Intensified modulation of the Pacific north equatorial current bifurcation by the southern annular mode since the early 1990s.

Long-term reanalysis data were used to assess inter-decadal to decadal modulations of the North Equatorial Current (NEC) bifurcation in the Pacific after the early 1990s. The wind stress curl anomaly (WSCA) in the region of 10° N-15° N and 160° E-170° E (C-BOX) had been found to excite Rossby waves and control NEC bifurcation along the Philippine coast. Our analysis revealed that the WSCA in the C-BOX has been remotely modulated by the Southern Annular Mode (SAM) since the early 1990s. It is shown that the SAM shifted to its positive phase at this transition and began strongly impacting the WSCA in the C-BOX and the NEC bifurcation. During the positive SAM phase after the early 1990s, strong climate variability occurred in the tropical to subtropical area of the North Pacific, with a clear footprint connected to the Antarctic region. Consistent with that finding, we determined that during the positive SAM phase, a dipole sea surface temperature pattern was generated in the South Pacific; this induced an atmospheric Rossby wave train in upper-level wind shear that propagated northward to the North Pacific. Such effects further enhanced downward motion and divergence at the surface, intensifying the easterlies in the equatorial area and the anticyclonic WSCA in the C-BOX. The anticyclonic WSCA in the C-BOX substantially excited downwelling oceanic Rossby waves at the surface, inducing an equatorward trend of NEC bifurcation after the early 1990s.

Continued weakening of the equatorial Pacific upwelling annual cycle in CMIP5 future projections.

This study explores the dynamics of the equatorial Pacific upwelling annual cycle under global warming using the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. Through a linear-weighted theory developed recently, the theoretical upwelling annual cycles under global warming helped reasonably characterize the patterns of the original upwelling annual cycles simulated in CMIP5 models; however, an apparent weakening in magnitude as compared to that during the present stage was observed. To verify the above, we divided 90-year outputs in the CMIP5 future projections into three 30-year windows and set side by side. The long-term evolution of the upwelling annual cycle reconfirmed an overall weakening tendency in the entire equatorial Pacific. Moreover, the weakening of the Ekman upwelling could most likely be attributed to the meridional surface wind stress divergence, while the gradually smoothing inclination in the overall equatorial thermocline depth was responsible for the weakening of the wave upwelling. The weakening of the wave upwelling in the east and the Ekman upwelling in the west jointly contributed to the gradual weakening of the equatorial Pacific upwelling annual cycle. The above projected changes are robust among the 19 chosen CMIP5 models. Equatorial upwelling largely influences the sea surface temperature, associated atmosphere-ocean interactions, and convection and precipitation in tropical areas; hence, a continuous weakening of the upwelling annual cycle over the equatorial Pacific Ocean could likely affect the major climate phenomena variability with strong seasonal-locking characteristics by modifying the background strength at their peak phases in the future. The theoretical results can provide us the equatorial upwelling annual cycle patterns based on the Ekman and wave dynamics, which would be a strong tool for our investigations on the climate variability under global warming.

Coherent Response of Vietnam and Sumatra-Java Upwellings to Cross-Equatorial Winds.

Upwelling off Vietnam in the South China Sea (SCS) and the Sumatra-Java upwelling in the Indian Ocean significantly modulate regional variation in climate. Although located in different hemispheres, these upwellings nearly concur during the boreal summer; both are the result of wind-induced Ekman divergence. Beyond seasonal time scales, the two upwellings were not synchronous in 1998. In the summer of 1998, upwelling off Vietnam was almost absent, generating the warmest summer on record in the SCS. We demonstrated that the El Niño-Southern Oscillation, which was highly correlated with the upwelling in previous studies, was not solely responsible for this variability. Wind trajectory analyses revealed that cross-equatorial winds, which had passed over the Sumatra-Java upwelling site about 2 weeks earlier, were a rapid force acting on SCS summer upwelling. In the summer of 1998, SCS winds were greatly perturbed due to an anomalous wind path that blew toward the SCS through the Sulu Sea. Our findings suggest that not only the resulting weakening but also the perturbation of the SCS winds prevented the formation of summer upwelling off central Vietnam in that year.