The relative atmospheric angular momentum (AAM) about the Earth's axis is a convenient global index of the intensity of the zonal circulation that can be monitored for climate purposes. It is a function of the zonal winds integrated through the depth of the atmosphere with higher weights given in the low latitudes due to the large radius arm there. Each year, the AAM undergoes a strong annual cycle, related in large part to the strong seasonal variability in the Northern Hemisphere subtropical jets. Superimposed on the seasonal signal are high-frequency fluctuations, often related to the tropical intraseasonal oscillation (Madden and Julian 1994). Higher variance tends to occur during the broad maximum during the Northern Hemisphere winter period. On the interannual scale, AAM variations are influenced by El Nio/Southern Oscillation (ENSO) signals. The AAM is also of interest to geodetic science because of its relation to changes in the Earth's rotation, and it is one of several quantities monitored by a specialized data center in this connection (Salstein et al. 1993).
A fifteen-year time series of AAM integrated between 50 and 1000 mb (Fig. 1.27) reveals the signals on the time scales described above. Of particular note are the strong peaks during the maximum phases of warm episodes, particularly in 1983, and to a lesser extent in 1987. Minima in AAM occurred in the cold episode years, such as 1988. During 1994, less variance occurred in AAM during the early part of the year compared with many earlier years. However, the last month of the year saw a dramatic increase in AAM, possibly related to the development of mature warm episode conditions in the tropical Pacific (see section 2). These signals are revealed in the anomaly plot of Fig. 1.28, representing the difference between the 1994 signal and the 15-year mean. The rapid mid-December rise is quite evident, as are other peaks in late February and mid-September. The minimum of AAM usually arrives during July, but in 1994 this low value did not occur until August, when it dropped to its lowest value since 1985.
Oceanic and Atmospheric Anomalies Related to ENSO -
Large-scale Conditions In The Tropics: 1990-1994
Climate and Global Change Issues: Cryosphere - Sea Ice
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