Connections Between Multi-Decadal SST Variability and the Inter-Tropical Convergence Zone

Abstract:

The connections between multi-decadal variability in North Atlantic and Pacific sea surface temperatures (SSTs; termed the AMO and PDO, respectively) and the inter-tropical convergence zone (ITCZ) are examined using reconstructions of Earth’s climate over the 20^th Century. In the context of a simple energetic framework, variations in SST in both basins are shown to modulate the ITCZ position through the exchange of energy between the surface ocean and the atmosphere. On multi-decadal timescales, the AMO and the PDO exert similar influences on the ITCZ, shifting it northwards when the northern Atlantic and Pacific basins are anomalously warm. Positive SST anomalies in the Northern Hemisphere mid-latitudes create inter-hemispheric thermal contrasts in the troposphere, which the tropical atmospheric circulation responds to by shifting the Hadley cells and the ITCZ. On inter-annual timescales, a North Atlantic Oscillation-like pattern of atmospheric forcing dominates North Atlantic sea-surface temperature variability, obscuring the connection between ocean circulation variability and the ITCZ position on those timescales. The differences between oceanic and atmospheric forcing of North Atlantic sea surface temperature on multi-decadal versus inter-annual timescales are discussed in terms of the ocean’s mixed-layer energy budget.