Summary of 19-20 Aug Pacific Implementation Panel meeting, Seattle
Chris Bretherton and Billy Kessler, PIP co-chairs
We reviewed the current status of Pacific CLIVAR and related programs. We discussed three proposed process studies (VEPIC, equatorial upwelling, subtropical subduction) and a pilot study to begin monitoring of equatorial western boundary currents. We agreed that all of these proposals had sufficient merit to proceed with further planning. We propose two combined science and planning workshops. These will help guide the further elaboration of the process studies. The first, to be held next April in conjunction with the next PIP meeting, addresses 'Equatorial and coastal upwelling in the tropical eastern Pacific'. We plan to hold our next (brief) PIP meeting in conjuction with this workshop. The second workshop, to be held later next year, involves 'Mixing processes and the initiation of subtropical subduction'. We also agreed on the need for atmospheric OSSEs to determine if additional upper-air measurements over the Pacific can meaningfully improve means and eddy statistics on monthly timescales derived from atmospheric analyses. We agreed that development of technology to enable regular measurement and study of the low-latitude western boundary currents should be a priority for CLIVAR. Lastly, we support both CPTs and phenomenon-oriented diagnostic analyses of ENSO and decadal variability in coupled models, the historical record, and emerging datasets from PBECS. These provide promising mechanism to integrate models, theories, and observation in support of the goals of CLIVAR. Note the action items at the end, including A3, Recommendations to SSC.
Chris Bretherton, U. Wash.
Billy Kessler, NOAA/PMEL
George Kiladis, NOAA Aeronomy Lab
Art Miller, Scripps
Sumant Nigam, U Md./NSF
Paul Robbins, Scripps
Dan Rudnick, Scripps
Paul Schopf, COLA
Max Suarez, NASA/GSFC
The other PIP members, Greg Johnson and Roger Lukas could not attend, but Roger provided extensive commentary via email.
Program review
Since 4 of the 9 attending panel members were new, we spent a considerable fraction of the first day reviewing the history, goals and current status of Pacific CLIVAR and related programs (e.g. THORPEX, EPIC, KESS), CPTs, coupled model results from the new CCSM2 integrations, possibilities for international collaboration, and discussing strategy for getting projects implemented within the CLIVAR framework.
Response to Held/Stammer SSC9 panel review letter (see WWW site above)
We are in agreement with the general points raised by the letter. We agree that the seasonal and ENSO cycles remain thorny modeling problems, and the PIP notes that the PBECS implementation plan calls these out as central issues. We agree that a central criterion for prioritizing process studies should be their importance to coupled modeling of seasonal and interannual as well as decadal variability. We discussed the importance of fostering new measurement technologies (e.g. gliders) for long-term monitoring goals of PBECS, particularly the tropical western boundary currents. We recognize that anthropogenic climate change may well be one of the dominant signals in the PBECS measurements of the next decade. The VEPIC program hopes to contribute to our understanding of low cloud feedbacks on climate sensitivity, and to coordinate closely with any CPTs working on this issue. While ocean data assimilation efforts do not appear to require any special attention from our panel at present, we do recognize the need to coordinate a CLIVAR program in diagnostics and modeling of Pacific interannual to decadal variability that takes advantage of these emerging observational products.
