Posted on October 3rd, 2015 in Isaac Held's Blog
A proxy for the strength of the trade winds in the North Pacific: nitrogen isotope records from three sediment cores off the west coast of North America (blue = 33oN, green =25oN , red = 24oN). More 15N is interpreted as stronger trades. From Deutsch et al. 2014.
The warming of the globe over the last couple of decades has been slower than the forced warming predicted by most GCMs, due to some combination of internal variability, incorrectly simulated climate responses to the changes in forcing agents, and incorrect assumptions about the forcing agents themselves. A number of studies have implicated the tropical Pacific as playing a central role in this discrepancy, specifically a la Nina-like trend — with eastern equatorial Pacific cooling and strengthening trade winds. If you intervene in a climate model by imposing the observed near-surface ocean temperatures in the eastern equatorial Pacific (Kosaka and Xie, 2013) or by imposing the observed surface equatorial Pacific wind fields (England et al 2014; Delworth et al 2015), the rest of the simulation falls into place– not just the global mean temperatures but the spatial pattern of temperature trends over the past two decades — as well as the California drought. The wind and ocean surface temperatures are tightly coupled on annual and longer time scales in this region, so these different studies tell a consistent story. The implication is that explanations for the discrepancy in global warming rate need to simultaneously explain this La Nina-like trend to be convincing. Based on its importance in recent decades, it is tempting to assume that the tropical Pacific has played an important role in modulating the rate of global warming throughout the 20th century. But if the nitrogen isotope record on the eastern margins of the subtropical north Pacific provided by Deutsch et al. 2014 and shown above is a good proxy for trade wind strength, it is interesting that it isn’t dominated by quasi-periodic multi-decadal variability. Instead it looks like a long term trend towards weaker trades until the last 20 years or so — a trend that happens to be roughly consistent with the forced response of the tropical winds to greenhouse warming in most models — which is then interrupted by an event that is unique in the context of the last 150 years. (This is admittedly less clear for the red record in the figure than for the green and blue.)
In regions of the ocean where oxygen is scarce (oxygen minimum zones (OMZs)) bacteria utilize nitrate as a substitute, taking up lighter N preferentially, leaving the water enriched in 15N. The more extensive or intense the OMZ, the more enriched the water is in this heavier isotope. This water makes its way up closer to the surface where most of the biological activity is and most of the falling detritus that reaches the ocean floor is produced, so the sediments become enriched in 15N as the OMZ’s expand. It is the surface productivity and the utilization of oxygen as this falling detritus decomposes that generates the OMZs in the first place, so much of the variability in the OMZs is itself thought to be produced by variations in the productivity in the surface water. The productivity variations in turn are produced in large part by variations in the upwelling of nutrient rich waters. So more upwelling results in heavier N in the sediments. Finally, on these slow time scales you expect the variations in upwelling to be tied to large scale variations in the winds over the tropical and subtropical Pacific. If these variations in the wind stress are sufficiently coherent spatially maybe we can think of this proxy as a measure of the overall strength of the trades, with stronger trade winds resulting in more upwelling on the eastern boundary of the basin and increasing 15N in the sediments. (The circulation and biology provide a natural low-pass filter, so you don’t see much of an imprint of internanual ENSO-driven variability.)
I am far from an expert on the ways in which this argument can break down or how this proxy is related to others, such as coral records. But this picture of the evolution of the tropical Pacific winds is consistent with direct surface pressure measurements. On time scales of seasons and longer, surface pressure is a very smooth field in the tropics, and the pressure difference across the basin in low latitudes is strongly correlated with the strength of the equatorial winds. Deutsch et al also provide the following figure, based on HadSLP2 (Allan and Ansell, 2006) and ERA40 (Uppala et al 2005) (yellow and cyan respectively), of the near equatorial difference in sea level pressure between the Indian Ocean/ western Pacific and the central/eastern Pacific. The black line is a 10 year running mean and an average over the two datasets where they overlap.
The 15N proxy and the direct estimates of the equatorial east-west surface pressure gradient give the same qualitative picture, supporting the idea that the wind field variations are coherent and large scale enough that we can think of the proxy as a trade wind index. Of course the proxy then opens up the possibility for extending the record farther back in time.
I was involved in a paper with GFDL colleagues (Vecchi et al 2006) in which we were looking at the slow downward pressure gradient trend as of 2005, before the extended strengthening trend in recent years became such a dominant part of the record. Zhang and Song 2006 independently focused on the same downward trend at the same time. We felt that we could attribute this trend to increasing greenhouse gas forcing, given that models simulate a trend of this sign and magnitude in their forced response. We also felt that this weakening could be related to the overall weakening of the convective mass exchange in the tropical atmosphere between the surface boundary layer and the deeper troposphere, a weakening that is expected to accompany warming (post #52). Understandably, the extension of this record since 2005 has led others to cast doubt on this attribution.
Given the emphasis on quasi-periodic variability in the Pacific, the 15N figure from Deutsch et al caught my eye. But my comfort level in talking about proxies is pretty low (and as a result I have pestered more colleagues than usual while writing this post), and perhaps I am reading too much into it. How unique an event is the recent strengthening of the trades? The coincidence between the slow weakening trend that preceded it and the response expected from increasing greenhouse gases suggests that there may be something distinctive going on (post #45 expands on this possibility a bit). Or is this just a rather big instance of run-of-the-mill background variability?
[The views expressed on this blog are in no sense official positions of the Geophysical Fluid Dynamics Laboratory, the National Oceanic and Atmospheric Administration, or the Department of Commerce.]