1. Introduction
The split flow of the east Asian jet in the western North Pacific forms a preferred region of blocking in the downstream side of the jet between the Alaskan North Pacific and the Pacific Northwest (Rex 1950; Austin 1980; Lejenäs and Økland 1983). It is revealed from this jet split-flow structure that two basic factors are required for the formation and maintenance of blocking: the large-scale environmental flow and the internal dynamics of the atmosphere. As compiled by Benzi et al. (1986), blocking mechanisms include the index cycle (Kidson 1985), multiple equilibrium in the winter circulation (Charney and DeVore 1979; Wiin-Nielsen 1979), amplification of planetary waves by resonance (Tung and Lindzen 1979), and nonlinear interactions between planetary and cyclone waves (e.g., Colucci et al. 1981; Hansen and Chen 1982; Shutts 1983; Chen and Shukla 1983; Colucci 1985; Metz 1986). All of these mechanisms involve planetary-scale waves. It is likely that any variation in the large-scale environmental flow over the Alaskan North Pacific region may result in a possible change in the internal dynamics of blocking and eventually the blocking activity in this region.
The interannual climate variability caused by the Pacific–North American (PNA) teleconnection pattern associated with the El Niño–Southern Oscillation (ENSO) influences the winter atmospheric circulation in the Alaskan North Pacific. Negative (positive) geopotential height anomalies of the PNA pattern appear in this region during the warm (cold) ENSO phase (van Loon and Madden 1981; Horel and Wallace 1981; Wallace and Gutzler 1981). Some numerical experiments have been performed to explore the possible effect of the PNA-like teleconnection (induced by the tropical Pacific heating) on aspects of the North Pacific blocking, such as the preferred formation location (Mullen 1989) and the occurrence frequency (Ferranti et al. 1994). Based upon the so-called Alaskan-pattern index and 500-mb height variance, Renwick and Wallace (1996) searched in terms of the statistical approach for the relationship between the North Pacific winter blocking activity and ENSO and found that 69% more days of blocking are observed during winters of the cold ENSO phase than during those occurring during the warm phase.
It has been observed by numerous studies (e.g., Douglas et al. 1982; Shabbar et al. 1990; Chen et al. 1992, 1996; Graham et al. 1994) that the Aleutian low deepened and the Pacific Northwest ridge amplified during the first four decades after World War II. This interdecadal circulation variation is named the Pacific decadal oscillation (PDO; e.g., Mantua et al. 1997; Zhang et al. 1997; Power et al. 1999). Much as the PNA anomalous circulation affects the blocking activity in the North Pacific Alaska–Pacific Northwest region, the PDO anomalous circulation over this region likely exerts some impact on the blocking activity. To substantiate this conjecture, the conventional approach of identifying blocking highs (Lejenäs and Økland 1983; Dole and Gordon 1983; Dole 1986; Tibaldi and Molteni 1990; Tibaldi et al. 1994) was adopted to compile the climatology of the North Pacific winter blocking highs with the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data (Kalnay et al. 1996) for the 1954–97 period. Despite the existence of numerous sophisticated diagnostic schemes used to explore the formation and maintenance mechanisms of blocking, the simple streamfunction budget suggested by Sanders (1984) is adopted to illustrate how the dynamic processes maintaining the PDO anomalous circulation may facilitate the interdecadal change in the North Pacific blocking activity over the longitudinal sector between the peninsula of Kamchatka and the Pacific Northwest.
2. Data and analysis
- As proposed by Tibaldi and his collaborators, the atmospheric flow is declared to be blocked at a given longitude if the following two conditions are met at least by one of the three Δ values:
The duration (i.e., life cycle) of a blocking event is equal to or longer than 5 days. This criterion is verified with an x–t diagram of ΔZ
ϕs The amplitude (or intensity) of a block at its center indicated by the eddy component of 500-mb height ZE(500 mb) (i.e., departure from its zonal-mean value) is equal to or greater than 150 m.
One may argue that the interdecadal variation in the North Pacific blocking activity is attributed to anomalous circulations associated with the PDO. Such an argument concerning the effect of anomalous circulation on the blocking activity is oversimplified. It is beyond the scope of this study to explore how the formation and maintenance mechanisms of blocking are possibly modulated by the aforementioned anomalous circulations. However, blocking highs may be well portrayed by streamfunction ψ like geopotential height through the geostrophic relationship (e.g., Holton 1992). Thus, it would be informative to apply the simplified streamfunction budget analysis proposed by Sanders (1984) to infer the possible dynamic effect of the anomalous circulation on the blocking over the Alaskan North Pacific.
