3.1. Intraseasonal Variation of AL
shows the climatological field of MSLP and 20 to 80-day standard deviations of MSLP during the winters of 1979–2014. The large value of MSLP’s intraseasonal variance is located over the mid-North Pacific basin. Its center is slightly on the eastern part of the climatological AL. Thus, it is reasonable to use the red box domain to define the ALI. We obtained the daily ALI for winters of 1979–2014 according to Section 2.3
. Figure 2
a displays the time series of the daily ALI, as well as an identified AL intraseasonal oscillation event for the winter of 2007. The averaged power spectrum of ALI in winters 1979–2014 is shown in Figure 2
c. The peak periodicity is about 40 days, according to Figure 2
c. Meanwhile, a periodicity shorter than 40 days is also seen in Figure 2
c. As examples, the power spectra of ALI in four individual winters (1986, 1996, 2007, and 2012) are plotted in Figure 2
d–g. Figure 2
d,f show a similar peak periodicity as Figure 2
c, while Figure 2
e,g show a periodicity that is robustly shorter than 40 days.
Each AL oscillation event is divided into eight phases. Figure 2
b shows the eight phases. Phase three represents the strongest stage of AL with the minimum value of ALI. Phase seven is corresponding to the weakest stage of AL with the maximum value of ALI. Phase five denotes the transition from the strong AL to the weak AL period. Phases two, four, six, and eight are defined at the time when the oscillation reaches half of its minimum or maximum amplitude. According to above eight phases, the structure and evolution of AL on an intraseasonal time-scale can be examined by a phase composition. For the composition of phase
, we use the average of three days: one day leading into phase
, the day of phase
, and one day after phase
. The life cycle of an oscillation that is shown above is commonly used for examining the atmospheric intraseasonal oscillation, similar to the Madden–Julian Oscillation (MJO) [28
] and the boreal summer intraseasonal oscillation over the Asian summer monsoon region [30
plots the life cycle composite of MSLP anomaly based on the ALI in eight phases for 43 AL intraseasonal oscillation events. The signals are concentrated over the pan-North Pacific region, especially over the North Pacific basin region. From phases one to three, the continuous enhancement in the AL’s intensity is seen (Figure 3
a–c). The AL’s circulation gets its strongest in phase three. From phases four to five, the cyclonic circulation anomaly over the North Pacific basin fades away, and an anticyclonic circulation occupies over the basin region. In phase seven, the AL is in its weakest stage.
3.2. IVT and AR Changes Associated with the Intraseasonal Variation of AL
In this section, we reveal the changes in atmospheric water vapor transport and its divergence, and the ARs associated with the intraseasonal variation of AL over the North Pacific in winter. To demonstrate the fluctuation clearly, we show the actual value of IVT, ∇·IVT, and ARs, rather than the anomalies.
depicts the life cycle composite of IVT and ∇·IVT based on ALI in eight phases. A total of 43 AL intraseasonal oscillation events were used for composition. Overall, the observed IVT over the North Pacific region showed cyclonic transports for all the phases. However, the magnitude and direction of the IVT showed considerable differences between the AL’s strengthening stage (phases two, three, and four) and its weakened stage (phases six, seven, and eight). The IVT is featured by eastward transport to the west of 150° W, and poleward transport between 150° W–125° W during phases one to four. During phases five to eight, especially in phase seven, the eastward transport is slightly weaker, and mainly located to the west of the dateline, compared with the earlier stage. Meanwhile, a weak poleward transport is mostly concentrated over the western and central areas of the oceanic basin. The large moisture source is located over the western North Pacific, and the large moisture sink is over the northeastern North Pacific, which is the west coast of North America in phase three. In contrast, both the moisture source and moisture sink are weaker in phase seven. The moisture sink region shifts to the center basin in phase seven.
