Two leading modes of Northern Hemisphere blocking variability in the boreal wintertime and their relationship with teleconnection patterns

Abstract

Two principal modes of Northern Hemisphere (NH) wintertime blocking variability are identified to examine the possible relationships between regional blocking activities and to understand how they are linked. The first mode of NH blocking variability is characterized by regional blocking activities including the North Pacific (PA), Greenland, European, and Ural-Siberian regions. There exists dominant PA blocking associated with the negative North Pacific Oscillation (NPO) pattern. The second mode shows a zonally dipole pattern between PA and North Atlantic (AT) blockings. It is more related to AT blocking, such that there is strong coupling with the negative North Atlantic Oscillation (NAO) pattern. Correlation analysis with major climate variability patterns revealed that both the NPO and NAO may modulate the two leading modes of NH blocking variability. Also, the NPO and Arctic Oscillation (AO) can simultaneously be associated with the blocking occurrence over both the PA and AT basins. The negative phase of the NPO (AO) is favorable for the in- phase (out-of-phase) relationship between west of (south of) PA and south of (west of) AT blocking sectors. This occurs because NH blocking occurrence is dependent on the phase of the AO, especially in the negative NPO pattern. If the NPO and AO are in phase (out of phase), NH blocking occurrence is more enhanced (weakened), in particular, having higher frequency over the Pacific and higher (lower) frequency over the Northwest Atlantic.

Appendix

1.1 A hybrid blocking index

To obtain the two-dimensional spatial distribution of atmospheric blocking, the following five-step procedure was applied.

Step 1: Compute the anomaly field, ${\text{Z}}^{{\prime}}$, as described by Sausen et al. (1995).

$$ Z^{{\prime }} = Z - \bar{Z} - \hat{Z} $$

where ${\text{Z}}$ is the 500-hPa geopotential height divided by the sine of latitude (Dole and Gordon 1983), $\bar{Z}$ is the running annual-mean of Z centered on a given day, and $\hat{Z}$ is the mean seasonal cycle derived from Z ^*, which is the running-monthly mean of ${\text{Z}} - \bar{Z}$ centered on a given day.

Step 2: Identify the closed positive contours of Z ^′, satisfying the minimum amplitude (A) and spatial scale (S).

Step 3: Track the blocking anomalies in time, ensuring the minimum overlap (O) in blocking area within 2 days. As such, it remains only quasi-stationary systems.

Step 4: If the reversal of the meridional gradient of absolute geopotential height is satisfied around the blocking anomalies, it can be seen that they (the detected anomalies) block the flow.

Step 5: If all of these conditions are satisfied for a duration criterion (D), the anomalies are finally labeled as blocking highs (See Dunn-Sigouin et al. (2013) for further details of the index).

For the threshold values used in this study, (A) was assumed to be 1.5 standard deviations of the geopotential height anomalies over 30–90°N for a 3-month period centered at a given month, (S) = 2.5 × 10⁶ km², (O) = 50 %, and (D) = 5 days.