Vuille, M., Burns, S.J., Taylor, B.L., Cruz, F.W., Bird, B.W., Abbott, M.B., Kanner, L.C., Cheng, H. and Novello, V.F. 2012. A review of the South American monsoon history as recorded in stable isotopic proxies over the past two millennia. Climate of the Past 8: 1309-1321.
Noting that "the stable water isotopic composition of an air mass is affected by processes that take place far upstream from the site of the proxy has led to the concept of interpreting water isotopes as recorders of atmospheric circulation or modes of climate variability," the authors employ δ18O data to identify monsoon intensity over the core region of convective activity in South America: the tropical Andes and Southeast Brazil. And hypothesizing that "these centennial-scale climate anomalies were at least partially driven by temperature changes in the Northern Hemisphere and in particular over the North Atlantic, leading to a latitudinal displacement of the Intertropical Convergence Zone and a change in monsoon intensity (amount of rainfall upstream over the Amazon Basin)," they found large excursions during three key periods: the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the Current Warm Period (CWP), during which "the South American Summer Monsoon's mean state was significantly weakened (MWP and CWP) and strengthened (LIA), respectively." And with their noting that the intensity of the South American Summer Monsoon "today appears on par with conditions during the MWP," it can logically be concluded that the peak temperatures of the MWP and the CWP over the North Atlantic Ocean were also likely on a par with each other.