Southern Hemisphere westerlies and South Pacific Split Jet
Shih-Yu Lee and I were motivated by this intriguing paleoproxy study to explore how North Atlantic stadial conditions could lead to southern hemisphere (SH) westerly changes. From prior work, we knew that ITCZ shifts could lead to knockoff effects on the Hadley circulation and subtropical jets. This turned out indeed to be how that connection worked. In Lee et al. 2011, we showed that North Atlantic cooling (representing stadial conditions) shift the ITCZ southwards, weakening the SH subtropical jet; the latter then leads to an increase in the SH eddy-driven midlatitude jet (as explained by this study), and thus the SH surface westerlies. We also showed that increased surface westerlies could lead to increased CO2 fluxes out off the southern ocean, thus providing a mechanistic link between stadial conditions and increasing CO2 concentrations.
Paleoclimate literature typically conceptualizes Southern Hemisphere (SH) westerly changes in terms of zonal mean changes to its position or strength (for example, this perspective), motivated by the prevailing annular mode thinking. The SH westerly changes in Lee et al. 2011 however exhibited both seasonal and longitudinal preferences, leading me to explore zonally asymmetric changes. In today’s climate, there is a pronounced zonal asymmetry of the austral winter SH westerlies called the South Pacific Split Jet, where (as the name suggests) the jet splits into a strong subtropical branch and a weaker subpolar branch at longitudes around Australia and extending across the Pacific. Moreover, the dominant interannual variability of the austral winter SH westerlies is the modulation of this Split Jet: in some years, the split is weaker making the jet more zonally symmetric. This Split Jet is an interesting quirk in the SH midlatitude circulation, and one that challenges the dominant annular mode view of SH westerly changes.
In Chiang et al. 2014 we auditioned the Split Jet modulation as an alternative model for SH westerly paleoclimate changes. This model can readily explain temperature and precipitation changes across Chile and New Zealand, two key locations where paleoclimate proxies are linked to zonal mean westerly changes. The hypothesis also makes predictions for changes in West Antarctica. In Chiang et al. (2020), we explored the implications of this ‘Split Jet’ hypothesis on SH ocean circulation, contrasting its changes against those predicted by SH annular-mode (SAM) like changes. A weakened Split Jet leads to a strengthened South Pacific subtropical gyre and increased formation of Subantarctic Mode water, whereas a positive SAM leads to a strengthened Antarctic Circulpolar Current and increased formation of Antarctic Intermediate Water. In Lamy et al. 2019, we showed that the precessional modulation of the Split Jet explains the marked precessional changes in rainfall over subtropical Chile.
Lee, S.Y., Chiang, J.C.H., Matsumoto, K. and Tokos, K.S., 2011. Southern Ocean wind response to North Atlantic cooling and the rise in atmospheric CO2: Modeling perspective and paleoceanographic implications. Paleoceanography, 26(1).
Chiang, J.C.H., Lee, S.Y., Putnam, A.E. and Wang, X., 2014. South Pacific Split Jet, ITCZ shifts, and atmospheric North–South linkages during abrupt climate changes of the last glacial period. Earth and Planetary Science Letters, 406, pp.233-246.
Chiang, J.C.H., Tokos, K.S., Lee, S.Y. and Matsumoto, K., 2018. Contrasting impacts of the South Pacific Split Jet and the Southern Annular Mode modulation on Southern Ocean circulation and biogeochemistry. Paleoceanography and Paleoclimatology, 33(1), pp.2-20.
Lamy, F., Chiang, J.C.H., Martínez-Méndez, G., Thierens, M., Arz, H.W., Bosmans, J., Hebbeln, D., Lambert, F., Lembke-Jene, L. and Stuut, J.B., 2019. Precession modulation of the South Pacific westerly wind belt over the past million years. Proceedings of the National Academy of Sciences, 116(47), pp.23455-23460.