I’ve circled a dark region where SST are perhaps -5C below normal, due no doubt to strong local upwelling. (The other dark areas are land.)

I also noticed the ECMWF subsurface anomaly cross-section for 140W (the smaller circle on the SST map is at 140W):

This shows another upwelling pocket of cold water, with anomalies of 5C or greater just 30 meters below the surface. This pocket has been rising for several months.

I don’t know how unusual this is. I’ve observed a few other such cold pockets in recent years but they tend to be small and disappear quickly. That may prove to be true of these, too – we’ll see. But, if they are more widespread and longer-lasting, I wonder if they’ll affect the trade wind strength and cloud cover, which in turn may affect ENSO behavior. Something to watch.

From the alkenone proxy data in Lorenz (2006), I calculated average tropic and extratropic temperature changes to present and came up with: tropics (30N to 30S) change from 7000 years BP (the Holocene Optimum) to present = 0.23 degrees C and for the extratropics (all other areas of the globe) this change = -1.45 degrees C. Since the tropics and extra tropics have nearly equal areas we can say that from the alkenone data alone that 7000 years BP was globally about 0.61 degrees C warmer than present.

For the ensemble of computer simulations, the Lorenz paper presents, not data for the change from Holocene Optimum to present, but color coded global maps showing the estimated temperatures changes. I eye-balled these maps to arrive at the following temperature changes: For the maps showing changes for annual temperatures I estimated changes of +0.1 degree C for the tropics and -0.7 degrees C for the extra tropics (most of this change occurring in the northern hemisphere). So without stating this conclusion directly the computer simulations showed that the Holocene optimum was approximately 0.3 degrees C warmer than present.

Now the authors of Lorenz (2006) do not appear to be content with this as a conclusion. They go on to attempt to show that computer model climate simulations can provide seasonal change resolution for the Holocene Optimum period to present that the alkenone results cannot. It appears to me that the interest is not so much quantitative at this point but qualitative in that they want to direct attention to a point that the seasonal variations might be of such a magnitude that alkenone proxy indications of a warmer Holocene than present might be reversed with more precise and seasonal resolved data.

The Lorenz paper shows two color coded maps of Holocene to present temperature changes from computer simulations one for the Dec-Jan-Feb months (DJF) and the other for the June-July-Aug months (JJA). The global change for DJF shows – by eye ball – a change of 0.14 degree C and for JJA it shows a change -0.6 degrees C.

Lorenz et al. (2006) give a less than confident observation of phytoplankton growth and the resulting relationship of the alkenone proxy relationship to seasonal temperatures:

[39] Although the seasonal cycle of SST in the tropics is small, phytoplankton production is not constant throughout the year. It is reasonable that a change of seasonal insolation on the order of 10% is able to impact marine biological productivity. If the alkenone
production is thought to be highest during the month with the warmest water temperature in the mixed layer, then the resemblance of reconstructed trends with the simulated trends of local summer can be taken as an indication that the time of maximum production may have changed with the insolation signal.

Lorenz’s case is less than clear when from the link we have:

Like their land-based relatives, phytoplankton require sunlight, water, and nutrients for growth. Because sunlight is most abundant at and near the sea surface, phytoplankton remain at or near the surface.

..The atmosphere is a rich source of carbon dioxide, as millions of tons of this gas settle into the ocean every year. However, phytoplankton still require other nutrients, such as iron, to survive. When surface waters are cold, deeper depths are allowed to upwell, bringing these essential nutrients toward the surface where the phytoplankton may use them. However, when surface waters are warm (as during an El Niño), they do not allow the colder, deeper currents to upwell and effectively block the flow of life-sustaining nutrients. As phytoplankton starve, so too do the fish and mammals that depend upon them for food. Even in ideal conditions an individual phytoplankton only lives for about a day or two. When it dies, it sinks to the bottom. Consequently, over geological time, the ocean has become the primary storage sink for atmospheric carbon dioxide. About 90 percent of the world’s total carbon content has settled to the bottom of the ocean, primarily in the form of dead biomass.

Also from this link we can understand better what is not clear from Lorenz et al. (2006).

In the tropics, there is not much in the way of seasonality, and it’s generally pretty warm. That means that the surface waters can get pretty warm, and there’s not a lot of wind to mix that warm water down very deep. Because the density of water depends on its temperature, the warming at the surface can cause the surface layer to become less dense than the water underneath it (which is cooler), and it begins to “float” on top of the cool water. Because these two layers are no longer of the same density, there is an interface between the two layers (the thermocline), which acts as a barrier to mixing, and to the movement of things like nutrients. So, the surface waters, once they become depleted of nutrients, can only be resupplied by whatever recycling of nutrients can happen in the surface layer, and by mixing of nutrient-rich deep water can happen across the thermocline (nutrients in deep water are also regenerated, but all you need to know is that deep water is generally high in nutrients). This doesn’t amount to very much, and it results in phytoplankton growing slowly, and not a lot of growth overall; because the tropics are warm all the time this pattern persists over the entire year. Therefore, phytoplankton in the tropics are generally nutrient limited all the time.

