1. Effect of the Gulf Stream on Winter Extratropical Cyclones Jill Nelson* and Ruoying He Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695 *jsnelso2@ncsu.edu COAWST MODELING SYSTEM Coupled-Ocean-Atmosphere-Wave-Sediment-Transport Model WRF  Atmospheric Model. The Weather Research and Forecasting Model (WRF) is a mesoscale numerical weather prediction system designed to serve both atmospheric operational forecasting and research needs (www.wrf-model.org). ROMS  Ocean Model. The Regional Ocean Modeling System (ROMS) is a free-surface, hydrostatic, primitive equations model for coastal and open ocean applications (www.myroms.org). MCT  Coupler. The Model Coupling Toolkit (MCT) is a set of open- source software tools that handles data exchange between WRF and ROMS (www.mcs.anl.gov/research/projects/mct/). Weather Research & Forecasting Model (WRF) Regional Ocean Modeling System (ROMS) τ and net heat flux SST Variables exchanged in the two-way coupled WRF/ROMS system. SlopeWater Sargasso Sea Gulf Stream CapeHatteras Photo credit: NOAA Figure 1. The model domain for the coupled WRF and ROMS simulation. The dashed box highlights our study region. Also shown are locations of three NDBC marine buoys and three cross-shelf transects along which vertical structures of coastal wind fields are diagnosed. Buoy Wind Comparison SLP Comparison Air Temp Comparison SST Comparison r / Θ / Regr / RMS Error 440090.92 / -2.5 / 1.060.99 / 1.450.96 / 1.960.97 / 0.59 410010.93 / -9.0 / 0.990.98 / 1.620.96 / 2.650.02 / 1.75 410040.84 / -3.9 / 0.910.98 / 1.700.97 / 2.690.96 / 0.47 Table 1. Model/Data Comparisons. Decent agreements are found between modeled and observed winds, SLP, air temperature, and sea surface temperature.  WRF has 15-km horizontal resolution and 48 vertical levels  ROMS has 5-km horizontal resolution and 18 vertical levels  COAWST simulates 5 ETCs from January 13-31, 2005 The Gulf Stream meanders northeastward from southern Florida to Newfoundland. In winter, large heat fluxes and sharp SST gradients associated with the Gulf Stream directly influence local atmospheric circulations. We use a coupled air-sea model to examine the effect of the Gulf Stream on low- level wind convergence during an outbreak of extratropical cyclones (ETC) off the U.S. east coast in January 2005. RESULTS I. Surface Wind Convergence  Strong synoptic surface wind convergence prevails in winter over the Gulf Stream.  Synoptic surface wind convergence is proportional to the Laplacian of SLP and –SST.  SLP anomalies across horizontal SST gradients of the Gulf Stream generate the surface wind convergence zone.  Enhanced surface convergence induces upward vertical motions throughout the marine atmospheric boundary layer, particularly near Cape Hatteras (T2).  Our results suggest air-sea interactions with the Gulf Stream are important for generating and sustaining winter ETCs. Minobe, S., M. Miyashita, A. Kuwano-Yoshida, H. Tokinaga, and S.-P. Xie (2010), Atmospheric Response to the Gulf Stream: Seasonal Variations, Journal of Climate, 23, 3699—3719. Nelson, J. S., and R. He (submitted for review), Effect of the Gulf Stream on Winter Extratropical Cyclone Outbreaks. Geophysical Research Letters. Warner, J. C., B. Armstrong, R. He, and J. B. Zambon (2010), Development of a Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System, Ocean Modelling, 35(3), 230-244. Figure 2. Simulated synoptic mean (13- 31 January) 10-m wind convergence (a), mean SLP Laplacian (b), mean – SST Laplacian (c). In (a)—(c) the contour interval for SST is 4°C. Also presented are the relationship between wind convergence and SLP Laplacian (d), and the relationship between wind convergence and –SST Laplacian (e). In (d) and (e), the linear regression line and correlation coefficient are shown. CONCLUSIONS REFERENCES Figure 3. Cross-sections of vertical wind velocity [shaded; upward positive, m s -1 ] in relation to –SST Laplacian [red; 10 -10 K m -2 ] and SLP Laplacian [blue; 10 -9 Pa m -2 ] along three cross-shelf transects (see locations on Figure 1). Enhanced ocean heat loss over the Gulf Stream due to persistent cooling associated with the passage of ETCs warms the marine atmospheric boundary layer and generates an area of low pressure offshore. Thus, SLP anomalies across ocean and air temperature gradients of the cause winds to accelerate offshore and converge over the Gulf Stream. Vertical motion in the MABL shows good correspondence with the Laplacian of SLP and –SST. Strong vertical velocities anchored over the Gulf Stream (T2 and T3) are co-located with peak SLP Laplacian. INTRODUCTION II. Vertical Winds