How does atmospheric jet stream position and blocking affect the North Atlantic Ocean?

Extra Supervisors: Prof. Dr Vassil Roussenov (University of Liverpool), Dr Tim Woollings (University of Oxford)
External Supervisor: Prof. Susan Lozier (Duke University, USA)

Introduction:

This studentship examines how different atmospheric regimes affect the ocean heat storage and overturning in the North Atlantic Ocean and the wider climate system.

The prevailing view is that much of the ocean variability in heat storage in the North Atlantic is understood in terms of slowly varying climate modes, i.e. fixed pressure patterns such as the North Atlantic Oscillation. Instead we wish to explore how the ocean heat uptake and overturning is controlled by the position and frequency of individual weather systems, in particular atmospheric blocks, which determine the path of the jet stream.

The jet stream can be in a variety or regimes, such as involving undisturbed flow, wave-like undulations, persistent deflections to the south or north linked to atmospheric blocks.

The ocean storage of heat and ocean circulation is strongly affected by the atmospheric forcing. In particular, stronger Trade winds enhance the northward heat transport into the subtropical ocean, while stronger heat loss over the Labrador Sea enhances the ocean overturning and northward heat transport into the subpolar North Atlantic [Williams et al., 2014]; see our animations of changing heat storage at "http://www.ukosnap.org/results-and-publications".

Project Summary:

The studentship aims to investigate the mechanisms by which the jet stream position and atmospheric blocks affect the storage and transport of heat in the North Atlantic Ocean.

The studentship will examine the following research questions:

• The jet stream regime and position of atmospheric high pressure systems affects the ocean heat content changes;
• The ocean subtropical warming is linked to the strength of the Trade winds, while the ocean subpolar warming is linked to the wind forcing and air-sea fluxes over the Labrador Sea.

The plan of work for the student involves:

1. Identifying different atmospheric regimes using two century long weather centre reanalyses from (i) NCEP (20CR) from 1870 to the present day and (ii) ECMWF (ERA-20C) from 1900 to the present day. Separate out the European and Greenland blocking events, and diagnose indices for how strong and stable the jet stream is versus how variable it is.

2. Compare the atmospheric regimes for blocking and jet indices with our estimates of ocean heat storage based upon reanalyses of historical data in the North Atlantic using the UK Met Office Statistical Ocean Reanalysis [Smith et al., 2015] from 1950 to 2013. This analysis extends the comparison of blocking with sea surface temperature variability by Hakkinen et al. [2011].

3. Test our ideas of how the atmospheric regimes control the ocean heat content response by forcing ocean circulation models.

4. Assess how the different atmospheric regimes then affect the wider climate response, altering the rate of sea surface warming and the storage of heat in the upper and deep ocean.

This work plan can be revised and modified according to the input and aptitude of the student.

The studentship will be part of the NERC funded UK-OSNAP programmme: Overturning in the Subpolar North Atlantic,http://www.ukosnap.org/, designed to understand how the heat storage and transport in the high latitude North Atlantic is controlled. The student will have repeated short visits to work with Dr Tim Woollings. In addition, the student will have the opportunity to visit and work with Professor Susan Lozier (Duke University, USA), who leads the international OSNAP programme, as well as attending national meetings and having the opportunity to participate in fieldwork.

Applicants should have a strong academic track record with a science degree, such as including Ocean Sciences, Meteorology, Mathematics, Physics or Engineering. The project involves analysing data and integrating ocean models, so that the student needs to have an aptitude for quantitative work. Prior experience in computational and numerical work is though not required, as training in developing simple models and analysing data is provided in the first year of the PhD.