Do periodic consolidations of Pacific countercurrents trigger global cooling by equatorially symmetric La Niña?

• Main Author: Duke, J. H.
• Format: Text
• Language: English
• Published: 2018
• Subjects: Pacific; Ice Sheet
• Online Access: https://doi.org/10.5194/cpd-6-905-2010; https://cp.copernicus.org/preprints/cp-2010-28/

A sporadic phenomenon of internal tide resonance (ITR) in the western equatorial Pacific thermocline is shown to precede 11 of 12 major upturns in the Niño 3.4 index between 1992 and 2008. Observed ITR has up to 9 °C semidiurnal temperature excursions indicating thermocline heave, but is invisible in time resolution longer than one day. It is independent of westerly wind bursts (WWB). A hypothesis is advanced that (1) ITR dissipates vorticity, leading to Pacific countercurrent consolidation (PCC) by reducing the vortex stretching term in Sverdrup balance. The consequence of lost vorticity survives ephemeral ITR events; (2) The specific surface area of countercurrents is reduced by PCC, which reduces frictional opposition to zonal gradient pressure, which triggers eastward advection at El Niño onset; (3) PCC also accelerates transfer of potential energy to the "pycnostad" below the Equatorial Undercurrent. This shoals the equatorial thermocline, leading to a distinct mode of equatorially symmetric La Niña (ESLN) characterized by a winter monsoon cell above a "cold eye" that is separated from the South American continent, as in 1998; (4) Precessional southward intertropical convergence zone migration (ITCZ) is an alternate PCC trigger, but its effect is modulated by obliquity; and (5) ESLN causes global cooling in all timescales by (a) reduced Hadley cell water vapor production when its rising branch is above the cold eye, (b) equatorward shift in southern circumpolar westerlies due to Hadley cell constriction, (c) possible CO 2 sequestration by increased EUC iron fertilized export production on the equator, and (d) possible adjacent cloud seeding by biogenic dimethyl sulphide. Surprising coincidences of WWB with perigean eclipses suggest a parallel atmospheric tide influence. Proposed PCC-ESLN forcing operates in multiple timescales, beginning where the annual cycle of strong equinoctial tides coincides with the minimum perigee cycle. This forcing corresponds with El Niño Southern Oscillation (ENSO) events in 1997, 2002, and 2006. Next, extreme central eclipses that perturb perigee-sysygy intervals also correspond with extreme ENSO events, notably in 1877, 1888, and 1982, and a 586 year cycle in the frequency of these eclipses corresponds with known stadial events in the past 4 thousand years. Contrast in the 586 year cycle increases with Earth eccentricity because it is the result of shorter synodic months at aphelion. Longer timescale forcing is by orbital control of the east-central Pacific ITCZ position, yielding a 10 thousand year fast ice sheet melt interval between March and September perihelion. But default ESLN is only interrupted when perihelion in March coincides with rising obliquity. A change in the phase relation between obliquity and precession from 1:2 to 3:5 or 2:5 may therefore explain skipped obliquity cycles after the mid-Pleistocene transition. A secular improvement in eclipse commensurability that parallels Cenozoic cooling is noted.