In temperate latitudes, there is stong seasonality. In winter, it gets good and cold, and there’s lots of storms, which means lots of wind-driven mixing. So, in higher latitudes, the thermocline is seasonal, and does not occur in winter. This means that there is lots of mixing to very great depths (winter mixing in the Labrador and Norwegian seas reach depths greater than 2000m!). So, the deep, nutrient-rich water is mixed up to the surface. There isn’t a lot of phytoplankton growth (or biomass) in the winter, because they become light limited, because they are mixed to such depths. In the spring, when it starts to warm up, a thermocline develops, again separating the phytoplankton from the deep water, but this time they are in the presence of abundant nutrients, and in the surface water where there’s lots of light. When this happen, the phytoplankton begin to grow like crazy in a phenomenon called the spring bloom. Phytoplankton biomass can become extremely high during these spring blooms, which is why overall phytoplankton biomass is very high in temperate latitiudes. Of course, the phytoplankton generally use up all those nutrients eventually, and phytoplankton biomass is smaller in the summer once the phytoplankton become nutrient-limited. There is also often a smaller fall bloom as well, caused when winter mixing starts to break down the thermocline and nutrient-rich deep water beings to be mixed upward.

The above links make the phytoplankton growth considerably more complicated than the Lorenz paper seems to in looking for supporting evidence for their conclusions. The following links: http://www.whoi.edu/science/GG/paleoseminar/pdf/rosell04.pdf and http://www.mit.edu/~jsachs/Sachs_G3.pdf flat out state that alkenone proxies calibrate with the annual temperatures and not seasonal ones.

Global sedimentary values of U37K0 have a higher correlation factor with annual SST at 0-m water depth than for any other depth and season [e.g., Mu¨ller et al., 1998]. Therefore, SST estimates from U37K0 can be interpreted as reflecting annual mean values at the ocean surface.

Global core top sediment calibrations of the alkenone paleothermometer [Miller et al., 1998] are most consistent with mean annual SSTs at 0-M water depth. This is enigmatic since much of the ocean is characterized by seasonal or episodic maxima in haptophyte production [Brown and Yoder, 1994].

Pronounced occurrence of long-chain alkenones and dinosterol in a 25,000-year lipid molecular fossil record from Lake Titicaca, South America
Kevin M. Theissen1, David A. Zinniker1, J. Michael Moldowan1, Robert B. Dunbar1 and Harold D. Rowe2

Presumably Lorenz did not include this site beacuse it wasn’t an “ocean” site.

From the abstract:

Using these criteria, the U37K unsaturation indices suggest relatively warmer temperatures in the mid-Holocene. In contrast to previous speculation, lipid analysis provides little evidence of a greatly increased presence of aquatic plants during the mid-Holocene. Instead, it appears that a few algal species were dominant in the lake. Based on the dramatic rise in abundances of LCAs and dinosterol during the early to mid-Holocene, we suspect that the algal producers of these compounds rose in response to a combination of physical and chemical changes in the lake. These include temperature, salinity, and alkalinity changes that occurred as lake level dropped sharply during a multi-millennial drought affecting the Central Andean Altiplano.

As Gator observed, Lorenz et al did not actually say that the sites were tropical and extratropical and so this arrangement in Figure 3 was merely rhetorical rather than according to their actual latitudes.

That was my impression also and in line with my earlier stated impressions that the article tends, in my view, to be very fuzzy in making points. I asked myself why the authors did not otherwise summarize all the trends in the tropics and extratropics and compare them with the seasonal computer simulations. That is why I am interested in seeing other proxy data from this period that is not as seasonal as the alkenone proxy is claimed to be and something that might test the computer simulations results for seasonal variations. The Lorenz paper seems to me to be constructed as a very quotable source for the Team and others — without having to deal with the issues with much specific detail or with regard to countervailing evidence. That is of course my opinion and I await further information and details to clarify it.

Where does it specify in the paper which cores are “tropical”? The colour coding in figure 3 seems to refer to the trend, not the location.

It, I think, all derives from your statement below. I agree that the authors appear to be using temperature trend and not location — even though the trends are not totally consistent with locations. I think that there are many interesting points to be made about this article, so if this relatively unimportant point, to me anyway, can be resolved perhaps the discussion can continue.

A small point first: core 11 with decreasing SST is shown as “tropical” while core 19 with increasing SST is shown as extra-tropical. Yet they seem to be very close together – why the difference?