Can We Detect Changes In The Amoc Is The Amoc Changing Already

Previously, direct estimates of the vigour of the AMOC have been obtained from transatlantic hydrographic (density profile) sections, assuming the geo-strophic balance to hold on the ocean interior. Five such sections have been carried out along 24.5°N in the Atlantic over the last 50 years [50]. Taken together the five snapshots, each of which is assumed to be representative of the annual mean strength of the AMOC of the year in which they where taken (i.e. intra-seasonal variations assumed to be small), implies an AMOC slowdown of 30% (or 8 Sv) since 1957 [50]. Other measurements, focusing on single components of the AMOC gave rather inconclusive results regarding long-term AMOC changes. Using a combination of direct and indirect transport measurement techniques a gradual 1 2 Sv decrease in the amount of cold, dense inflow of deep waters from the Nordic Seas through the Faroe Bank Channel (feeding the NADW) has been found since 1970 [51], implying a long-term AMOC weakening. However, the continuation of the direct measurements showed an increase over the last few years back to the levels of the mid 1990s [52,53].

At the same time measurements in the Deep Labrador Current which represents a major pathway for the export of NADW from the deep water formation regions seems to have strengthened by 15% when comparing the 1996 1999 to 2000 2005 periods [54]. However, measurements in the DWBC further south off Grand Banks gave no significant change over roughly the same period [55]. It is uncertain how representative the strength of the DWBC off Grand Banks is, for the basin wide AMOC. Hydrographic measurements in the mid and high-latitude North Atlantic suggest that a substantial part of the southward export of NADW might be accomplished along a pathway in the ocean interior that feeds into the DWBC only in the subtropical North Atlantic [56]. Kanzow et al. [57] showed from observations and model simulations that fluctuations in the strength of the DWBC may not be a good indicator of AMOC changes in the tropical North Atlantic either, due to the presence of time-variable deep offshore recirculations.

A pilot system to measure the strength of the AMOC continuously at 26.5°N (i.e. the zonally integrated meridional transport profile between Florida and Morocco) has been operating since April 2004 [9,16,58]. Figure 5 shows a 1-year long time series of the AMOC between April 2004 and April 2005, exhibiting a time mean of 18.5 Sv and arms variability of ±5.6 Sv [16]. The range of values the AMOC assumed within one year spans roughly 30 Sv (varying between 5 and 35 Sv). The observed intra-seasonal variability raises concerns

Atlantic Meridional Overturning Circulation at 26.5°N

1 Apr 1Jun 1Aug 10ct 1Dec 1Feb lApr

Time from 29 March 2004 to 31 March 2005

FIGURE 5 Time series of the strength of the Atlantic meridional overturning at 26.5°N, based on the continuous transport measurements within the RAPID/MOCHAexperiment [16], defined as the vertical integral of the transport per unit depth down to the deepest northward velocity (~1100 m) on each day. It represents the sum of the Florida Current, Ekman and upper mid ocean transports [16]. This figure was published by Kanzow et al. [59].

1 Apr 1Jun 1Aug 10ct 1Dec 1Feb lApr

Time from 29 March 2004 to 31 March 2005

FIGURE 5 Time series of the strength of the Atlantic meridional overturning at 26.5°N, based on the continuous transport measurements within the RAPID/MOCHAexperiment [16], defined as the vertical integral of the transport per unit depth down to the deepest northward velocity (~1100 m) on each day. It represents the sum of the Florida Current, Ekman and upper mid ocean transports [16]. This figure was published by Kanzow et al. [59].

whether the hypothesised 30% slowdown of the AMOC [50] may represent aliasing effects (as a consequence of not resolving the large intra-seasonal variations) rather than a sustained change of the ocean circulation [16,59].