There is one point that we disagree with. In contrast to the reviewers, we feel that the KESS program does not address midlatitude atmosphere-ocean interactions of potential importance to decadal climate variability because of its exclusive focus on ocean dynamics. **Given the importance of this region to some mechanisms of coupled Pacific decadal variability, it would be an appropriate use of CLIVAR funding to encourage diagnostic studies of the local and remote atmospheric effects of oceanic SST anomalies seen during KESS using all available data, as well as encouraging enhanced air-sea flux measurements in this region during KESS. This would be a remote place for US scientists to do a focussed atmospheric process study, and in fact several relevant atmospheric studies of cold air outbreaks and rapidly developing polar lows have been done in the NW Atlantic already. Nevertheless, CLIVAR has an interest in the scientific community considering whether there are creative and cost-effective ways of piggybacking on KESS measurements to better document and understand atmosphere-ocean feedbacks in the NW Pacific.**
Enhanced atmospheric monitoring and modeling considerations
George Kiladis led a discussion on the long-term atmospheric measurements proposed as part of PBECS and endorsed by a working group at the International Pacific CLIVAR planning workshop of Feb. 2001 in Honolulu. OSSEs, THORPEX update, etc. We need atmospheric OSSEs to address the utility of enhancing upper-air observations over the tropical and/or midlatititude north Pacific for improving atmospheric analyses on monthly and longer timescales.
We discussed a concern raised by Sumant Nigam that we were overly focused on the oceanic elements of understanding Pacific climate variability. We view the CPTs as one way to bridge that gap, and diagnostic/modeling studies of decadal variability such as proposed by Ed Sarachik in a letter to the SSC as another productive integrating approach. The present balance of the panel seems appropriate for considering the full complexity of the coupled problem.
http://www.atmos.washington.edu/~breth/VEPIC/VEPIC_Science_Plan.pdf
Goal
Better understand and model cloud-topped boundary layers and atmosphere-ocean-land feedbacks in the subsidence regions of the E Pacific, leading to better modeling of seasonal and ENSO variability in this region.
Relevance to coupled modeling
Important coupled model region due to combined steep topography, PBL cloud and nearby deep convection; relevant to other subtropical Sc regions but attractive due to ENSO feedbacks, so good location to focus on model and parameterization development and sensitivity studies. Feedbacks with Americas are not fully understood, and likely occur on all timescales from diurnal up. NE Pacific also relevant.
Coupled models consistently have equatorial warm bias along W coast of S America, decoupling coastal and equatorial upwelling and ENSO response. Ocean and atmospheric model horizontal resolution part of the problem, but probably not the entire story. Stratus now present in many models but not a solved problem.
Strategy:
1. Large-scale modeling studies and cloud-topped boundary layer parameterization development focussed on the E Pacific region. system.
2. Diagnostic studies using the rapidly increasing diversity of and satellite data as well as reanalysis data to better understand cloud-topped boundary layer mean structure, spatiotemporal variability, and ocean and land feedbacks.
3. Targeted enhancements of routine observing network in this region (a) Fairall remote sensing/surface fluxes routinely deployed on buoy maintenance cruises in this region. (b) Ceilometer, vertically pointing radar, IOPs with sondes, on San Felix Is (27S 80W)
4. Close coordination with potential low cloud/ climate sensitivity CPT. (Column comparisons of GCMs with Weller buoy observations, boundary layer and cloud param. development)
Cost estimate: $1M/yr
Leadership: C. Bretherton, U. Wash.
R. Mechoso and B. Stevens, UCLA
C. Fairall, NOAA/ETL
J. Ruttland and R. Garreaud, U. Chile
Timeline: Sustained low-level activity in 2003-2006
Potential IOP focussed on anthropogenic cloud/albedo/aerosol
feedbacks and/or SE Pacific coastal oceanography, 2007. This would
be a separately funded partnership between VEPIC and other
scientists, not a core requirement of the VEPIC plan.
Historical background
Sparse in-situ measurements in this region:
EPIC2001 (reviewed as part of this meeting) in Sep-Oct 2001,
CIMAR cruises documenting Sc off central Chilean coast in Mar 1999 and 2000.
Recommendations/discussion:
VEPIC planning should continue under VAMOS oversight.
Still need to fully digest EPIC2001 results to focus science aims.
