Argo Float Pressure Offset Adjustment Recommendations Taiyo Kobayashi 1, 2 and Gregory C. Johnson 3 1: IORGC/JAMSTEC, 2-15, Natsushima-cho, Yokosuka, 237-0061, Japan taiyok@jamstec.go.jp 2: Now visiting at National Oceanography Centre, Southampton, Empress Dock, Southampton, SO14 3ZH, UK 3: NOAA/PMEL, 7600 Sand Point Way NE Seattle, WA, 98115, USA gregory.c.johnson@noaa.gov 6 March 2007 ABSTRACT We strongly suggest the adjustment of all known pressure drifts in Argo data. These adjustments will improve the consistency and accuracy of the hydrographic dataset obtained by Argo Program. Even small errors should be corrected because the potential impact of a small bias in Argo data pressures from uncorrected pressure sensor drifts could be quite significant for global ocean heat content anomaly estimates. In the worst case, a bias could introduce an artifact comparable in magnitude to ocean heat content changes estimated for the later half of the 20th century. Float pressure measurements at the sea surface are a good standard for data adjustment due to the relatively small variability of marine surface atmospheric pressure. Nevertheless, we must continue to investigate characteristics of pressure sensor biases to enhance Argo data accuracy. 1
1. The case for data consistency Float CTD sensors are subject to calibration drift after laboratory calibration. With a target of an average 4-year lifetime untended in the ocean, the potential for sensor calibration drift is significant. Argo DMQC work first focused on conductivity sensor drift, because it was thought that calibration drift of this sensor could be the largest. With a lot careful work on this topic, systems have been built, and are being refined, for assessing and correcting those drifts statistically. These systems generally exploit the relative stability of the T-S relationship. Little has been done to detect subtle (not obvious from visual inspection by an experienced researcher) temperature sensor calibration drift. The way forward on such detection, should it be needed, is not clear. Temperature sensors are thought to be sufficiently stable to meet Argo accuracy targets even with typical calibration drift over a 4-year lifetime. Recently DMQC attention has turned to pressure. All the specifics on the operations of and procedures used for different instrument (float/ctd combination) are to the best of our understanding, and we are not experts in how all of these floats work, so please regard the following summary with caution, and correct us if we are wrong. Argo uses several types of profile floats, among of which APEX, SOLO, and PROVOR are dominantly used now (as of Feb. 2007). Pressure errors are treated in different manners by different floats. SOLO floats with SBE-41CP CTDs now internally correct for pressure errors using the surface pressure as a calibration point at each profile by adjusting pressure to be zero at the surface and calculating salinity from the temperature, conductivity, and adjusted pressure. PROVOR floats with SBE-41CP CTDs do the same. The SOLO floats report each incremental adjustment from one profile to the next, although unfortunately with a precision (0.5 dbar for SIO SOLO and 0.1 dbar for WHOI SOLO) that makes detection of calibration drifts difficult or impossible because of round-off error. The PROVOR floats report the accumulated correction (to 1-dbar resolution), so larger (> 1 dbar) pressure calibration drifts can be diagnosed from PROVOR data. APEX floats with APF-8 controllers do not make any internal pressure corrections, and report surface pressures with 5 dbar added. Unfortunately any negative surface pressures measured by the floats are set to zero before the 5-dbar is added and the result is transmitted. In the discussions below, the 5 dbar offset added by the float before transmission has been subtracted again. APEX float with APF-9 controllers do not make any internal pressure corrections, but telemeter back the recorded surface pressure whether it is positive or negative. So what to do about pressure corrections about the APEX floats with either version of the controller? This question is complicated by the fact that APF-8 floats currently set negative surface pressure values to zero before transmission. If pressure sensor offset drifts are even on both sides of zero, correcting only the positive drifts could result in a biased dataset. Hopefully, in the future, the APF-8 firmware will be updated so that negative pressure drifts can be corrected. The firmware modification has been requested of the manufacturer, and they are bench testing it presently. The first thing to remember in considering the bias problem that the largest pressure drifts we know about in SBE-41 (and SBE-41CP?) CTDs are for those that were equipped with Paine and Ametek pressure sensors. In these early instruments, the drifts 2
