Atmospheric Conditions for Oceanic Convection in the South-East Labrador Sea

1. Atmospheric conditions associated with oceanic convection in the south east Labrador Sea Dave Sproson, Ian Renfrew, Karen Heywood School of Environmental Sciences, UEA, UK

2. Open ocean convection From Rahmstorf 1999, Nature

3. Open ocean convection I -Pre-conditioning II - deep convection III - lateral exchange & spreading From Rahmstorf 1999, Nature From Marshall and Schott 1999, Rev. Geophys.

4. Open ocean convection A new site for open ocean convection? - the Irminger Sea Pickart et al. 2003, Nature Pickart et al. 2003, Deep Sea Res. Bacon et al., 2003, GRL Våge et al., 2008, JPO From Rahmstorf 1999, Nature

5. Open ocean convection A new site for open ocean convection? - the Irminger Sea Pickart et al. 2003, Nature Pickart et al. 2003, Deep Sea Res. Bacon et al., 2003, GRL Våge et al., 2008, JPO Labrador Sea convection • two locations Lavender et al. 2000, Nature Lavender et al. 2002, JPO From Rahmstorf 1999, Nature

6. Preconditioning in the South East Labrador Sea • Cyclonic recirculations help precondition the ocean, isolating water masses allowing them to be repeatedly modified by the atmosphere, and doming isopycnals, exposing more weakly stratified water to the atmospheric forcing. • High heat fluxes associated with tip-jets are thought to cause relatively deep convection in the Irminger Sea. Lavender et al., 2000, Nature

7. GFD: QuikSCAT climatology Mean wind speed for DJF 1999-2004 From Moore and Renfrew 2005, J. Climate

8. GFD: QuikSCAT climatology • Reverse Tip Jets From Moore and Renfrew 2005, J. Climate

9. Surface sensible and latent heat fluxes from a ‘reverse tip jet’ case study (Martin and Moore 2007, GRL)

10. Preconditioning in the South East Labrador Sea Lavender et al., 2002

11. Preconditioning in the South East Labrador Sea Floats winter 96-97 Lavender et al., 2002

12. 1D mixed-layer ocean model (Price-Weller-Pinkel) constructed. • Meteorological timeseries were created by averaging ERA-40 surface fields over the area where float data were collected. • Reverse tip-jets were objectively identified as times when the wind speed was greater than 15 m/s and the wind direction was between 45 and 135 degrees from north.

13. High NAO year

14. Low NAO year

15. Composites of combined (latent + sensible) heat flux were created for the 20 highest flux events in each winter. • Every year shows a similar pattern, with a low pressure system in between Greenland and Iceland, with generally westerly flow over the Labrador Sea. • This is the same synoptic situation which triggers convection in the central Labrador, and possibly Irminger Seas.

16. Conclusions • Moderately deep convection, up to around 800 m, does occur in the recirculation in the south-east Labrador Sea. • This deepening is reasonably well reproduced in a one-dimensional mixed layer model. • Deepening of the mixed layer is not related to occurrences of reverse tip-jets, but rather to westerly flow over the Labrador Sea, consistent with convection in the central Labrador and Irminger Seas. D. A. J. Sproson, I. A. Renfrew and K. J. Heywood: 2008. Atmospheric Conditions Associated with Oceanic Convection in the South-East Labrador Sea, Geophys. Res. Lett.35, L06601, doi:10.1029/2007GL032971.