Atmospheric response to SST fronts: a Review

1. Atmospheric response to SST fronts: a Review Justin Small and many contributors

2. Direct Effects – Surface Stress • Change in surface stress due to change in surface stability AND change in wind speed • (Sweet et al MWR1981, Chelton et al JCLI2001) • AND surface current variation • (Kelly et al GRL2001, Cornillon and Park GRL2001). • Positive correlation between wind stress anomalies and SST anomalies. • Hashizume et al JGR2001, Xie BAMS2004, Liu et al GRL2000 • Wind stress curl and divergence related to crosswind and alongwind components of SST gradient. • Chelton et al JCLI2001, SCI2004

3. Surface roughness over Gulf Stream SAR image showing convective cells over Gulf Stream: smooth waters over cool water. Sikora et al 1995. Photo taken over Gulf Stream looking towards cooler shelf waters. Courtesy P. Chang and D. Chelton.

4. Positive correlation between SST and wind speed on ocean mesoscales Small et al 2007, accepted: “A review of air-sea interaction over ocean fronts and eddies.” Dyn. Atm. Ocean.

5. Schematic showing the relationship of wind stress curl (hashed) and divergence (stipled) to flow across an SST front (thick isotherm). (From Maloney and Chelton (2006)). SST effects on wind stress divergence and curl. From Chelton et al (2004). Shown are binned scatter plots of spatial high-pass filtered fields of the wind stress divergence from QUIKSCAT as a function of the downwind SST gradient (top row) from AMSR-E and the wind stress curl as a function of the crosswind SST gradient (bottom row) for four geographical regions: the Southern Ocean (60°S to 30°S, 0° to 360°E), the eastern tropical Pacific (5°S to 3°N, 150°W to 100°W), the Kuroshio Extension (32°N to 47°N, 142°E to 170°W), and the Gulf Stream (35°N to 55°N, 60°W to 30°W).

6. Direct Effects - Stratification • Change in surface heat flux due to change in surface stability AND change in wind speed • Over 1200Wm-2 in Gulf Stream (Doyle and Warner MWR1990, • Correlation of heat fluxes with TIWs (Thum et al JCLI2002). • Changes in stratification – sometimes formation of internal boundary layer • (Hsu 1984, Rogers 1989, Anderson 2001) • Change in boundary layer height – several hundred meters higher on warm sideof Gulf Stream • (Sweet et al MWR1981, Wayland and Raman BLM1989). • Change in cloud height • (Holt and Raman MWR1992).

7. Eastern Pacific Investigation Climate Processes (EPIC) EPIC 2001 observations of the MABL across the Equatorial Front at 95° W. Potential temperature (a) from representative dropwinsoundes in the cold tongue (dot-dash) and north of the front (solid). C), Cross-section composited from in situ data from 8 flights by the NCAR C-130 aircraft. Adapted from deSzoeke et al. (2005). WARM WATER COLD TONGUE

8. Direct Effects – Winds and secondary circulations • Change in wind profile due to momentum mixing and pressure gradients • Changes in turbulent momentum flux as air crosses front, momentum from upper levels passed to surface(Sweet et al MWR1981, Hayes et al JCLI1989, • Changes in thermal structure lead to hydrostatic pressure anomalies which force winds (Small et al JCLI2003, Cronin et al JCLI2003) • Secondary circulations (akin to land-sea breeze) • (Hsu JGR1984, Wai and Stage QJRMS1989, Warner et al MWR1990, Sublette and Young MWR1996)

9. Tropical Instability Waves Small et al 2003, JCLI. Model/satellite data intercomparison.

10. Tropical Instability Waves Observations from TAO moorings (Cronin et al 2003, JCLI) confirmed the downstream pressure response.

11. Indirect Effects: Observations • Atmospheric fronts can form over Gulf Stream thermal gradient and can influence cyclogenesis • (Doyle and Warner MWR1990). • Holt and Raman MWR1992 • Rapidly growing synoptic storms (‘bombs’) can intensify over, and track along, the Gulf Stream • (Colucci BAMS1977, Sanders MWR1986, Businger et al 2005., Jacobs et al MWR2005) • Intense storms (‘bombs’) can have warm cores with bent-back warm fronts • (Neiman and Shapiro MWR1993, Businger et al MAP2005).

12. Atmospheric shallow Fronts and Gulf Stream Holt and Raman 1992 – a coastal front aligned with Gulf Stream

13. Indirect Effects: Theory and Models • Synoptic storm development optimally comprises an upper level trough (or potential vorticity anomaly) and surface temperature gradient (baroclinicity). • Cyclonic flow induced by upper vorticity anomaly will form a temperature anomaly at surface via advection (Holton Textbook 2004). • Can induce mutually reinforcing PV anomalies at surface and upper levels (Hoskins, McIntyre and Robertson QJRMS1985, Stoelinga MWR1996).

14. Indirect Effects: Theory and Models • Synoptic storm development optimally comprises an upper level trough (or potential vorticity anomaly) and surface temperature gradient (baroclinicity).

15. Indirect Effects: Theory and Models • Differential diabatic heating can enhance storm growth • Surface fluxes very important at very beginning of growth stage (Kuo et al MWR1992, • Latent heating gives rise to a low level (~800 mb ) PV anomaly (Stoelinga MWR1996). • Diabatic heating induces an upper level mass flux divergence to enhance storm and can destabilise atmosphere (Businger MAP2005.) • Surface sensible heating can act to enhance or reduce low level thermal structure (fronts) • (Doyle and Warner MWR1990, Stoelinga MWR1996). q is Ertel’s PV,  is absolute vorticity,  is density and  is potential temperature. d /dt is diabatic heating. (Stoelinga 1996)