North Atlantic's Transformation Pipeline Chills and Redistributes

The pathways as- 60˚E sociated with the 30˚E transformation of 60˚W warm subtropical waters into colder 30˚W subpolar and polar waters in the northern North Atlantic. 80˚N Along the subpolar gyre pathway the red to yellow transi- tion indicates the cooling to Labrador Sea Water, which R GREENLAND flows back to the Greenland subtropical gyre Sea in the west as an intermediate depth Labrador 70˚N current (yellow). In L A B R A D O Sea the Norwegian and N Greenland Seas the EW FO U N red to blue/purple D LA N Norwegian D transitions indicate Sea the transformation t to a variety of cold- n e er waters that spill rr u southwards across C n the shallow ridge ia eg 60˚N system connecting rw northern Europe, No Iceland, Greenland, and northern North America. These No rth overflows form up A into a deep current tlant 40˚N ic Current IRELAND also flowing back 50˚N to the subtropics Jack Cook (purple), but beneath the Labrador Sea Water. The green pathway also indicates cold waters—but so North Atlantic’s influenced by continental runnoff as to remain light and Transformation Pipeline Chills near the sea surface on the continental and Redistributes Subtropical Water shelf. But It’s Not A Smooth Process And It Mightily Affects Climate able oceanic heat to the atmosphere. This is the Michael S. McCartney Senior Scientist, Physical Oceanography Department first stage of “warm water transformation” within the North Atlantic, a process that culminates in the Ruth G. Curry Research Associate, Physical Oceanography Department high latitude production of cold and fresh waters that return to the South Atlantic in deep reaching Hugo F. Bezdek Director, Atlantic Oceanographic and Meteorological Labora- currents beneath the warm waters of the subtropics tory, National Oceanic and Atmospheric and tropics. Administration This article focuses on the part of this warm wa- arm and salty waters from the upper ter transformation that occurs northwards of about part of the South Atlantic flow north- 45°N, the subpolar realm of the North Atlantic. Here ward across the equator and then prog- the warm waters brought to the area by the Gulf W Stream flow eastward across the basin and then ress through the tropical and subtropical North Atlantic to reach high latitudes. Beginning with the sweep northwards in the eastern Atlantic, continu- intense northward flow of the Gulf Stream off the ing to cool, and freshening as precipitation and East Coast of the United States, these waters are continental runoff exceed evaporation. This trans- exposed to vigorous cooling, liberating consider- formation occurs along two distinct pathways. The OCEANUS ◆ 19 thickens progressively through consecutive annual reexposures to the atmosphere as it simultaneously Greenland flows counterclockwise around the subpolar gyre. In winter data the seasonally exposed mode waters form a smoothly varying ring of progressively colder and more deeply convecting waters (see Irminger Basin figure at left). Estimates of flow speeds in the subpolar gyre suggest a transit time of about a decade Labrador for a parcel of water that enters the transformation Basin pipeline east of Newfoundland with a temperature of 12° to 14°C, travels counterclockwise, and emerges from the pipeline in the Labrador Basin at temperatures colder than 4°C. The figure below left illustrates the great thickness of the regional con- vection in this mode water ring with temperature/ depth profiles from the Newfoundland Basin where Newfoundland the ring begins, west of Ireland in the northward Basin flow of the eastern subpolar gyre, in the Irminger Basin east of Greenland where flow is westward, and in the Labrador Basin where the mode water ring ends. The process of heat liberation from ocean to atmosphere by water flowing along the pipeline acts Temperature (oC) of Norwegian Current carries part of the warm water as a regional radiator, particularly for northern Eu- the deep mixed lay- flowing northward past Ireland into the Norwegian rope where the westerlies carry the heat extracted er near the end of winter in the areas and Greenland (Nordic) Seas, while the subpolar from the ocean. There is considerable evidence for where that depth gyre carries the rest westwards towards and past interdecadal variability in this climate process. Our exceeds 200 meters. Greenland to the Labrador Basin. first evidence comes from the Labrador Sea where It is based on hydro- The transformation from warm to colder water deep wintertime convective overturning consti- graphic survey data recorded from 1957 is a multi-year process: Wintertime winds cool the tutes the last stage of cooling along the transfor- to 1967. surface waters, causing them to convect or verti- mation pathway and vertically