the westerly jet stream. That the midlatitude westerlies play a role in the dynamics of the PJ-pattern (and hence EOF-1) is clear from the work of Kosaka and Nakamura (2006) and Sampe and Xie (2010). However, the role of the midlatitude westerly in the dynamics of EOF-2 remains a topic for future research.…”

“…At 200 hPa (not shown), the model captures anomalous cross-equatorial flow between 90 • E and 120 • E found in the reanalysis. However, in the ERA-40 data the circulation anomalies become increasingly equivalent barotropic in the middle latitudes (see Kosaka and Nakamura (2006), who note the northward tilt of the anomalies in middle latitudes), a feature that is much less evident in the model, presumably because the model lacks the eddy-mean flow interaction with the upper tropospheric westerly jet stream. We have also forced the model by the diabatic heating anomalies confined to the regions shown in Figure 7.…”