Climate Variability Ocean Heat Content Role in Past Climate Change

Over the past several years I have become increasingly interested in variations in ocean heat content (OHC) in the context of past climate change. The advantage of this approach is that OHC records integrate over time and thus provide additional perspective on climate response to perturbations in the Earth’s energy budget. My aim is to provide a comprehensive record of these changes and understand the mechanisms behind them on different time scales. In doing so, the obvious challenge is to find locations that provide the largest oceanographic coverage of these changes that can also be linked to a certain source/mechanism.

Taking advantage of the unique position of the Indonesian Seaways as a crossroads for Pacific and Indian Ocean intermediate water masses, and new foraminiferal calibrations (Rosenthal et al., 2011), we have documented a substantial cooling trend in Pacific intermediate water temperatures (IWT) from the early Holocene Thermal Maximum to the present as well as significant temperature variations in response to the Medieval Climate Anomaly and Little Ice Age (Rosenthal et al., 2013). We have speculated that the observed trends in Indonesian IWT reflect surface cooling of the high-latitudes in both hemispheres. To estimate the oceanic extent of these variations and assess their sources/causes we have been studying other regions in the Pacific and Atlantic Oceans. Using cores from the Peru margin collected on the R/V KNR195 cruise, we show that the eastern equatorial Pacific thermocline cools in the early Holocene in accord with surface temperature and ice core records from the Southern Ocean suggesting a close link to the Southern Ocean climate (Kalansky PhD thesis 2014 and papers in prep). In contrast, during the Common Era, eastern Pacific OHC is dominated by internal variability. A further test to the extent of these changes and the effects from both hemispheres will be offered by a new set of cores that we collected during the recent cruise to the WPWP (R/V Revelle 2013), which collected high-sedimentation cores from regions dominated by the Antarctic and North Pacific Intermediate water masses.

As a major source for deep water formation, the North Atlantic is key to understanding changes in OHC and its implication for climate. Over the past several years my group has been using intermediate and deep ocean temperature records from this region to evaluate the influence on and response of ocean-atmosphere interactions to climate change. These studies provide new insights on the sensitivity and response of meridional ocean circulation to melt water inputs to the North Atlantic high latitudes (e.g., Bamberg et al., 2010; Irvali et al., 2012; Morley et al., 2011) and their potential role in amplifying small radiative variations into large a climate response through dynamic changes in ocean-atmosphere interactions (e.g., Morely et al., 2011; Irvali et al., 2012; Morley et al., 2014). For example, we recently showed that NADW formation may become unstable under slightly warmer climate, such as occurred during the last integlaciation ~125 kyr B.P. (Galaasen et al., 2014).

While the emerging picture is incomplete it suggests a complex response of OHC to climate change possibly with high sensitivity ocean-atmosphere dynamics at the high latitudes. Based on the initial observations we argued that large changes in OHC throughout the Holocene, might have provided a thermal buffer for global climate change. Furthermore, if validated the record suggests that the ocean interior is still recovering from the strong anomaly of the Little Ice Age, which should influence current and near future climate change. We are currently in process of testing both the observations and hypotheses using new cores and intermediate complexity model simulations.

Sea interview with Andrew Revkin of the New York Times: