Oceanus REPORTS ON RESEARCH FROM THE WOODS HOLE OCEANOGRAPHIC INSTITUTION Vol. 39, No. 2 • Fall/Winter 1996 • ISSN 0029-8182 Oceans & Climate ^ The Ocean Conveyor Belt Flows Around the World PACIFIC SAMW Subantarctic Mode Water AAIW Antarctic Intermediate Water RSOW Red Sea Overflow Water AABW Antarctic Bottom Water NPDW North Pacific Deep Water AAC Antarctic Circumpolar Current CDW Circumpolar Deep Water NADW North Atlantic Deep Water UPPER IW Upper Intermediate Water IODW Indian Ocean Deep Water One of the keys to understanding how and on what time scales the vast volume of water in the ocean interacts with the atmosphere and modifies Earth's climate is determining how water moves from the surface of the ocean into the interior, how it returns from the depths, and how it flows between the ocean basins. While many of the articles in this issue focus on the movement of water within the North Atlantic, these two recent figures from WHOI Senior Scientist Bill Schmitz provide a global synthesis of the present understanding of the movement of water between ocean basins and across the depths. The numbers in the top figure are flow rates or transports in units called Sverdrups (after Nor- wegian oceanographer Harald U. Sverdrup), which represent flow at 1,000,000 cubic meters per second. The red arrows show flow paths and rates in the shallow and intermediate depths. Green and blue arrows and numbers show the paths and rates for the deep ocean and for bot- tom flows, respectively. The figure at left provides a three-dimensional perspec- tive, labeling the different water types moving along the pathways and adding the color orange for the very salty, warm water that flows out of the Red Sea, along with the color purple indicating near-surface circulations. These two figures are part of what Bill Schmitz, who holds the W. Van Alan Clark, Jr., Chair for Excellence in Oceanography, calls his "final report," a summary (in a somewhat speculative vein, he says) of what he has learned over the past 35 years about large-scale, low-frequency ocean currents. This two volume work is being published as part of the WF1O1 Technical Report series. Oceanus REPORTS ON RESEARCH FROM THE WOODS HOLE OCEANOGRAPHIC INSTITUTION Vol. 39, No. 2 • Fall/Winter 1996 • ISSN 0029-8182 Cover: R/V Oceania weathers a North Atlantic storm during a March 1981 study of a warm core ring spawned by the Gulf Stream. Inset: Researchers aboard R/V Endeavor (University ot Rhode Is- land), an Oceania sister ship, wresde with a rosette water sampler in the southern Labrador Sea during a spring 1991 investigation of the origins of the deep western boundary current. Urge photo by lames McCarthy. Harvard University Inset by Peter Undry. WHOI Oceanus is published semi-annually by the Woods Hole Oceanographic Institution, Woods Hole, MA 02543. 508-289-3516. http //www whoi edu/oceanus Oceania and its logo are « Registered Trademarks of the Woods Hole Oceanographic Institution, All Rights Reserved. A calendar-year Oceanus subscription is available for $15 in the US, $18 in Canada. The WHOI Publication Package, including Oceania magazine and Woods Hole Currents (a quarterly publication for WHOI Associates and Friends) is available for a $25 calendar- year fee in the US, $30 in Canada. Outside North America, the annual fee for Oceanus magazine only is $25, and the Publication Package costs $40 To receive the publications, please call (toll free) 1-800-291-6458, or write: WHOI Publication Services, P.O. Box 50145, New Bedford, MA 02745-0005. To purchase single and back-issue copies of Oceanus, please contact: lane Hopewood, WHOI-MS#5, Woods Hole MA 02543. Phone: 508-289-3516. Fax: 508-457-2182 Checks should be drawn on a US bank in US dollars and made payable to Woods Hole Oceanographic Institution. When sending change of address, please include mailing label. Claims for missing numbers from the US will be honored within three months of publication; overseas, six months. Permission to photocopy for internal or personal use or the internal or personal use of specific clients is granted by Oceanus to libraries and other users registered with the Copyright Clearance Center (CCC), provided that the base fee of $2 per copy of the article is paid directly to: CCC, 222 Rosewood Drive, Danvers, MA 01923 Special requests should be addressed to theOcomus editor Oceans & Climate Oceans & Climate 2 Tlie Ocean's Role in Climate & Climate Change By Michael S. McCartney If Rain Falls On the Ocean, Does It Make a Sound? 4 Fresh Water's Effect on Ocean Phenomena By Raymond W. Schmitt ALACE, PALACE, Slocum 6 A Dj'misrr of Free Floating Oceanographic Instruments By Raymond W. Schmitt Alpha, Bravo, Charlie... 9 Ocean Weather Ships 1940-1980 By Robertson P. Dinsmore A Century of N. Atlantic Data Indicates Interdecadal Change 11 Surface Temperature, Winds, & Ice in the North Atlantic By Clara Deser North Atlantic Oscillation 13 By Michael S. McCartney The Bermuda Station S 14 A Long-Running Oceanographic Show By Terrence M. loyce and Lynne Talley Sedimentary Record Yields Several Centuries of Data 16 Tlie Little Ice Age and Medieval Warm Period in the Sargiisso Sea By Lloyd D. Keigwin North Atlantic's Transformation Pipeline 19 It Chills and Redistributes Subtropical Water By Michael S. McCartney, Ruth G. Curry, and Hugo F. Bezdek Labrador Sea Water Carries Northern Climate Signal South 24 Subpolar Signals Appear Years Later at Bermuda By Ruth G. Curry and Michael S. McCartney Transient Tracers Track Ocean Climate Signals 29 By William I. lenkins and William M. Smethie, Jr. New Data on Deep Sea Turbulence 33 Shedding Light on Verticil/ Mixing By John M. Toole Computer Modelers Simulate Real and Potential Climate 36 Combining Equations and Data Pushes Computers' Limits By Rui Xin Huang and liayan Yang The El Nino/Southern Oscillation Phenomenon 39 Seeking /te "Trigger" and Working Toward Prediction By Lewis M. Rothstein and Dake Chen 1930 Editor: Vicky Cullen • Designer: Jim Canavan Woods Hole Oceanographic Institution Robert B. Gagosian, Director Frank V. Snyder, Chairman of the Board of Trustees lames M. Clark, President of the Corporation Robert D. Harrington, Jr., President of the Associates Woods Hole Oceanographic Institution is an Equal Employment Opportunity and Affirmative Action Employer Printed on recycled paper OCEANUS 90N 60N 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 90S 120E Annual surface temperature change in degrees Centigrade for the period 1975-1994 relative to 1955- 1974. This figure, prepared for the 1996 Intergovern- mental Panel on Climate Change, indicates that Earth's surface has been, on average, warmer (predomi- nating orange) over the past 20 years compared to the preceding 20 years. The cooler blue areas show, however, that the warming has not been universal. 