Armin Köhl

Initializing Decadal Climate Predictions with the GECCO Oceanic Synthesis: Effects on the North Atlantic

This study aims at improving the forecast skill of climate predictions through the use of ocean synthesis data for initial conditions of a coupled climate model. For this purpose, the coupled model of the Max Planck Institute (MPI) for Meteorology, which consists of the atmosphere model ECHAM5 and the MPI Ocean Model (MPI-OM), is initialized with oceanic synthesis fields available from the German contribution to Estimating the Circulation and Climate of the Ocean (GECCO) project. The use of an anomaly coupling scheme during the initialization avoids the main problems with drift in the climate predictions. Thus, the coupled model is continuously forced to follow the density anomalies of the GECCO synthesis over the period 1952–2001. Hindcast experiments are initialized from this experiment at constant intervals. The results show predictive skill through the initialization up to the decadal time scale, particularly over the North Atlantic. Viewed over the time scales analyzed here (annual, 5-yr, and 10-yr mean), greater skill for the North Atlantic sea surface temperature (SST) is obtained in the hindcast experiments than in either a damped persistence or trend forecast. The Atlantic meridional overturning circulation hindcast closely follows that of the GECCO oceanic synthesis. Hindcasts of global-mean temperature do not obtain greater skill than either damped persistence or a trend forecast, owing to the SST errors in the GECCO synthesis, outside the North Atlantic. An ensemble of forecast experiments is subsequently performed over the period 2002–11. North Atlantic SST from the forecast experiment agrees well with observations until the year 2007, and it is higher than if simulated without the oceanic initialization (averaged over the forecast period). The results confirm that both the initial and the boundary conditions must be accounted for in decadal climate predictions.

Estimating air-sea fluxes of heat, freshwater, and momentum through global ocean data assimilation

Spectral properties of whitecaps are of importance for color ocean remote sensing and aerosol optical thickness probing from satellite-based instruments. They also influence planetary albedo and climate. In particular, whitecaps may affect the response of the climate system to changes in greenhouse gases and other atmospheric constituents. Several experimental measurements of whitecap spectral reflectance have been performed both in the surf zone and in the open ocean, which indicate that oceanic foam cannot be considered as a gray body (e.g., for satellite remote sensing techniques). This paper is devoted to the interpretation of experiments performed in terms of the radiative transfer theory. Only the case of a semi-infinite foam is studied in detail. However, results can be easily extended to the case of finite foamed media having large optical thickness. The model introduced is capable of explaining main features observed, like a sharp decrease of the foam spectral reflectance in the infrared as compared with the visible part of the electromagnetic spectrum and a high correlation of the foam reflectance R and the water absorption coefficient a. A simple method to retrieve the spectral dependence of a from the spectral foam reflectance R is proposed.

Projecting twenty-first century regional sea-level changes

We present regional sea-level projections and associated uncertainty estimates for the end of the 21st century. We show regional projections of sea-level change resulting from changing ocean circulation, increased heat uptake and atmospheric pressure in CMIP5 climate models. These are combined with model- and observation-based regional contributions of land ice, groundwater depletion and glacial isostatic adjustment, including gravitational effects due to mass redistribution. A moderate and a warmer climate change scenario are considered, yielding a global mean sea-level rise of 0.54 ±0.19 m and 0.71 ±0.28 m respectively (mean ±1σ). Regionally however, changes reach up to 30 % higher in coastal regions along the North Atlantic Ocean and along the Antarctic Circumpolar Current, and up to 20 % higher in the subtropical and equatorial regions, confirming patterns found in previous studies. Only 50 % of the global mean value is projected for the subpolar North Atlantic Ocean, the Arctic Ocean and off the western Antarctic coast. Uncertainty estimates for each component demonstrate that the land ice contribution dominates the total uncertainty.

Variability of the Meridional Overturning in the North Atlantic from the 50-Year GECCO State Estimation

Abstract The German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) provided a dynamically consistent estimate of the time-varying ocean circulation over the 50-yr period 1952–2001. The GECCO synthesis combines most of the data available during the entire estimation period with the ECCO–Massachusetts Institute of Technology (MIT) ocean circulation model using its adjoint. This GECCO estimate is analyzed here for the period 1962–2001 with respect to decadal and longer-term changes of the meridional overturning circulation (MOC) of the North Atlantic. A special focus is on the maximum MOC values at 25°N. Over this period, the dynamically self-consistent synthesis stays within the error bars of H. L. Bryden et al., but reveals a general increase of the MOC strength. The variability on decadal and longer time scales is decomposed into contributions from different processes. Changes in the model’s MOC strength are strongly influenced by the southward communication of d...

Interannual to Decadal Changes in the ECCO Global Synthesis

Abstract An estimate of the time-varying global ocean circulation for the period 1992–2002 was obtained by combining most of the World Ocean Circulation Experiment (WOCE) ocean datasets with a general circulation model on a 1° horizontal grid. The estimate exactly satisfies the model equations without artificial sources or sinks of momentum, heat, and freshwater. To bring the model into agreement with observations, its initial temperature and salinity conditions were permitted to change, as were the time-dependent surface fluxes of momentum, heat, and freshwater. The estimation of these “control variables” is largely consistent with accepted uncertainties in the hydrographic climatology and meteorological analyses. The estimated time-mean horizontal transports of volume, heat, and freshwater, which were largely underestimated in the previous 2° optimization performed by Stammer et al., have converged with time-independent estimates from box inversions over most parts of the World Ocean. Trends in the mode...

