Changes in ITCZ location and cross-equatorial heat transport at the Last Glacial Maximum, Heinrich Stadial 1, and the mid-Holocene

摘要

Tropical paleoclimate records provide important insights into the response of precipitation patterns and the Hadley circulation to past climate changes. Paleo-records are commonly interpreted as indicating north-south shifts of the Intertropical Convergence Zone (ITCZ), with the ITCZ's mean position moving toward the warmer hemisphere in response to changes in cross-equatorial temperature gradients. Though a number of records in tropical Central and South America, North Africa, Asia and the Indo-Australian region are consistent with this interpretation, the magnitudes and regional variability of past ITCZ shifts are poorly constrained. Combining estimates of past tropical sea surface temperature (SST) gradients with the strong linear relationship observed between zonally averaged ITCZ position and tropical SST gradients in the modern seasonal cycle and in models of past climates, we quantify past shifts in zonally averaged ITCZ position. We find that mean ITCZ shifts are likely less than 1° latitude during the Last Glacial Maximum (LGM), Heinrich Stadial 1 (HS1) and mid-Holocene (6 ka) climates, with the largest shift during HS1. The ITCZ's position is closely tied to heat transport between the hemispheres by the atmosphere and ocean; accordingly, these small mean ITCZ shifts are associated with relatively large (~0.1-0.4 PW) changes in cross-equatorial atmospheric heat transport (AHT_(EQ)). These AHT_(EQ) changes point to changes in cross-equatorial ocean heat transport or net radiative fluxes of the opposite sign. During HS1, the increase in northward AHT_(EQ) is large enough to compensate for a partial or total shutdown in northward heat transport by the Atlantic Ocean's meridional overturning circulation. The large AHT_(EQ) response for small changes in mean ITCZ position places limits on the magnitude of past shifts in the globally averaged ITCZ. Large (≥5°) meridional displacements of the ITCZ inferred from regional compilations of proxy records must be limited in their zonal extent, and ITCZ shifts at other longitudes must be near zero, for the global mean shift to remain ≤1° as suggested by our results. Our examination of model results and modern observations supports variable regional and seasonal changes in ITCZ precipitation. This work thus highlights the importance of a dense network of tropical precipitation reconstructions to document the regional and seasonal heterogeneity of ITCZ responses to past climate changes.