Climate feedbacks induced by the North Atlantic freshwater forcing in a coupled model of intermediate complexity

Revista Brasileira de Meteorologia, v.25, n.1, 103 - 113, 2010 Climate Feedbacks Induced by THE NORTH ATLANTIC FRESHWATER Forcing in a Coupled Model of Intermediate Complexity FLÁVIO BARBOSA JUSTINO E JEFERSON PRIETSCH MACHADO Universidade Federal de Viçosa (UFV), Viçosa, MG, Brasil , Received October 2008 - Accepted October 2009 ABSTRACT Based on coupled model simulations (ECBilt-Clio), we investigate the atmospheric and oceanic response to sustained freshwater input into the North Atlantic under the glacial maximum background state. The results demonstrate that a weakening of the thermohaline circulation triggered by weaker density flux leads to rapid changes in global sea-ice volume and reduced poleward heat transport in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), however, the oceanic heat transport increases substantially. This in turn leads to strong cooling over the North Atlantic whereas the SH extratropical region warms up. The suppression of the NADW also drastically changes the atmospheric circulation. The associated northward wind anomalies over the North Atlantic increase the warm air advection from the tropics and induce the transport of tropical saltier water to mid-latitudes. This negative atmospheric-oceanic feedback should play an important role to resume the NADW, after the freshwater forcing ends up. Keywords: Thermohaline circulation, climate changes, baroclinic activity RESUMO: ANOMALIAS CLIMÁTICAS INDUZIDAS POR UM AUMENTO DE ÁGUA DOCE NO ATLÂNTICO NORTE EM UM MODELO ACOPLADO DE COMPLEXIDADE INTERMEDIÁRIA Baseado em simulações numéricas realizadas com um modelo acoplado (ECBilt-Clio), investigamse as anomalias atmosféricas e oceânicas associadas ao aumento de água doce no Atlântico Norte sob condições glaciais. Os resultados demonstram que um enfraquecimento da corrente termohalina provoca rápidas mudanças no volume global de gelo marinho, bem como uma redução no transporte de calor em direção ao Hemisfério Norte (HN). No Hemisfério Sul (HS), no entanto, o transporte oceânico de calor aumenta substancialmente. Como resultado, ocorre um forte esfriamento ao longo do Atlântico Norte, enquanto que a região extratropical do HS aquece. A inibição da circulação termohalina também muda drasticamente a circulação atmosférica. Intensas anomalias de ventos da região equatorial induzem um transport de águas quentes e salinas, assim como são associadas com um aumento na advecção de ar quente dos trópicos. Esta interação oceano-atmosfera opõe-se à tendência climática inicial imposta pelo aumento de água doce no Atlântico Norte. Palavras chave: Circulação termohalina, mudanças climáticas, atividade baroclínica 1. INTRODUCTION Centennial to millennial scale changes in global climate over the last 100 kyr (thousand years before present) were identified in the Greenland Ice Core Project (GRIP). Further analyzes demonstrated that instability mechanisms of the ice sheets around the North Atlantic and of the thermohaline circulation are prime candidates to trigger such changes (Schäfer et al., 2001). Moreover, additional investigations demonstrated that those climate shifts may be also related to freshwater anomalies into the Arctic/Labrador Seas (Peltier et al., 2006; Blunier and Brook, 2001) or into the Antarctica Peninsula (Seidov et al., 2001). The series of rapid oscillations called DansgaardOeschger events (warm periods) and Heinrich events (cold periods), were associated with local temperature changes 104 Flávio Justino e Jeferson Machado. over Greenland of up to 15°C (Lang et al., 1999), within a decade. The cause of these glacial events is still under debate. Currently, the leading hypothesis involves a slowdown of the ocean’s thermohaline circulation. Studies have hypothesized that these freshwater dumps reduced ocean salinity enough to slow deepwater formation and the thermohaline circulation. Since the thermohaline circulation plays an important role in transporting heat northward, a slowdown would cause the North Atlantic to cool (MacAyeal, 1993). Many of these swings in the climate system were not just a local phenomenon, but had global impact with evidence in South America (Lowell et al., 1995), the North Pacific (Kotilainen and Shackleton, 1995), Southwest Pacific (Pahnke and Zahn, 2005), the Santa Barbara Basin (Behl and Kennet, 1996), the Arabian Sea (Schulz et al., 1998) and the South China Sea (Wang, 1999). These events are perhaps the most pronounced climate changes that occurred during the past 100 kyr. Recently, based on U/Th-dated stalagmite from subtropical southern Brazil, -variations of U/Th are likely related to the intensity of precipitation-, Cruz et al. (2005) demonstrated that the Dansgaard-Oeschger cycles affected the tropical hydrological cycle, but in southern Brazil, millennialscale climate changes are not as dominant as they are in the Northern Hemisphere. The Thermohaline Circulation (THC) is a key element of the global climate system because it carries a substantial amount of heat and saline waters poleward (Knutti et al., 2004). Climate changes associated with perturbation of the THC have been inferred from paleo-climatic records as well as from model simulations forced by increasing concentration of atmospheric greenhouse gases. Recently, Bryden et al. (2005), based on measurements from ships along a transatlantic section along latitude 25°N suggested that the Atlantic meridional overturning circulation has slowed by about 30 per cent, mostly between 1992 and 1998. Climate changes triggered by perturbation of the THC have been extensively documented in high latitudes, more recently evidence has also been put forward on the role of the THC upon tropical climate. Timmermann et al. (2005) demonstrated that sea level fluctuations and thermocline depth in the North Atlantic vary in opposite phase with salinity changes in the tropical Pacific, this raises the possibility that a distinct temporal variability and magnitude of the El-Niño-Southern Oscillation (ENSO) phenomenon may occur in the future, due to changes in the North Atlantic precipitation/evaporation regime. Based on coupled model simulations, Dahl et al. (2005) provided additional evidence that extreme cold conditions over the North Atlantic linked to weaker THC can lead to a southward displacement of the Intertropical Convergence Zone (ITCZ) to allow for greater heat transport across the equator. Given this, the northeast trade winds intensify and precipitation patterns throughout the tropical Atlantic are altered. In particular, Volume 25(1) precipitation in Northeast Brazil changes significantly. Hence, this study addresses the impact of the stationary freshwater inflow into the North Atlantic on the atmosphere-seaice-ocean system based on 5 sensitivity experiments. It has also been argued that model simulations conducted under anomalous deep water formation rate can improve our understanding of climate feature characteristic of stadial per (...truncated)