60E 120E Our thanks to Senior Scientist Robert A. Weller for editorial assistance with this issue. Oceans & Climate The Ocean's Role In Climate & Climate Change Michael S. McCartney Senior Scientist, Physical Oceanography Department The past decade has brought rapid scientific progress in understanding the role of the ocean in climate and climate change. The ocean is involved in the climate system primarily because it stores heat, water, and carbon dioxide, moves them around on the earth, and exchanges these and other elements with the atmosphere. Three important premises of the oceans and cli- mate story are: • The ocean has a huge storage capacity for heat, water, and carbon dioxide compared to the atmosphere. • Global scale oceanic circulation transports heat, water, and carbon dioxide horizontally over large distances at rates comparable to atmospheric rates. • The ocean and atmosphere exchange as much heat, water, and carbon dioxide between them as each transports horizontally. The ocean and atmosphere are coupled — their "mean states," evolution, and variability are linked. Ocean currents are primarily a response to exchanges of momentum, heat, and water vapor between ocean and atmosphere, and the resulting ocean circulation stores, redistributes, and releases these and other properties. The at- mospheric part of this coupled system exhibits variability through shifts in intensity and loca- tion of pressure centers and pressure gradients, the storms that they spawn and steer, and the associated distributions of temperature and water content. Oceanic variability includes anomalies of sea surface temperature, salinity,* and sea ice, as well as of the internal distribution of heat and salt content, and changes in the patterns and intensities of oceanic circulation. These coupled ocean-atmosphere changes may impact the land through phases of drought and deluge, heat and cold, and storminess. One example of coupled ocean-atmosphere variability is the El Nino/Southern Oscillation or ENSO (see article on page 39). The appearance of warm water at the ocean's surface in the east- ern tropical Pacific off South America has a dra- *Many of this issue's articles discuss the physical properties of seawater. The density of seawater changes with tempera- ture (measured in °C), salinity (measured in parts per thousand or grams of salt per kilogram of water — typically given without units, such as simply 34.9), and pressure. The density of seawater (p) in kilograms per cubic meter is close to and slightly larger than 1,000 kilograms per cubic meter. "Potential density," (a), is the value of the relative density if the seawater is brought to the surface without ex- changing heat on its way up. This expression helps ocean- ographers understand the water column's stability. FALL/WINTER 1996 matic impact on weather and seasonal-to- interannual climate. Considerable effort has been dedicated to developing the ability to pre- dict ENSO, including deployment and mainte- nance of buoys and other observational systems in the tropical Pacific and sustained attention to improving models of ENSO. However, ENSO is but one of the mechanisms by which the ocean and atmosphere influence one another. Such coupling occurs on many time scales, even over centuries (see "Sedimentary Record" article on page 16). There is growing interest among the oceanographic community in developing a bet- ter understanding of the ocean's role in climate changes on decadal to centennial time scales, and many of the articles in this issue focus on such variability in the North Atlantic Ocean. There are, as yet, no continuing observations dedicated, as the observing network in the tropi- cal Pacific is to ENSO, to monitoring, under- standing, and predicting decadal climate variabil- ity involving ocean-atmosphere interaction. Our challenges are to learn from what observations and modeling have been done and to develop strategies for future work. Sustained observations allow scientists to detect climatic spatial patterns. For example, the figure opposite shows interdecadal change in land and sea surface temperatures. This figure is taken from the 1996 Intergovernmental Panel on Climate Change (IPCC) report, a huge effort of the international climate research community to assess Earth's climatic state every five years. The predominating orange indicates that the earth's surface has been, on average, warmer the past 20 years compared to the preceding 20 years. Signifi- cant blue areas, principally over the oceans, show that the warming has not occurred everywhere: Large areas of the subpolar North Atlantic are cold, sandwiched between warm northern North America and northern Eurasia, and the North Pa- cific is also cold, but with a subtropical emphasis rather than a subpolar emphasis. The figure above right puts a longer time perspec- tive on the warming by showing the hemispheric and global average tempera- ture over the past 135 years, the rough limit of useful sustained measurements. These curves show the overall global warming beginning with the industrial age, but note the roughly 60 year oscillation this century, par- 04 5 o.o i -02 ^ -0.4 -0.6 04 0.2 00 -02 -04 -0.6 Southern Hemisphere Globe 1860 1880 1900 1920 Year ticularly in the north- ern hemisphere, show- ing steeper warming trends 1910-1940/ 1945 and 1975-1995. Time series like these lie at the heart of controversies about global warming as a trend versus as a phase of some mode of "natural" climate variability. Continued sus- tained measurements of a broad array of climate indicators will eventually directly answer key questions: Is the steep temperature rise of the past 20 years the portent of a crisis: a rise that will continue through the next century and evolve into an increasingly major climate perturbation? Or is the steep rise "just" a phase of a natural oscillation of the climate system super- imposed on a less severe warming? Or is the entire warming trend of the past 135 years itself just the warming phase of a still longer natural oscillation? There is a preponderance of scientific judgement, as carefully compiled and described by the IPCC, that the answer will be somewhere between the first two possibilities, and that this is caused by human impact on the climate system. This issue of Oceania emphasizes the North Atlantic Ocean, but, to answer these scientific questions, we must also take on the challenges of filling in many sparsely sampled regions, build- ing on the ENSO work in the Pacific and decadal variability research in the North Atlantic, and working toward understanding on a global basis. 1940 1960 1980 2000 Hemispheric and global average tem- perature for the past 135 years. Scientists aboard R/V Knorr launch a rosette water sam- pler and conduc- tivity/temperature/ depth instrument. Much of the data discussed in this issue was collected by such equip- ment. Author McCartney is the fellow getting wet at top left. The Great Salinity Anomaly, a large, near-surface pool of fresher-than- usual water, was tracked as it trav- eled in the subpo- lar gyre currents from 1968 to 1982. If Rain Falls On the Ocean Does It Make a Sound? Fresh Water's Effect on Ocean Phenomena Raymond W. Schmitt Senior Scientist, Physical Oceanography As with similar questions about a tree in the forest or a grain of sand on the beach, it may be hard to imagine that a few inches of rain matters to the deep ocean. After all, the ocean's average depth is around 4 kilometers and only 1 to 5 centimeters of water are held in die atmosphere at any one time. But it does matter, in part because the ocean is salty. The effect of rain diluting the salts in the ocean (or evaporation concentrating them) can be greater than the effect of heating (or cooling) on the density of seawater. 