Visualising changes in the North Atlantic ocean

by Ric Williams – University of Liverpool

Animations of the observed changes in the physical properties of the North Atlantic can really help us to understand their geographical distribution and their evolution over time. We’ve created a series of animations of different properties, and here show a few to give you a flavour of the insight they provide. You can see more on our YouTube Channel.

The North Atlantic Ocean anomaly animations are based on a reanalysis of historical temperature and salinity data by Dr Doug Smith at the UK Met Office, where the sparse observations are mapped using model covariances from a Hadley Centre model (Smith and Murphy, 2007). The plots are made by Dr Vassil Roussenov and the videos by Dr Andy Heath, and Prof. Ric Williams (all from the University of Liverpool) is the lead investigator; this work was supported by the UK Natural Environment Research Council.

This is an animation of annual anomalies of ocean heat content (10^8Jm^-2) over the North Atlantic, where red is warm and blue is cool. The annual anomalies are defined relative to a time average from 1950 to 2010, and are evaluated for the full ocean depth. The animations reveal the strong interannual and decadal variability. There are often different responses occurring in the subtropical and subpolar gyres (Williams et al., 2014), as well as occasions when thermal anomalies appear to pass from one gyre to the other. At the same time, there is an overall warming trend over this 60 year period, represented by the anomalies becoming warmer over the last decade. The different decadal responses are emphasized by the pictures of the time-averaged thermal anomalies for 1950-1970, 1980-2000 and 2000-2010.

The mechanisms forming these ocean heat content anomalies involve the imprint of changes in wind forcing, physical transport and redistribution of heat within the basin, and local and far field changes in air-sea heat fluxes. The dominant mechanisms probably vary with the location and timescale of interest.

Sea surface height varies for a range of reasons. Atmospheric forcing induces waves and even storm surges, while gravitational effects of the Moon and Sun induce the predictable tidal undulations in sea level. The sea level also responds to the heating and cooling of the ocean, sea level increasing from the water column expanding when the water warms or freshens. The sea level likewise increases from the addition of mass from more water added to the global ocean from river runoff and melting of ice on land.

The animation shows the annual sea surface height anomalies in sea level (mm) in the North Atlantic calculated from how the water column expands when there is warmer or fresher water. The sea level varies by -50 mm (blue) to +50 mm (red) due to these volume changes. The animation reveals a similar response to ocean heat content change, regions of heat gain associated with a higher sea level (red) versus regions of heat loss associated with a lower sea level (blue). Again there is strong decadal variability for the ocean gyres, as well as a background rise in sea level over the entire record.

The mechanisms forming these sea surface height anomalies involves the imprint of changes in wind forcing, physical transport and redistribution of heat and freshwater within the basin, and local and far field changes in air-sea heat and freshwater fluxes. The dominant mechanisms probably vary with the location and timescale of interest, although much of the local variability is likely to involve a redistribution of warmer and lighter waters.

Sea surface temperature varies due to solar heating and air-sea exchange of heat, as well as due to transport of warm waters and mixing with cooler, deeper waters.  The animation shows the annual anomalies in sea surface temperature (C) in the North Atlantic, varying from -1.5C (blue) to +1.5C (red). The animation is broadly similar to the ocean heat content change, regions of higher sea surface temperature generally coinciding with greater ocean heat content, although there are detailed differences, particularly in the subtropical latitudes. Again there is strong decadal variability for the ocean gyres, as well as a recent surface warming for 2000 to 2010.

The sea surface temperature (SST) anomalies are generally viewed as being driven by anomalies in air-sea fluxes on interannual timescales (greater heat input from the atmosphere leads to warmer SST), but might feedback back and determine the air-sea fluxes on decadal or longer timescales (a warmer SST leads to a greater loss of heat from the ocean to the atmosphere). There is also a physical transport of the SST anomalies on all timescales.

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