Ocean mixed layer depth: A subsurface proxy of ocean‐atmosphere variability

A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, h mix. Using historical global hydrographic profile data, including conductivity-temperature-depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used Δ criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an h mix/c that is more reliable than the one of the Δ criterion. The uncertainty for h mix/c is ±5 m for high- (<5 m) and ±8 m for low- (<20 m) resolution profiles. A quality index, QImix, which compares the variance of a profile above h mix to the variance to a depth of 1.5 × h mix, shows that for the 70% of the profile data for which a clearly recognizable well-mixed zone exists near the surface, our criterion identifies the depth of the well-mixed zone in all cases. The standard deviation of anomalous monthly h mix/c is typically 20–70% of the long-term mean h mix/c . In the tropical Pacific the monthly mean anomalies of h mix/c are not well correlated with anomalies of sea surface temperature, which indicate that a variety of turbulent processes, other than surface heat fluxes, are important in the upper ocean there. Comparisons between observed h mix/c and Massachusetts Institute of Techonology/ocean general circulation model/Estimating the Circulation and Climate of the Ocean model simulated mixed layer depth indicate that the KPP algorithm captures in general a 30% smaller mixed layer depth than observed.

Decadal Sea Level Changes in the 50-Year GECCO Ocean Synthesis

Abstract An estimate of the time-varying ocean circulation, obtained over the period 1952–2001, is analyzed here with respect to its decadal and longer-term changes in sea level. The estimate results from a synthesis of most of the ocean datasets available during this 50-yr period with the Estimating the Circulation and Climate of the Ocean/Massachusetts Institute of Technology (ECCO/MIT) ocean circulation model. Over the period 1992 through 2001, the increase in thermosteric sea level rise on average amounts to 1.2 mm yr−1 over the top 750 m and 1.8 mm yr−1 over the total water column. This corresponds to an increase in upper-ocean heat content of 1.5 × 1022 J yr−1 and is in agreement with the estimates of Willis et al. However, over the period 1962 through 2001 the global net thermosteric sea level rise is estimated as 0.66 mm yr−1 over the top 750 m, which is twice the recent estimate from Antonov et al. (0.33 mm yr−1). The corresponding trend over the total water column of 0.92 mm yr−1 is also about t...

NASA supercomputer improves prospects for ocean climate research

Estimates of ocean circulation constrained by in situ and remotely sensed observations have become routinely available during the past five years, and they are being applied to myriad scientific and operational problems [Stammer et al., 2002]. Under the Global Ocean Data Assimilation Experiment (GODAE), several regional and global estimates have evolved for applications in climate research, seasonal forecasting, naval operations, marine safety, fisheries, the offshore oil industry coastal management, and other areas. This article reports on recent progress by one effort, the consortium for Estimating the Circulation and Climate of the Ocean (ECCO), toward a next-generation synthesis of ocean and sea-ice data that is global, that covers the full ocean depth, and that permits eddies.

Coastal sea level changes: Observed and projected during the 20th and 21st century.

Timeseries of observed and projected sea level changes for the 20th and 21st century are analyzed at various coastal locations around the world that are vulnerable to climate change. Observed time series are from tide gauges and altimetry, as well as from reconstructions over the last 50 years. CMIP5 coupled atmosphere-ocean model output of regional sea-level and associated uncertainty estimates are merged with scenario-independent contributions from GIA and dynamic ice to provide time series of coastal sea-level projections to the end of the 21st century. We focus on better quantifying the regional departure of coastal sea level rise from its global average, identify the reasons for the regional departure, and quantify the reasons for the uncertainty in these regional projections. Many of these coastal sea level projections are lower than the global mean change in sea level due to glacial isostatic adjustment, and gravitational changes from loss of land ice and terrestrially stored ground water. In most coastal regions, local deviations from the global mean vary up to ±20 cm which, depending on the location, differ substantially in their underlying causes.

Generation and Stability of a Quasi-Permanent Vortex in the Lofoten Basin

Abstract In the Nordic seas the Lofoten Basin is a region of high mesoscale activity. The generation mechanism and the conditions for the stability of a quasi-permanent vortex in the center of the Lofoten Basin are studied with a high-resolution ocean circulation model and altimeter data. The vortex and its generation mechanism manifest themselves by a pronounced sea surface height (SSH) signature and variability, which are found to be in agreement with altimeter data. The vortex results primarily from anticyclonic eddies shed from the eastern branch of the Norwegian Atlantic Current, which propagate southwestward. The large-scale bottom depression of the Lofoten Basin plays a crucial role for attracting anticyclones into the trough and for enabling the dynamical stability of the vortex. The water mass characteristics of the anticyclone lead to enhanced atmospheric interaction (heat loss) during wintertime. The cold water trapped in the upper part of the vortex preconditions convection in the following wi...