Jiacan Yuan

The Relationship between the North Atlantic Jet and Tropical Convection over the Indian and Western Pacific Oceans

AbstractBoreal winter jet variability over the North Atlantic is investigated using 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) data, where the variability is defined by the first EOF of the zonal wind on seven vertical levels. The principal component time series of this EOF is referred to as the jet index. A pattern correlation analysis indicates that the jet index more accurately describes intraseasonal North Atlantic zonal wind variability than does the North Atlantic Oscillation (NAO). A series of composite calculations of the jet index based on events of intraseasonal convective precipitation over the tropical Indian and western Pacific Oceans reveals the following statistically significant relationships: 1) negative jet events lead enhanced Indian Ocean precipitation, 2) positive jet events lag enhanced Indian Ocean precipitation, 3) positive jet events lead enhanced western Pacific Ocean precipitation, and 4) negative jet events lag enhanced western Pacific Ocean p...

The East Pacific Wavetrain: Its variability and impact on the atmospheric circulation in the boreal winter

The East Pacific wavetrain (EPW) refers to here the intense stationary wave activity detected in the troposphere over the East Pacific and North America in 45 northern winters from 1958 to 2002. The EPW is generated in the lower troposphere over the East Pacific, propagating predominantly eastward into North America and slightly upward then eventually into the stratosphere. The intensity of the EPW varies from year to year and exhibits apparent decadal variability. For the period 1958–1964, the EPW was in its second maximum, and it was weakest for the period 1965–1975, then it was strongest for the period 1976–1987. After 1987, the EPW weakened again.The intensity and position of the members (i.e., the Aleutian low, the North American trough, and the North American ridge) of the EPW oscillate from time to time. For an active EPW versus a weak EPW, the Aleutian low deepens abnormally and shifts its center from the west to the east of the date line, in the middle and upper troposphere the East Asian trough extends eastward, and the Canadian ridge intensifies at the same time. The opposite is true for a weak EPW. Even in the lower stratosphere, significant changes in the stationary wave pattern are also observed.Interestingly the spatial variability of the EPW assumes a Pacific-North American (PNA)-like teleconnection pattern. It is likely that the PNA low-frequency oscillation is a reflection of the oscillations of intensity and position of the members of the EPW in horizontal direction.

Wintertime North Pacific Teleconnection Patterns: Seasonal and Interannual Variability

AbstractThe teleconnections of the wintertime North Pacific are examined from the continuum perspective with self-organizing map (SOM) analysis. Daily ERA-Interim data for the 1979–2011 period are used. It is found that most of the North Pacific teleconnections can be grouped into several Pacific–North American (PNA)-like, western Pacific (WP)-like, and east Pacific (EP)-like SOM patterns. Each of the SOM patterns has an e-folding time scale of 7–10 days.The WP-like SOM patterns undergo a decline in their frequency from early to late winter, and vice versa for the EP-like SOM patterns, corresponding to an eastward seasonal shift of the North Pacific teleconnections. This seasonal shift is observed for both phases of the WP and EP patterns, and is only weakly sensitive to the phase of El Nino–Southern Oscillation. It is shown that the interannual variability of the PNA, WP, and EP can be interpreted as arising from interannual changes in the frequency of the corresponding SOM patterns. The WP- and EP-like ...

The Linkage between the Eastern Pacific Teleconnection Pattern and Convective Heating over the Tropical Western Pacific

AbstractThe eastern Pacific (EP) pattern is a recently detected atmospheric teleconnection pattern that frequently occurs during late winter. Through analysis of daily ERA-Interim data and outgoing longwave radiation data for the period of 1979–2011, it is shown here that the formation of the EP is preceded by an anomalous tropical convection dipole, with one extremum located over the eastern Indian Ocean–Maritime Continent and the other over the central Pacific. This is followed by the excitation of two quasi-stationary Rossby wave trains. Departing from the subtropics, north of the region of anomalous convection, one Rossby wave train propagates eastward along the East Asian jet from southern China toward the eastern Pacific. The second wave train propagates northward from east of Japan toward eastern Siberia and then turns southeastward to the Gulf of Alaska. Both wave trains are associated with wave activity flux convergence where the EP pattern develops. The results from an examination of the E vecto...

Amplified subtropical stationary waves in boreal summer and their implications for regional water extremes

The linkage between climate change and increased frequency/magnitude of weather extremes remains an open question in the scientific field.Here we investigate such a dynamical linkage by focusing on an amplification trend of the northern subtropical stationarywaves found in recent decades. Specifically, we show that inmultiplemodern reanalysis products, a robust positive trend exists in awave amplitude index defined through the summer-mean tropospheric stream function field. Pronounced changes in the subtropical atmospheric circulation accompany this wave amplification, including an intensified SouthAsianmonsoon and strengthened subtropical highs over theNorth Pacific andNorthAtlantic oceans. Throughmodifying the characteristics of large-scale moisture transport, these circulation changes are coupled to changes in the regional precipitation amount and the occurrence of water extremes including both droughts and heavy rainfall events. Given this connection, amplified stationarywaves have likely contributed to the elevated occurrence probabilities of droughts in the central United States,Mexico, Japan, and northernChina, as well as those of heavy rainfall events in SouthAsia, southeasternChina, and the easternUnited States. These results suggest that as climatewarming continues, the amplification of subtropical stationarywaves will increase the risk of water extremes over the above-mentioned regions.