In this section we focus on comparisons of the timing and amplitude of the maximum MLD between various climatologies. The deepest extent of the winter mixed layer, also called the “bowl” [Guilyardi et al., 2001], defines the boundary between the interior ocean and the surface ocean which is, at least once in a year, in direct contact with the atmosphere through vertical mixing. It represents the MLD on a yearly timescale. The ventilation of the thermocline and the volumes and characteristics of water masses formed at the surface strongly depend on it, and eventually yield the large-scale distributions of properties in the interior ocean.
5.1. North Atlantic Comparison With Other Climatologies
 In comparing our climatology with the two available published global products, ML97 and KRH03, we concentrate on an analysis of the temperature-based MLD in the North Atlantic. Compared to the density-based product and to other regions, a greater amount of data is available, giving the highest confidence possible in the comparison. Since the temperature-based MLD is misleading in locations where a barrier layer occurs, the comparison is also made with our optimal estimate of the MLD, based both on temperature and salinity. Figures 12 and 13 show the timing and maximum value of MLD in the seasonal cycle for the four products. Note that even in the North Atlantic, some grid boxes must be left without value in the optimal MLD, due to a lack of salinity profiles. Since the seasonal cycle is difficult to define in the tropics, this region is not shown in Figure 12.
Figure 13. (a, b, c, d) Same as Figure 12, but for the maximum of MLD, and (e) the median deviation of the maximum MLD from the ΔT = 0.2°C climatology, estimated with at least four values per grid box.
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 On the basin scale, both our temperature-based and optimal MLDs are characterized by a beginning of restratification (Figure 12) in January/February, about 1 month earlier than ML93 and KRH03, and in contradiction with the commonly cited February/March time frame for restratification [e.g., Stommel, 1979; Williams et al., 1995]. This difference in the timing of the maximum MLD likely originates in the choice of MLD criterion. As ML97 and KRH03 estimate the MLD from a temperature-salinity climatology, they use the larger criteria of 0.5°C or 0.8°C (section 4.2). In regions of weak surface stratification, such as the winter high latitudes, the larger criteria in fact measure changes deeper in the water column near the top of the thermocline, and are unable to detect weaker restratification events. This induces a delay in the timing of the seasonal MLD maximum, as seen in Figure 4. There are, however, modeling [Lazar et al., 2002] and observational [Takeuchi and Yasuda, 2003] analyses that support the early restratification seen in our climatology.
 On smaller scales, other differences appear, especially in the Arctic seas (i.e., east of Greenland and in the Labrador Sea), where the deepest MLDs often appear much later in the year in ML97 and KRH03 as compared to the current climatology. Again, this is likely an artifact of the larger temperature criteria, which in polar regions marked by weak vertical temperature gradients are even more likely to pick out the main thermocline rather than changes in mixing. This is supported by the slight delay in KRH03 as compared with ML97 in these regions, corresponding to the larger criterion used. Between our temperature-based and optimal MLD, the small differences appear by construction in regions of barrier layers (see Figure 9, JFM) like the North Sea, around Newfoundland, and east of the Caribbean islands.
 As the climatologies represent a bulk monthly value of MLD, the actual timing of the peak MLD and therefore of the last ventilation with the atmosphere may be masked. If the daily maximum in MLD over the year occurs near the beginning of a month, and is followed by spring restratification events, the median MLD of this month may be smaller (though it will have greater variability) than the previous month. An example is seen in the time series shown in Figure 4a, where the maximum daily MLD (red curve) is reached at the beginning of February, while the maximum monthly median MLD is found in January. A better estimate where data are available would be to increase the time resolution of the climatology.
 A striking dynamical pattern evident from the maximum yearly MLDs (Figure 13) is the main deep convection sites in the Labrador and GIN Seas. The deep maxima (550 m in the Labrador Sea and 740 m in the GIN Sea) are more clearly identified in our climatology, and their depth, variability, and location are well placed [Lavender et al., 2002]. They are also shallower than reported by MLD97 and KRH03, where the deep convection sites are found within larger areas of deep constant MLD of about 1000 m. Again, the difference in MLD criterion plays a role in the differences between climatologies. In the northern North Atlantic the ΔT = 0.2°C climatology yields values of around 350 m with a median deviation between 100 m and 150 m, while ML97 and KRH03 reach values over 600 m. The larger temperature criteria in these latter two appears to be capturing deeper thermocline gradients instead of the base of the mixed layer, particularly in these low temperature stratification situations.