The maintenance of blocking (depicted by ψ) by the counterbalance between ψA and ψχ will be used to illustrate the possible PDO effect on the North Pacific blocking activity. Because the ψχ analysis involves horizontal divergence, the streamfunction budget analysis is performed at 200 mb rather than at the 500-mb level (except for the identification of blocking events). The streamfunction budget analysis is performed with the 12-h NCEP–NCAR reanalysis data; every term in the vorticity equation was computed first on the 2.5° × 2.5° grid mesh. Then, we solve the Poisson equation in terms of the spectral method with a T31 truncation to obtain ψ, ψA, and ψχ in Eq. (3).
3. Results
a. Occurrence
Based on sea surface temperature anomalies over the National Oceanic and Atmospheric Administration Niño-3.4 region and the Southern Oscillation index issued by the Climate Prediction Center (CPC) of NCEP, the following winters are determined by the CPC to be warm and cold winters of the ENSO cycles, respectively:
warm: 1957/58, 1958/59, 1963/64, 1965/66, 1968/69, 1969/70, 1972/73, 1976/77, 1977/78, 1982/83, 1986/87, 1987/88, 1990/91, 1991/92, 1992/93, 1994/95
cold: 1954/55, 1955/56, 1964/65, 1970/71, 1971/72, 1973/74, 1974/75, 1975/76, 1984/85, 1988/89, 1995/96
More (fewer) blocking days occur in cold (warm) winters of the ENSO cycles. To be more quantitative, averaged blocking days over all 43 winters (NM), and over cold (NC) and warm (NW) winters, are displayed in Table 1 along with their standard deviations. On average, slightly more blocking days appear in the present study as compared with Renwick and Wallace (1996). This contrast is interesting, because blocking identification with the conventional approach adopted in this study may have less restrictive criteria than Renwick and Wallace's statistical one. There are about 86% more blocking days during cold winters than there are during warm winters; this ratio (NC/NW) is somewhat larger than Renwick and Wallace's measurement (∼70%).
A least squares fit trend (solid line) is added on histograms of blocking days; an interdecadal increasing trend of blocking days is discernable. This trend was computed with a statistical scheme outlined in Chen et al. (1996). The ENSO event occurs once every 2–7 yr. To reduce the possible bias toward the interdecadal trend, we first developed a 7-yr running-mean time series of winter blocking days (Fig. 1). This time series is modeled as a linear deterministic trend plus fifth-order autoregression [AR(5)] noise (e.g., Bloomfield 1992). The AR(5) noise was chosen to give the minimum value of Akaike's information criterion (Priestly 1981) on average for the 7-yr running-mean time series. The slope of trend and confidence interval are obtained using the proprietary statistical software “AUTOREG” procedure (SAS Institute 1988). Numerical values of these two variables are 6.5 days (40 yr)−1 and −1.2 to +14.2 days (40 yr)−1 (at 95% confidence level), respectively. The interdecadal trend of the North Pacific blocking activity presented in Fig. 1 is consistent with the PDO depicted by our previous studies (Chen et al. 1992; 1996).
Despite the fact that more blocking days on average are observed by the present study, the contrast of blocking days between warm and cold ENSO winters revealed from Fig. 1 is consistent with Renwick and Wallace's analysis. They argued that this contrast of blocking days is caused by the alternation of the PNA anomalous circulation pattern over the Alaskan North Pacific on the North Pacific blocking. The anomalous circulation associated with the PDO exhibits the deepening of the Aleutian low and the amplification of the Pacific Northwest ridge (e.g., Chen et al. 1996). Can this PDO anomalous circulation pattern modulate the North Pacific blocking activity? This question, originating from Fig. 1, will be explored in the next section.
