If Rain Falls on the Ocean— Does It Make a Sound? Fresh Water’S Effect on Ocean Phenomena

If Rain Falls On the Ocean— Does It Make a Sound? Fresh Water’s Effect on Ocean Phenomena Raymond W. Schmitt It also matters because rainfall and evapora- Senior Scientist, Physical Oceanography tion are not evenly distributed across and among s with similar questions about a tree in the ocean basins—some regions continuously gain forest or a grain of sand on the beach, it water while others continuously lose it. This leads Amay be hard to imagine that a few inches of to ocean current systems that can be surprisingly rain matters to the deep ocean. After all, the ocean’s strong. The processes of evaporation and precipi- average depth is around 4 kilometers and only 1 to tation over the ocean are a major part of what is 5 centimeters of water are held in the atmosphere called “the global water cycle;” indeed, by all esti- at any one time. But it does matter, in part because mates, they dominate the water cycle over land by the ocean is salty. The effect of rain diluting the salts factors of ten to a hundred. The addition of just one in the ocean (or evaporation concentrating them) percent of Atlantic rainfall to the Mississippi River can be greater than the effect of heating (or cool- basin would more than double its discharge to the ing) on the density of seawater. Gulf of Mexico. As discussed previ- 70˚ ously in Oceanus, our 1979 1978 knowledge of the water 1979 cycle over the ocean is 1981- extremely poor (see the 1982 Spring 1992 issue). Yet Greenland Norway we now realize that it is 1968 one of the most impor- 1977 1978 tant components of the climate system. One of 60 ˚ the significant pieces of 1969- Iceland evidence for this comes 1970 1976 1976 from a description of the 1978 “Great Salinity Anomaly” put together by Robert 1975 Dickson (Fisheries Labo- Canada 1979 ratory, Suffolk, England) 50˚ with other European oceanographers. The 1971- Great Salinity Anomaly 1972 1974 (GSA) can be character- ized as a large, near- surface pool of fresher water that appeared The Great Salinity Spain off the east coast of Anomaly, a large, 40˚ near-surface pool of Greenland in the late fresher-than-usual 1977 1960s (see figure at left). water, was tracked It was carried around as it traveled in the Greenland and into the subpolar gyre currents from 1968 to Labrador Sea by the pre- 1982. 50˚ 40˚ 30˚ 20˚ 10˚ vailing ocean currents, 4 ◆ FALL/WINTER 1996 in the counterclockwise 35 1,000 meters Salinity as a func- circulation known as tion of time at 10 meters, 200 meters, the subpolar gyre. It 200 meters and 1,000 meters hovered off Newfound- depth as recorded land in 1971–72 and at Ocean Weather Station Bravo (see 00 was slowly carried back / 0 map on page 10) in toward Europe in the 10 meters the Labrador Sea. North Atlantic Current, Deep convection is which is an extension of Salinity possible when the the Gulf Stream. It then salinity difference between shallow completed its cycle and Salinity in the Labrador Sea and deep water is was back off the east from Ocean Weather Station Bravo small. This normally occurs every win- coast of Greenland by 34 the early 1980s, though ter. However, from from Lazier, 1980 1968 to 1971, the reduced in size and presence of the intensity by mixing with 1964 1966 1968 1970 1972 fresh, shallow, Great surrounding waters. Salinity Anomaly prevented deep The origin of the Great Salinity Anomaly is thought However, when the GSA passed through a region, convection. Unfor- to lie in an unusually large discharge of ice from the surface waters became so fresh and light that tunately, Weather the Arctic Ocean in 1967. Its climatic importance even strong cooling would not allow it to convect Station Bravo is no arises from the impact it had on ocean–atmo- into the deeper waters. Thus, the deep water re- longer maintained. Scientists will need sphere interaction in the areas it traversed. mained isolated from the atmosphere, which could to use new technol- The GSA derives its climate punch from the not extract as much heat as usual from the ocean. ogy like the PAL- strong effect of salinity on seawater density, with The GSA acted as a sort of moving blanket, insulat- ACE float (see Box salty water being considerably denser than fresh ing different parts of the deep ocean from contact overleaf) in order to reestablish such water. That is, these northern waters normally ex- with the atmosphere as it moved around the gyre. time series. Such perience strong cooling in the winter, which causes Its impact in the Labrador Sea has been particularly data is essential for the surface water to sink and mix with deeper well documented (see “Labrador Sea” article