atmospheric angular momentum
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<p><strong>Angular momentum is fundamental to the structure and variability of the atmosphere and hence regional weather and climate. Total atmospheric angular momentum (AAM) is also directly related to the rotation rate of the Earth and hence the length of day. However, the long-range predictability of fluctuations in the length of day, atmospheric angular momentum and the implications for climate prediction are unknown. Here we show that fluctuations in AAM and the length of day are predictable out to more than a year ahead and that this provides an atmospheric source of long-range predictability of surface climate. Using ensemble forecasts from a dynamical climate model we demonstrate predictable signals in the atmospheric angular momentum field that propagate slowly and coherently polewards into the northern and southern hemisphere due to wave-mean flow interaction within the atmosphere. These predictable signals are also shown to precede changes in extratropical surface climate via the North Atlantic Oscillation. These results provide a novel source of long-range predictability of climate from within the atmosphere, greatly extend the lead time for length of day predictions and link geodesy with climate variability.</strong></p>

Abstract The atmospheric surface pressure time series of Madras, Darwin, and Tahiti together with non-tidal length-of-day (LODR) variations and axial component of atmospheric angular momentum (AAM) were analyzed by wavelet transform as well as the combination of the Fourier transform band pass filter with the Hilbert transform (FTBPF + HT) to detect interannual and intra-seasonal oscillations in them. It was found that annual oscillations in the atmospheric surface pressure variations of Darwin and Tahiti stations are in phase and are about 180o out of phase in the atmospheric surface pressure variations of Madras station. The phase of the annual oscillation in atmospheric surface pressure variations of Madras station is slightly greater (~ 20o) than the phase of the annual oscillation in the LODR time series. The amplitude and phase variations of the annual and semi-annual oscillations computed by the FTBPF + HT combination in LODR and the axial component of AAM are very similar. The mean amplitudes of the semi-annual oscillation in the atmospheric surface pressure variations of Madras and Tahiti are of the order of 0.4 hPa, the phases of these oscillations are stable and the amplitude of the semi-annual oscillation in the atmospheric surface pressure variations of Darwin is negligible due to unstable phase of this oscillation. The atmospheric surface pressure variations of Madras, Darwin, and Tahiti stations show similar amplitude wideband signals with a central period of ~ 4 years (cutoff periods ranging from about 2.2 to 20 years) related to El Niño phenomenon. The amplitude maxima of this signal corresponding to the strongest El Niño events in 1982-83, 1997-98, and 2014-15 are also present in amplitude variations of this signal in the LODR and AAM χ3 time series.