Effects of fluctuating daily surface fluxes on the time-mean oceanic circulation

The effect of fluctuating daily surface fluxes on the time-mean oceanic circulation is studied using an empirical flux model. The model produces fluctuating fluxes resulting from atmospheric variability and includes oceanic feedbacks on the fluxes. Numerical experiments were carried out by driving an ocean general circulation model with three different versions of the empirical model. It is found that fluctuating daily fluxes lead to an increase in the meridional overturning circulation (MOC) of the Atlantic of about 1 Sv and a decrease in the Antarctic circumpolar current (ACC) of about 32 Sv. The changes are approximately 7% of the MOC and 16% of the ACC obtained without fluctuating daily fluxes. The fluctuating fluxes change the intensity and the depth of vertical mixing. This, in turn, changes the density field and thus the circulation. Fluctuating buoyancy fluxes change the vertical mixing in a non-linear way: they tend to increase the convective mixing in mostly stable regions and to decrease the convective mixing in mostly unstable regions. The ACC changes are related to the enhanced mixing in the subtropical and the mid-latitude Southern Ocean and reduced mixing in the high-latitude Southern Ocean. The enhanced mixing is related to an increase in the frequency and the depth of convective events. As these events bring more dense water downward, the mixing changes lead to a reduction in meridional gradient of the depth-integrated density in the Southern Ocean and hence the strength of the ACC. The MOC changes are related to more subtle density changes. It is found that the vertical mixing in a latitudinal strip in the northern North Atlantic is more strongly enhanced due to fluctuating fluxes than the mixing in a latitudinal strip in the South Atlantic. This leads to an increase in the density difference between the two strips, which can be responsible for the increase in the Atlantic MOC

Storm Surges: Phenomena, Forecasting and Scenarios of Change

AbstractStorm surges are behind the geophysical risk of short term and abrupt inundating low-lying coastal regions known along most coasts of the world. They are related to meteorological phenomena, mostly wind storms. Storm surges represent a challenge for science and risk management with respect to short term forecasts of specific events but also with long-term changes of the statistics of storm surges due to anthropogenic global climate change, sinking coasts and estuarine water works. Storm surges are expected to become more severe in the coming decades and centuries because of ongoing and expected accelerated mean sea level, and much less so because of more energetic wind storms

On equilibrium fluctuations

This paper considers a dynamical system described by a multidimensional state vector x. A component x of x evolves according to dx/dt = f(x). Equilibrium fluctuations are fluctuations of an equilibrium solution x(t) obtained when the system is in its equilibrium state reached under a constant external forcing. The frequencies of these fluctuations range from the major frequencies of the underlying dynamics to the lowest possible frequency, the frequency zero. For such a system, the known feature of the differential operator d(·)/dt as a high-pass filter makes the spectrum of f to vanish not only at frequency zero, but de facto over an entire frequency range centered at frequency zero (when considering both positive and negative frequencies). Consequently, there is a non-zero portion of the total equilibrium variance of x that cannot be determined by the differential forcing f. Instead, this portion of variance arises from many impulse-like interactions of x with other components of x, which are received by x along an equilibrium solution over time. The effect of many impulse-like interactions can only be realized by integrating the evolution equations in form of dx/dt = f(x) forward in time. This integral effect is not contained in, and can hence not be explained by, a differential forcing f defined at individual time instances

From Decoding Turbulence to Unveiling the Fingerprint of Climate Change

This open access book serves as a reference for the key elements and their significance of Klaus Hasselmann's work on climate science and on ocean wave research, all based on a rigorous and deeply physical thinking. It summarizes the original articles (mostly from the 1970 and 1980s; some of which are hard to find nowadays) and brings them in a present-day context. From 1975 until 2000, he was (founding) Director of the Max Planck Institute of Meteorology, which he made to one of the world-leading academic institutions. He first made the issue of anthropogenic climate change accessible to analysis and prediction and later transformed climate science into a significant factor in forming public policy. The book is written by co-workers and colleagues of Klaus Hasselmann, who—many under his immediate supervision—joined him in this effort. With this background, they present the key achievements and assess the significance of these for the present state of knowledge and scientific practice

Utility of Coastal Science

Keynote Lecture

Is greenhouse gas forcing a plausible explanation for the observed warming in the Baltic Sea catchment area?

We investigated whether anthropogenic forcing is a plausible explanation for the observed warming in the Baltic Sea catchment area. Therefore, we compared the most recent trends in the surface temperature over land with anthropogenic climate change projections from regional climate model simulations. We analyzed patterns of change with different spatio-temporal resolutions. The observed annual area-mean change in the daily-mean temperature was consistent with the anthropogenic climate change signal. This finding was robust to the removal of the signal of the North Atlantic Oscillation. In contrast to the annual area-mean change, we found little consistency in both annual cycle and spatial variability of the observed and projected changes

Piecewise evolutionary spectra: A practical approach to understanding projected changes in spectral relationships between circulation modes and regional climate under global warming

Regional climate variability is strongly related to large-scale circulation modes. However, little is known about changes in their spectral characteristics under climate change. Here, we introduce piecewise evolutionary spectra to quantify time-varying variability and co-variability of climate variables, and use ensemble periodograms to estimate these spectra. By employing a large ensemble of climate change simulations, we show that changes in the variability and relationships of the North Atlantic Oscillation (NAO) and regional surface temperatures are disparate on individual timescales. The relation between NAO and surface temperature over high-latitude lands weakens the most on 20-year timescales compared to shorter timescales, whereas the relation between NAO and temperature over subtropical North Africa strengthens more on shorter timescales than on 20-year timescales. These projected evolution and timescale-dependent changes shed new light on the controlling factors of circulation-induced regional changes. Accounting for them can lead to the improvement of future regional climate predictions. © 2021. The Authors