Ocean influence on climate variability

How might the ocean affect climate variability? Indeed, might it even effect climate variability? The ocean has one unambiguous influence on midlatitude climate variability and a number of more ambiguous influences. The unambiguous influence stems from the fact that the heat capacity of the ocean is much greater than that of land, as we discussed in chapter 5. Thus, changes in surface temperatures over land, especially in locations far from the ocean, are much larger than the changes over the ocean, and so the variations over land tend to dominate any hemisphere-wide measure of the variability. Changes in the temperature associated with changes in the NAO pattern from positive to negative are distinctly more marked over land than over the ocean.

The more ambiguous effect concerns the relationship between fluctuations in the sea-surface temperature (SST) and the state of the atmosphere in midlatitudes, and in particular the state of the NAO. Certainly, variations in the SST and the overlying atmosphere are related. To be specific, let us focus on the North Atlantic and the NAO, but similar concerns and effects almost certainly apply to other regions of the world's ocean. It turns out that a common pattern of SST variability in the North Atlantic winter is a tripole. Rather like the NAO itself, the tripole commonly exists in one of two phases. In one phase, the pattern consists of a cold anomaly in the subpolar North Atlantic around Greenland, a warm anomaly in the middle latitudes off the Atlantic seaboard of the United States, and a cold subtropical anomaly between the equator and 30° N, concentrated most in the eastern Atlantic. Is the SST pattern created by the pattern of variability of winds and temperature in the atmosphere, or does the SST pattern determine the variability of the atmospheric winds and temperature? This is the $64,000 question! It may of course be a chicken-and-egg problem, with one pattern leading to the other, which then reinforces the first pattern, and so on. How are we to determine the answer? One way is to see if we can determine if variations in the atmosphere unambiguously lead those in the ocean, or vice versa. If it is the former case, then it seems likely that the atmosphere is driving the ocean, rather than vice versa (although of course the reader may be able to come up with perverse counterexamples where this is not the case).

Careful observational analysis in fact suggests that the basic pattern of SST anomalies is created by air-sea heat exchanges and the wind-induced near-surface ocean currents associated with the NAO. It seems that the correlations between atmosphere and ocean are strongest for atmospheric patterns that exist before the SST variability by a few weeks, suggesting that large-scale SST patterns are responding to atmospheric forcing, rather than causing the atmospheric patterns. Put simply, the atmosphere leads the ocean. However, rather intriguingly, that may not be the whole story, although the reader should be warned that the situation is far from settled and is an active topic of research. At still longer timescales, there is some evidence that a large-scale, pan-Atlantic SST pattern actually precedes the atmospheric NAO pattern by up to about six months.1

What is going on? We can explain these apparently differing observations as follows. First of all, let's be clear that we are talking about patterns of variability in both the atmosphere and the ocean. The mean ocean gyres and the meridional overturning circulation are set up, climate variability as we discussed in chapter 4, in response to the mean atmospheric circulation. Now, as we know, the atmosphere varies on timescales of days to a few weeks, and on space scales of a few thousand kilometers, and we call this variability weather. It seems that on timescales from days to a few months, the atmosphere does indeed drive anomalies in the ocean. That is to say, if the atmospheric winds and temperature happen to have a certain configuration for a few days, then (depending on that configuration) cold or warm SST anomalies can be generated, typically also on space scales of a few thousand kilometers. Because of the much larger thermal inertia of the ocean (as well as the fact that the ocean currents are roughly a hundred times smaller than the winds in the atmosphere), these patterns may persist much longer than typical atmospheric patterns. Thus, the ocean smooths away the smaller scale, day-to-day variability and leaves behind larger scale and more persistent patterns. The ocean is really a passive partner in this activity—the origins of patterns of SST variability on monthly timescales primarily lie in the atmosphere— and in this sense we may say the atmosphere drives the ocean. The persistence of the ocean anomalies does feed back on the atmosphere, leading to somewhat more persistent and less extreme weather than might otherwise be the case, so that one might also say the ocean damps the atmosphere. This effect is obviously rather similar to the one we discussed in the previous chapter, where we discussed the generally moderating influence of the ocean on the climate.

However, the ocean does have dynamics of its own. The gyres contain smaller eddies that produce considerable variability, just as the atmosphere has weather, and the large-scale gyres themselves may vary on interannual to decadal timescales. The meridional overturning circulation also varies, and its sluggish nature suggests that the variability may include timescales of decades and longer. If this variability is able to produce large-scale, long-lived SST anomalies, it is possible that these anomalies may, over time, affect the average behavior of the atmosphere. Even though the impact of mid- and high-latitude SST anomalies on the atmosphere seems to be small, if the anomalies persist for long enough, they will have a cumulative effect. Such an effect may be responsible for the seeming persistence of the NAO pattern in certain decades that we alluded to previously, but the evidence is not definitive.

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