50 40 It also matters because rainfall and evapora- tion are not evenly distributed across and among ocean basins — some regions continuously gain water while others continuously lose it. This leads to ocean current systems that can be sur- prisingly strong. The processes of evaporation and precipitation over the ocean are a major part of what is called "the global water cycle;" indeed, by all estimates, they dominate the water cycle over land by factors often to a hundred. The addition of just one percent of Atlantic rainfall to die Mississippi River basin would more than double its discharge to die Gulf of Mexico. As discussed previ- ously in Oceanus, our knowledge of the water cycle over the ocean is extremely poor (see the Spring 1992 issue). Yet we now realize that it is one of the most impor- tant components of the climate system. One of the significant pieces of evidence for this comes from a description of die "Great Salinity Anomaly" put together by Robert Dickson (Fisheries Laboratory, Suffolk, England) with other European ocean- ographers. The Great Salinity Anomaly (GSA) can be character- ized as a large, near- surface pool of fresher water that appeared off the east coast of Greenland in the late 1960s (see figure at 1 left). It was carried 5 around Greenland and 10° into the Labrador Sea FALL/WINTER 1996 34 from Lazier, 1980 _J I by the prevailing 35 ocean currents, in the counterclockwise cir- culation known as the subpolar gyre. It hov- Q ered off Newfound- 0^? land in 1971 -72 and £ was slowly carried 5 back toward Europe in *° the North Atlantic Current, which is an extension of the Gulf Stream. It then com- pleted its cycle and was back off the east coast of Greenland by 1964 1966 the early 1980s, though reduced in size and intensity by mixing with surrounding waters. The origin of the Great Salinity Anomaly is thought to lie in an unusu- ally large discharge of ice from the Arctic Ocean in 1967. Its climatic importance arises from the impact it had on ocean-atmosphere interaction in the areas it traversed. The GSA derives its climate punch from the strong effect of salinity on seawater density, with salty water being considerably denser than fresh water. That is, these northern waters normally experience strong cooling in the winter, which causes the surface water to sink and mix with deeper waters. This process, called deep convec- tion (see figure below), is a way for the ocean to Salinity in the Labrador Sea from Ocean Weather Station Bravo 1968 1970 1972 Normal Ocean in Winter large heat loss release heat to the atmosphere, heat that then helps to maintain a moderate winter climate for northern Europe. However, when the GSA passed through a region, the surface waters became so fresh and light that even strong cooling would not allow it to convert into the deeper waters. Thus, the deep water remained isolated from the atmosphere, which could not extract as much heat as usual from the ocean. The GSA acted as a sort of moving blanket, insulating different parts of the deep ocean from contact with the atmo- sphere as it moved around the gyre. Its impact in the Labrador Sea has been particularly well docu- mented (see "Labrador Sea" article on page 24). When the surface waters were isolated from deep waters, they became Ocean in Winter with Fresh Surface Anomaly Deep convection is a key component of the ocean's role in Earth's climate. Strong win- ter cooling of surface waters causes them to become denser than water below them, which allows them to sink and mix with deeper water. This process releases heat from the overturned water to the atmosphere and maintains northern Europe's moderate winter climate. The Great Salinity Anomaly interrupted this process as its pool of fresher water prevented convection. cooler. Changing sea surface temperature patterns can affect at- mospheric circulation, and may possibly rein- force a poorly under- stood, decades-long variation in North At- lantic meteorological conditions known as the North Atlantic Os- cillation (see box on page 13). For it is the ocean that contains the long-term memory of the climate system. By comparison, the atmo- sphere has hardly any thermal inertia. It is 3 difficult to imagine | how the atmosphere alone could develop a regular decadal oscilla- tion, but the advection of freshwater anomalies by the ocean circulation Salinity as a func- tion of time at 10 meters, 200 meters, and 1,000 meters depth as recorded at Ocean Weather Station Bravo (see map on page 10) in the Labrador Sea. Deep convection is possible when the salinity difference between shallow and deep water is small. This nor- mally occurs every winter. However, from 1968 to 1971, the presence of the fresh, shallow, Great Salinity Anomaly prevented deep convection. Unfortunately, Weather Station Bravo is no longer maintained. Scien- tists will need to use new technol- ogy like the PAL- ACE float (see Box overleaf) in order to reestablish such time series. Such data is essential for understanding the role of freshwater anomalies in the climate system. OCEANUS could be an important key to this climate puzzle. Unfortunately, we have no ready means of detecting freshwater pulses like the GSA. While surface temperature can be observed easily from space, surface salinity, so far, cannot. The salinity variations important for oceanography require high precision and accuracy, so there is no quick and inexpensive method of measurement. We have had to rely on careful analysis of sparse historical records from mostly random and unre- lated surveys gleaned from several nations to piece the GSA's story together. But how many other "near-great" salinity anomalies have we missed because the signal was not quite large enough? Is there a systematic way to monitor salinity so that we know years in advance of an- other GSA's approach? In addition to variability within an ocean ba- ALACE, PALACE, Slocum A Dynasty of Free Floating Oceanographic Instruments Autonomous diving floats have been developed by Doug Webb of Webb Research, Inc. in Falmouth, MA, in conjunction with Russ Davis of the Scripps Institution of Oceanography. The Profiling Au- tonomous LAgrangian Circulation Explorer (PALACE) is a free float that drifts with the currents at a selected depth, much like a weather balloon drifts with the winds. At preset time intervals (typically one or two weeks) it pumps up a small bladder with oil from an internal reser- voir, which increases its volume, but not its mass, and causes it to rise to the surface. On the way up it records temperature and salinity as a function of depth. Once at the surface it transmits the data to a satellite system that also determines its geo- graphical position. The drift at depth between fixes provides an esti- mate of the "Lagrangian" velocity at that time and place (as Doug Webb was photographed on a catwalk