Endorse close coordination with any low cloud/ climate sensitivity CPT
Oceanography during EPIC has been an afterthought. Equatorial oceanography (currents, upwelling, T/S structure) off Ecuador near equator an intriguing issue in this area with which ocean models have great trouble. PIP contact to explore this - Art Miller. Explore this issue as part of proposed CLIVAR workshop on 'Equatorial and coastal upwelling in the tropical eastern Pacific' (see Action Items)
upwelling-experiment1.html
Goal
Identify spatial (10+ km, 10m vertical) and temporal (diurnal cycle and longer) structure of upwelling and corresponding shears, mixed layer structure and turbulence, over an annual cycle. This would enable both improvement in model parameterizations of vertical mixing along the equator and detailed diagnosis of model evolution in this region.
Relevance to coupled modeling
Upwelling appears to be a strongly diapycnal process, thus sensitive to model parameterizations.
Mean SST, currents and ENSO amplitude in OGCMs and coupled models are sensitive to mixing params in this region (e.g. less vertical diffusivity leads to stronger ENSOs, stronger mean currents.) Present-generation OGCMs and CGCMs simulate a disturbing variety of heat balances in the region, implying considerable doubt whether model representations of the relevant processes can be relied on for diagnostic purposes.
Diurnal cycle probably important to vertical mixing, but underdocumented in this region and not included in many coupled models.
Strategy
1. Diagnostic studies of existing (satellite, moored and in situ) time series to help determine the structure of the needed field observations.
2. Array of ~10 velocity, water property, surface met moorings parked for an annual cycle.
3. IOP (at least one, probably during the cooling time of year) with detailed ship surveys of the vertical structure and microstructure, in the context of the moored array.
4. Compare observed structure to output of appropriately forced fine-scale ocean models and equatorial structure of coarse-res ocean models.
Cost estimate: $5M/yr for two years, plus planning/data analysis costs
Leadership
Kessler, PMEL (mooring obs)
Gregg, U. Wash. (microstructure)
P. Schopf, U Md. (modeling)
?A. Miller, Scripps (modeling)
Timeline: Field phase 2006-7 (hope for a neutral/cold ENSO phase),
Location
TBD, but 0, 110 and 140 E are attractive choices because of the existing 2 decades of TAO mooring velocities and temperature time series.
Historical background
There have been a few experiments over the past two decades to sample the equatorial microstructure, and (separately) to estimate vertical velocity from horizontal divergence. The present proposal would build on those by embedding the microstructure measurements in a detailed meso- and large-scale observational context, and by deploying sufficient moorings to enable taking horizontal derivatives on a variety of scales. Observations would be taken on scales suitable for direct comparison with forced ocean model runs.
Recommendations/discussion:
Broad PIP support for need for this type of process study. Can be done with existing technology and builds nicely on TAO array.
Further consideration needs to be given regarding the most useful location for the study. Current OGCMs appear to have more difficulty in simulating the vertical structure at 110W than at 140W, however 110W may be a more difficult place to sample because of the very short scales in the vertical.
This study should be explored at an open planning meeting as part of proposed CLIVAR workshop on 'Equatorial and coastal upwelling in the tropical eastern Pacific' (see Action Item A1)
Goal
Focus on diabatic processes causing temperature/salinity anomalies in the initial stages of subduction. Processes controlling the production of T/S anomalies include surface fluxes caused by winter storms, restratification at fronts, stirring and mixing in the mixed-layer and in the turbulent region below the mixed-layer base.
Relevance to coupled modeling
The experiment will concentrate on the variety of mixing processes that produce T/S anomalies and are parameterized in coupled models. The slumping of fronts, a powerful mechanism of subduction, may be reasonably represented in the thermocline (for example, by aGent-McWilliams formulation), but its extension to the surface needs improvement. Between the forced winter mixed layer and the relatively quiescent thermocline lies a highly turbulent, sheared region where better parameterizations are sorely needed. The anomalies in subducted water are influenced by the strongest individual storm of the year, thus limiting the predictive ability of climate models.
Strategy
1. Intensive observational periods in two consecutive winters will aim to identify T/S anomalies for subsequent tracking by ARGO, and determine processes creating anomlies.
