Abstract
Dynamical understanding of the Madden–Julian Oscillation (MJO) has been elusive, and predictive capabilities therefore limited. New measurements of the ocean's response to the intense surface winds and cooling by two successive MJO pulses, separated by several weeks, show persistent ocean currents and subsurface mixing after pulse passage, thereby reducing ocean heat energy available for later pulses by an amount significantly greater than via atmospheric surface cooling alone. This suggests that thermal mixing in the upper ocean from a particular pulse might affect the amplitude of the following pulse. Here we test this hypothesis by comparing 18 pulse pairs, each separated by <55 days, measured over a 33-year period. We find a significant tendency for weak (strong) pulses, associated with low (high) cooling rates, to be followed by stronger (weaker) pulses. We therefore propose that the ocean introduces a memory effect into the MJO, whereby each event is governed in part by the previous event.
Highlights
Dynamical understanding of the Madden–Julian Oscillation (MJO) has been elusive, and predictive capabilities limited
The Madden–Julian Oscillation (MJO) is a tropical intraseasonal oscillation that forms in the Indian Ocean and whose dominant surface signature in the equatorial Indian and western Pacific Oceans is a pulse of intense winds and heavy precipitation coupled with deep atmospheric convection lasting 2–3 days at a given location, covering an area of 50,000 km[2] and recurring at intervals of 30–90 days[1]
A large-scale quantification of the intensity of each pulse is provided by the real-time multivariate MJO (RMM) index[15]
Summary
Dynamical understanding of the Madden–Julian Oscillation (MJO) has been elusive, and predictive capabilities limited. New measurements of the ocean’s response to the intense surface winds and cooling by two successive MJO pulses, separated by several weeks, show persistent ocean currents and subsurface mixing after pulse passage, thereby reducing ocean heat energy available for later pulses by an amount significantly greater than via atmospheric surface cooling alone This suggests that thermal mixing in the upper ocean from a particular pulse might affect the amplitude of the following pulse. Since it was at least possible that the greater upper ocean heat content (HC) remaining after the weak pulse might have contributed to the stronger following pulse, this anecdotal result prompted us to pose a hypothesis that stronger (weaker) pulses always follow weak (strong) pulses To address this hypothesis we exploited a set of reanalysis data products that begins in 1980 and includes sufficient information to identify MJO pulses, and to quantify their intensity as well as pre- and post-pulse SST. These suggest negative feedback between successive MJO pulses, supporting our stated hypothesis
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