 The limitations of a climatology with its bulk time and space resolution in studying these episodic deep convection events are clear. Some complementary information is found in the MLD median deviation (see Appendix A) shown in Figure 13e, representing the variability of the estimated MLD over the month. The sum of the MLD and median deviation, while not statistically rigorous, gives an order of magnitude for the maximum depth reached during the month, a quantity which may be more important for ventilation and water mass formation than the median MLD. The sums of the MLD and median deviation are 840 m for the Labrador Sea and 1200 m for the GIN Seas, in agreement with previous studies [Lavender et al., 2002; Schott et al., 1993]. MLDs from individual profiles using the ΔT = 0.2°C criterion can be found greater than 1000 m in these regions, but in the same grid box one finds MLDs of 200 or 300 m. The small spatial scales and timescales of deep convective events make them hard to capture in a climatology; however, the same methodology has been applied at 0.5° resolution in the Mediterranean Sea, yielding well-known MLDs of around 1000 m in the Gulf of Lions (F. d'Ortenzio et al., On the Mediterranean mixed layer variability from a new climatology based on individual profiles, manuscript in preparation, 2004). The slight smoothing in our climatology compounds this limitation, reducing the MLD maximum in the Labrador Sea from 775 m to 550 m, for example. One final bias may come from shallow observations, particularly MBTs, which end before the base of the mixed layer in deep convection and deep MLD situations. This may introduce a shallow bias in our climatology of up to 50 m in winter in the northern North Atlantic.
 The shallow MLD maximum regions (less than 50 m) found along North America and northern Europe, within the Mediterranean Sea, and widespread in the tropics, are another important feature of this comparison. They correspond to major barrier layer regions (see Figure 9, JFM), and are major regions of difference between our climatologies and ML97 and KRH03. These differences are especially pronounced in the midlatitudes, with both ML97 and KRH03 missing the extended regions of shoaling observed toward the coasts. Within the tropics the three temperature-based products are fairly similar. Only the optimal MLD captures the coherent barrier layer signal centered at 60W between 10N and 30N, and the more tropical barrier layers. This optimal product, with its good estimate of MLD in both barrier layer and compensated layer regions, is the most reliable one, though it is somewhat hampered by the still-evident areas where the density-based correction cannot be calculated due to the sparsity of salinity data.
5.2. Global Comparisons With Oxygen MLD
 The 95% oxygen saturation limit from CTD data is a useful proxy in determining the maximum annual MLD [Reid, 1982]. It also provides another way to estimate wintertime MLD, especially in the Southern Ocean where temperature and salinity data are very sparse (Figure 1). For instance, oxygen saturation data have been used to estimate the convection depth in southeast Pacific Ocean [Tsuchiya and Talley, 1998]. Figure 14 presents the depth of the bowl estimated from both the oxygen saturation limit and the temperature-based climatology.
 Several regions of discrepancy of the estimates are found. The estimate of the bowl based on oxygen is shallower at high latitudes in the Southern Hemisphere, in the Antarctic divergence. Taking into account density-derived MLD (see Figures 8 and 9), we find that the influence of salinity in these regions cannot completely explain the observed discrepancy. Oxygen-based estimates from data collected during the austral summer could be biased because of the intense vertical movement of layers caused by the positive Ekman pumping. Other regions of discrepancy are the middle and high latitudes in the Northern Hemisphere, especially in the North Atlantic. In this regard, we note that the assumption of Reid  is that 95% oxygen saturation corresponds to the oxygen dissolved during ventilation at the surface at the time of the deepest convective mixing during the year. The choice of the 95% threshold takes into account the moderate respiration occurring in the water column during the season. In areas where the export production is very high at specific times of the year (e.g., the North Atlantic spring bloom or the bloom in the Malvinas confluence), the oxidation of material exported from the surface respiration may consume enough oxygen to drive its concentration below the 95% value, thus altering the estimated maximum MLD. The oxygen-based estimate of the bowl is larger than the temperature-based one in all the subtropical gyres. One reason is the persistent downwelling Ekman pumping. On the other hand, it is interesting to find a good correspondence in the southeastern corner of the Pacific, suggested formation region of Antarctic Intermediate Water (AAIW) [e.g., Hanawa and Talley, 2001].