Oceans and Internal Climate Variability William Kininmonth Melbourne, Australia “The warming over the past 100 years is very unlikely to be due to internal variability alone, as estimated by current models.” IPCC, 2001: Climate Change 2001: The Scientific Basis Oceans and Internal Climate Variability

Global Temperature Anomalies have their Origins in the Tropics. Why? ENSO Oceans and Internal Climate Variability

Wind Action and Ocean Dynamics Change Upwelling and Surface Temperature Patterns July 1997 July 1998 Oceans and Internal Climate Variability

ENSO is Internal Variability on the Inter-Annual Timescale Radiation to space varies with global surface temperature Is there Potential for Ocean Circulations to Regulate Global Temperature (Climate) on Longer Timescales? We look to the deep oceans and their longer circulation times Oceans and Internal Climate Variability

How Does the Return Flow Overcome Stable Stratification? The Thermohaline Circulation: Sinking cold saline polar water drives the circulation and can be tracked by temperature patterns How Does the Return Flow Overcome Stable Stratification? Oceans and Internal Climate Variability

The Warm Tropical Ocean is a Thin Lens Overlying the Cold Abyss Thermohaline Circulation: The return circulation is bringing cold interior water to the surface. The cold interior water mixes across the thermocline into the warm surface mixed layer. Solar radiation is absorbed at the surface and a portion of the energy is used to warm cold water mixing across the thermocline. ↑ Depth (metres) ↓ ← 10,000 km → across the equatorial Pacific Ocean Oceans and Internal Climate Variability

How is the Thermohaline Circulation linked to Global Radiation Fields? Annual average TOA net radiation Poleward moving surface water loses heat as longwave radiation to space Upwelling interior water consumes solar energy as it mixes across the thermocline and warms Trenberth and Caron, 2001 The timescale of overturning of the thermohaline circulation is constrained by the availability of excess solar radiation necessary to overcome ocean stratification. Oceans and Internal Climate Variability

How much of the excess tropical solar radiation is required to sustain the thermohaline circulation against the stable stratification? Assumptions: The circulation time is 1,000 years The average ocean depth is 5,000 m In the vertical column water warms 25oC There is need for continuous input of solar energy to the tropical ocean surface mixed layer at about 25 Wm2 to maintain the current steady state thermal stratification. That is, about 35% of the net TOA solar radiation excess is utilised in maintaining the thermohaline circulation. Oceans and Internal Climate Variability

How is the Thermohaline Circulation linked to Global Radiation Fields? Poleward moving surface water loses heat as longwave radiation to space Upwelling interior water consumes solar energy as it mixes across the thermocline and warms ~ 25 Wm2 Annual average TOA net radiation Trenberth and Caron, 2001 The current circulation time of the thermohaline circulation is not constrained by the TOA net radiation distribution. Excess solar radiation of the tropics can sustain the ocean overturning and there is additional heat available for export to higher latitudes by the atmospheric circulation. Oceans and Internal Climate Variability

What is the effect of a slowing of the overturning circulation? Trenberth and Caron, 2001 Annual average TOA net radiation Less solar radiation is required to warm cold water mixing across the thermocline. More solar radiation is available to warm the surface mixed layer and for export to higher latitudes by the atmospheric circulation. We can deduce: A slowing thermohaline circulation will lead to global warming An accelerating thermohaline circulation will lead to global cooling Oceans and Internal Climate Variability

How can the overturning period of the thermohaline circulation vary to regulate global temperature? Gravitational effects of relative planetary motions affect ocean circulations on a range of timescales Oceans and Internal Climate Variability

SUMMARY Global temperature and climate can vary independently of radiation forcing Radiation forcing is not necessary for climate change The amount of energy available for transport to higher latitudes by the atmospheric circulation is regulated by the ocean overturning rate: If the circulation were to speed up then less energy would be available for transport and Earth would cool Similarly, if the circulation were to slow then Earth would warm Net TOA radiation distribution can accommodate significantly faster overturning, leading to global cooling. Proposition It is the thermodynamics of ocean overturning circulations, driven by the relative planetary motions, that determine climate change, not radiation forcing. Oceans and Internal Climate Variability