Meridional Modes in the Atlantic and Pacific

A theory of tropical ocean-atmosphere interactions in the meridional direction emerged when I was in graduate school (for example, this classic paper). This was partly what motivated me to examine Tropical Atlantic variabilty for my thesis, as its dominant mode of variability - the Atlantic Meridional Mode - exemplifies this type of interaction. What impressed me in particular was the acute sensitivity of its Intertropical Convergence Zone (ITCZ) to its latitudinal position to very small changes in the meridional SST gradient: a 0.5K change in the SST difference between the north and south tropical Atlantic made the difference between a drought and good rainfall year for Northeast Brazil.

In Chiang et al. 2001 we decomposed Atlantic ITCZ variations into a north-south shift that is controlled by meridional SST gradient changes, and a suppression of rainfall from an ‘anomalous Walker circulation’ influence from El Nino. The decomposition allowed us to isolate the observed ‘meridional mode’ behavior of the Atlantic and to show that the associated tropical wind-evaporation-SST feedback was relatively weak and confined to the deep tropics; it meant that the meridional SST gradient anomaly essentially damped down after its peak in boreal spring. The Atlantic Meridional Mode was thus a damped response to external forcing, rather than being self-sustaining.

When I went to the University of Washington for my postdoc, I saw a talk given by Dan Vimont on his ‘seasonal footprinting mechanism’ for the North Pacific. I was struck by how closely it resembled how the North Atlantic Oscillation forced the Atlantic Meridional Mode, except that in his case it was the North Pacific Oscillation. We started comparing notes, and that was how the ‘Pacific Meridional Mode’ of Chiang and Vimont 2004 came about. We showed that the two meridional modes closely resembeled each other in spatial structure and temporal evolution of the SST, precipitation and trade wind changes, and also their respective forcing by midlatitude atmospheric variability. Because of this resemblance, we argued that this behavior was a ubiquitous feature of ITCZ-cold tongue climates, later borne out though the identification of a similar feature in the South Pacific. The Pacific Meridional Mode has since been linked to climate impacts in the Pacific sector (e.g. tropical cyclones in the western Pacific and rainfall over the maritime continent), the forcing of ENSO and to the existence of the ‘Central Pacific El Nino’. In Chiang al. 2008, we argued that ENSO activity in the mid-Holocene reduced because of a similar reduction in Pacific meridional mode activity, arising from a less stormy North Pacific (Chiang and Fang 2010).

It was during the tail end of my graduate studies that a number of paleoclimate papers on the tropical Atlantic emerged, claiming ITCZ variations on millennial and orbital timescales. These records were intriguing in that the ITCZ variations were connected to distant northern high latitude processes. I thought that the Atlantic Meridional Mode could provide a useful model for connecting high to low latitude processes, and this became the subject of Chiang et al. 2003 and Chiang 2004. This line of thinking eventually led to the work linking ITCZ shifts to interhemispheric gradients and with atmospheric energetics that began with Chiang and Bitz 2005

Chiang, J.C.H., Kushnir, Y. and Giannini, A., 2002. Deconstructing Atlantic Intertropical Convergence Zone variability: Influence of the local cross‐equatorial sea surface temperature gradient and remote forcing from the eastern equatorial Pacific. Journal of Geophysical Research: Atmospheres, 107(D1), pp.ACL-3.

Chiang, J.C.H. and Vimont, D.J., 2004. Analogous Pacific and Atlantic meridional modes of tropical atmosphere–ocean variability. Journal of Climate, 17(21), pp.4143-4158.

Chiang, J.C.H., Biasutti, M. and Battisti, D.S., 2003. Sensitivity of the Atlantic intertropical convergence zone to last glacial maximum boundary conditions. Paleoceanography, 18(4).

Chiang, J.C.H., 2004. Present-Day Climate Variability in the Tropical Atlantic. In The Hadley Circulation: Present, Past and Future (pp. 465-488). Springer, Dordrecht.

Chiang, J.C.H. and Bitz, C.M., 2005. Influence of high latitude ice cover on the marine Intertropical Convergence Zone. Climate Dynamics, 25(5), pp.477-496.

Chiang, J.C.H., Fang, Y. and Chang, P., 2009. Pacific climate change and ENSO activity in the mid-Holocene. Journal of Climate, 22(4), pp.923-939.

Chiang, J.C.H. and Fang, Y., 2010. Was the North Pacific wintertime climate less stormy during the mid-Holocene?. Journal of climate, 23(14), pp.4025-4037.