above a test tank used to put the Slocum glider through its paces, opposed to "Eulerian" measurements of the velocity past a fixed point. These names derive from 18th century mathematicians who originated these ways of looking at fluid flows). The basic technology of the float has been used for several years in the nonprofiling ALACE, which simply provides velocity information. Hundreds of ALACES have been successfully deployed in the Pacific and Indian Oceans. A program to release a large number of PALACES in the Atlantic is just getting underway. The use of the ALACE as a platform for salinity mea- surements is not without problems. The slow rising mo- tion, and low power available, limit the type of sensor that can be deployed. The problem of sensor drift due to biological fouling may be severe in some regions, and methods to prevent fouling are just being developed. However, because the float spends most of its life in a deep and climatically stable water mass, not subject to near-surface atmospheric variations, we should be able to compensate for any drifts. But the fact that these floats move around is something of a drawback if the objective is to monitor ocean tempera- ture and salinity. That is, in the long run, we would rather that they stayed put and measured the properties in one place. Such a task could be achieved if the float were capable of gliding _ horizontally and turning I as it rose. The horizontal ~\ displacement achieved - could be directed to maintain one position, with each excursion compensating for the drift caused by ocean currents. With Navy funding, Webb, Davis, and Breck Owens (WHOI) are currently working on such a gliding float (see photo). All these floats depend on batteries to power the elec- tronic sensors, the pump that varies ballast, and the trans- mitter that sends data to the satellite. The battery life is around two years, depending on the frequency of profil- ing and transmitting. One way to extend its life is to use the ocean's vertical temperature differences to run a simple heat engine. Doug Webb has another type of float FALLWINTER 1996 sin, we would like to understand the large dillc-i- ences in salt concentration among ocean basins, (see figure on next page ) For example, the Pacific Ocean is significantly fresher than the Atlantic and, because it is lighter, stands about halt a meter higher. This height difference drives the flow of Pacific water into the Arctic through the Bering Strait. The salinity difference between these two major oceans is thought to be caused by the transport of water vapor across Central America: The trade winds evaporate water from the surface of the Atlantic, carry it across Central America, and supply rainfall to the tropical Pa- cific. This water loss is the major cause of the Atlantic's greater saltiness and its propensity to form deep water. The extra rainfall on the Pacific makes it fresher and prevents deep convection. How does this atmospheric transport vary with with such a propulsion system. It uses a waxy material that expands when it melts at around 50 degrees, a tem- perature the float encounters at several hundred meters depth on each trip to and from the surface. This expan- sion is used to store energy to pump ballast when needed. Use of this "free" energy for propulsion reduces the load on the batteries and extends the life of the float. The ther- mal ballasting engine has been tested extensively in the lab and recently deployed off Bermuda in a nongliding float, where it performed over 120 depth cycles. Doug Webb's dream is to marry the thermal engine with the glider, and thus make a long-lived, roving (or station-keeping) autonomous profiler possible. Years ago he described the technical possibilities to the late Henry Stommel, who developed a vision of how such an instru- ment might be deployed in large numbers around the globe (see Oceanus, Winter 1989/90). They called the instrument Slocum, with the idea that it could circumnavi- gate the globe under its own power, like New Englander Joshua Slocum, the first solo sailor to perform that feat. The Internet could allow scien- tists to monitor Slocum data from their home laboratories around the world. If we deploy enough Slocums, their data should be as valuable for predicting global climate on seasonal to decadal time scales as satel- lites and weather balloons are for forecasting the daily weather. Indeed, one of Slocum's key attractions is that it is inexpensive enough to deploy in large numbers. Per-profile costs for both temperature and salinity are expected to be $50 or less, once a mature system is oper- ating— vastly cheaper than anything possible using ships. A globe-spanning array of 1,000 Slocums would cost less than a new ship, yet provide an unprecedented view into the internal workings of the global ocean. — Ray Schmitt MIT/WHOI loint Program student Steve Jayne holds an ALACE (Autonomous LAgrangian Circulation Explorer) float aboard R/V Knorr during a 1 995- 1 996 (yes, Christmas at sea) cruise for the World Ocean Circulation Experiment in the Indian Ocean. About 1 day at surface Recording temperature and salinity as it rises 1,000m Drifting 1 week During a data collection and reporting cycle, a PALACE (Profiling Autonomous LAgrangian Circu- lation Explorer) float drifts with the current at a programmed depth, rises every week or two by in- flating the external bladder (recording temperature and salinity profiles on the way up), spends a day at the surface transmitting data, then returns to drift at depth by deflating the bladder. The average surface salinity distribu- tion in the global ocean, as compiled from many indi- vidual ship mea- surements, mostly during this century. The figure also shows the approxi- mate coverage ob- tainable with an ar- ray of about 1,000 Slocums or PAL- ACES. These would resolve the large scale features of the salinity field and provide completely new information on its variability with time. The ar- ray would be an early warning sys- tem for the Great Salinity Anomalies of the future. time? Since salinity is a good indicator of the history of evaporation or precipitation, perhaps if we had sufficient data, we could see changes in the upper ocean salt content of the two oceans that reflect variations in atmospheric transports. How many years does it take for salinity anoma- lies in the tropical Atlantic to propagate to high- latitude convection regions and affect the sea- surface temperature there? What is the impact on the atmospheric circulation? These and other climate problems will con- tinue to perplex us until we make a serious at- tempt to monitor salinity on large space and time scales. One approach would be to maintain ships in certain places to sample the ocean continually. A modest effort along these lines was made after World War II when weather ships were main- tained at specific sites by several nations (see following article). The data they collected provide nearly the only long time-series measurements available from deep-ocean regions. However, the weather ships are all but gone; there is only one now, maintained seasonally by the Norwegians. Today's