were almost always toward positive errors in surface pressure, which are reported by all APEX floats, even those with APF-8 controllers. The mechanism for this problem is known for the Ametek sensors, in that there was supposed to be a reference volume for the sensors at absolute vacuum, but that volume quickly leaked and after leaking would drift towards the internal float pressure (only a partial vacuum), resulting in positive reported surface pressures. Of the 22 PMEL APEX floats equipped with APF-8 controllers and SBE-41 CTDs that used Ametek pressure transducers, all drifted very quickly (within 1-3 profiles of deployment) to reporting surface pressures from 2 to 5 dbar high of correct, and then more slowly to reporting surface pressures as much as from 3 to 6 dbar high of correct (Fig. 1). Thus it would appear that correcting only positive biases in Ametek-equipped floats would only improve the accuracy and bias in data from these floats. Figure 1. Reported sea surface pressure (SSP) vs. profile number from Argo Float 4900167, an APEX 260 with APF8 controller and an Ametek pressure sensor located in the Bering Sea. Note that even in the Bering Sea, the RMS variation about the long-term drift is a few tenths of a dbar. The JAMSTEC experience with data from floats equipped with Ametek sensors is similar to the PMEL experience: a rapid initial shift to anomalously high readings of 2-7 dbar for SSP, followed by a much slower drift towards higher values (Fig. 2). 3
Figure 2. Reported sea surface pressure (SSP) vs. profile number from JAMSTEC Argo Floats equipped with Ametek pressure sensors. JAMSTEC also deployed floats equipped with Paine pressure sensors. These sensors show large drifts towards anomalously positive SSP measurements. Their behavior is somewhat different from those of Ametek (Fig. 3). During the first 10 cycles after float deployment SSP values increase rapidly. Thereafter, they increase much more slowly, but eventually some SSP values approach 15 dbar during the lifetime of the float. JAMSTEC has no information about the causes (or mechanisms) of this sensor bias. Figure 3. Reported sea surface pressure (SSP) vs. profile number from JAMSTEC Argo Floats equipped with Paine pressure sensors. The other 191 PMEL APEX floats that reported data at the time of this analysis were all equipped with Druck pressure sensors. These floats had executed varying numbers of cycles, from 4 to 92, with a mean and median of 49 cycles. Of these 191 floats, 51 floats showed evidence of negative surface pressure drift. That is to say, they either always reported zero surface pressures, or after reporting a few slightly positive surface pressures, began and continued reporting zero surface pressure. Another 36 floats reported surface pressures consistent with being near zero (some slightly positive pressures throughout the record, interleaved with some zero pressures throughout the 4
record). The remaining 104 floats showed evidence of drift towards reporting positive surface pressures (Fig. 4). These positive pressure drifts were generally very slight, almost always < 1 dbar. If the negative drifts are of similar magnitude to the positive ones, then correcting even just the positive drifts would tend to increase the accuracy of individual profiles, and even reduce the overall bias, although only slightly. JAMSTEC floats equipped with Druck sensors behave similarly (Fig. 5). Figure 4. Reported surface pressure vs. profile number from Argo Float 5900669, an APEX 260 with APF8 controller and a Druck pressure sensor located in the southwest tropical Pacific Ocean. These sensors appear to have generally small (< 1 dbar) surface pressure drifts but they are often measurable. Float with pressures high of correct significantly outnumber floats with reported pressures low of correct, at least for PMEL Argo floats. In the tropics the RMS variability about the mean drift is about 0.1 dbar. 5
