Mixed-layer water oscillations in tropical Pacific for ENSO cycle

Abstract

The main modes of interannal variabilities of thermocline and sea surface wind stress in the tropical Pacific and their interactions are investigated, which show the following results. (1) The thermocline anomalies in the tropical Pacific have a zonal dipole pattern with 160°W as its axis and a meridional seesaw pattern with 6-8°N as its transverse axis. The meridional oscillation has a phase lag of about 90° to the zonal oscillation, both oscillations get together to form the El Niño/La Niña cycle, which behaves as a mixed layer water oscillates anticlockwise within the tropical Pacific basin between equator and 12°N. (2) There are two main patterns of wind stress anomalies in the tropical Pacific, of which the first component caused by trade wind anomaly is characterized by the zonal wind stress anomalies and its corresponding divergences field in the equatorial Pacific, and the abnormal cross-equatorial flow wind stress and its corresponding divergence field, which has a sign opposite to that of the equatorial region, in the off-equator of the tropical North Pacific, and the second component represents the wind stress anomalies and corresponding divergences caused by the ITCZ anomaly. (3) The trade winds anomaly plays a decisive role in the strength and phase transition of the ENSO cycle, which results in the sea level tilting, provides an initial potential energy to the mixed layer water oscillation, and causes the opposite thermocline displacement between the west side and east side of the equator and also between the equator and 12°N of the North Pacific basin, therefore determines the amplitude and route for ENSO cycle. The ITCZ anomaly has some effects on the phase transition. (4) The thermal anomaly of the tropical western Pacific causes the wind stress anomaly and extends eastward along the equator accompanied with the mixed layer water oscillation in the equatorial Pacific, which causes the trade winds anomaly and produces the anomalous wind stress and the corresponding divergence in favor to conduce the oscillation, which in turn intensifies the oscillation. The coupled system of ocean-atmosphere interactions and the inertia gravity of the mixed layer water oscillation provide together a phase-switching mechanism and interannual memory for the ENSO cycle. In conclusion, the ENSO cycle essentially is an inertial oscillation of the mixed layer water induced by both the trade winds anomaly and the coupled ocean-atmosphere interaction in the tropical Pacific basin between the equator and 12°N. When the force produced by the coupled ocean-atmosphere interaction is larger than or equal to the resistance caused by the mixed layer water oscillation, the oscillation will be stronger or maintain as it is, while when the force is less than the resistance, the oscillation will be weaker, even break.tropical Pacific, thermocline and sea surface wind stress anomaly, ENSO cycle, mixed layer water oscillation

Two modes of dipole events in tropical Indian Ocean

“…Its time coefficients were best correlated with the first mode when there was an eight to nine month delay (r=0.78) and it had two significant periods: 56 and 44 months (Figure 2(d)), which is so called the second mode of ENSO. A research [16] pointed out that it was a combination of these two modes of the ENSO event that constituted an EI Niño/La Niña cycle. After removal of the ENSO signal, the spatial distribution of the first mode in the tropical Indian Ocean (see the area of the tropical Indian Ocean in Figure 2(b)) is extremely similar to that of a typical dipole (Figure 1(a)), with its spa- tial correlation coefficient being 0.90.…”

Bifurcations in a low-order nonlinear model of tropical Pacific sea surface temperatures derived from observational data

“…of ENSO completely. 14,15 So it is hard to accurately establish a SST dynamical model of an idiographic ENSO event. Because of the complexity of physical mechanisms of SST and ENSO, as well as the inexact nonlinear dynamics and chaotic nature of the ocean-atmosphere coupled system, climate forecasts have gotten limited success beyond a season during the early half of a calendar year.…”