Gulf Stream: la fin d'un mythe

To understand the origin of the mild winters in Europe, Richard Seager and his colleagues have studied how winds and ocean currents distribute heat around the globe. Based on weather observations and data collected by ships, they first showed that the main conveyor of heat to Europe is not the ocean but the atmosphere ... According to their simulations, the two main phenomena involved are part of a large atmospheric flow associated with the presence of the Rockies, which cools the eastern United States, and also the dissipation in the atmosphere of heat stored during summer in surface waters of the ocean. The Gulf Stream there is virtually nothing. Pour comprendre l‘origine des hivers doux européens, Richard Seager et ses collègues ont étudié la façon dont les vents et les courants marins distribuent la chaleur autour du Globe. Á partir d‘observations météorologiques et des données collectées par les bateaux, ils ont d‘abord montré que le principal convoyeur de chaleur vers L‘Europe n‘est pas l‘océan mais l‘atmosphère... Selon leurs simulations numériques, les deux phénomènes principaux en jeu sont d‘une part les grands écoulements atmosphériques liés à  la présence des Rocheuses, qui refroidissent l‘Est américain, et d‘autre part la dissipation dans l‘atmosphère de la chaleur stockée pendant l‘été dans les eaux superficielles de l‘océan. Le Gulf Stream n‘y est pratiquement pour rien

Mark Cane: 2014 Fellow of The Oceanography Society

Mark Cane, who was honored in 2014 as a Fellow of The Oceanography Society, is the G. Unger Vetlesen Professor of Earth and Environmental Sciences at Columbia University, based at Columbia’s Lamont-Doherty Earth Observatory in Palisades, New York. He received a bachelor’s degree from Harvard in 1965 and his PhD from MIT in 1975. He moved to Lamont in 1985 and has made his research home there ever since. His unusual career has ranged from theoretical equatorial ocean dynamics to studying links between climate variability and social conflict. In all cases, he has applied his piercing intellect, deep intuition, and methodological rigor to make major advances in understanding of the ocean, the climate system, and how climate variability and change impact human society. In particular, Mark Cane is a founding father of seasonal-to-interannual climate prediction, a revolutionary field in ocean and climate science

Decadal Drought Variability Over North America: Mechanisms and Predictability

The physical mechanisms and potential predictability of North American drought on decadal timescales are reviewed in a simple and straightforward manner amenable to a wide audience. During decadal droughts, the tropical oceans, most notably cold states of the Pacific but also warm states of the Atlantic, provide forcing that continually nudges the atmosphere toward circulation anomalies that favor high pressure over southern North America and dry conditions. However, even in these regions, and even more so in the northwest and northeast, the oceans exert less than dominant control and actual drought onset, evolution and termination can deviate due, presumably, to potent internal atmosphere variability. The ocean influence, however, justifies efforts to determine if the driving sea surface temperature anomalies in the tropical Pacific and Atlantic are predictable beyond the seasonal to interannual timescale. Evidence to date, based on initialized predictions with coupled models, is tantalizingly suggestive that useful predictability on these timescales may exist within the atmosphere-ocean system although relevance to North American decadal drought has not yet been demonstrated. These recent advances in drought science and prediction warrant continued research aimed at developing useful long term predictions of drought that can guide adaptation and minimize the associated widespread social and economic disruptions

A Mechanisms-Based Approach to Detecting Recent Anthropogenic Hydroclimate Change

Both naturally occurring La Niña events and model-projected anthropogenic-driven global warming are associated with widespread drying in the subtropics to midlatitudes. Models suggest anthropogenic drying should already be underway but climate variability on interannual to multidecadal time scales can easily obscure any emerging trend, making it hard to assess the validity of the simulated forced change. Here, the authors address this problem by using model simulations and the twentieth-century reanalysis to distinguish between natural variability of, and radiatively forced change in, hydroclimate on the basis of the mechanisms of variations in the three-dimensional moisture budget that drive variations in precipitation minus evaporation (P 2 E). Natural variability of P 2 E is dominated by the El Niño–Southern Oscillation (ENSO) cycle and is "dynamics dominated" in that the associated global P2E anomalies are primarily driven by changes in circulation. This is quite well reproduced in the multimodel mean of 15 models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/Coupled Model Intercomparison Project 3 (CMIP3). In contrast, radiatively forced P 2 E change is "thermodynamics mediated" in that the rise in specific humidity leads to intensified patterns of moisture transport and P 2 E. But, as for ENSO, the poleward shift of the storm tracks and mean meridional circulation cells also contribute to changes in P 2 E. However, La Niña and radiatively forced changes in the zonal mean flow are distinct in the tropics. These distinctions are applied to the post-1979 record of P 2 E in the twentieth-century reanalysis. ENSO-related variations strongly influence the observed P 2 E trend since 1979, but removal of this influence leaves an emerging pattern of P 2 E change consistent with the predictions of the IPCC AR4/CMIP3 models over this period together with, to some extent, consistent contributions from dynamical and thermodynamical mechanisms and consistent changes in the zonal mean circulation. The forced trends are currently weak compared to those caused by internal variability