Figure 5. Reported sea surface pressure (SSP) vs. profile number from JAMSTEC Argo Floats equipped with Druck pressure sensors. As noted before, the SOLO datasets are not suitable for looking at surface pressure offsets because of accumulated truncation errors. PROVOR surface pressure offset data (telemetered with a resolution of 1 dbar) provided by Virginie Thierry are clearly skewed toward positive values (Fig. 6). Of 18,405 profiles from 329 floats, 11,600 read zero to within the resolution, 4,473 read surface pressures high of correct with a positive value of 2.64 dbar on average, and only 2332 read low of correct with a value of -2.19 on average. Again, correcting the data, (even if only for the positive offsets were to be corrected), results in more accurate individual profiles and a data set with a slightly smaller overall bias as well. 6
Figure 6. Histogram of the Surface Pressure reported, binned at the 1-dbar resolution of the telemetered data. There are 11,600 out of 18,405 values at zero, but the remaining values are significantly skewed towards positive pressures. 2. Sea level pressure variations The "calibration standard" for the float pressure correction is the surface marine atmosphere, which is usually constant (and known) to within better than 1 dbar. Annual means in Sea Level Pressure (SLP) over the ocean range is from 9.8 to 10.2 dbar, a total range of 0.4 dbar (Fig. 7). 7
Figure 7. Mean Sea Level Pressure for 2005 from 6-hourly values of the NCEP/NCAR reanalysis (dbar). The global record low sea level pressure is 8.70 dbar (Typhoon Tip) and the record high is 10.86 dbar (Mongolia on a very cold, -40 C day), so the global range of extreme values (for all recorded years) is about 2 dbar. However, the standard deviation of 4xdaily NCEP seal-level pressure values for the year 2005, (Fig. 8) is generally < 0.05 dbar over much of the globe, and > 0.15 over the ocean only in a few locations around 60 N and 60 S. Figure 8. Standard Deviation of Sea Level Pressure for 2005 from 6-hourly values of the NCEP/NCAR reanalysis (dbar). Over much of the ocean the difference of the maximum and minimum sea level pressure at each location for all of 2005 (Fig. 9) is < 0.2 dbar. Values of this quantity 8
exceed 0.8 dbar only in a few locations at high latitudes. Figure 9. Difference of the maximum and minimum values of sea level pressure for 2005 from 6-hourly values of the NCEP/NCAR reanalysis (dbar). Sea level pressure provides an excellent calibration point for Argo floats that report surface pressure, being generally stable to < 1 dbar. Both PROVOR and SOLO floats take advantage of this calibration point, and internally correct their profile pressures (and salinities) prior to telemetry. This stability is reflected in the short-term RMS variability of data from floats that telemeter surface pressure data at sufficiently fine resolution (e.g. the 0.1 dbar resolution for most APEX floats with APF8 controllers). In all the data sets examined here, an error at the 1-dbar level or less appears easily detectable, and carefully correcting profile pressure data using the reported surface pressure (even for instrument configurations where only positive drifts are telemetered) would result in a more accurate data set in terms of individual profiles and a less biased data set in terms of overall averages. These corrections can easily be accomplished for APEX floats that report surface pressures. Steps should be taken to ensure that in the future APEX floats correctly report negative surface pressures, rather than setting them to zero before telemetry. However, even for the current configuration of APEX, the benefits of profile pressure (and salinity) correction on the basis of surface pressure drifts at any level detectable (no just > 5 dbar) seem clear, and the "Make no adjustment for pressure offsets of magnitude < 5 dbar " recommendation from DMQC2 does not seem justified. 3. Pressure biases and ocean heat content estimation So far in this document the subject of pressure drift has been discussed from the viewpoint of data accuracy and consistency. However, even small (5 dbar) pressure biases can have a very significant impact on ocean heat (and freshwater) content estimates. Assessment of ocean heat content is one of the primary goals, if not the primary goal, of Argo Large changes of heat content (Figs. 10-12; 0-300m, 0-700m, and 0-2000m) can be caused by a spurious 5-dbar pressure bias. The calculations presented here use the 9