b. Interdecadal variations
For the convenience of presentation of the streamfunction budget analysis, let us define the following symbols used in this study: [(
The composite 200-mb eddy streamfunction departures of blocking, Δ
It was inferred by Renwick and Wallace (1996) that the interannual variation of blocking over the Alaskan North Pacific is caused by the anomalous circulation pattern of the PNA teleconnection pattern in this region. It is conceivable that the PDO anomalous circulation over the Alaskan North Pacific–Pacific Northwest region is responsible for the interdecadal enhancement and eastward shift of the North Pacific blocking activity. The PDO anomalous circulation depicted by D
So far the discussion only focuses on the suggestive PDO impact on the North Pacific blocking from a morphological perspective of this blocking activity and the PDO anomalous circulation. Spatial coincidences between D
As inferred from Eq. (3), the aforementioned streamfunction anomaly cells should be maintained by the counterbalance between corresponding streamfunction tendencies induced by vorticity advection and stretching. Therefore, the spatial structure of these two dynamical processes should be opposite. This is actually the case in our analysis. To avoid redundancy, only streamfunction tendency anomalies of vorticity advection are displayed in Fig. 4 for discussion: Δ
4. Concluding remarks
With the NCEP–NCAR reanalysis data for the 1954–97 period, it was observed that the North Pacific blocking activity underwent a noticeable interdecadal change over the past four decades caused by the Pacific decadal oscillation: blocking days increased and blocking highs migrated eastward. Several climate/weather implications may be derived from the interdecadal change of blocking:
The major precipitation in the Pacific Northwest occurs during the winter. A decreasing precipitation trend in this region emerged over the past four decades (e.g., Chen et al. 1996). Owing to large-scale subsidence, weather conditions near the blocking are unusually warm and dry. The interdecadal change in the North Pacific blocking activity may contribute to the decreasing rainfall trend in the Pacific Northwest.
The formation and maintenance mechanisms of blocking (as pointed out in section 1) are involved with the development of planetary-scale waves and the interaction between synoptic- and planetary-scale waves. Apparently, the proper simulation of planetary-scale waves is important for successful blocking forecasts. Over a period of years, the structure and behavior of planetary-scale waves in the North Pacific can be modulated by the PDO. Blocking forecasts by a global operational forecast system over a period of years may possibly be affected by the model PDO bias.
The deepening of the Aleutian low (by the PDO) may enhance the cyclogenesis over the North Pacific Alaska region. In turn, these synoptic storms are channeled farther north and south, away from the more east–west original storm track, by the blocking high. Therefore, local weather conditions north and south of the Pacific Northwest region may be affected by the interdecadal change in the blocking activity. Some effort to explore this impact is urged.
It is an acceptable consensus that the PNA pattern associated with the ENSO cycle regulates the storm track in the North Pacific (e.g., Lau 1988). In contrast, some previous studies (e.g., Hoerling and Ting 1994; Hurrel 1995) have shown that transient eddies may contribute toward maintaining the PNA structure. The modulation of the PDO on the North Pacific blocking activity was stressed in this study. It is likely that North Pacific blockings may conversely affect the PDO maintenance. The possible feedback effect of synoptic disturbances on the low-frequency PDO is worthy of future research.
Acknowledgments
This study is supported by NSF Grant ATM9906454 and NASA Grant NAG 58293. We appreciate the typing support provided by Mrs. Reatha Diedrichs and the editing assistance provided by Ms. Kathryn J. St. Croix. Comments and suggestions offered by two reviewers and by Dr. John R. Gyakum were helpful in improving the presentation of this paper.
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Histogram of blocking days during each winter; cold (warm) ENSO winter is denoted by shaded (dotted) histogram. The averaged blocking days (dashed line) and a least squares fit line (solid line) of histograms [with a slope of 6.5 days (40 yr)−1 and 95% confidence interval of −1.2 to 14.2 days (40 yr)−1] are added
Citation: Monthly Weather Review 130, 12; 10.1175/1520-0493(2002)130<3136:IVOTNP>2.0.CO;2
Eddy streamfunction (ψE) anomalies at 200 mb: (a) composite departures of blocking [Δ
Citation: Monthly Weather Review 130, 12; 10.1175/1520-0493(2002)130<3136:IVOTNP>2.0.CO;2
The x–t diagram of longitude locations (dots) of composite blocking high centers during each winter. A least squares fit line [with a slope of 8.7° of longitude (40 yr)−1 and a confidence interval of −1.1° to 18.6° of longitude (40 yr)−1] is added
Citation: Monthly Weather Review 130, 12; 10.1175/1520-0493(2002)130<3136:IVOTNP>2.0.CO;2
Same as Fig. 2 but for anomalies of the streamfunction tendency induced by vorticity advection (ψA). Contour intervals of Δ
Citation: Monthly Weather Review 130, 12; 10.1175/1520-0493(2002)130<3136:IVOTNP>2.0.CO;2
Mean values and standard deviations of blocking days averaged over all 43 winters (NM, σM) and over the cold (NC, σC) and warm (NW, σW) winters. Renwick and Wallace's (1996) estimates are also displayed