satellites provide information on ap- proaching storm systems, but, unfortunately, they cannot tell us what we need to know about ocean salinity distributions. It now appears that new technology will pro- vide the key to the salinity monitoring problem, at a surprisingly modest cost. The Box on pages 6 and 7 describes how we might obtain tempera- ture and salinity profiles from data collected by autonomous diving floats. It should be quite feasible to deploy an array of these station-keep- ing "Slocums" that would intercept and monitor the progress of the "Great Salinity Anomalies" of the future. In the next two years, a large number of profiling ALACE (precursor to the Slocum) floats will be deployed in the Atlantic in a pre- liminary test of the general concept. In addition to measuring temperature and salinity, Slocums might some day measure rain. It turns out that rain falling on the ocean does make a sound, and work is underway to record that sound with hy- drophones and develop algorithms to convert the measured sound level to rain rates.The remaining technical obstacles to development of a globe- spanning array of station-keeping Slocums are small. The only thing lacking is a strong societal commitment to the support of such fundamental research on the climate system of the earth. This research was sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration's Climate and Global Change Program. Mast of Ray Schmitt's career has been focused on very small- scale processes in the ocean related to mixing by turbulence and "salt fingers." Hoivevcr, he has been driven toward studies of the global-scale hydrologic cycle by a desire to contribute to im- proved weather and climate prediction, so that he can better plan to take advantage of the rare good weather in Woods Hole. ,-. FALL/WINTER 1996 Alpha, Bravo, Charlie Ocean Weather Ships 1940-1980 Robertson P. Dinsmore WHOI Marine Operations The ocean weather station idea originated in the early days of radio communications and trans-oceanic aviation. As early as 1921, the Director of the French Meteorological Service proposed establishing a stationary weather observing ship in the North Atlantic to benefit merchant shipping and the anticipated inauguration of trans-Atlantic air service. Up to then, temporary stations had been set up for special purposes such as the US Navy NC-4 trans- Atlantic flight in 1919 and the ill-fated Amelia Earhart Pacific flight in 1937. The loss of a PanAmerican aircraft in 1938 due to weather on a trans-Pacific flight prompted the Coast Guard and the Weather Bureau to begin tests of upper air observations using instru- mented balloons. Their success resulted in a recommendation by Commander E. H. Smith of the International Ice Patrol (and future Director of the Woods Hole Oceanographic Institution) for a network of ships in the Atlantic Ocean. World War II brought about a dramatic in- crease in trans-Atlantic air navigation, and in January 1940 President Roosevelt established the "Atlantic Weather Observation Service" using Coast Guard cutters and US Weather Bureau ob- servers. Most flights at this time were using south- ern routes. On February 10, 1940, the 327-foot cutters Bibb and Duane occupied Ocean Stations 1 and 2 — the forerunners of Stations D and E (see chart on next page). With the US entering the war, Coast Guard cutters were diverted to anti-submarine duties, and the weather stations were taken over by a motley assortment of vessels ranging from con- verted yachts to derelict freighters, mostly Coast Guard operated. As trans-Atlantic air traffic in- creased, so did the number of weather and plane guard stations. The role of weather during the Battle of Coral Sea and trans-Pacific flights re- sulted in stations being set up in that ocean also. At the service's peak, there were 22 Atlantic and 24 Pacific stations. At war's end, the Navy intended to discontinue weather ship operations, but pressure from sev- eral sources resulted instead in establishment of a permanent peacetime system of 13 stations. These are shown on the next page, with the posi- tions and operating nations listed in the accom- panying table. Costs of the program were shared by nations operating transoceanic aircraft. A typical weather patrol was 21 days on-sta- tion. A "station" was a 210-mile grid of 10-mile squares, each with alphabetic designations. The center square, which the ship usually occupied, was "OS" (for "on-station"). A radio beacon Coast Guard Cutter Sebago was photographed on Station A in lanuary 1949. OCEANUS Ocean Weather Stations 1 94O - 1 98O Sta. Position A 62W W; 33~J00' W 56"30'N;51'00'W 52"45' N; 3930' W WOO' N; 4100' W 35°00' N; 48 00' W 36°00' N; 7000' W 6VOO'N;15°00'W 52*30' N; 20*00' W 45=00' W; 76 00' W 66*00' N; 02*00' E B C D E H I ) K M Operator U.S. & Wet/i. U.S. U.S. U.S. U.S. U.S. U.K. U.K. France Norway PACIFIC Map shows the 13 permanent weather stations established in 1946 by the United Nations Civil Aviation Orga- nization. Program costs were shared by nations operat- ing transoceanic aircraft. Letters missing from the alphabetical se- quence were those used for stations occupied during World War II but not included in the postwar weather station program. Weather balloons were released from weather ships every six hours to gather data from eleva- tions as high as 50,000 feet. transmitted the ship's location. Overflying aircraft would check in with the ship and receive position, course and speed by radar tracking, and weather data. Surface weather observations were transmitted every three hours, and "upper airs" — from instrumented balloon data — every six hours. Using radiosonde transmitters and radar tracking, balloon observers obtained air temperature, humidity, pressure, and wind direc- tion and speed to elevations of 50,000 feet. Oceanographic observations were recom- mended for weather ships almost from the start. Beginning in 1945 and continuing to the end, US Sta. Position Operator N 30 N; 140 W U.S. P 50° N; 145" W Canada V 34 N; 164 f U.S. ships made bathyther- mograph (B/T) observa- tions that today consti- tute the largest B/T archive in existence. Many specific, short- term programs were carried out with ocean- ographers frequently riding the ships. In addition to serving as weather reporters and navigation aids, weather ships occasion- ally rescued downed aircraft and foundering ships. Dramatic weather station rescues include the Bermuda Sky Queen in 1947 (Station C), Pan-American 943 (Station N) in 1956, and SS Ambassador (Station E) in 1964. By 1 970, new jet aircraft were coming to rely less on fixed ocean stations, and satellites were beginning to provide weather data. In 1974, the Coast Guard announced plans to terminate the US stations, and, in 1977, the last weather ship was replaced by a newly developed buoy. The international program ended when the last ship departed Station M in 1981. Gipt. Dinsmore commanded the weather ship USCGC Cook Inlet. During /»'» 28-year Coast Guard career, he seri'ed on four North Atlantic weather ships and was weather ship program manager before joining the WHO/ Staff in 1971. This article is e\erpted from a