3.1 Mean patterns and changing frequencies of occurrence
The characteristic patterns in 500 hPa height anomalies across the
domain for all months are displayed in the master SOM matrix
(Fig. 1 ). Adjacent nodes are most similar to each other while
those farthest apart are most dissimilar. Nodes along the edges of the
matrix, and especially in the corners, occur most frequently. The
percentages of winter (JFM) and summer (JAS) days belonging in each node
are indicated over each map in the matrix. Winter days are more likely
to exhibit patterns with large height contrasts, while summer days are
fairly evenly distributed across the SOM.
Patterns in the upper-left part of the matrix feature positive height
anomalies (ridging) in northern and western areas of the domain along
with negative anomalies (troughing) in the east. The total number of
days belonging in node #1 has increased during recent years
(Figs. 3, S7 ), though not in winter (Fig. 3b ).
Anomalous temperature and precipitation patterns associated with each
node can be used to ascertain weather conditions associated with each
node. Figure 4a displays the number of days at each gridpoint
in which anomalously high (> 1.5 σ) temperatures occurred,
and Fig. 4b indicates the occurrence of extremely low
(< -1.5 σ) temperatures associated with each pattern. The
weather associated with node #1, for example, favors relatively dry,
warm conditions in California, below-average snowpacks in western
mountain ranges, and prolonged cold, wet spells in the Midwest and East
(Figs. 4, 5 ). Nodes in the bottom-right part of the matrix
feature the opposite anomaly pattern, with negative (positive) values
spanning much of the high (middle) latitudes. Western states tend to be
relatively cool and wet in these patterns, while the east experiences
abnormally warm conditions. The total number of days belonging in these
nodes, especially node #12, has declined in recent decades,
particularly during winter (Fig. 3 ). In the upper-right and
lower-left corners are opposing patterns featuring a strong zonal
anomaly gradient, indicative of an amplified ridge/trough in each half
of the domain. In terms of weather conditions, the strong western trough
depicted in node #4, for example, tends to bring anomalously cold
temperatures to Alaska along with stormy weather to the U.S. west coast.
The opposite pattern in node #9 produces abnormally warm conditions
along the west coast, cold temperatures in much of the central and
eastern continent, and unsettled conditions in the eastern states.
As high latitudes warm disproportionately faster than midlatitudes, we
find that patterns exhibiting positive (negative) high-latitude height
anomalies are occurring more (less) frequently, especially in the most
recent decades (Fig. 3a and S7 ). During winter months, the
pattern featuring anomalous troughing in the west and ridging in the
east (#4) is the only one that has occurred significantly more
frequently (Fig. 3b ), which is consistent with the region of
strongest AAW during JFM presented in Fig. 1b of Francis and Vavrus
(2012). During summer, nodes #1 and #3 are more common. Spring and
fall also exhibit significant increases node #1 (not shown).