This post is prompted by an article published by Richard Seager et al. at AMS Journal Is There a Role for Human-Induced Climate Change in the Precipitation Decline that Drove the California Drought? Excerpts in italics with my bolds.
The recent California drought was associated with a persistent ridge at the west coast of North America that has been associated with, in part, forcing from warm SST anomalies in the tropical west Pacific. Here it is considered whether there is a role for human-induced climate change in favoring such a west coast ridge. The models from phase 5 of the Coupled Model Intercomparison Project do not support such a case either in terms of a shift in the mean circulation or in variance that would favor increased intensity or frequency of ridges. The models also do not support shifts toward a drier mean climate or more frequent or intense dry winters or to tropical SST states that would favor west coast ridges. However, reanalyses do show that over the last century there has been a trend toward circulation anomalies over the Pacific–North American domain akin to those during the height of the California drought.
First we plot together the history of California winter precipitation and Arctic sea ice anomaly in terms of area covered by ice at the annual minimum month of September and also as the November through April winter average (Fig. 9, top). While all three are of course negative during the drought years there is no year to year relationship between these quantities. Next we composite 200-mb height anomalies, U.S. precipitation, and sea ice concentration for, during the period covered by sea ice data, the driest 15% of California winters and subtract the climatological winter values (Fig. 9, bottom). As in Seager et al. (2015), the composites show that when California is dry the entire western third of the United States tends to be dry and that there is a high pressure ridge located immediately off the west coast, which does not appear to be connected to a tropically sourced wave train. There also tends to be a trough over the North Atlantic, similar to winter 2013/14. There are notable localized sea ice concentration anomalies with increased ice in the Sea of Okohtsk, reduced ice in the Bering Sea, and increased ice in Hudson Bay and Labrador Sea, though the anomalies are small. These ice anomalies are consistent with atmospheric forcing. The Sea of Okhotsk and Hudson Bay/Labrador Sea anomalies appear under northerly flow that would favor cold advection and increased ice. The Bering Sea anomaly appears under easterly flow that would drive ice offshore. As shown by Seager et al. (2015), the dry California winters are also associated with North Pacific SST anomalies forced by the atmospheric wave train and the sea ice anomalies appear part of this feature rather than as causal drivers of the atmospheric circulation anomalies.
These analyses do not support the idea that variations in sea ice extent influence the prevalence of west coast ridges or dry winters in California.
On the basis of the above analysis we conclude that the occurrence of persistent ridges at the west coast is more connected to SST anomalies than it is to sea ice anomalies. The CMIP5 model ensemble lends no support to the idea that ridge-inducing SST patterns become more likely as a result of rising GHGs. However, the models could be wrong so we next examine whether trends in observed SSTs lend any support to this idea. Trends were computed by straightforward linear least squares regression.
A number of features stand out in these trends regardless of the time period used.
- Amid near-ubiquitous warming of the oceans the central equatorial Pacific stands out as a place that has not warmed.
- The west–east SST gradient across the tropical Pacific has strengthened as the west Pacific has warmed.
- Increased reanalysis precipitation over the Indian Ocean–Maritime Continent–tropical west Pacific and reduced reanalysis precipitation over the central equatorial Pacific Ocean were found.
- Tropical geopotential heights have increased at all longitudes.
- A trend toward a localized high pressure ridge extending from the subtropics toward Alaska across western North America.
These associations in the trends—a strengthened west–east SST gradient across the tropical Pacific and localized high pressure at the North American west coast—are in line with every piece of evidence based on observations and SST-forced models presented so far that there is a connection between drought-inducing circulation anomalies and tropical Pacific SSTs. The mediating influence is seen in the precipitation trends that show enhanced zonal gradients of tropical Indo-Pacific precipitation and a marked increase centered over the Maritime Continent region.
