The inability of forecasters to track hurricanes accurately is fundamentally related to the inability to model tropical behavior such as ENSO

> Professor Phil Klotzbach of Colorado State University’s Department of Atmospheric Science told “The primary reason for NOAA’s increase in their forecast was due to the weakening of El Niño.

> “El Niño is warmer than normal water in the central and eastern tropical Pacific.

> “Typically, when you have El Niño conditions, it increases vertical wind shear in the Caribbean into the tropical Atlantic, tearing apart hurricanes.

> “With El Niño going away, they anticipated less vertical wind shear and consequently more conducive conditions for hurricanes.”


Consider the simplicity and the symmetry of the forcing model I am using for ENSO. The essential tidal forcing is the sidereal (tropical) and synodic lunar cycles. The combination of this pair of fortnightly cycles leads to a semi-annual symmetry in the time-series. Even though the analysis started with a comprehensive forcing model, just a couple of the main factors provide the basic pattern.


From the amplitude spectrum, one can see how these are the strongest tidal factors, labelled Mf (fortnightly cycle doubled from the monthly sidereal) and Msf (from the synodic), with the Mf+Ssa (solar semi-annual) peak generating the semi-annual beat pattern. (The inset is an earlier model that gave a hint of the beat pattern)


The tricky fluid dynamics aspect is in determining the LTE modulation, both for the major ENSO standing wave and the higher harmonics. The major cycle represents the primary dipole, while the higher frequency noise is actually constructive in that it provides harmonics that shape the peaks in the time series.


Considering the simplicity and low dimensionality of the model, the good fit suggests that it's a very plausible mechanism.


The LTE modulation derives from an assumed wiggle in the equatorial latitude. Imagine that the cyclic forcing impacts the location of the equatorial latitude, pushing it north & south in a deterministic pattern. This is the ansatz that leads to the solution to Laplace's Tidal Equations, and thus providing a means to create a non-linear modulation of the tidal forcing.

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