WOA05 climatology, and the 5-dbar bias used here is the threshold for pressure drift adjustment recommended for the present at the Second Delayed-Mode Quality Control Workshop (DMQC2, held at WHOI in October 2006). DMQC2 recommended that pressure drift exceeding the threshold should be adjusted. This 5-dbar threshold may have been based on the target of pressure measurement accuracy for individual floats of the Argo Program. However, systematic biases present in the float data, their potential detrimental impacts on Argo science goals, and the fact that we have the means to correct them, call this 5-dbar threshold into question. If the pressure sensor of a float has a 5-dbar bias, the float reports the profiles with a warm surface layer that is falsely thick, and a thermocline that is falsely deep, both of which tend to increase the oceanic heat content in the most of the ocean. The heat content of water column (Figs. 10-12, upper panel, annual average, per unit area, relative to 0 C) and the increase falsely generated by a 5-dbar bias (Figs. 10-12, middle panels) can be compared. For those unfamiliar with heat content, corresponding changes of temperature averaged in the water-column are shown (Figs. 10-12, lower panels). The false changes of water temperature averaged in 0-300m and 0-700m exceed 0.1 C in wide regions, especially in the tropics and subtropics. In the subarctic and subantarctic regions, the false warming is much smaller due to the weaker thermoclines in those regions. Even in the 0-2000 m averages, the false water-column warming exceeds 0.03 C over large regions. Changes in global oceanic heat content have recently been widely studied (e.g., Willis et al., 2004; Levitus et al., 2005; Lyman et al., 2006). These studies use historical datasets, and many of them include Argo data. As an example, ocean warming signals (changes of the total heat content and the averaged temperature for the recent 50 years) estimated by Levitus et al. (2005) are shown (Table 1; Note that the rightmost two columns are the estimations for 0-3000m). These estimates are obtained by linear fits to annual values. False ocean warming caused by a 5-dbar pressure bias exceeds the estimated warming of the global ocean in the tropics and subtropics. In the subarctic and subantarctic regions, the false warming is about half of the observed warming signal for this 50-year period. A potential bias of this size can not be ignored in climate-change investigations. The actual warming signals estimated by Levitus et al. (2005) show large variations for each ocean, more warming occurs in the Atlantic and less in the Pacific. The warming signal observed in the Pacific is about 0.1 C for the upper 300m water column during this 50-years, which may make it very difficult to assess climate changes in the Pacific by the current Argo Project, in which there are many profiles with pressure drift. This calculation gives an upper limit on the size of the effect because not all pressure sensors are biased. However, considering the history of Argo Project, the situation appears quite serious. False warming and cooling signals could appear in certain areas and periods due to accretions of the following factors: 1. Among the major 3 types of Argo floats (APEX, PROVOR, and SOLO), only APEX still needs additional operation for pressure drift adjustments. However, it is likely that earlier versions of the PROVOR and SOLO floats both had problematic pressure sensors and did not effect this correction automatically. 2. Large pressure sensor drifts (more than 5 dbar) are found in Paine and Ametek 10