Conclusions and discussion
We have examined whether there is any evidence, observational and/or model based, that the precipitation decline that drove the California drought was contributed to by human-driven climate change. Findings are as follows:
- The CMIP5 model ensemble provides no evidence for mean drying or increased prevalence of dry winters for California or a shift toward a west coast ridge either in the mean or as a more common event. They also provide no evidence of a shift in tropical SSTs toward a state with an increased west–east SST gradient that has been invoked as capable of forcing a west coast ridge and drought.
- Analysis of observations-based reanalyses shows that west coast ridges, akin to that in winter 2013/14, are related to an increased west–east SST gradient across the tropical Pacific Ocean and have repeatedly occurred over past decades though as imperfect analogs.
- SST-forced models can reproduce such ridges and their connection to tropical SST anomalies. Century-plus-long reanalyses and SST-forced models indicate a long-term trend toward circulation anomalies more akin to that of winter 2013/14.
- The trends of heights and SSTs in the reanalyses also show both an increased west–east SST gradient and a 200-mb ridge over western North America that, in terms of association between ocean and atmospheric circulation, matches those found via the other analyses on interannual time scales.
- However, SST-forced models when provided the trends in SSTs create a 200-mb ridge over the central North Pacific and, in general, a circulation pattern that cannot be said to truly match that in reanalyses.
So can a case be made that human-driven climate change contributed to the precipitation drop that drives the drought? Not from the simulations of historical climate and projections of future climate of the CMIP5 multimodel ensemble.
These simulations show no current or future increase in the likelihood or extremity of negative precipitation, precipitation minus evaporation, west coast ridges, or ridge-forcing tropical SST patterns. However, when examining the observational record a case can be made that the climate system has been moving in a direction that favors both a ridge over the west coast, which has a limited similarity to that observed in winter 2013/14, the driest winter of the drought, and a ridge-generating pattern of increased west–east SST gradient across the tropical Pacific Ocean with warm SSTs in the Indo–west Pacific region. This observations-based argument then gets tripped up by SST-forced models, which know about the trends in SST but fail to simulate a trend toward a west coast ridge. On the other hand, idealized modeling indicates that preferential warming in the Indo–west Pacific region does generate a west coast ridge.
To make the argument we outline above requires rejecting the CMIP5 ensemble as a guide to how tropical climate responds to increased radiative forcing since this tropical ocean response is at odds with what they do. To do so follows in the footsteps of Kohyama and Hartmann (2017, p. 4248), who correctly point out that “El Niño–like mean-state warming is only a ‘majority decision’ based on currently available GCMs, most of which exhibit unrealistic nonlinearity of the ENSO dynamics” (see also Kohyama et al. 2017). The implications of changing tropical SST gradients would extend far beyond just California and include most regions of the world sensitive to ENSO-generated climate anomalies.
We believe that the current state of observational information, analysis of it, and climate modeling does not allow a confident rejection of the CMIP5 model responses and/or a confident assertion of human role in the precipitation drop of the California drought. We also believe that for the same reasons a human role cannot be excluded.
The researchers set out to prove man-made global warming contributes to droughts in California, but their findings put them in a quandry. The models include CO2 forcings, yet do not predict the conditions resulting in west coast droughts, They have to admit the models are wrong in this respect (what else do the models get wrong?). They cling to the hope that global warming can be tied to droughts, but have to admit there is no evidence from the failed models.
A separate study is Global patterns of change and variation in sea surface temperature and chlorophyll by Piers K. Dunstan et. al.
The blue tongue shows up as an equatorial pacific region that shows little variability over the 14 year period of study. From the article:
The interaction between annual variation in SST and Chl-a provides insights into how and where linkages occur on annual time scales. Our analysis shows strong latitudinal bands associated with variation in seasonal warming (Fig. 4a). The equatorial Pacific, Indian and Atlantic Oceans are all characterised by very low annual RMSE for both SST and Chl-a. The mid latitudes of each ocean basin have higher variance in SST and/or Chl-a.