on understanding the waters. This process, called deep convection (see page 24). When the surface waters were isolated role of freshwater anomalies in the figure below), is a way for the ocean to release heat from deep waters, they became cooler. Changing climate system. to the atmosphere, heat that then helps to main- sea surface temperature patterns can affect at- tain a moderate winter climate for northern Europe. mospheric circulation, and may possibly reinforce a poorly understood, Normal Ocean in Winter Ocean in Winter with decades-long variation Fresh Surface Anomaly in North Atlantic me- teorological conditions cold known as the North air large small Atlantic Oscillation (see heat loss heat loss box on page 13). For it is the ocean that contains surface the long-term memory of the climate system. By comparison, the cold fresh water atmosphere has hardly any thermal inertia. It is deep difficult to imagine how warm no the atmosphere alone deep convection could develop a regular water decadal oscillation, but the advection of fresh- convection water anomalies by the ocean circulation could 1,000m be an important key to this climate puzzle. Deep convection is a key component of the ocean’s role in Earth’s climate. Strong winter Unfortunately, we cooling of surface waters causes them to become denser than water below them, which have no ready means allows them to sink and mix with deeper water. This process releases heat from the of detecting freshwater 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 pulses like the GSA. prevented convection. While surface tempera- OCEANUS ◆ 5 ture can be observed easily from space, surface anomalies have we missed because the signal was salinity, so far, cannot. The salinity variations im- not quite large enough? Is there a systematic way portant for oceanography require high precision to monitor salinity so that we know years in ad- and accuracy, so there is no quick and inexpensive vance of another GSA’s approach? method of measurement. We have had to rely on In addition to variability within an ocean basin, careful analysis of sparse historical records from we would like to understand the large differences mostly random and unrelated surveys gleaned in salt concentration among ocean basins. (see fig- from several nations to piece the GSA’s story to- ure on next page ) For example, the Pacific Ocean is gether. But how many other “near-great” salinity significantly fresher than the Atlantic and, because ALACE, PALACE, Slocum A Dynasty of Free Floating Oceanographic Instruments utonomous diving floats have been developed by low power available, limit the type of sensor that can be Doug Webb of Webb Research, Inc. in Falmouth, deployed. The problem of sensor drift due to biological foul- AMA, in conjunction with Russ Davis of the Scripps ing may be severe in some regions, and methods to prevent Institution of Oceanography. The Profiling Autonomous fouling are just being developed. However, because the float LAgrangian Circulation Explorer (PALACE) is a free float spends most of its life in a deep and climatically stable water that drifts with the currents at a selected depth, much like mass, not subject to near-surface atmospheric variations, we a weather balloon drifts with the winds. At preset time should be able to compensate for any drifts. intervals (typically one But the fact that these or two weeks) it pumps floats move around is up a small bladder with something of a drawback oil from an internal reser- if the objective is to moni- voir, which increases its tor ocean temperature volume, but not its mass, and salinity. That is, in the and causes it to rise to the long run, we would rather surface. On the way up it that they stayed put and records temperature and measured the proper- salinity as a function of ties in one place. Such a depth. Once at the sur- task could be achieved face it transmits the data if the float were capable to a satellite system that of gliding horizontally also determines its geo- and turning as it rose. graphical position. The The horizontal displace- drift at depth between ment achieved could fixes provides an estimate be directed to maintain of the “Lagrangian” veloc- one position, with each ity at that time and place excursion compensating (as opposed to “Eulerian” for the drift caused by measurements of the ve- ocean currents. With Navy locity past a fixed point. Doug Webb was photographed on a catwalk above a test tank used to put funding, Webb, Davis, and the Slocum glider through its paces. These names derive from Breck Owens (WHOI) are 18th century mathemati- currently working on such cians who originated these ways of looking at fluid flows).

Load more