pressure sensors, which were used for Argo floats with equipped with SeaBird CTDs before 2003. For floats deployed during and after 2003, the pressure sensors for SeaBird CTDs were changed to those made by Druck, in which drift is usually within 1 dbar (Figs. 4-5). All floats equipped with Falmouth Scientific Instruments CTDs used Druck pressure sensors (W. B. Owens, personal communication, 2007). 3. In 2003, the Argo array was still largely a collection of regional arrays, not a global one, and there were many regions devoid of floats. In addition, many regions were occupied by floats of a single country or PI (Fig. 13). 4. At the present, the operation of pressure drift adjustments depends on each country/organization/pi. Thus, it is possible that many of the profiles obtained in certain regions and time periods are Argo float profiles with significant pressure drift and the false bias discussed above may be erroneously interpreted as observed signals. Table 1: Change in ocean heat content (10 22 J) and mean temperature ( C) as determined by the linear trend for the world ocean and individual basins. Estimates are from supplementary documentation of Levitus et al. (2005), available on an NODC website (http://www.nodc.noaa.gov/oc5/pdf/papers/grlheat05.pdf). 0-300m 0-700m 0-3000m (1955-2003) (1955-2003) (1955-59) to (1994-1998) Heat Content Mean temp. Heat Content Mean temp. Heat Content Mean temp. (10 22 J) ( o C) (10 22 J) ( o C) (10 22 J) ( o C) World Ocean 7.029 0.171 11.192 0.118 14.473 0.037 N. Hem. 3.137 0.188 5.781 0.153 7.317 0.048 S. Hem. 3.891 0.159 5.411 0.095 7.156 0.031 Atlantic 3.373 0.297 5.656 0.221 7.683 0.075 N. Atl. 2.109 0.354 3.606 0.274 4.808 0.095 S. Atl. 1.264 0.233 2.050 0.165 2.875 0.056 Pacific 2.343 0.112 3.558 0.073 3.344 0.017 N. Pac. 0.875 0.093 1.826 0.084 1.624 0.018 S. Pac. 1.468 0.127 1.732 0.064 1.721 0.016 Indian 1.319 0.150 1.987 0.098 3.457 0.041 N. Ind. 0.159 0.125 0.358 0.122 0.896 0.076 S. Ind. 1.159 0.154 1.629 0.093 2.561 0.035 11
Figure 10: Estimation of ocean heat content (0-300m). Top: Oceanic heat content of water column (unit is Giga (10 9 ) J/m 2, annual mean value relative to 0 C based on WOA05). Middle: False anomaly of heat content caused by a 5-dbar (positive) profile bias (unit is G J/m 2 ). Bottom: False change of averaged temperature ( C) of water column (0-300m) caused by this bias. 12
Figure 11: As is shown in Fig. 10 except for 0-700m. 13
Figure 12: As is shown in Fig. 10 except for 0-2000m. 14
Figure 13: Distribution of Argo floats by country in April 2003. References Levitus, S., J. Antonov, and T. Boyer, 2005: Warming of the world ocean, 1995-2003, Geophys. Res. Lett., 32, L02604, doi:10.1029/2004gl021592. Lyman, J. M., J. K. Willis, and G. C. Johnson, 2006: Recent cooling of the upper ocean, Geophys. Res. Lett., 33, L18604, doi:10.1029/2006gl027033. Oka, E., 2005: Long-term Sensor Drift Found in Recovered Argo Profiling Floats. J. Oceanogr., 61, 775-781. Ueki, I., and T. Nagahama, 2005: Evaluation of property change of pressure sensor installed on TRITON buoys, JAMSTEC report of Research and Development, 1, 51-55. Willis, J. K., D. Roemmich, and B. Cornuelle, 2004: Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales, J. Geophys. Res., 109, C12036, doi:10.1029/2003jc002260. Appendix 1: Mechanics of pressure corrections, APEX floats that do not make it to the surface during some profile (call it N) for some reason (e.g. stuck in the mud or encountered a very light surface layer), will transmit an erroneous surface pressure for profile N+1. Use of the surface pressure telemetered during the transmission of profile N+1 to correct pressure and salinity from profile N eliminates these problematic data from the analysis. The reason for this is that the surface pressure for profile N+1 is actually measured at the end of profile N, and when the float doesn't make it to the surface for profile N, no data from profile N are available to be corrected with the erroneous surface pressure from profile N+1. Occasionally a float SSP may be wildly erroneous for various reasons. JAMSTEC has set a threshold of 20 dbar for SSP values; and values exceeding the threshold are not used in the pressure adjustment procedure. Similarly, PMEL uses differences between a raw 15
