2015 SPA - Maria Jesus Rubio de Inglés. The Evolution of the North Atlantic Oscillation for the last 700 years inferred from D/H isotopes in the sedimentary record in Lake Azul (Azores Archipelago, Portugal)

[This post is participating at the 2015 Student Presentation Awards at ICTJA]

Spanish people are concerned about the existence of the Azores High Pressure since Mariano Medina (“weather man”) introduced this term on the TV weather forecast around 1958. At these southern latitudes, this expression has been associated to pleasant weather conditions but, a persistence of these conditions are linked to severe droughts in the westernmost areas of the Iberian Peninsula. But, what does it mean?

The Azores high pressure is a part of a complicated atmospheric system formed by 2 centers of action. This dipole is formed by a high pressure cell in Azores and a low pressure cell in Iceland. This atmospheric pattern is called the North Atlantic Oscillation (NAO) and it is defined as the pressure difference between Azores and Iceland. The NAO is responsible of the winter weather in Europe and North America.

Owing to the importance of this climatic phenomenon in Europe and nearby areas, we have gone to the crux of the southern center of action to reconstruct the NAO index for the last 700yr from lake sediments. If the high pressure cell is intensified and over Azores archipelago, the precipitation decreases. And, the precipitation increase with a weaker or shifted high pressure cell.

The precipitation gets recorded in the sediment in many ways but, in the present work the hydrogen isotopes have been used. Water molecule is composed by two atoms of hydrogen and one atom of oxygen. The hydrogen contained in the water can be lighter or heavier depending on the number of neutrons. The gravity force makes heavy water molecules fall first than light molecules. But, if the rainy episode continues, the light molecules will fall. In other words, short rainy periods record heavy hydrogen isotope signal and long rainy periods record light signal. Since the positive phase of NAO is related to driest periods, these periods will be marked by a heavy hydrogen isotope signal.

The analyses every half centimeter in the sediment core retrieved from Lake Azul disentangle the NAO effects over Azores. This reconstruction shows a multidecadal oscillation of the NAO phase for the last 700 year. Other authors (such as Trouet et al., 2009) found a persistence of a positive phase during the Medieval Climate Anomaly (MCA) followed by a trend towards negative phase during the Little Ice Age (LIA). We do not observe these patterns in our record despite that, similar fluctuations are observed. This could be because, since all the NAO reconstruction reflects the effects not the NAO itself (which is defined as pressure), those effects can vary between sites or be affected by other patterns. Then, we can conclude saying that we are reconstructing the NAO effects for the southern dipole of the climatic phenomenon.  

This is part of the project PaleoNAO. The supervisors of this phD thesis are Santiago Giralt (ICTJA- Environmental changes in the Geological Record department) and Alberto Sáez (UB- Stratigraphy, paleontology and marine geoscience department)