| Summary: | 博士 === 國立臺灣大學 === 大氣科學研究所 === 105 === In this study, we investigate the Pacific interannual variability to clarify the phase transitions of El Nino-Southern Oscillation (ENSO) phenomenon. During the ENSO evolution, the equatorial variations are closely connected to off-equatorial processes through zonal and meridional mass transports associated with the thermocline anomalies. The meridional transport associated with tropical wind stress curl plays a role in determining the equatorial zonal-mean thermocline variations which could transit the phase of ENSO via thermocline feedback. Compared with meridional transport, the zonal transport plays an important role in affecting the ENSO evolution through advection feedback associated with SST anomalies in the eastern Pacific.
The meridional transport in the Pacific is composed of equatorward geostrophic flow within the interior pycnocline subtropical cells (STCs), surface Ekman transport and western boundary current. The Simple Ocean Data Assimilation (SODA 2.2.4) analysis for the period of 1960-2010 is used to study the Subtropical Cells (STCs) variability and its causal relation with tropical climate variability. Result shows that the interior STCs transport into the equatorial basin through 9°S and 9°N is well connected with equatorial SST (9°S-9°N, 180°-90°W). The highest correlation at interannual timescales is contributed by the western interior STCs transport within 160°E and 130°W. It is known that the ENSO recharge-discharge cycle experiences five stages, i.e., the recharging stage, recharged staged, warmest SST stage, discharging stage and discharged stage. A correlation analysis of interior STCs transport convergence, equatorial WWV, wind stress curl and SST identifies time interval between the five stages, which are 8, 10, 2 and 8 months, respectively. A composite analysis for El Nino and La Nina developing events is also performed. The composited ENSO evolutions are in accordance with the recharge-discharge theory and the corresponding time lags between the above denoted five stages are 4~12, 6, 2, and 4 months. Those results clarify subsurface transport processes and their time intervals, which are useful for refinement of theoretical models and for evaluating couple ocean-atmosphere general circulation model results.
Zonal transport affects the ENSO evolution through advection feedback associated with SST anomalies in the eastern Pacific. Our result shows the sudden basin-wide reversal of anomalous equatorial zonal transport above the thermocline at the peaking phase of ENSO triggers rapid termination of ENSO events. The anomalous equatorial zonal transport is controlled by the concavity of anomalous thermocline meridional structure across the equator. During developing phase of ENSO, opposite zonal transport anomalies form in the western-central and central-eastern equatorial Pacific, respectively. Both are driven by the equatorial thermocline anomalies in response to zonal wind anomalies over the western-central equatorial ocean. At this stage, the anomalous zonal transport in the east enhances ENSO growth through zonal SST advection. In the mature phase of ENSO, off-equatorial thermocline depth anomalies become more dominant in the eastern Pacific due to the reflection equatorial signals at the eastern boundary. As a result, the meridional concavity of the thermocline anomalies is reversed in the east. This change reverses zonal transport rapidly in the central-to-eastern equatorial Pacific, joined with the existing reversed zonal transport anomalies further to the west and forms a basin-wide transport reversal throughout the equatorial Pacific. This basin-wide transport reversal weakens the ENSO SST anomalies by reversed advection. More importantly, the reversed zonal transport reduces the existing zonal tilting of equatorial thermocline and weakens its feedback to wind anomalies effectively.
Further, the oceanic processes associated with zonal transport are separated into low-frequency ENSO cycle and high-frequency oceanic wave process. Both processes can be represented by the concavity of meridional thermocline anomalies and generate the reversal of equatorial zonal current at the peaking phase which be a trigger to the rapid termination of ENSO events. For low-frequency process, the zonal transport presents slower and basin-wide evolution. During the developing phase of El Nino (La Nina), the eastward (westward) transport prevails in the central-eastern Pacific and enhances the ENSO through zonal SST advection by anomalous zonal current. At the peak of ENSO, a basin-wide reversal of transport resulted from recharge-discharge process is occurred and weakens the SST anomalies through advection damping. The high-frequency zonal transport presents obvious eastward propagation related to the Kelvin wave at equator. The major wester wind bursts (WWBs)/easterly wind surges (EWSs) occur in boreal summer and fall with coincident downwelling (upwelling) Kelvin waves for El Nino (La Nina) events. After the peak of El Nino (La Nina), the signal of Kelvin waves reaches eastern boundary in boreal winter and reflect as off-equator Rossby waves, then the zonal transport just switches from eastward (westward) to westward (eastward). The high-frequency equatorial zonal transport can be definitely represented by equatorial wave dynamics captured by first three EOFs based on high-pass filtered equatorial thermocline. The transport anomaly during decaying phase is more dominated by low-frequency process in El Nino events; however, the transport anomaly is caused by both low- and high-frequency process during La Nina decaying phase. Those results clarify the sudden phase transition of ENSO and provide an additional remark of phase-locking
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