Understanding the role of stochastic atmospheric forcing in tropical Pacific decadal variability and ENSO modulation

Interdecadal variability of the tropical Pacific affects global hydroclimate and ecosystem and is suggested to modulate global mean temperature in the past century. It has been long acknowledged that Pacific decadal variability can be generated in the extratropics through integration of stochastic atmospheric forcing by the ocean mixed layer. However, it remains unclear how the extratropical signal propagates into the tropical Pacific and forms a basin-wide anomaly pattern and how the resultant tropical Pacific decadal variability (TPDV) interacts with the El Niño-Southern Oscillation (ENSO), the dominant mode of interannual climate variability. To answer this question, a suite of climate model experiments is conducted with small time-invariant surface heat flux anomalies associated with the leading atmospheric modes in the extratropical South and North Pacific. The results show that subtropical ocean-atmosphere anomalies driven by the surface heat flux forcing propagate into the tropical Pacific mainly through thermodynamic ocean-atmosphere interactions, with ocean dynamics playing an important role in modifying the equatorial sea surface temperature (SST) response. The associated changes in the tropical Pacific mean state not only extend climate anomalies into the other hemisphere through atmospheric teleconnections, but also significantly affect the properties of ENSO. In particular, the ENSO-like TPDV affects the relative frequency of El Niño and La Niña. To investigate the causality and mechanism of this linkage, a separate set of climate model experiments is conducted by imposing surface heat flux anomalies associated with the ENSO-like TPDV in the tropical Pacific. The forced mean state change affects the frequency of El Niño and La Niña events by modulating the SST contrast between the tropical Pacific and the Indian/Atlantic Oceans and associated zonal wind anomalies in the western Pacific. This study presents in detail the atmospheric and oceanic mechanisms by which extratropical atmospheric forcing induces basin-wide coherent patterns of TPDV that can further affect ENSO characteristics, which has not been systematically studied in fully coupled climate models. The results also show that the tropical Pacific is more strongly influenced by the South Pacific through both thermodynamic processes and ocean dynamics. The strong oceanic linkage to the South Pacific allows delayed negative oceanic feedback to effectively affect the equatorial Pacific SST, which may be important for setting up the time scales of TPDV.

Author dc:creator

• Sun, Tianyi, Ph. D.

Contributors dc:contributor

• Okumura, Yuko M.
• Quinn, Terrence M
• Di Nezio, Pedro N
• Heimbach, Patrick
• Yang, Zong-liang

• Decadal climate variability
• El Niño-Southern Oscillation
• Tropical Pacific climate

Language dc:language

en

Identifier

http://dx.doi.org/10.26153/tsw/3301

OAI identifier oai:identifier

oai:tdl-ir.tdl.org:2152/76212