time-series of SSP and one passed through median filter to identify wild outliers in SSP. Appendix 2: Are pressure errors mainly offsets? At present, pressure drifts are adjusted in Argo by subtracting SSP value from all the pressure measurements of profile. This means the pressure sensor drift is assumed to be an offset that can be measured using SSP and then corrected. Is the evidence sufficient to support this assumption? JAMSTEC has recovered 7 Japanese Argo floats, with 6 of them operating normally at time of recovery. Pressure sensors from 3 of these floats were post-calibrated at a Mutsu Institute, JAMSTEC, laboratory seven months after float recoveries (Oka, 2005). All pressure sensors subject to post-calibration were from Ametek. The postcalibration for pressure sensors clarified that the pressure drifts were almost constant over a 0 to 2000dbar range, being slightly higher under high pressure (Fig. A1). This result largely supports the assumption that pressure drift occurs as an "offset". At JAMSTEC, many mooring buoys have been deployed in western tropical Pacific and the Indian Ocean during a period of 8 years. These mooring buoys (named TRITON buoys) are replaced every year, and all the sensors on the SBE37IM instruments, equipped with Druck pressure sensors (we have not determined that the sensors are the same model of Druck sensor used in the SBE41 CTD for Argo floats), are also post-calibrated at Mutsu Institute (summarized by Ueki and Nagahama, 2005). These post-calibrations are carried out just after each cruise within about 3 months after buoy recoveries. The statistics based on the calibration results for 99 of these instruments are very different from the findings of Oka (2005) for three floats. For the mooring pressure sensors, the pressure errors increase exponentially with higher pressure (Fig. A2), from which we can expect the bias of pressure sensor at 2000 dbar may exceed 20 dbar. These differences in pressure sensor error characteristics raise serious concerns regarding how best to correct for pressure drifts in Argo float data. Fig. A3 shows the calculation of false ocean warming caused by an exponential pressure bias expressed as follows: p pressure bias = 1.5 exp( ) 1 1000 In this case, the sensor has a pressure bias of 4 dbar at 1000 dbar and about 10 dbar at 2000 dbar. False warming is limited to up to 0.02 C in all cases: these effects may be ignored when considering features of the shallower water-column (e.g., 0-300m), but in 0-2000m, the effect seems up to a half or a third of the actual ocean warming during this 50-years. Ueki and Nagahama (2005) also described the Druck pressure sensors of SBE37IM that have biases of about 2-4 dbar on the deck (under atmospheric pressure) after mooring recovery. The biases were found in the sensors moored for only 10 days and they seem increased after longer mooring deployments. This feature is different from our experience with Druck sensors on floats that exhibit almost no bias at the sea surface (Figs. 4-5). We have no idea whether the difference may be caused by the mode of sensor deployment, the possible use of different models of Druck pressure sensor, or some other difference. 16
In any event, we should accumulate knowledge about pressure sensor error characteristics, especially for the Druck sensors that are used on most of the recent Argo floats. Just recently, a JAMSTEC float with a Druck pressure sensor was beached and recovered in Hawaii. JAMSTEC will post-calibrate the pressure sensor as soon as possible to obtain more information on Druck pressure sensor errors. 17
Recovered on Jun. 06, 2003 Recovered on Jun. 14, 2003 Recovered on Aug. 06, 2003 Figure A1: Post-calibration results of pressure sensors (Ametek) from recovered floats (top) WMO 29045 (at 84 th cycle), (middle) 2900056 (at 73 rd cycle), and (bottom) 29051 (at 90 th cycle) reported by Oka (2005). Stars at 0 dbar in each panel are SSP values measured just before float recovery. Dates of post-calibration for the pressure sensors are noted on each panel. Figure A2: Post-calibration results of Druck pressure sensor used in TRITON buoys (SBE37IM, from Ueki and Nagahama, 2005). Pressure sensors are moored at the depth of 300m or 750m for about 1 year generally. 18
Figure A3: False change of average temperature of water column for a pressure sensor bias with the form: 1.5*( exp(pressure/1000) -1 ), which generates about a 4dbar error at 1000dbar and a 10dbar error at 2000dbar. 19