Mechanisms of Climate Variability in the North Atlantic Ocean

Johns Hopkins University, Baltimore MD

Investigators

Abstract

0136327/Haine In an effort to understand some of the oceanic processes that may be involved in decadal climate variability, this project will apply adjoint sensitivity analysis to 1-degree models of the N. Atlantic Ocean circulation to look at mechanisms that control sea-surface temperature (SST). The behavior of the 1-degree model will also be compared to forward integrations with 1/3-degree and 1/6-degree models in order to investigate the importance of unpredictable internal processes such as transport by mesoscale eddies. The adjoint study will be used to determine the sensitivity of SST to prior surface forcing. One of the hypotheses underlying this work is that changes to surface forcing can lead to changes in the depth of the thermocline which subsequently propagate through the ocean on baroclinic time scales and which may affect the SST above them in subsequent years. The work will be based on a z-coordinate ocean circulation model developed at MIT. The approach involves first conducting a 50-year hind-cast experiment for 1950-2000 with a 1-degree resolution version of the model. The fidelity of this will be assessed by comparing model SST with COADS SST. Three sets of 20-year long adjoint sensitivity calculations will then be made using the a linearization of the model system around the fiducial trajectory given by the hind-cast run. The three adjoint calculations will correspond to during different phases of the NAO -- low - early 60s, high - early 90s, near zero - mid-70s. The variable whose sensitivity will be examined will be the end-of-winter SST spatially averaged over O(10,000 Km^2) regions of the ocean surface. Model calculations will be performed on a Beowulf cluster at JHU and at one of the national supercomputing centers.