text about twice this length. Interested readers may request the longer account from the Oceanus office by calling 508-289-3516 (email: oceanusmag@whoi.edu). FALL/WINTER 1996 A Century of North Atlantic Data Indicates Interdecadal Change Surface Temperature, Winds, & Ice in the North Atlantic Clara Deser Research Associate, University of Colorado For hundreds of years mariners have re- corded the weather over the world ocean. Some 100 million marine weather reports have accumulated worldwide since 1854, when an international system for the collection of meteorogical data over the oceans was estab- lished. These reports include measurements of sea surface temperature, air temperature, wind, cloudiness, and barometric pressure. In the 1980s, the National Oceanic and Atmospheric Administration (NOAA) compiled these weather observations into a single, easily accessible digi- tal archive called the Comprehensive Ocean- Atmosphere Data Set. This important data set forms the basis for our empirical knowledge of the surface climate and its variability over the world's oceans: One example of a variable sys- tem is the phenomenon known as El Nino in the tropical Pacific (see article on page 39). A major challenge in climate research is to use these data to document and understand the role of the oceans in long-term — decadal and centennial — climate change. The figure at right shows the geographical distribution of weather observations over the oceans for three periods: 1880-1900, 1920- 1940, and 1960-1980. Before the turn of the century, marine weather reports were largely restricted to shipping lanes in the North Atlantic and western South Atlantic. The North Pacific was not well sampled until after World War II, and the tropical oceans not until after about 1960; the southern oceans are still largely un- measured. Due to the irregular sampling, we focus on describing climate variations over the North Atlantic back to the turn of the century. Fortunately, the North Atlantic plays an impor- tant role in world-ocean circulation. Two parameters are of key importance to the physical interaction between ocean and atmo- sphere: sea-surface temperature and near-surface wind. They control the rates of heat and momen- tum transfer between the two media. The top figure on page 12 displays the long-term average distributions of sea-surface temperature and near-surface wind over the North Atlantic. These charts are based upon all available observations since 1900. The prevailing westerly winds or "westerlies" are a well-known feature of the wind distribution. Sea surface temperatures are gener- ally warmer in the East Atlantic than in the West Atlantic at the same latitude, reflecting the mod- erating influence of the Gulf Stream. How have the wind and temperature distribu- tions changed with time? A statistical technique called empirical orthogonal function analysis aids in identifying regions of coherent temporal Geographical dis- tribution of weather reports over the world. Colored areas show the average number of weather reports per month in each 2° latitude by 2" longitude square over the world oceans for each of the time periods indicated. White areas indi- cate there are no reports. OCEANUS Average distribu- tions of sea surface temperature ("C) (top) and near sur- face wind climatol- ogy (bottom) over the North Atlantic since 1900. The longest wind arrow corresponds to 8 meters per second. change. The results of the statistical analysis point to the area directly south and east of Newfound- land as a site of pronounced sea surface tempera- ture variability. The figure below shows the his- tory of sea surface temperatures in this region since 1900. There is a notable tendency for cold and warm periods to be spaced approximately one decade apart, as well as longer-term warming and cooling trends that span several decades. When the near-surface wind field is analyzed in a similar manner (but independently from the sea surface temperatures), similar decadal-scale oscilla- tions and longer term trends are evident. As noted in the box on the opposite page, this basin scale pattern of \ variability has been | labeled the North At- - lantic Oscillation. What is the nature of these decadal and multi-decadal fluctua- tions? Are they surface signatures of oscilla- tions inherent to the deep ocean circulation? Are they global or confined to the North Atlantic? What is the role of the atmosphere? There is mounting evidence from mathematical models that the North Atlantic Ocean's thermohaline (heat and density driven) circulation may be- have as a damped oscillatory system at decadal- to-multidecadal frequencies, with the atmo- sphere supplying the energy to maintain the oscillations against dissipation. In order to test the relevance of hypotheses generated from the modeling work, further description of the ob- served climate record is needed. A composite picture of the decadal-scale varia- tions can be formed by averaging all of the cold (or warm) periods from the figure below left and subtracting the long-term mean. The figure di- rectly below shows such an "anomaly" compos- ite of the cold events. When sea surface tempera- tures to the east of Newfoundland are colder than normal, the near-surface westerly winds are stronger than normal. This relationship may be indicative of positive feedback between atmo- sphere and ocean: Stronger winds cool the ocean surface by enhancing evaporation and heat loss, while colder surface temperatures shift the lati- tude of the storm track and prevailing westerlies southward. Thus, the decadal swings in wind and temperature may be a manifestation of a coupled air-sea interaction process, in line with recent modeling results. What determines the time scale of the fluctuations, as well as their amplitude, are unsolved issues at this time. The decadal fluctuations in sea surface tem- perature show an intriguing relation to the amount of sea ice in the Labrador Sea, as the top figure opposite shows. While information on sea ice dates back only to 1953, it is evident that each of the decadal swings of colder-than-nor- mal temperatures was preceded by a period of greater-than-normal amounts of sea ice. The mechanism for this association is not well un- derstood, although it is plausible that the cold, stable water mass resulting from melting ice could be carried by ocean currents into the re- i surface temperatures for the region ind east of Newfoundland since •''•p.irtures from normal in de- i il curve is a low-pass filtered version I, curve, emphasizing fluctua- tions li i .1 few years. Composite ot decadal-scale cold events in the North Atlantic using sea surface temperature and wind anomaly patterns since 1900 Blue (red) contours indi- cate colder (warmer) than normal sea surface tempera- tures. The longest wind arrow is 1 meter per second. FALLWINTSR1996 gion east of Newfoundland. Some researchers have hypothesized a complex feedback loop involving Arctic precipitation, runoff, salinity, and ocean circulation to explain the decadal- scale sea ice variations. The lack of understanding of observed, long- term climate events in the North Atlantic