homogenizes the cally overturn and mix progressively more deeply water column to depths sometimes exceeding into the cooler waters beneath. This seasonal over- 2,000 meters, creating the so-called Labrador Sea turning creates large volumes of vertically ho- Water (coolest profile in figure below left). mogenized water, called mode waters. In summer, The following article discusses the history of the sun heats the surface waters, forming a cap of Labrador Sea Water (LSW). The figure on the op- warmer water that effectively isolates the mode wa- posite page shows the LSW temperature record ter from contact with the atmosphere. Surface cool- overplotted with a smoothed version of the North ing in the following winter removes the cap, and Atlantic Oscillation index, an expression of the rela- reexposes the mode water, which then undergoes tive strength of the atmospheric westerlies (see box another round of winter cooling in which it gains on page 13). The LSW temperature history shows more thickness. The mode water thus cools and a long period of warming from the 1930s to 1971 followed by cooling from 1972 to 1993, culminating Temperature (˚C) in the 1990s with the coldest, freshest, and thickest 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 LSW ever observed. This cooling trend, however, Irminger West was interrupted by a brief warming in the late Labrador Basin of 1970s and early 1980s. Thus the first piece of evi- Basin Ireland dence for climate change in the warm water trans- 500 formation system is that the system’s end product, Depth (meters) LSW, which in the figure opposite maps in winter 1962 as 3.3°C, shows interdecadal variability with Newfoundland 1000 temperatures as warm as 3.5°C in 1970, compared Basin to as cold as 3.1°C forty years earlier and as cold as 2.7°C twenty-three years later. Examples of the Several agents interact to produce LSW. The LSW variation of tem- 1500 within the Labrador Basin has a “residence time.” perature with depth The total volume of LSW is only partially replaced from the stations used for the each year through the addition of transformed figure above. 2000 warm water and export of LSW from the Labrador 20 ◆ FALL/WINTER 1996 Basin to the rest of the North Atlantic. To visualize temperature history shows provocative parallels: this, imagine the Labrador Basin as a jacuzzi. Water The post-World War II LSW warming trend occurs is supplied to the tub at some temperature and at while the warm SST anomaly travels along the some rate, is well stirred and mixed in the tub, and transformation pathway. The LSW cooling period that mix is drained away at a rate that matches the beginning in 1971 coincides with substantial cold supply rate. The residence time is the volume of the SST areas in the subpolar gyre—but that general tub divided by the rate of supply and is a measure coldness is interrupted by warmer SST anomalies of the time a given parcel of water spends in the in 1980–81, when the LSW cooling trend was also tub before going down the drain, and thus is the interrupted. Thus the second piece of evidence for average time the water in the tub loses heat to the climate change in the warm water transformation overlying colder air. The temperature of the mixed system: There are significant interdecadal winter water in the tub depends on the temperature his- SST anomalies in the subpolar gyre as a whole and tory of the warm water being fed into the tub, in particular moving along the warm water trans- the rate of that water supply, the residence time, and the rate of heat loss from the tub to the air 3.5 C) above it. LSW temperature similarly depends first ° ( on the temperature history of the product emerg- ing from the warm water transformation pipeline 3.0 into the Labrador Basin plus the rate of that flow, and second on the history of the heat exchange between the Labrador Basin waters and the overly- Temperature 2.5 ing atmosphere and surrounding ocean (sort of 1930 1940 1950 1960 1970 1980 1990 an uninsulated tub!). Other factors can influence such histories: Ice atop the jacuzzi alters how its tub The history of tem- temperature evolves; similarly, sea ice and upper formation pathway. Periods of relatively warm SST perature (circles and black line) of the ocean freshwater circulation can modify convective anomalies along the transformation pipeline corre- Labrador Sea Water cooling in the Labrador Basin. spond to periods when LSW warmed, and periods convecting in the The second kind of evidence for climate change of relatively cold SST anomalies to periods when central Labrador Ba- signals in the northern North Atlantic comes from LSW cooled. These observations suggest that the sin to depths sometimes exceeding sea surface temperature (SST) data.

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