What is atmospheric angular momentum? According to NOAA the AAM is useful as an index of the large scale zonal flow since it is highly correlated with independent length-of-day measurements and with common climate diagnostics used by forecasters to predict climate such as the quasi-biennial oscillation (QBO), El Nino southern oscillation (ENSO) and the Madden Julian oscillation (MJO) plus (potentially) the influence of CO2 on global climate.
Discussion: The projected AAM for January 2018 is -2.2 (Fig. 1) which would tie the record for the most negative monthly value in January previously set in 1974 according to NOAA data. The AAM climatology presented by NOAA begins in 1958. Previous to 2018 the strongest negative anomalies for the month of January occurred in 1963, 1965, 1974, 1984 and 1989. Each year a version (some weak, some strong) of La Nina was present.
During –AAM regimes the zonal wind is weaker than normal and the susceptibility to amplified long wave troughs and ridges increases (Fig. 2 and 3). Troughs produce storms and if they are stronger than normal the risk of extreme weather events increases as observed many times during January in both hemispheres. Similarly, upper ridge patterns become more amplified and lead to extreme heat risk during summer as observed in Australia. Troughs are made more potent if arctic air is involved as observed in the U.S. during January. The strong troughs lead to propagating wind fields within the jet stream flow much stronger than normal which can down-well to ground level causing extreme wind events as witnessed in Europe this month.
The –AAM regime was driven by La Nina and magnified by the more intense than normal character of the MJO. The “character” meaning the heavy equatorial convection caused by MJO was strongest over the warmer than normal waters of the ocean surface in either side of Indonesia from the eastern Indian Ocean to the western Pacific Ocean.
Forecast: The analog years each indicate that the strong –AAM in January was followed by similarly intense negative values in February. Note how all the analog years occurred in the 1960’s, 1970’s and 1980’s before CO2 increase affected global climate in a meaningful way. One would expect the 2018 version of –AAM to react differently. According to forecasts that lead into February the strong –AAM of January fades to near neutral. If correct, we should lose the January 2018 ability of the atmosphere to produce high and low pressure systems that create weather extremes.
Fig. 1: The projected atmospheric angular momentum for January of 2018 is -2.2 which would tie an all-time record. The previous 5 most –AAM years are also indicated. Note that each analog year the strong –AAM continued in February. But in 2018 the –AAM is expected to recover to near normal.
Fig. 2: January 1-18, 2018 ridge (orange/red) and trough (blue) areas in the northern hemisphere. The pattern is highly amplified including storm/high wind producing troughs in the Aleutians and Western Europe plus cold air producing troughs over the eastern U.S. and central Russia. The ridge areas are just as strong producing record warmth in the western U.S.
Fig. 3: In the southern hemisphere the January 1-18, 2018 trough pattern is also impressive especially considering summertime with 3 super long wave troughs indicated (in blue). Also note the hot weather producing ridge over eastern Australia.
Fig. 4: The atmospheric angular momentum forecast to early February indicates the super intense –AAM eases.