Convection in the Western Irminger

Two aspects of the forward Greenland tip jet are of particular importance to the ocean. The first is the meridional length scale of the jet, and in particular the sharp gradient in wind speed to the north of the jet axis. Often times the wind decreases significantly over a very short distance. For example, in Fig. 26.6a, to the east of Cape Farewell, the westerly wind speed diminishes to the north by 15 m s-1 in just 50 km. Such sharp gradients result in very large synoptic values of cyclonic windstress curl, nearly three orders of magnitude larger than the broad-scale curl of the North Atlantic (Pickart et al. 2003b). Figure 26.8 shows the composite wind stress curl from 7 years of QuikSCAT data, where the year has been divided into winter (November-April) and summer (May-October). The frequent storms in winter result in a band of strong cyclonic curl along southeast Greenland (Fig. 26.8a), due largely to the barrier winds. The strongest positive curl occurs east of Cape Farewell and is clearly the result of the forward tip jet (as is the enhanced negative curl west of Cape Farewell). Note that this curl signature largely vanishes during the summer (Fig. 26.8b). The forward tip jet is thus a major contributor to the enhanced seasonal cyclonic curl pattern near southern Greenland.

QuikSCAT 1898-2006 (a) Novemlier-April (b) May-Oclober

QuikSCAT 1898-2006 (a) Novemlier-April (b) May-Oclober

■60' -50' -40' -30' -20' _ -50' -50' -40' -30' -20"

■60' -50' -40' -30' -20' _ -50' -50' -40' -30' -20"

Fig. 26.8 Climatological average surface wind vectors and wind stress curl (color) over the period 1999-2006 from QuikSCAT. The year has been split into two 6-month averages. (a) November-April; (b) May-October

This vorticity distribution in turn has a significant impact on the circulation of the sub-polar gyre. According to the numerical model study of Spall and Pickart (2003), the enhanced positive curl drives the cyclonic recirculation in the western Irminger and Labrador Seas (Fig. 26.3). Even though the wind forcing is seasonal, a steady circulation develops because of the slow baroclinic wave response at this latitude (wave speeds roughly 1 cm s-1), together with the effect of the bottom topography which causes the deep circulation to dampen the seasonal response. The bottom topography also helps to form the multiple areas of closed streamlines along the lower continental slope (Kvaleberg and Haine 2008). Pickart et al. (2003b) showed that frequent tip jets alone (i.e. without any barrier winds) can drive the Irminger gyre. Hence, the tip jet is largely responsible for the trapping of water near the region of southern Greenland, as well as for the doming of the isopycnals in this area. Both of these factors help facilitate deep convection (Marshall and Schott 1999).

The second crucial aspect of forward tip jets is the large heat flux that results from the cold air being advected over the warm ocean. This was recognized by Doyle and Shapiro (1999), and subsequently studied by Pickart et al. (2003b), Centurioni and Gould (2004), and Vage et al. (2008). Using a numerical model forced by a sequence of tip jets associated with a strong winter, Pickart et al. (2003b) showed that deep convection can occur in the southwest Irminger Sea. The area of overturning in the model corresponded with the observed extremum in mid-depth potential vorticity to the east of Cape Farewell (Fig. 26.9). This provided compelling evidence that the Labrador Sea is not the sole source of the sub-polar

Model mixed-layer depth and heat flux Objen/sd mid-deptfl PVand COAMPS htitfluK

Model mixed-layer depth and heat flux Objen/sd mid-deptfl PVand COAMPS htitfluK

Fig. 26.9 Results from the study of Pickart et al. (2003b). (a) Final depth of the winter mixed-layer (color) in a regional ocean model forced by repeated occurrences of the forward tip jet. The heat flux of the tip jet is shown by the contours (W m-2). (b) Observed potential vorticity (color) showing newly ventilated water east of Cape Farewell. The heat flux of a forward tip jet event from the COAMPS model is shown by the contours

Fig. 26.9 Results from the study of Pickart et al. (2003b). (a) Final depth of the winter mixed-layer (color) in a regional ocean model forced by repeated occurrences of the forward tip jet. The heat flux of the tip jet is shown by the contours (W m-2). (b) Observed potential vorticity (color) showing newly ventilated water east of Cape Farewell. The heat flux of a forward tip jet event from the COAMPS model is shown by the contours mode water of the western North Atlantic, and solved the puzzle regarding the unrealistically fast travel times into the Irminger Sea deduced from measurements during the 1990s high phase of the NAO. However, the model configuration as well as the forcing used by Pickart et al. (2003b) were idealized, and direct wintertime measurements of deep convection in the Irminger Sea are still lacking today.

