North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is a prominent pattern influencing both temperature and precipitation across portions of the United States. The index can be derived from a rotated Empirical Orthogonal Function Analysis or a station-based approach. The station-based NAO index describes the pressure difference between the sub-tropical Azores High and the Subpolar Icelandic Low. In general,positive NAO phases feature above average heights across the eastern United States and below average heights at high latitudes.
When using the NAO index as guidance for temperature and precipitation forecasts, it is important to note that NAO correlation patterns change from one month to another. During the winter, the correlation between the NAO index and monthly temperature anomalies across the Southeast United States is positive; for the summer, the correlation between the NAO index and monthly temperature anomalies across the Southeast United States is negative. Temperature relationships are very weak in the spring. More details of the correlation patterns associated with the NAO can be found on the NAO map page.

Ridge-trough Dipole Pattern

The ridge-trough dipole pattern is the dominant pattern governing mean monthly temperature variability across the Northeast United States and portions of the Southeast United States in the winter (Schulte and Lee, 2018). For some locations, the ridge-trough dipole pattern can explain up to 80% of mean monthly temperature variability.
the strength and evolution of the ridge-trough dipole pattern is measured using the dipole index. The dipole index is defined as the difference between 500-hPa geopotential height anomalies at 42.5N and 75W and 70N and 157.5W. Positive phases of the ridge-trough dipole pattern feature above average heights over the Great Lakes region and below average heights over Alaska. As shown by Schulte and Lee (2018), this pattern is closely related to the East- Pacific/North-Pacific pattern whose index is routinely calculated by the Climate Prediction Center using a rotated principal component analysis of 500-hPa geopotential height anomalies poleward of 20N.

Eastern North American Sea-level Pressure Dipole Pattern

The Eastern North American Sea-level Pressure Dipole Pattern (ENA; Schulte et al, 2017) is characterized by two main sea-level pressure (SLP) anomaly centers, one located near Novia Scotia and another one located over the Southeast United States. Positive ENA phases feature positive SLP anomalies near Novia Scotia and negative SLP anomalies across the Southeast United States. Negative phases feature negative SLP anomalies near Novia Scotia and positive SLP anomalies across the Southeast United States.
The ENA pattern is a dominant pattern governing precipitation variability across the Northeast and Southeast United States (Schulte et al., 2017). The strong relationships with precipitation suggest that droughts across the eastern United States may reflect a predominance of negative ENA phases.The ENA pattern is also related to precipitation variability across Texas and the western United States.
The ENA pattern can also explain a substantial amount of temperature variability across the eastern United States. For many locations, the ENA index can explain more temperature variability than the NAO.
As shown by Schulte et al. (2017), this pattern is related to the El Nino/Southern Oscillation (ENSO) during the late winter and early Spring. The winter and spring ENA positive phases are associated with warmer- than-normal conditions across the eastern equatorial Pacific. Enhanced tropical convection across the eastern equatorial Pacific is also associated with spring and winter positive ENA phases. Negative ENA phases in the spring and winter are associated with cooler-than-normal conditions across the eastern equatorial Pacific.As such, negative ENA phases are associated with convective precipitation deficits across the eastern equatorial Pacific. In the late summer and early fall, the pattern is related to sea surface temperature anomalies across the Caribbean Sea and tropical Atlantic Ocean.

The Pacific-North American Pattern

The Pacific-North American (PNA) pattern is an important mode of atmospheric variability influencing mean monthly temperature and precipitation across portions of the United States. The PNA index, which measures the strength and evolution of the PNA pattern, is often calculated from a rotated principal component analysis of 500-hPa geopotential height anomalies poleward of 20N. Positive PNA phases are associated with above average heights over the western United States and below average heights over the southeastern United States. Positive phases of the PNA pattern tend to occur more frequently during El Nino years. As such, the monthly PNA index is positively correlated with SST anomalies across the central and eastern equatorial Pacific.

The West Pacific Pattern.

The West Pacific Pattern is another leading mode of atmospheric variability. This pattern is characterized by a north-south anomaly dipole. One center of the dipole is located over the Kamchatka Peninsula, whereas the other oppositely signed center is located over parts of southeastern Asia and the western subtropical North Pacific. The position of the dipole centers implies that strong positive or negative phases are associated with an anomalous Pacific jet steam in terms of both position and intensity. The West Pacific index is not consistently correlated with temperature across the Northeast U.S on a month to month basis. The strongest association between temperature and the West Pacific index is in December.