The Role of Stochastic Forcing on the Behavior of Thermohaline Circulation

Abstract

The nonlinear nature of the climate system suggests that its reactions to unexpected perturbations could be different from the expected ones. In nonlinear science it is recognized as a promising paradigm that stochastic fluctuations can generate order or other counterintuitive effects. Thus, noise sources, adequately coupled to a nonlinear system, may give rise to a rich new phenomenology not present in a deterministic noiseless scenario. In this review we focus attention on thermohaline circulation (THC). THC presents two modes of operation; one state shows active THC and the other inactive. Previous episodes of transitions between both states of THC observed in paleoclimatic records and the influence of this circulation on climate have resulted in detailed investigations on the dynamics of the THC. A weakening or a collapse of this current could trigger the onset of a new Younger Dryas. In this review the introduction of stochastic forcing in key parameters, both in a simple box model and in an earth model of intermediate complexity, provokes a weakening and even a shutdown of the THC. The consequences of this weakening are observed in different variables. The surface air temperature and the sea surface temperature are dominated by cooling of the Northern Hemisphere. Changes in the position of the Intertropical Convergence Zone and in precipitation are observed. There is also an intensification of the North Atlantic Oscillation values during winter. These results reinforce the necessity to consider stochastic sources in climate models to improve our understanding of the climate.

The Role of Stochastic Forcing in Climate Models: The Case of Thermohaline Circulation

“…This section shows some results obtained from the model ECBilt-Clio after adding noise from freshwater input around Greenland (Lorenzo et al, 2008). This work corroborates the importance of considering the most likely processes in the development of a climate model to achieve more realistic results.…”

“…Other variables, such as the sea surface salinity, geopotential or the NAO index (Lorenzo et al, 2008) have been analyzed and notable changes have been measured when a decadal or multi-decadal variability were considered in the freshwater flux of the GIN Sea. This should alert us to the need to add to the climate models many of the scales of natural variability that are not considered in most models in the name of simplicity.…”

“…This forcing was introduced in the model through the addition of random Gaussian noise in the freshwater discharge around Greenland with a variability of 10 and 70 years (Lorenzo et al, 2008). The length of the experiments was 800 years, using initial conditions that were generated using a 1200 year spin-up run during which no stochastic forcing was applied.…”

Description of the Earth system model of intermediate complexity LOVECLIM version 1.2

“…They were mainly devoted to idealised process studies (e.g., Timmermann et al, 2005;de Vries and Weber, 2005;van der Schrier et al, 2007;Lorenzo et al, 2008), the LGM climate (e.g., Roche et al, 2007;Flückiger et al, 2008;Menviel, 2008;Menviel et al, 2008), the last deglaciation (e.g. Timm et al, 2009), the Holocene climate (e.g., Renssen et al, 2001Renssen et al, , 2003Renssen et al, , 2005Jiang et al, 2005), the previous interglacials (e.g., Duplessy et al, 2007;Yin et al, 2008), the last millennium (e.g., Goosse et al, 2005;van der Schrier and Barkmeijer, 2005), the present-day climate variability (e.g.…”

Astronomical forcing of sub-Milankovitch climate oscillations during the late Quaternary

“…Additionally, realisation that warming began some 2 ky prior to the strengthening of AMOC changed the line of causality (WAIS divide project members 2013). The thermohaline current [ Figure 2.4], also known as the Meridional Overturning Circulation (Lorenzo et al, 2008), is the major transporter of heat around the world (Toggweiler and Key, 2001). Wunsch (2010) suggests that the simple conveyor belt idea [ Figure 2.4] is much more complex than often presented and that "only a tiny minority" has attempted to understand the underlying physics.…”