under- scores the need for further research, particularly in relating the deep ocean circulation to the surface conditions. The work described by Michael McCartney, Ruth Curry, and Hugo Bezdek beginning on page 19 is one important step in this direction. This research was funded by a grant from the Atlantic Climate Change Program of the National Oceanic and Atmospheric Administration. Clara Deser ii'as introduced to oceanography in l')res and thermohaline emulation. When he was in high school, his dream was to become an inventor like Edison. Through a long and winding road, he came to Woods Hole, and found oceanography an exciting field. He also likes swimming, gardening, and, above all, pilules and games. During his graduate student and postdoctoral years, liayan Yang was interested mainly in tropical air-sea interaction. He decided to take "a short break" away from the tropics to do a small, high-latitude oceanography project when he was a postdoctoral fellow at the University of California, Los Angeles. He has stayed in high-latitude oceanography eivr since. He moved from Los Angeles to Neu* England to get a bit closer to (though still far way from) sea-ice margins. In his leisure time, he likes hiking, swimming, biking, and karaoke. 38 • FALL/WINTER 1996 The El Nino/Southern Oscillation Phenomenon Seeking Its"Trigger" and Working Toward Prediction Lewis M. Rothstein Associate Professor, Graduate School of Oceanography, University of Rhode Island Dake Chen Research Scientist, Lamont-Doherty Earth Observatory, Columbia University The El Nino/Southern Oscillation (ENSO) phenomenon, an eastward shift of warm water in the tropical Pacific and associated effects on the atmosphere, is at the heart of glo- bal interannual climate variability. The just com- pleted, decade-long Tropical Ocean/Global At- mosphere (TOGA) program was dedicated to understanding and working toward predicting ENSO by bringing together oceanographers and atmospheric scientists in a coordinated observa- tional and numerical modeling research pro- gram. TOGA has not answered all the questions: We have not uncovered the physical mechanisms of the elusive ENSO "trigger" nor have our best coupled air/sea numerical models been as suc- cessful in predicting the rather irregular ENSO signal of the 1990s as uhey were in predicting the regular events of the 1 980s and hindcasting the events of the late 1960s through the 1970s. Prediction is the ultimate goal of ENSO re- search. It is also the ultimate test for an ENSO model and the theory underlying the model. During the last decade, a number of forecast models have shown predictive skills in both ret- rospective and real time forecasting, and they are now being used for routine ENSO prediction. Nevertheless, the skill of even the best available models is far from perfect, and there is still con- siderable room for improvement in modeling, observation, and forecasting techniques. Factors that limit the current skill of ENSO forecasts include: • an inherent limit to predictability because of the chaotic and random nature of the natural system, • model flaws such as oversimplified physics, • gaps in the observing system, and • flaws in the way the data is used (data assimila- tion and initialization). It seems likely that the inherent predictability limit for ENSO is years rather than weeks or months, though more theoretical study is needed in this area. The observing system is improving, but still far from satisfactory. Thus a challenge facing the modelers is to improve model forecasts by making the most reasonable and efficient use of available data. In the past few years much effort has been devoted to assimilating various observational data into die initial state of forecast models. The most common approach is to improve die ini- tial ocean conditions by assimilating observa- tions of sea surface temperature, thermocline (region of rapid temperature decline) deptJi, or sea level into an ocean model prior to coupling it with an atmosphere model. One problem Time series of ob- served (red) and forecast (blue) El Nino sea surface temperature anomalies. Fore- casts with 0, 6, 12, and 18 month leads are shown in different panels, and the observed anomalies are re- peated from panel to panel. 13 O c I 2 I •E I1"!"1!"1!"1!"1!1"!1"!1"!1"!"1!1"!"1!" 0 Month Lead I"'!'"!'"!"'!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"!'"! 72 74 76 78 '80 '82 '84 '86 '88 '90 '92 '94 '96 Year OCEANUS with this approach is that no attention is paid to the ocean-atmosphere interaction during initialization, so the coupled system may not be well balanced initially and may experience a shock when the forecast starts. A new initializa- tion/assimilation procedure significantly im- proves the predictive skill of one of our most promising coupled models, which was con- structed by Mark Cane and Steve Zebiak (Lamont-Doherty Earth Observatory). In the new methodology the model is initial- ized in a coupled manner, using a simple data assimilation scheme in which the coupled model wind stress anomalies are "nudged" toward ob- 120 E This diagram illus- trates weather pathways in the North Pacific sub- tropical/tropical upper ocean and the main horizon- tal gyres and me- ridional-vertical cells of the region's ocean circulation. overpredicted, and the short warm episodes in 1993 and late 1994 are missed. Although the predictive skill of this model is most likely limited by its highly reduced physics, the skill of more sophisticated coupled ocean- atmosphere general circulation models presently does not exceed that of the model described above, at least in terms of the tropical Pacific sea surface temperature. In order to predict the glo- bal impact of ENSO, a two-tiered approach appears to be reasonable: A physics-simplified, coupled model is first used to predict tropical sea surface temperature fields, and these fields are then used as boundary conditions for a more- complete-physics, global atmospheric XModel..KurOShi0.... general circulation model to predict the global distribution of atmospheric disturbances. Scientists are rigorously pursuing this kind of research. A second area of progress concerns improved understanding of the cou- pling between different depths and different regions of the ocean. A popular ENSO paradigm that emerged in the late 1980s was based on the a observed, rather regular rythms of f ENSO conditions during a span of 25 = years before 1990. The "delayed oscil- ~ lator" mechanism emphasizes east- ward-propagating equatorial wave processes, * westward-propagating off- equatorial signals, and their asymmetric reflections at eastern and western bound- aries respectively. Despite the irregularities in the 1990s ENSO, this wave propagation/ reflection paradigm is still compelling; it can accomodate irregularities in the ENSO signal by combining the tropical signal with longer-term variability in the subtropics. A number of studies have sought to under- stand how tropical variability is linked to the mid latitudes. Ocean circulation may provide the links via several different pathways that are sum- marized schematically above. These are not