Unfortunately, during the period of active mode water formation in the early 1990s there were no wintertime cruises to the Irminger Sea. Furthermore, the PALACE/ARGO profiling float programs (e.g. Lavender et al. 2000, 2002; Centurioni and Gould 2004) had not yet begun. It was not until 1997 onward that the floats were able to measure the seasonal development of the mixed-layer in the western sub-polar gyre, but by this time the winters had become more moderate and convection had diminished considerably in the Labrador Sea (Lazier et al. 2002). Nonetheless, the float data were used by Bacon et al. (2003) and Centurioni and Gould (2004) to demonstrate that overturning to depths of 400700 m did occur during this period in the western Irminger Sea. Centurioni and Gould (2004) also used several 1D mixed-layer models, forced by an idealized representation of the forward tip jet, to show that tip jets were likely responsible for the observed convection.

In an effort to elucidate the role of the Greenland tip jet on convection in the Irminger Sea, a subsurface mooring was deployed east of Cape Farewell in August 2001, in the region of the low potential vorticity (see Fig. 26.4). The mooring contained a McLane CTD profiler with an acoustic current meter, and was programmed to return two vertical traces per day between 55 and 1,800 m. Unfortunately the profiler failed the first year, but did successfully profile through the winter during the next two deployments (2002-2003 and 2003-2004). Since these winters were characterized by a low value of the NAO index, it was not expected that deep convection would occur at the site. Nonetheless, using the CTD data together with a variety of atmospheric data sets, Vage et al. (2008) demonstrated that the Greenland tip jet plays a dominant role in the wintertime deepening of the mixed-layer in the western Irminger Sea.

As mentioned above, the small meridional scale of the tip jet (order 100 km or less) makes it difficult to resolve in the global meteorological fields. As such, Vage et al. (2008) constructed an improved heat flux time series at the mooring site using bulk formulae together with various surface data. For winds the QuikSCAT data were used, for sea surface temperature the (extrapolated) mooring time series was used, for air temperature the weather station data from Cape Farewell were used, and for relative humidity the NCEP data were used. The latter two time series were adjusted for the mooring site using meteorological buoy data collected at the site during the fall of 2004 (see Vage 2006). The resulting total heat flux, averaged over the winter of 2002-2003, was more than 30% larger than that from NCEP alone. The biggest discrepancy occurred during the tip jet events. For the 12 robust events between December and April, the average heat flux from NCEP was 267 W m-2, compared to a value of 413 W m-2 from the improved estimate -an increase of 55%.

Not surprisingly, this extra heat flux has a significant impact on the evolution of the mixed-layer. To demonstrate this Vage et al. (2008) ran the Price et al. (1986) 1-D mixed-layer model on a CTD profile from November 2002, forced with both the NCEP heat flux time series and the improved heat flux product. As seen in Fig. 26.10, the mixed-layer depth predicted from NCEP alone (red curve) is too shallow compared to the observations from the mooring, whereas the depth from the improved heat flux time series (blue curve) does a much better job tracking the envelope of deepest observed mixed-layer depth (black curve). (The high frequency signal in the observations is likely due to the effects of lateral advection, which is not captured by the 1D model.) To quantify the effect of the intermittent wind events, a third model run was performed in which the tip jets were removed from the improved heat flux time series (green curve in Fig. 26.10). It is clear that the heat loss due to the succession of tip jet events over the course of the winter had a sizable impact on the final depth of convection.

One of the remaining questions is, can the forward tip jet cause deep convection during high NAO winters? It will be impossible to address this with observations until the return of cold and stormy winters to the western North Atlantic. However, Vage et al. (2008) have shown that the answer is likely yes. During the early 1990s there were on average more robust tip jet events per winter and overall stormier conditions (Pickart et al. 2003b), plus the water column in the Irminger Sea was better preconditioned for overturning. Vage et al. (2008) initialized the mixed-layer model with a CTD profile from fall 1994, and forced the model with a similarly computed improved heat flux time series for the winter of 1994-1995. The predicted final depth of convection for this calculation was nearly 1,800 m (Fig. 26.11), consistent with hydrographic data collected in this

Fig. 26.10 Comparison of observed and modeled mixed-layers for winter 2002-2003 in the southwestern Irminger Sea, from Vage et al. (2008). Figure 26.4 shows the location of the mooring. See the key and the discussion in the text for an explanation of the different curves. The vertical lines denote the tip jet events

Fig. 26.10 Comparison of observed and modeled mixed-layers for winter 2002-2003 in the southwestern Irminger Sea, from Vage et al. (2008). Figure 26.4 shows the location of the mooring. See the key and the discussion in the text for an explanation of the different curves. The vertical lines denote the tip jet events

Fig. 26.11 Modeled mixed-layer for the winter of 1994-1995, from Vage et al. (2008). The different curves are the same as in Fig. 26.10 (see key)

region the following summer (Pickart et al. 2003a). This is roughly 1,000 m deeper than predicted using NCEP alone, and, as was true for the winter of 2002-2003, the presence of the tip jets had a significant impact.

Renewable Energy 101

Renewable Energy 101

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