Oceanic Convection in a Front: or Eighteen Degree Water (EDW) formation as observed in CLIMODE Terrence Joyce, Leif Thomas and Frank Bahr, WHOI, Presented at EGU, April 2008

SeaSoar studies on Knorr 188 The WHOI SeaSoar was towed typically at a speed of 8 kts, in an undulating saw-tooth pattern between depths of 30 & 450m. Sensors included dual pumped temperature & conductivity, pressure, dissolved oxygen, and fluorometer. Both lab and in situ calibrations were applied to the data. Fluorometer data are reported as voltages. Grid1_lines1-8 Grid2_lines1-3 Grid3_lines1-3 Grid4_lines1-3 Grid5_lines1-3

Cold Air Outbreak on 20 Feb 2007 A regional forecast model downscaled from NCEP [NAM] provided ‘data’ for Sea Level Pressure (white), Surface Air Temperature (colors), and winds (black barbs). Here we show the trailing cold air outbreak following a strong winter low (L) passing over the Gulf Stream within the winter storm track. L

 T, (T water -T air ) for 20 Feb With cold winds from the NW, large Sea/Air temperature differences occurred over the Gulf Stream and northward meanders/Rings. Selected SST contours delimit the temperatures of newly forming EDW. East of ca. 57W the outcrop window opens & EDW is widely ventilated within the Gulf Stream.

Surface Ocean on 22 Feb Following the cold air outbreak,  - wave satellite SSH and SST data for 22 Feb are shown with geostrophic surface currents and EDW outcrops for the day on which SeaSoar data (white lines) from the third of the 3 lines were collected across the Gulf Stream.

SeaSoar Survey 2, Line 3 The day following a cold air outbreak on 20 Feb 2007, this SeaSaor survey was begun ‘downstream’ where the surface SST max. warm core of the Gulf Stream had been completely eroded due to cooling. We show the vessel track as well as the SST contours from a microwave satellite system for 22 Feb., the day for which the third line (red line) of the grid was made. SeaSoar was towed at a ship speed of ca. 4 m/s.

Stream Coordinate System The vertically-averaged current from the ADCP along the ship track (thin black line) was used to define a stream coordinate system: the origin is the location of the maximum current & the positions are given in cross-stream coordinates (red dashed line) relative to that center) and projected at right angle to the major flow. Current are rotated into (u d,u c ). All plots are done looking ‘downstream’. x c =0

Detailed SeaSoar Section: Grid 2, Line 3 SeaSoar and ADCP data are plotted in stream coordinates relative to the core of the Gulf Stream [north on left]. The trace of the SeqSoar is shown in the lower left panel. For this third section from our second SeaSoar Grid. Pot. Temp., o C Oxygen,  mole/kg V d, m/s

Ertel Potential Vorticity Defined Where f is the vertical component of the coriolis vector,  is the projection of the northwards component of the coriolis vector along the cross-stream direction, x c, V dz is the vertical shear in the downstream velocity,  ≈ V dx is the vertical component of the relative vorticity, and  xc is the cross-stream density gradient. The above 4 terms on the rhs are defined in subsequent figures as T n, where n=1:

Detailed sections Grid 2 Line 3 (cont.) As for the previous figure, the gridded distribution of fN 2 and EPV (left) reveal incidence of vertical overturning near the surface and low EPV (but not stratification) deeper. % oxygen saturation and raw fluorometer data (right) indicate substantial variability across the center (x c =0) of the GS.

Detailed sections (cont.)

Mixing in the EDW: Grid2 Line2 - 1 The best example of active mixing of new EDW was made on this SeaSoar section, almost totally within the newly forming EDW just to the south of the GS (seen on extreme left of plot).

Mixing in the EDW: Grid2 Line2 - 2 Gravitational instabilities are marked by inversions of density (contours) in the planetary component of Potential Vorticity (right). These mixing zones extend beyond the max. depth of the SeaSoar (450 m) between 30:40 km in the cross- track direction.

Mixing in the EDW: Grid2 Line2 - 3 Significantly more inversions are found in the full Ertel PV for this section. Regions of EPV inversions are alighend with potential density surfaces suggesting active mixing. In what follows the various terms in the EPV will be diagnosed to determine the causes of the inversions.

Mixing in the EDW: Grid2 Line2 - 4 Considerable structure is found in the fluorescence, with high bands generally associated with low EPV regions. In a later figure, this will be replotted along with percent oxygen saturation.