The structure and evolution of actual IVT during eight phases are highly associated with the life cycle of the AL. During the AL’s strengthening stage, especially in phase three, the enhanced cyclonic circulation suggests strengthened westerly winds over the mid-basin and strengthened northward and northeastward winds over the northeastern North Pacific (Figure 3
c). Therefore, the AL’s circulation during phase three transports more moisture eastward over the mid-basin and poleward over the northeastern North Pacific (Figure 4
c). During the AL’s weakened stage, especially phase seven, the moisture transport to the east of the dateline becomes weak because of the weakened AL.
shows the distribution of AR frequency over the North Pacific during the AL’s eight phases. The numerical value in Figure 5
is actual value of the AR frequency (not the anomaly). Thus, there is no negative value in Figure 5
. The blue contour in Figure 5
means a climatological winter AR frequency of 30%, indicating the AR’s climatological main body region. From phases one to three, the AR frequency increases dramatically in its main body region. In phase three, the AR frequency in its main body region is larger than 40% (namely, larger than the climatological value). The region with 40% AR frequency stretches northeastward toward the western coastal region of North America, and even occupies the region of the Gulf of Alaska. The above enhancement of AR frequency is consistent with the strengthened AL and enhanced IVT over the same region. After phase four, the AR frequency decreases dramatically in its main body region. It enters its weakest stage during phase seven. The AR frequency in its main body region is robustly smaller than 30% in phase seven (namely, smaller than the climatological value). There is a slightly increased AR frequency north of 40° N over the basin region in phase seven (Figure 5
g), which is due to the occurrence of poleward transport in this stage (Figure 4
g). An AR judgment of Formula (2) has no threshold requirements for IVT or integrated water vapor (IWV). Therefore, more ARs might be detected in this study, especially over mid–high latitudes.
Composite fields of IVT and ∇⋅ IVT in phases three and seven for group AR days and non-AR days are illustrated in Figure 6
. More intensive moisture transportation is seen for the AR day group than that for the non-AR day group, for both phases three and seven. For example, the IVT magnitude along 30° N over the mid-basin for the AR day group (Figure 6
a) is about twice that for the non-AR day group (Figure 6
c). Similarly, the IVT magnitude averaged over the northwestern North Pacific region for the AR day group (Figure 6
a) is approximately two to three times larger than that of the non-AR day group (Figure 6
c). For phase 7, the poleward transport is performed during the AR days (Figure 6
b), but not on the non-AR days (Figure 6
d). Regarding the moisture source and sink aspect, the moisture source over the western North Pacific is hoarded during non-AR days, while the moisture sinks over the northeastern North Pacific in phase three and those over the center basin in phase seven are converged during AR days.
Previous studies have demonstrated the huge contribution of the amount of IVT (∇⋅IVT) produced in AR-days to the total amount of IVT (∇⋅IVT) in the whole winter [7
]. Figure 6
only highlights the intensity distribution of IVT for AR days and non-AR days. The quantitative contribution of IVT (∇⋅IVT) in AR days to the total amount of IVT (∇⋅IVT) for different phases is still not clear. To answer this question, we firstly calculate the accumulated amount of zonal component of IVT (denoted as IVT-X) that was performed on AR days in phase
for the 43 AL oscillation events for every grid cell. Secondly, the ratio of the accumulated amount of IVT-X performed on AR days to the total amount of IVT-X for total days in phase
for every grid cell is obtained. Similar processes are done for the meridional component of IVT (denoted as IVT-Y) and ∇⋅ IVT.
The above ratios are plotted in Figure 7
and Figure 8
. It is observed that the accumulated amount of IVT-X (IVT-Y) during AR days is important mostly within AR’s main body region in phase three, which finishes 50–90% of the total amount of IVT-X (IVT-Y) for the total number of days in phase three (Figure 7
a). The region with a ratio larger than one in Figure 7
c,d denotes the occurrence of the opposite direction of an accumulated amount of IVT-Y between AR days and non-AR days. The accumulated amount of IVT-Y in AR days is northward, while it is slightly southward in the non-AR days, and is around the region of (160° E–160° W, 25° N–40° N) in phase three (Figure 7
c). The poleward vapor transport in phase seven (Figure 4
g) is mostly done during AR days (Figure 7
d). Figure 8
shows that the moisture sink during AR days overwhelmingly dominates the pattern shown in Figure 4