Map (Positive Phase)
Plotted Historical Time Series : (January 1950-February 1997)
Table Archive of Indices Dating Back to 1950
Table showing Calendar months when specific teleconnection patterns are important
The Tropical/ Northern Hemisphere pattern was first classified by Mo and Livezey (1986), and appears as a prominent mode from November-February. The pattern consists of one primary anomaly center over the Gulf of Alaska and a separate anomaly center of opposite sign over the Hudson Bay. A weaker area of anomalies having similar sign to the Gulf of Alaska anomaly extends across Mexico and the extreme southeastern United States. This pattern reflects large-scale changes in both the location and eastward extent of the Pacific jet stream, and also in the strength and position of the climatological mean Hudson Bay Low. Thus, the pattern significantly modulates the flow of marine air into North America, as well as the southward transport of cold Canadian air into the north-central United States.
Pronounced negative phases of the TNH pattern are often observed during December and January when Pacific warm (ENSO) episode conditions are present (Barnston et al. 1991). One recent example of this is the 1994/95 winter season, when mature Pacific warm episode conditions and a strong negative phase of the TNH pattern were present. During this period, the mean Hudson Bay trough was much weaker than normal and shifted northeastward toward the Labrador Sea. Additionally, the Pacific jet stream was much stronger than normal and shifted southward to central California, well south of its climatological mean position in the Pacific Northwest. This flow pattern brought well above-normal temperatures to eastern North America and above-normal rainfall to the southwestern United States.
In contrast, positive phases of the TNH pattern tend to accompany Pacific cold events. An example is the very persistent positive phase of the TNH pattern during 1988/89 -1990/91, which developed in apparent association with the strong 1988/89 Pacific cold event.
