Role of the Ocean Conveyor Belt as a Cause of Global Multidecadal Climate
William M. Gray
Colorado State University
Fort Collins, Colorado 80523
gray@climo.atmos.colostate.edu
1. INTRODUCTION
The global circulation of the atmosphere has been observed to experience multi-decadal variations. For example, during the mid-1940s through the late 1960s the general circulation functioned distinctly differently than it did during the subsequent quarter century between 1970-1994. Prominent differences between these two periods include the strength of the middle latitude westerly circulation patterns, the Pacific (PNA), the Atlantic (NAO), the strength of the Azores high, African Sahel rainfall, frequency of intense (category 3-4-5) Atlantic hurricanes, frequency and intensity of El Niño events, hemispheric scale north-south ocean SST differences, changes in mean global temperatures and numerous other features. Similar but less extreme multi-decadal variations occurred during 1870-1899 (similar to 1940-1960) versus 1900-1920 (similar to 1970-1994) versus 1921-1943 (properties intermediate between these two extremes). Recent trends in ocean SST patterns, salinity in the North Atlantic and many other global circulation features suggest that we may now be shifting to a climate era more typical of the mid-1940s to late 1960s.
It is hypothesized that these long period circulation differences are all related and are primarily due to variations in the strength of the ocean's global "thermohaline circulation" and, in particular, to the Atlantic circulation termed "conveyor belt". Salinity variations in the North Atlantic, such as occurred during the late 1960s [the so-called Great Salinity Anomaly (GSA)] are hypothesized to be the primary forcing function for such circulation changes between the 1940s-1960s versus 1970-1994. Saline water is more dense than fresh water. Greenland Ice Core measurements going back thousands of years indicate that the North Atlantic SST patterns typically fluctuate back and forth on 20-50 year time scales and appear to fit this hypothesis. Warmer than normal North Atlantic SSTs occur with a stronger Atlantic thermohaline circulation and colder SSTs with a weaker circulation. A faster thermohaline circulation causes larger amounts of warm upper ocean mass to be advected to higher latitudes. It is observed that over broad areas of the North Atlantic, SSTs are 0.5 to 1.0 degree C warmer during multi-decadal periods of strong versus weak thermohaline circulation (Figure 1). These back and forth shifts are hypothesized to result for natually occurring North Atlantic salinity variations on time scales of 20-50 years. Figure 2 shows North Atlantic SST anomalies over the last 100 years. Note the distinctive multi-decadal periods of warm and cold water anomaly. Figure 3 shows inferred smoothed multi-decadal variations of the Atlantic thermohaline circulation over the last 130 years.
A major decrease of surface layer salinity appeared over portions of the North Atlantic during the late 1960s. This freshening reduced North Atlantic Ocean water density and slowed the North Atlantic deep water formation process. This trend, in turn, lead to diminished northward heat transport by the ocean, broad-scale cooling of the North Atlantic Ocean surface temperatures and warming of SSTs in much of the South Atlantic and the tropical Indian and West Pacific Oceans sea surface temperatures. Schmitz (1996) estimates that, on average, there is about 14 Sverdrup (106m3 of water) of North Atlantic deep water formation. This is equal to an amount of ocean water mass flux per 20 years equivalent to an ocean volume of 5000 km x 5000 km x 350 meters deep -- a very large amount of ocean mass. At present, there appears to be no other physical mechanism as attractive as variations in the Atlantic thermohaline for explaining these many and related multi-decadal global climate trends.
There are no direct measures of the strength of the thermohaline circulation. Rather, it must be inferred from alterations of similarly timed atmosphere and ocean variables. These include North Atlantic salinity and SSTA, the NAO, strength of the Azores High, Sahel rainfall anomaly, intense Atlantic hurricanes, El Niño frequency, Southern vs. Northern Hemisphere SST along with many other concurrently varying climate characteristics. The variations of these parameters are hypothesized to have occurred as a result of the changing strength of the Atlantic conveyor circulation which was driven in turn by naturally occurring North Atlantic salinity variations on 20-50 year time scales.
