Coordinator: Prof. Dr. Peter Brandt
(Deputy: Prof. Dr. Katja Matthes)

Mission:

To determine the processes controlling the coupling between the ocean and the atmosphere, with special emphasis on seasonal to decadal variations in the tropics, based on upper ocean/lower atmosphere observations, analysis of the instrumental climate record, and coupled ocean-atmosphere modelling. The overall aim is to advance seasonal to decadal climate prediction capability by improving the ocean components of the forecast systems and the representation of key atmospheric processes.

Scientific Questions

Ocean-atmosphere feedbacks give rise to various modes of climate variability with global teleconnections and significant predictability. The tropical ocean-atmosphere system is the most important driver of global climate variability on interannual to decadal timescales.

1. How do the oceans affect seasonal to decadal climate variations and their potential predictability?
2. What is the role of ocean-atmosphere interactions in modes of tropical variability?
3. What are the pathways of signal communication between the ocean, the troposphere and the stratosphere?

Contents and Goals

Large-scale ocean-atmosphere interactions are a key to understanding internal and external climate variability on a wide range of timescales. Since exchanges of heat and substances between the ocean and the atmosphere operate on timescales ranging from seasons to millennia, they are also among the most important factors in shaping future global climate change. Thus, understanding the nature of ocean-atmosphere interactions is a prerequisite for separating anthropogenic climate change from natural climate variability. An example is the current debate about a possible anthropogenic signal in weather extremes during the 20^th century, such as Atlantic hurricane activity, and heavy precipitation extremes and drought in Europe. One goal is to investigate how such extremes are affected by low-frequency climate variability.

To better understand the dynamics of ocean-atmosphere interactions, we need to enhance our knowledge of the individual climate system components and the way they interact with each other. A prime example of a coupled ocean-atmosphere mode on inter-annual timescales is the El Niño/Southern Oscillation (ENSO), originating in the equatorial Pacific Ocean. This is a phenomenon with global meteorological impact. The coupled nature of the phenomenon gives rise to its relatively high predictability, which is on the order of several months. Therefore, it is well beyond the internal predictability limit of the chaotic atmosphere, which is only two weeks on average.

Coupled phenomena also exist in the other tropical oceans. ENSO-like variations are observed, for instance, in the equatorial Atlantic. One goal is the quantification of seasonal to decadal tropical predictability and its implication for middle latitudes. Another goal is to quantify the degree of coupling between the ocean and the atmosphere in various mid-latitude phenomena, such as the Atlantic Multi-decadal Variability (AMV) or Pacific Decadal Variability (PDV), and to estimate the level of decadal-scale predictability related to these modes. On the decadal timescale, some predictability also arises from the anthropogenic forcing. We wish to quantify the contributions of the internal and forced sources to decadal predictability.

It is well known that the atmosphere, due to its chaotic nature, can produce variations on a wide range of timescales. Likewise, seasonal to interannual variability in the coupled system can arise from internal ocean dynamics alone, such as the vertically propagating equatorial deep jets. This can result, for instance, from instabilities of the mean ocean currents or wave-mean flow interaction. This is an important topic we wish to study jointly using theoretical concepts, observations, process modelling and coupled climate model simulations. Another goal is to understand the sensitivity of the atmosphere to anomalous sea surface temperature (SST) in tropical and mid-latitude regions. For instance, the influence of SST anomalies in the tropical Atlantic and Indian Oceans on the atmosphere is less well understood than the influence of those in the tropical Pacific. Moreover, we wish to investigate the role of SST fronts associated with the western boundary currents and their extensions (Kuroshio, Gulf Stream) in driving variability in the atmosphere. Furthermore, the role of the stratosphere in the signal communication from the ocean to the atmosphere, and back to the ocean, needs to be understood.

Finally, we need to understand how the coupled ocean-atmosphere system responds to external forcing, may it be natural (solar, volcanoes) or anthropogenic (greenhouse gases, aerosols) in nature. We must address in this context, for instance, the scale-interactions, e.g. the role of oceanic mesoscale eddies in the response of the coupled system to enhanced greenhouse gas concentrations and the implications for the global carbon cycle [see also WP1].

Research Highlight 1: Observed climate signals in the tropical Pacific associated with solar forcing can only be reproduced by climate models when they resolve stratospheric processes and also include coupled atmosphere-ocean feedbacks. Competing mechanisms for this “top-down” stratospheric response and “bottom-up” coupled ocean-atmosphere surface response have been proposed but require further testing. In order to understand the role of external forcing in driving climate variability and the pathways of signal communication between the ocean, the troposphere and the stratosphere, a suite of model studies will be used to investigate the different processes systematically.

Research Highlight 2: Multi-year moored observations in the equatorial Atlantic reveals strong interannual variability originating from internal deep ocean dynamics. The variability affects sea surface temperature and the climate in the tropical Atlantic region. Ongoing observations of hydrographic and velocity fields in the tropical Atlantic will allow us to distinguish between different modes of tropical ocean variability and their respective climate impact. Diapycnal mixing below the surface mixed layer represents the link between variations of the deep currents and the sea surface temperature. The study of mixing processes by state-of-the-art instruments like gliders with attached microstructure probes will complement the present observing system. This will aid climate model improvement and allow short-range climate predictions in the tropics.

Research Highlight 3: Natural climate fluctuations and anthropogenic climate change are concurrent phenomena. Global average surface air temperature during the 20^th century, for instance, displays a gradual warming on the order of 0.7°C, with superimposed shorter-term fluctuations on interannual to multi-decadal timescales. This makes climate change detection a challenge, especially on the regional scale. Such variability, which often originates in the ocean, impacts important societal parameters, such as Atlantic hurricane activity. We wish to systematically explore the predictability of the large-scale ocean circulation variability which gives rise to climate variability on timescales up to decadal, by means of a hierarchy of climate models and by analyzing long-term observations.