Jan 25, 2014

Translating Tropical ISO to the Midlatitudes

Studies have attempted to identify recurrent modes that contribute to the alternation between active and inactive periods of the Indian summer monsoon and the South China Sea summer monsoon. Generally, two components of variability were found to be dominant on an intraseasonal time scale: the “short-term” component, with a period of less than 1 month, and the “long-term” component, with a period of 1–2 months but apparently less than a season. Both can be referred to as the intraseasonal oscillations (ISOs) since they are clearly separated from the high-frequency synoptic-scale (less than 10 days) features and the slowly varying seasonal cycle as well. - BIN GUAN AND JOHNNY C. L. CHAN

We considered our cycle correlation a step in the right direction. Next was to match the cycle correlation to the "short-term" and "long-term" ISO components. The paper referenced above identified the years 1981, 1984, and 1990 as case studies of when either the 10-20 or 30-60 day ISO was strong. The paper states 1981 is strong in the "short-term" 10-20 day ISO, 1984 is strong in the "long-term" 30-60 day ISO, and 1990 is strong in the seasonal cycle.

For each of the case studies we think our cycle correlation matches their findings. 1981 does favor the "short-term" component. We see positive and negative correlation in the 15 day time period. 1984 does favor the "long-term" component. We see one strong lengthy correlation at days 41-47. The 1990 seasonal cycle is inconclusive, however. We could debate either way, short and long, but would need a wider time scale to make a seasonal cycle determination. What we label as the "harmonic", they label as the "short-term" 10-20 day ISO. What we label as the "cycle", they label as the "long-term" 30-60 day ISO. Because the findings match, we can assume that our cycle correlation method is valid and can piggy back the paper as something significant and scientifically valid.

Tropical intraseasonal oscillation (ISO), especially its dominant component at the equator—Madden–Julian oscillation (MJO), has been extensively studied over the past decades. ISO (or MJO) consists of large-scale coupled patterns in atmospheric circulation and deep convection [...] the influence of MJO can extend to the subtropics and midlatitudes. Associated with enhanced (reduced) tropical convection, the upper-level divergence (convergence) is often accompanied by a subtropical upper-level convergence (divergence) counterpart. - Li LIN-LIN PAN AND TIM LI

Extending the tropical ISO into the midlatitude we are able to find the "short-term" and "long-term" ISO (see image below). Applying our cycle correlation to the h5 pattern since October 1, 2013 we see a strong correlation in the "short-term" component around 15 days. Using principles of standing wave harmonics we can then solve the "long-term" component. Evolution of the sine wave shows peak to peak harmonics of around 15 days. More to come.

If there are any questions, comments, or suggestions on the material presented please let me know. Thanks for reading!