2. One or two flux reference moorings will help ensure high quality fluxes from reanalyses.
3. Observations in both the upper atmosphere and boundary layer (as by THORPEX) will identify the storms responsible for creating T/S anomalies. Also address the possibility that reemergent SST anomalies affect the storms.
4. Ocean observations to include gliders, enhanced floats and drifters, and ship surveys (SeaSoar, microstructure) for a complete characterization of the upper ocean.
5. Theory and process modeling to motivate observations, interpret results, and propose specific parameterizations for testing in coupled models.
Cost estimate: $5M/year for two years plus planning/data analysis costs
Leadership
D. Rudnick
R. Lukas
G. Kiladis
S. Nigam
Timeline: Field phase around 2007 or later
Location: NE Pacific, 20-40 N, roughly 140 W
Historical background
The last comprehensive effort to examine mid-latitude ocean subduction and atmospheric processes took place 1991-1993 in the North Atlantic through the Subduction Experiment, NATRE, and ASTEX. Perhaps the most memorable result from these experiments was the tracer-release confirmation of weak mixing in the thermocline (Ledwell et al. 1993, Nature). Among a number of technological firsts was the first deployment of a profiling ALACE float, whose predecessors comprise ARGO. Interpretation of results was ultimately constrained by the lack of knowledge of the evolving large-scale flow. A modern experiment would take full advantage of ARGO-provided context.
Recommendations/discussion:
Requires fully populated ARGO in subtropical NE Pacific. PIP notes the data gap in this region and encourages the deployment of floats in the subduction area.
The diabatic processes to be observed are high priorities of the ocean mixing CPT.
The combination of autonomous measurements like THORPEX and gliders is particularly attractive. There is a natural shared interest between atmospheric and oceanic scientists, though perhaps on different space and time scales.
Planning should progress though a workshop in 2003, and a science team should be formed (see Action Item A2).
See link 6 on pbecs/pbecs.html for a related earlier proposal by Roger Lukas
Goal
Demonstrate cost-effective capability for monitoring mass transport and T/S anomaly advection in the Mindanao current, with potential extensions to the other tropical western boundary currents.
Relevance to coupled modeling and PBECS
Forced hi-res ocean models disagree on Mindanao current mean transport (~30%) and annual cycle.
WBC monitoring central to PBECS:
'Piping' - mean and variability of the horizontal and vertical structure and property transports of the tropical WBCs (e.g. Mindanao, New Guinea currents) are poorly known, but these are essential to our goal of producing an ocean state analysis of the Pacific basin.
The tropical WBCs are a principal means of meridional heat and mass flux in the Pacific, and in the Indo-Pacific connection.
The dynamics that control the bifurcation of the NEC at the Philippine coast are implicated in gyre-scale tropical-mid-latitude exchange.
Possible Strategies and tradeoffs
1. Towed ADCP plus XBTs using fishing vessel chartered in the Philippines, section across current repeated at least monthly. This is a labor-intensive exercise, which would probably require training local technicians if it was to be carried out longterm. ADCPs would not sample the full depth range of the current, but the combination of ADCP plus XBTs (with some XCTDs) might provide much of the needed information. Cost estimate $200 K/yr.
2. Moored profilers at current boundaries. Relatively new technology would give high temporal and vertical resolution, but only the total geostrophic transport. Would give only mass, not property, transports, unless used in conjunction with other in situ measurements. Possible interpretation problems because of single-point offshore mooring. Other types of mooring and tomographic arrays have also been proposed, all of which are relatively expensive. Cost estimate $500K/yr per pair of moorings.
3. Glider sections. Unproven technology, especially in strong, thick currents. May be available on the 3-5 year timescale. Potentially very attractive for low cost and longterm continuity. Should be fostered and investigated but we should not rely on this possibility to the exclusion of considering other technologies. Cost estimate $200 K/yr.
Note: any of these programs would require clearance and cooperation from the Philippines government, since the observations would be entirely within the EEZ.