2. DISCUSSION
A step-by-step physical interpretation of how linkages between variations in the ocean's thermohaline circulation and associated North Atlantic SST anomalies bring about multi-decadal global circulation changes is attempted as follows:
Salinity alterations in the North Atlantic are caused by variations in 1) fresh water (ice) flux from the Arctic, and by the 2) strength of the thermohaline circulation itself. The rate of sea particle transport through the Atlantic conveyor is slower when the thermohaline circulation is running slow and is shortened when it is running faster. The time available for Atlantic salinity particle buildup from positive evaporation minus precipitation (E-P) is thus greater for a slow than for a fast thermohaline circulation. This inverse association between the rate of salt buildup and thermohaline strength is part of the explanation for the thermohaline's back-and-forth multi-decadal variations over many thousands of years as inferred by recent Greenland ice-core measurements.
A strong multi-decadal thermohaline circulation leads to a general multi-decadal cooling of the tropical oceans. This association is a consequence of the sinking of North Atlantic Ocean water at temperatures of approximately 2 to 3oC while the mass balancing upwelling water in the tropical Indian Ocean and Pacific has been raised to temperatures of 25 to 28oC (Figure 4). For cold ocean deep water to upwell from bottom levels, it is necessary that this water be warmed by diffusion heating from its surrounding water masses. This is necessary so that the deep ocean water can become buoyant enough to rise from deep to upper levels. The energy to warm the ocean bottom water and make it buoyant enough to rise must come from the ocean itself. This buoyancy warming requirement of the tropical oceans causes a general overall energy consuming cooling over the broad regions where the upwelling occurs. The tropical oceans of the Eastern Hemisphere (EH) are the primary locations for upwelling water -- principally the tropical Indian Ocean.
A weak thermohaline circulation takes less energy out of Eastern Hemisphere oceans while a strong thermohaline circulation takes more energy out of the tropical Eastern Hemisphere waters. The physical linkage which causes variations of the thermohaline circulation of approximately ±2-4 Sverdrup (Sv) units (roughly 17 Sv when strong versus 11 Sv when weak) to bring about atmospheric-ocean circulation variations on multi-decadal time scales are hypothesized to occur in the following approximate manner. For an anomalously weak thermohaline circulation (due to reduced Atlantic salinity), there will be less warm ocean advection to the North Atlantic. This will have the following effects:
a. North Atlantic (50-60 oN, 10-50 oW) SST will become colder (Figure 2).
b. Atlantic middle-latitude westerly winds will become stronger to balance the increase of the North--South sea and air temperature gradients. The speed of the Atlantic air and ocean gyre will be increased. More cold ocean water mass will be advected to the southeast in the middle latitudes of the eastern Atlantic. This process lowers SSTs off coastal southwestern Europe. These colder eastern Atlantic temperatures will lead to a higher eastern Atlantic surface pressure (the Azores High will become stronger). A stronger Azores High will cause stronger trade wind flow and more upwelling off of the Northwest African coast. More cold-water upwelling will engender yet higher pressure and increase the Atlantic trade winds and a general reduction in sub-tropical Atlantic SSTs.
c. Higher surface pressure off the Northwest African coast leads to reduced southwesterly geostrophic winds over the Sahel and reduced summertime rainfall conditions.
d. Colder SSTs in the trade wind belts lead to stronger equator-to-subtropics air temperature gradients and stronger upper troposphere westerly winds and tropospheric vertical wind shear. Consequently, Atlantic hurricane activity is reduced.
e. Reduced sinking of North Atlantic 2-3 oC water leads to a smaller requirement for Asian tropical deep water warming to balance the buoyancy requirement of upwelling mass. These reduced ocean heating requirements lead to a more rapid buildup of the heat content in tropical Eastern Hemisphere. This greater heat buildup leads to an enhancement of El Niño's frequency and strength.
f. Less downwelling in the North Atlantic and, consequently, a smaller requirement to sustain upwelling leads to the Eastern Hemisphere tropical ocean becoming warmer. A warmer tropical ocean leads to a generalized tropical and a weak global warming. The hypothesized linkages between these physical processes are shown in Figure 5. By contrast, when the thermohaline circulation is running faster, the sign of the above described physical linkages are opposite.