Mixing in the EDW: Grid2 Line2 - 5 Between 100:300m, the EPV balance suggest (upper left) a gravitational instability near 31 km. Large vertical shear (low geostrophic richardson numbers, lower left) is causing a symmetric instability at cross-stream distances of 43, 63 & 88 km. Inertial instability, due to large anti-cyclonic shear is apparent at 52 km. A low EPV value near 18 km is due to a ‘mix’ of causes. EPV variations are anti-correlated with fluorescence & oxygen (lower right). Ri g -1 Ro Geostrophic balance

Mixing in the EDW: Grid2 Line2 - 6 Generally a symmetric instability (slantwise convection) is more likley to contribute to EPV inversions than any other single cause, although low stratification and anticyclonic vorticity are certainly important factors

Mixing in the EDW: Grid2 Line2 - 7 Analysis of all data below 100m on this section indicates that high fluorescence and high % oxygen saturation are both more likely for low EPV (near zero) than for large values, where no instability is likely. Both of these tracers are associated with recent contact with the atmosphere or, in the case of fluorescence, with the upper ocean where phytoplankton can grow.

Mixing in the EDW: Grid2 Line2 - 8 Contours of absolute momentum (v d +f*x c, solid black contours) are plotted with potential density (cyan lines in upper, black dashed below). The regions of likely gravitational instability (31 km, black arrow), symmetric instability (43, 63, & 88 km, green arrows) and inertial instability (52 km, red arrow) are all indicated.

EPV diagnosis for selected lines from all SeaSoar Grids  Regions of EPV inversions were most homogeneous in density space for the EDW during the second grid of leg 1. Before then we see active mixing at a range of densities (generally mixed instabilities) and later (right figures), we see the appearance of both denser EDW modes as well as lighter modes associated with mixing of the warm core.

Some conclusions Substantial formation of EDW occurs in the anticyclonic region of the Gulf Stream, south of max. velocity As a measure of convective activity, EPV is more indicative than stratification Instabilities [gravitational, inertial, symmetric] all act to remove regions of EPV<0. Low EPV waters are clearly associated with recent ventilation near the surface (high % sat. for O 2 & high fluorescence) Vertical and lateral shear of the basic state plays a critical & essential role in EDW formation The total percentage of new EDW formation in the GS front is an active area of our ongoing study in CLIMODE

Initial SeaSoar Survey: Grid 1 line 2 Initially, EDW was not forming: densities south of GS core tht were being ventilated were <26.3    These had high oxygen: significantly higher than the older EDW found on the section.

Grid 1 line 2 -2

Grid 1 line 2 -3

EPV Diagnosis, Grid 1 line 2 EPV inversions were for the lighter modes (upper left) later cooled into EDW. One can see that here, the inclusion of the anticyclonic vorticity from the south side of the GS (term T2) was instrumental in accounting for most of the EPV inversions (lower right). Only few were associated with density inversions (upper right).

EPV Diagnosis, grid 2 line The EPV was integrated vertically between 100:300 dbars and the various terms dignosed (upper left). The Rossby number and inverse Richardson Number (lower left) reveal regions of strong cross-front and vertical shear. The flow is fairly ‘balanced’ for this section (upper right), while the anomalies of EPV and ventilation are strongly anti-correlated (lower right).

EPV Diagnosis, grid 2 line All of the EPV inversions on this SeaSoar leg have been diagnosed for their potential density (upper left), stratification (upper right, t1), stratification plus vertical shear (lower left, t1+t4), and then including also relative vorticity term (t4+t2+t1, lower right). The ‘symmetric instability’ criterion is met (t1+t4<0) most often, with gravitational (t1<0) and mixed (t1+t2+t4<0) instabilities also common.

EPV for Grid 2, Line 3 Absolute Momentum [v d +f*x c, solid black contours] is plotted along with % saturation & EPV=0 (upper) and Fluorometer & EPV=0 (yellow, lower) for grid2, line 3. High Fl. and O 2 plumes are aligned well with momentum surfaces. In regions where momentum & potential density (thin lines) are parallel, EPV is generally near zero (see red arrows for two ‘sheets’ of slantwise convection & green arrow for region of possible inertial instability.

Final SeaSoar survey

Final SeaSoar survey - 2

Final SeaSoar survey - 3 In this survey, the warm core is starting to re-form due to advection with low (high) PV and high (low) fluorescence above (below) a barrier layer. Both to south AND the north of this core, newly formed EDW can be found with a dense (26.6) winter mode, though a low PV layer of 26.7 can be seen to the north.

Final SeaSoar survey - 5 From this layer average we can see that the low stratification, gravitational instabilities are found on the south side (xc=55-60 km) while mixed or symmetric ones come from the north side (xc=-15 km). However, in this layer average, there are no ‘critical’ regions to the north.

Air-Sea Exchange Cruise Summary, Winter 2007 SeaSoar Surveys were taken at the 5 times indicated by arrows. Leg 1 Leg 2

Final SeaSoar survey - 4 WC EDW SW On this section, we see at least three different varieties of actively overturning waters (EPV<0): those in the warm core (WC), in the EDW, and in the Slope Water (SW). The dominant mechanisms for mixing are grav. & symm. instabilities.