Leadership: TBD (needs a champion-this is an important issue!)
Timeline: Field phase around 2005 or later
Mike Johnson of NOAA/OGP may have funding for this in the short term.
Location
Mindanao current, possibly extending to the New Guinea Current system.
Historical background:
There have been short-duration studies and sections across the Mindanao Current, and also of the New Guinea current system. These suggest strong annual cycle and ENSO variability that is connected to fluctuations of the west Pacific warm pool. Repeated meridional sections along 137E over more than three decades indirectly suggest simultaneous changes in the NEC and NECC via the Mindanao Current associated with the ENSO cycle.
Recommendations/discussion:
Panel agreed to need for further panel discussion of different measurement strategies and appropriateness of fast-tracking some form of monitoring.
Need to ask modelers what the state of the art regarding Eq WBCs is (to get oriented, to get the scale of the current).
Such measurements could provide context for a more intensive process study at a later date.
Action Items emerging from PIP meeting
A1 Within next 1-2 months, PIP should propose an open science workshop to SSC on 'Equatorial and coastal upwelling in the tropical eastern Pacific'. Target workshop date: Apr 03 (to precede Intl CLIVAR SSG meeting schedule). CLIVAR sponsorship to be requested to pay for meeting logistics and to pay travel for up to 2-3 invited international participants. At this meeting there will be an evening or one-day additional planning meeting to elaborate plans for an equatorial upwelling experiment, and an evening meeting to discuss the utility of a process study for upwelling, mixing processes and currents near where the equator intersects the S American coast. The latter would be incorporated into VEPIC plans if there was enthusiasm for it, especially at the international level. Publicize through EOS, US and Intl. CLIVAR email lists. Lastly, the next PIP meeting will be held at this workshop. Location suggestions: NCAR, Oregon State. Short white paper as framework for this WG meeting to be formulated by PIP members before SSC10 and presented to SSC at that time.
A2 Propose to CLIVAR an open-attendance subduction workshop later in 2003 to focus on state of the science regarding vertical and lateral mixing processes conditioning the water undergoing subtropical subduction. An informal planning meeting for a CLIVAR subduction experiment would be held at this workshop. Utility of coordination with related Atlantic CLIVAR activities should be addressed at such a workshop.
A3 Recommendations to SSC
1. An important issue for PBECS is whether reinstatement of daily rawinsondes at selected Pacific island sites and/or regular THORPEX type driftsonde deployments would measurably benefit atmospheric reanalysis of mean flow and eddy statistics on monthly and longer timescales. We strongly recommend that upcoming AOs put out in the name of CLIVAR should call for proposals for atmospheric OSSEs aimed at this issue.
2. CLIVAR should explicitly foster model intercomparison and diagnostic studies and workshops aimed at interannual and decadal phenomena in the Pacific (e.g. the PDO), and try to ensure that they are well integrated with panel activities. Particular attention should be paid to diagnostic studies using the emerging ocean data assimilation products incorporating ARGO floats and other new subsurface measurements.
3. The PIP supports having an open workshop sponsored at least in part by CLIVAR on understanding errors in the climatology and variability of tropical convection in coupled models and AGCMs, especially if a CLIVAR CPT is formed to focus on that subject.
4. Three PIP members, Greg Johnson (PMEL), George Kiladis (NOAA Aeronomy), and Roger Lukas (U Hawaii), rotate off at the end of 2002. We'd like to maintain roughly our current balance of atmospheric and oceanic expertise. we suggest some potential nominees for new members to serve 4 year terms starting in 2003:
Grant Branstator, NCAR (midlatitude low-frequency atmospheric dynamics)
Nate Mantua or Ed Sarachik, U. Wash. (PDO phenomenology, human impacts)
Jim Moum, OSU or Mike Gregg, U Wash. (ocean microstructure)
Niklas Schneider or Shangping Xie, U Hawaii (Ocean decadal modeling)