simple, direct north-south flows; the existence of vigorous zonal current systems complicate the picture. In the upper layers of the ocean, up- welled waters along the equator flow into the subtropics, mainly through the mid-latitude western boundary current (the Kuroshio). There is an additional interior ocean pathway, through the eastward Subtropical Countercurrent, that more directly feeds subtropical sites where sur- *Flow along the equator tends to be trapped there. The Coriolis force, due to the earth's rotation, turns water that flows south back to the north and water that flows north back to the south. Because of this trapping, physical ocean- ographers recognize the equator as a waveguide, where co- herent signals or waves can be seen to propagate east-west for long distances. Model Eastward Subtropical Countercurrent Re-circulating Tropical Gyre 300m servations. The new procedure improves the model's predictive ability as measured by a vari- ety of statistical scores. It also eliminates the so- called "spring prediction barrier," a marked drop of skill in forecasts that try to predict across the boreal spring, lound in many previous ENSO forecast systems. The success of the new initial- ization procedure is attributed to its explicit consideration of ocean-atmosphere coupling, and the associated reduction of initialization shock and random noise. As an example, the forecasts made by the im- proved model are compared to observations in the figure on page 39 in terms of the sea surface temperature anomaly averaged over an area in the eastern/central equatorial Pacific (5° S to 5° N and 90° W to 150° W). The model is capable of forecasting ENSO more than one year in advance. The large warming and cooling events in the 1980s are particularly well predicted. However, the model does a poorer job for the 1970s and 1990s: The 1976-77 event is largely 40 « FALL/WINTER 1996 face water moves deeper into the ocean. These interior pathways are associated with a recirculat- ing tropical gyre in and just helow the mixed layer in the northeastern tropics. Below the mixed layer, thermocline water from the suhtropics to the tropics zigzags almost zonally across the basin, succeeding in flowing toward the equator only along zonally narrow, southward flowing conduits. The low-latitude western boundary currents serve as the main southward circuit for the subducted (water moving from the surface to depth), subtropical thermocline water. A model constructed by the authors also indi- cates important direct flow of thermocline water through the ocean interior, confined to the far western Pacific (away from the low-latitude west- ern boundary currents) along 10° N. These south- ward flowing waters are then swept eastward by the North Equatorial Countercurrent, finally penetrating to the equator in the central and eastern Pacific. The water pathways in the sub- tropical thermocline essentially reflect the surface gyre circulation. Along with our colleagues Ronghua Zhang (University of Rhode Island) and Antonio I. Busalacchi (NASA Goddard Space Flight Center), we have examined the interannual variability of these subtropical/tropical pathways and found important propagating subsurface ENSO signa- tures in the subtropical Pacific. There appears to be continual movement of subsurface, basin-scale anomalies that can then affect sea surface tem- perature (SST) anomalies, especially in sensitive regions where the thermocline is shallow. These SST anomalies can then trigger coupled air/sea interactions. A clear pattern of moving anomalies is less obvious at the sea surface. The systematic subsurface propagation is reminiscent of the delayed oscillator: eastward along the equator, westward off the equator with apparent further propagation along the eastern and western boundaries. Off the equator, subsurface propaga- tion of anomaly patterns initiates an SST anomaly in the North Equatorial Countercurrent regions of the western Pacific, which then intensi- fies and moves into the equatorial waveguide, consistent with the mean water pathways found above. We speculate that this could be a mecha- nism for initiating coupled, air-sea interactions that can begin to evolve as an ENSO event. The cycling time of the subsurface anomaly patterns may determine the ENSO's frequency. We look forward to continuing our investigations to so- lidify these assertions. One challenge for the newly established Cli- mate Variability (CLIVAR) program will be to uncover the ENSO triggering mechanism and enable intelligent design of a long-term ocean and atmosphere monitoring system. CLIVAR is MBI. WHOI LIBRARY the oceanographic and atmospheric scientific community's new program of climate prediction. Its focus is on understanding the coupled air/sea system's variability on seasonal-to-interannual- to-interdecadal time scales for the purpose of determining predictability, and then prediction. Those observations would then feed into coupled air/sea numerical models for the purpose of long lead time forecasting, much like the present-day weather forcasting systems. However, the interannual ENSO signal does not exhibit a simple ryhthm; there are clearly influences on longer (decadal) time scales that need to be con- sidered. There are clues as to what those signals might be (for example, the North Atlantic Oscil- lation— see page 13), but we are still in the early stages of identifying these signals. The natural system is not easily divided ac- cording to time scales; it is a fully nonlinear sys- tem. If we are to understand and eventually pre- dict global interannual variability, we must not limit ourselves to monitoring the air/sea system over a few interannual cycles. Permanent moni- toring systems are needed. It is the primary charge of CLIVAR to help design such a monitor- ing system while, at the same time, supporting the evolution of the numerical prediction systems that will issue the forecasts. The authors' ENSO research is supported by the National Oceanic and Atmospheric Administration, the TOGA Program on Seasonal to Interannual Prediction, and the National Aeronautics and Space Administration Lew Rothstein started his career as a physical oceanographer on the beautiful campus of the University of Hawaii. He is now a professor at the University of Rhode Island and an editor of the Journal of Geophysical Research. Dake Chen is also a physi- cal oceanographer by training. He worked with Lew on various tropical ocean models at the University of Rhode Island before he joined the senior staff of the Lamont-Doherty Earth Observa- tory last summer. Both of them are fond of building computer models of the ocean and atmosphere, not only for scientific research but also for fun. Servicing an Auton- omous Tempera- ture Line Acquisi- tion System (ATLAS) mooring of the Tropical Ocean Global At- mosphere (TOGA) program's Tropical Atmosphere-Ocean (TAO) Array in the Pacific Ocean. ATLAS moorings measure surface winds, air tempera- ture, relative hu- midity, sea surface temperature, and subsurface temper- ature to depths of 500 meters. OCEANUS 1930 Woods Hole Oceanographic Institution Woods Hole, MA 02543 508-457-2000