Assessing and Understanding Oceanic Climate Forcing on Decadal Climate Variability from Surface Heat Flux

Ohio State University, The, Columbus OH

Investigators

Abstract

The North Atlantic and North Pacific are both home to large-scale sea surface temperature (SST) variations that last for years and even decades. An example is the Atlantic Multidecadal Oscillation (AMO), in which a large portion of the Atlantic north of the equator warms and cools over periods of perhaps 20 to 40 years. The low-frequency variability of North Pacific and North Atlantic SST must be driven by some combination of oceanic and atmospheric forcing, but their roles are not well understood and there is still some debate as to which is dominant. An important consideration is that low-frequency SST variability can be generated by changes in evaporation and surface heat exchange accompanying the passage of weather systems even though the movement of weather systems is much faster than the SST variability. If the slow variations of SSTs are driven by the "weather noise" of fast-moving systems the prospects for long-term SST prediction are somewhat limited, while a strong role for ocean dynamics, perhaps involving slow fluctuations of the global overturning circulation, could mean that SST anomalies can be predicted years in advance. Work under this award seeks to quantify the contributions of atmospheric and oceanic forcing to low-frequency SST variability using a simple stochastic model. The gist of the model is that atmospheric and oceanic forcing can be distinguished by looking at the timing of SST and surface heat flux anomalies, where the surface heat flux refers to both the evaporation and heat exchange occurring at the ocean surface and changes in surface sunlight and infrared radiation caused by changes in cloud cover. If an SST change is driven by the atmosphere it should be preceded by the surface heat flux anomaly that caused it. On the other hand an SST change driven by the ocean is likely to produce a change in surface heat flux that acts to damp the SST anomaly, for instance an ocean-driven warm anomaly would likely produce a surface heat flux that has a cooling effect. In that case the heat flux anomaly would be roughly synchronous with the SST anomaly but with opposite sign. The model used here includes explicit representations of both atmospheric and oceanic damping and treats the oceanic forcing as a red noise process. The model is used to analyze SST variability in observational datasets and output from climate models using standard and enhanced horizontal resolution. Simulations with modified versions of the Community Earth System Model (CESM) are then used to identify mechanisms of SST variability. The work has societal value as it addresses the question of long-range SST prediction. The slow variations of SST have a number of human impacts, for instance the number of severe Atlantic hurricanes roughly doubles from the cold phase of the AMO to the warm phase, and the AMO is implicated in the great Sahel drought of the mid-20th century. The extent to which the AMO and other forms of low-frequency SST are predictable is not known, and an understanding of the driving mechanisms is essential for an assessment of predictability and to provide useful guidance as to how predictive models might be developed. In addition, the project provides support and training for two graduate students, thereby providing for the future workforce in this research area. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.