Kirk Bryan

Global seiching of thermocline waters between the Atlantic and the Indian‐Pacific Ocean Basins

[1]xa0Proxy climate data from the Greenland icecap and marine deposits in the Pacific indicate that warm conditions in the North Atlantic are linked to cool conditions in the Eastern Equatorial Pacific, and vice versa. Our ocean models show that the surface branch of the overturning circulation connecting the North Atlantic to the Equatorial Pacific adjusts by exchanging thermocline water between ocean basins in response to changes in deep water formation in the northern North Atlantic. Planetary ocean waves give rise to a global oceanic seiche, such that the volume of thermocline water decreases in the Pacific-Indian Ocean while increasing in the Atlantic Ocean. We conjecture that the remotely forced changes in the thermocline of the Eastern Equatorial Pacific may trigger El Nino events. These global seiches have been previously overlooked due to the difficulty of integrating high-resolution climate models for very long time-scales.

The steric component of sea level rise associated with enhanced greenhouse warming : a model study

Climate change due to enhanced greenhouse warming has been calculated using the coupled GFDL general circulation model of the atmosphere and ocean. The results of the model for a sustained increase of atmospheric carbon dioxide of 1% per year over a century indicate a marked warming of the upper ocean. Results of the model are used to study the rise in sea level caused by increase in ocean temperatures and associated changes in ocean circulation. Neglecting possible contributions due to changes in the volume of polar ice sheets and mountain glaciers, the model predicts an average rise in sea level of approximately 15 ± 5 cm by the time atmospheric carbon dioxide doubles. Heating anomalies are greatest in subpolar latitudes. This effect leads to a weakening of the ocean thermohaline circulation. Changes in thermohaline circulation redistribute heat within the ocean from high latitudes toward the equator, and cause a more uniform sea level rise than would occur otherwise.

Redistribution of sea level rise associated with enhanced greenhouse warming: a simple model study

Future sea level rise from thermal expansion of the World Ocean due to global warming has been explored in several recent studies using coupled ocean-atmosphere models. These coupled models show that the heat input by the model atmosphere to the ocean in such an event could be quite non-uniform in different areas of the ocean. One of the most significant effects predicted by some of the models is a weakening of the thermohaline circulation, which normally transports heat poleward. Since the greatest heat input from enhanced greenhouse warming is in the higher latitudes, a weakening of the poleward heat transport effectively redistributes the heat anomaly and the associated sea level rise to lower latitudes. In this study, the mechanism of ocean circulation spindown and heat redistribution was studied in the context of a much simpler, linearized shallow water model. Although the model is much simpler than the three-dimensional ocean circulation models used in the coupled model experiments, and neglects several important physical effects, it has a nearly 10-fold increase in horizontal resolution and clearer dynamical interpretations. The results indicated that advanced signals of sea level rise propagated rapidly through the action of Kelvin and Rossby waves, but the full adjustment toward a more uniform sea level rise took place much more slowly. Long time scales were required to redistribute mass through narrow currents trapped along coasts and the equatorial wave guide. For realistic greenhouse warming, the model showed why the sea level rise due to ocean heating could be far from uniform over the globe and hence difficult to estimate from coastal tide gauge stations.

Climate variability and the Atlantic Ocean

Climate models indicate global warming ranging from 1.5°C to 4°C in response to a doubling in greenhouse gases. Whether this warming occurs in 50 years or 100 years is not certain, due in part to our limited understanding of natural climate variability. Apart from its intrinsic merit, the study of natural climate variability is motivated by the societal need to address climate change issues. From this perspective, the interannual to interdecadal time scales are of particular importance. These are precisely the time scales at which coupling to the ocean circulation becomes of paramount significance.

The role of mesoscale eddies in the poleward transport of heat by the oceans: a review

Abstract The poleward transport of heat by the ocean circulation plays a major role in the global heat balance, but many details of this process remain unclear. In particular it is difficult to determine from observations what role energetic mesoscale eddies play in poleward heat transport. In place of missing long-term buoy measurements over extensive areas, eddy resolving numerical ocean circulation simulations offer some means to gain insight. For ocean models with specified meridional density distributions at the upper boundary it is possible to compare the poleward heat transport in eddy resolving and non-eddyresolving simulations. While a change of vertical diffusion in models is known to be very important for poleward heat transport, the reduction of horizontal viscosity and diffusion, which allows mesoscale eddies to appear in the simulation, seems to have little effect. A possible explanation appears to be that the models for normal parameter ranges are very weakly driven thermal systems. The time dependent mesoscale eddies appear to set up nearly adiabatic flows in which eddy transport of heat is compensated by induced mean flows which transport heat in the opposite direction. For extremely strong forcing of the density field in the upper ocean this is no longer true.

The Atlantic Climate Change Program

The Atlantic Climate Change Program (ACCP) is a component of NOAAs Climate and Global Change Program. ACCP is directed at determining the role of the thermohaline circulation of the Atlantic Ocean on global atmospheric climate. Efforts and progress in four ACCP elements are described. Advances include (1) descriptions of decadal and longer-term variability in the coupled ocean-atmosphere-ice system of the North Atlantic; (2) development of tools needed to perform long-term model runs of coupled simulations of North Atlantic air-sea interaction; (3) definition of mean and time-dependent characteristics of the thermohaline circulation; and (4) development of monitoring strategies for various elements of the thermohaline circulation. 20 refs., 4 figs., 1 tab.

EFFICIENT METHODS FOR FINDING THE EQUILIBRIUM CLIMATE OF COUPLED OCEAN-ATMOSPHERE MODELS

Climate involves the interaction of the atmosphere, hydrosphere and cryosphere, with an extremely wide range of significant timescales. Straightforward numerical integration of the time-dependent equations of a climate model is not a practical way to find equilibrium solutions of a more complete climate model. Methods that have been found useful for finding climate equilibrium in low-resolution coupled atmosphere-ocean models are briefly described.

A review of observations of the effect of bathymetry on ocean circulation in recent decades

In pioneering studies, Academician A. S. Sarkisyan, pointed out the importance of ocean bathymetry in determining the ocean circulation (Sarkisyan and Ivanov, 1971). Many decades later improved technology for measuring the ocean circulation provides overwhelming empirical evidence to support his idea. Academician Sarkisyan, my friend for over 50 years, is to be congratulated in having a long enough life to see this day! My brief review is an account of some of these new measurements. By “new” is meant within the last few decades. There is controversy over the optimum way to analyze this new data, but we believe this controversy is less important than the field evidence, which speaks for itself. Measurements included will be principally for the North Atlantic, the part of the World Ocean the author is most familiar with.

Ocean Circulation in Warm and Cold Climates

The response of a coupled atmosphere-ocean model is determined over a wide range of atmospheric CO2 levels. The solutions indicate the type of ocean circulations expected over a very wide range of climatic conditions. In climates warmer than the present one, the north-south gradient of temperature is significantly reduced, but, owing to the elevation of the thermal expansion coefficient with increased temperature, the meridional buoyancy gradient remains at about the same level in a wide range of warm climates. A weakening of the thermohaline circulation takes place only in the climates corresponding to reduced atmospheric CO2 levels.