Diseño experimental

Para conseguir reproducir la situación que tendría lugar, necesitamos colocar una fina capa de agua dulce sobre otra profunda de agua salada, representando la situación de fusión de los polos. Con un tinte seremos capaces de diferenciar ambas capas.

A su vez debemos generar una célula convectiva a través de un gradiente de temperatura vertical sobre todo el fondo de manera uniforme. Ello simulará la corriente del Golfo en la superficie oceánica (nótese que la corriente del Golfo NO es el resultado de una macro célula convectiva generada por un gradiente de temperaturas).

Para evitar la turbulencia que pudiera provocar la mezcla de ambas capas de agua, necesitamos que la recreación de las condiciones se produzca de la forma más lenta posible (o al menos de forma natural). Optamos por dejar al hielo derretirse a temperatura ambiente sobre una solución salina y calentarlo con vapor de agua hervida.

Polar Melting

Basics

This experiment aims at showing the effect of ice melting over geostrophic currents. This idea popped up after reading a paper in Scientific American by Stephen C. Riser and M. Susan Lozier, physical oceanography professors in the universities of Washington and Duke, respectively. The paper addresses the validity of the classical theory accepted that relates the Gulf Stream with winters in Europe, since new numerical models and researchers support alternative theories.

Figure 1: Gulf Stream in America. Red indicates the regions with the highest temperatures, down to yellow, green, blue and purple, that are the coldest ones.

The classical theory

It goes like this: European winters are milder than those in other regions of the Earth at the same latitude because the Gulf Stream transports wam tropical waters toward these areas that release heat when reaching the European coasts, allowing the westerly winds in the area to transport this heat toward the continent.

Figure 2: Gulf Stream in America. In red are shown the regions with highest temperature, down to yellow, green, blue and purple, that are the coldest ones.

The alternative theory 

It introduces a subtle difference to the classical theory: the Gulf Stream follows its path from America toward Europe but the jet stream produces turbulent mixing of the ocean surface that releases the heat stored in the ocean. It is thought that this mixing strengthens as both currents approach Europe.

Figure 2: The Gulf Stream opens up its way toward Europe where the jet stream (blue: cold winds; orange: warm winds) stirs up the surface of the ocean (purple clouds). This releases heat close to the European continent. Important efforts of the  global ARGO array allow to evaluate such theories by comparing the results of numerical models with data from buoys spread world-wide. Although this is not the only alternative theory, it is the most important one.

Ice melting at the poles

It is well known that salty water is more dense than freshwater. This, in the absence of mixing, it will sink below. How would the Gulf Stream (and the rest of the ocean currents) be affected through ice melting at the poles? Could this impact the winters in Europe. This is the question that we assess in this experiment.

Experimental Design

To reproduce the situation that would take place we need to place a thin layer of surface freshwater over a layer of salty water, thereby mimicking ice melting at the poles. A color dye allows identifying each layer.

At the same time we need to generate a convective cell through a vertical temperature gradient. This will mimic the Gulf Stream flow at the surface (note, however, that the Gulf Stream is not the result of a macroscopic convective cell generated through a temperature gradient).

To prevent turbulent mixing between the two layers we need that the process tales place as slowly as possible, or in a natural manner. To this end we allow the ice to melt at room temperature over a saline solution and warm it up with boiling water vapour.