Diagnostic Computation of Moisture Budgets in the ERA-Interim Reanalysis with Reference to Analysis of CMIP-Archived Atmospheric Model Data

The diagnostic evaluation of moisture budgets in archived atmosphere model data is examined. Sources of error in diagnostic computation can arise from the use of numerical methods different from those used in the atmosphere model, the time and vertical resolution of the archived data, and data availability. These sources of error are assessed using the climatological moisture balance in the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) that archives vertically integrated moisture fluxes and convergence. The largest single source of error arises from the diagnostic evaluation of divergence. The chosen second-order accurate centered finite difference scheme applied to the actual vertically integrated moisture fluxes leads to significant differences from the ERA-Interim reported moisture convergence. Using daily data, instead of 6-hourly data, leads to an underestimation of the patterns of moisture divergence and convergence by midlatitude transient eddies. A larger and more widespread error occurs when the vertical resolution of the model data is reduced to the 8 levels that is quite common for daily data archived for the Coupled Model Intercomparison Project (CMIP). Dividing moisture divergence into components due to the divergent flow and advection requires bringing the divergence operator inside the vertical integral, which introduces a surface term for which a means of accurate evaluation is developed. The analysis of errors is extended to the case of the spring 1993 Mississippi valley floods, the causes of which are discussed. For future archiving of data (e.g., by CMIP), it is recommended that monthly means of time-step-resolution flow–humidity covariances be archived at high vertical resolution

Western boundary currents and climate change

A recent paper in Journal of Geophysical Research-Oceans connects recent changes in atmospheric circulation to poleward movement and intensification of western boundary currents. Causes and characteristics of past and future trends in surface wind stress and western boundary currents are discussed here

Arabian Sea Response to Monsoon Variations

This study aims to quantify the impact of strong monsoons on the mixed layer heat budget in the Arabian Sea by contrasting forced ocean general circulation model simulations with composite strong and weak monsoon winds. Strong (weak) monsoons are defined as years with zonal component of the Somali Jet being greater (smaller) by more than a standard deviation of the long-term mean of the National Centers for Environmental Prediction reanalysis winds. Coastal upwelling is shown to be demonstrably stronger for strong monsoons leading to significant surface cooling, shallower thermoclines, and deeper mixed layers. A coupled ecosystem model shows that surface chlorophyll, primary, and export production are indeed higher for strong monsoons compared to weak monsoons driven by the supply of colder, nutrient-rich waters from greater than 100 m depths. The surprising result is that a strong monsoon results in stronger negative wind stress curl away from the coasts and drives Ekman pumping that results in a deeper thermocline. The weaker stratification and larger turbulent kinetic energy from the winds drive deeper mixed layers leading entrainment cooling with some contribution from the advection of colder upwelled waters from the coastal upwelling regions. Thus the strong monsoons, in fact, enhance oceanic heat uptake indicating that ocean dynamics are cooling the surface and driving the lower atmosphere which has implications for the interpretation of monsoon variability from paleorecords

Physical Mechanisms of the California Drought

At the time of writing in February 2016, California has enjoyed some heavy rain and snow, as expected given the massive El Nino of 2015 and 2016. However, California experienced less-than-normal precipitation in each of the previous four winters and, despite some relief, almost the entire state remains in drought. As a four-winter average, 2011-15 was the driest California has experienced since statewide records, as reported by the National Oceanic and Atmospheric Administration (NOAA), which began recording in 1895. There is no long-term trend, either wetting or drying, in California precipitation, but instead a tremendous amount of variability both year-to-year and decade-to-decade. For example, a silar multiyear drought occurred in the late 1980s to the early 1990s while the 1920s were an overall dry decade and the 1990s an overall wet decade. Evidence from tree rings and lake levels also indicate truly long, multi-decadal, droughts during the medieval era (Stine 1994, Cook et al. 2010). So, given how variable California precipitation is, what caused this particular drought? Also, was human-driven climate change caused by rising greenhouse gases (GHGs) in any way involved? The results reported here largely follow from a study conducted by the NOAA Drought Task Force, a multi-institution group, and uses analysis of the instrumental record, simulations with atmosphere models forced by the observed sea surface temperature (SST) history and model simulations of the climate system response to changing atmospheric composition (e.g. GHGs) conducted for IPCC Assessment Report Five (Seager et al. 2014, 2015). Climate variability that brings droughts and foods can arise in two fundamental ways. The first is by internal atmospheric variability in which the chaotic flow of the atmosphere can give rise to long periods of drier or wetter, or warmer or colder, than normal weather. The second method is via anomalies in the SST around the world which cause changes in the flux of heat between the ocean and atmosphere that force changes in atmospheric circulation. SST anomalies in the tropics are most effective in this regard—the El Niño-Southern Oscillation is a prime example. Coupled atmosphere-ocean interactions lead to changes in heat transport by ocean currents that generate SST anomalies that alter the intensity and location of tropical rain systems. This induces atmospheric circulation anomalies across the globe, including over North America