While oceanographers have not yet been able to document a statistically significant trend in the strength of the AMOC, it is worth asking, how much time it would take to detect a possible long-term trend from continuous measurements at 26.5°N. Making assumptions about the short term noise level of the AMOC, Baehr et al. [60] concluded from the analysis of an AMOC future projection, that a 0.75 Sv per decade decline could be detected after three decades. A more abrupt (than currently expected) AMOC change would be detectable earlier. The detectability could most likely be shortened significantly if several continuously observing AMOC monitoring system were operated simultaneously at different latitudes.

7. CONCLUSION

Observations have revealed that patterns of present-day regional and large-scale ocean circulation may display strong changes on intra-seasonal to multi-decadal time scales. Physical oceanographers have developed a variety of tools to quantify circulation changes, which involve direct and indirect measurement techniques and numerical simulations. While most of the documented present-day circulation changes are believed to fall within the class of natural (ocean climate) variability even at decadal and longer time scales, it is a non-trivial task to disentangle climate variability from presently possibly ongoing climate shifts.

The AMOC has been in the focus of climate change research. The interpretation of palaeo-climate records in the light of findings from numerical climate models reveals that the AMOC has undergone large changes in the Earth's past and that these went along with climate shifts in the North Atlantic sector and beyond. In the present day climate, the AMOC represents the major oceanic mechanism of meridional heat transport. The AMOC moves volumes of cold waters (having sunk at high latitudes) southward throughout the Atlantic at depth and keeps them out of contact with the atmosphere for centuries, until the waters rise to the upper ocean eventually. Thereby the AMOC ventilates the deep ocean with oxygen rich waters. The sinking of waters in the Nordic Seas and the Labrador Sea (push) and their eventual rising (pull) are necessary ingredients for the existence of the AMOC, both of which are thought to change in a changing climate.

Model projections imply that the AMOC might slow down between 0 and 50% by the end of the twenty-first century. This is thought to be due to an increase in vertical density stratification at high latitudes (both due to warming and freshening of surface waters) as a result of global warming. However, none of the present-day climate models have a sufficiently fine spatial resolution to resolve the processes that govern either the sinking or the rising, and have to rely on parametrisations instead. Additionally, the climate model projections that produced the range of 0 50% in AMOC decline all rely on the same greenhouse gas forcing scenario, which will inevitably differ from the actual one. Thus, the true range of uncertainty of the future evolution of the AMOC is even larger. There is clearly a need to monitor the state of the AMOC continuously over coming decades.

To date there is no clear evidence that the AMOC has started to decrease in strength, partly because it has only very recently been a subject of continuous monitoring. Indeed a reliable time series of the strength of the AMOC spanning the last 50 year (or so) does not exist. The recent continuous measurements at 26.5°N suggest that the amplitude of intra-seasonal variations of the strength of AMOC is larger than previously thought. This makes it doubtful whether reliable estimates of long-term AMOC changes can be inferred from the few sporadic attempts to estimate the strength of the AMOC, that have been done in the past. Measurements that have focused on the observation of one particular aspect rather than the whole AMOC (such the strength of the DWBC or the deep Nordic Sea inflow into the Atlantic) do not show clear, uniform trends either. In addition it is difficult to assess how a change in one of the components translates into a change of the whole

AMOC. However, even with the recently started suitable continuous AMOC observations now well under way, the detection of a possible ongoing, global-warming-induced decline in the vigour of the AMOC may still be decades away, unless more observing systems are put into place. This will strongly depend on how fast the decline actually comes about, if at all.

The major difficulty for scientists to document ocean circulation changes on climate relevant time scales arises from the sparseness of historical in situ observations both in space and time. Over the last decades it has been (and partly still is) a technical, logistical and financial challenge to maintain ocean observatories at key locations continuously for more a few years and/or to repeat measurement campaigns at a frequency that is sufficient to detect trends with a high level of confidence. However, the awareness that understanding the processes that govern ocean circulation changes may be vital for present and future societies has triggered dedicated, internationally coordinated field programmes and along with them technical developments (such as autonomous in situ profilers or advances in remote sensing). As a consequence physical oceanography is currently undergoing a step change in capacity, capability and understanding, from which future generations will certainly profit.

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