Background
The authors report that between the mid-1970s and mid-1990s, sea surface temperatures in the eastern and central equatorial Pacific Ocean (9°N-9°S, 90°W-180°W) rose by approximately 0.7°C in response to a slowdown of the shallow meridional overturning circulation, and that some scientists have suggested these phenomena were the result of greenhouse gas forcing, specifically citing, in this regard, Meehl and Washington (1995) and Knutsen and Manabe (1998).  However, they also note the existence of evidence for a late 1990s "regime shift" in the North Pacific (Chavez et al., 2003; Peterson and Schwing, 2003) that could temper, or even refute, this interpretation of the data.

What was done
Since year-to-year fluctuations associated with El Nino and La Nina conditions can greatly influence the state of earth's climate system, McPhaden and Zhang compared mean conditions in the eastern and central equatorial Pacific Ocean for the six-year period July 1992-June 1998 with the more recent five-year period July 1998-June 2003 (both of which periods span at least one complete ENSO warm and cold phase cycle) in order to gain some insight into the relative merits of these two differing views of the issue, i.e., greenhouse gas-induced warming vs. decadal-scale warming associated with a regime that switched to cooling in the late 1990s.  In addition to focusing on sea surface temperatures, they also utilized hydrographic and wind data spanning the period 1992-2003 to calculate geostrophic meridional volume transports in the upper pycnocline of the tropical Pacific Ocean.

What was learned
The data and the authors' analyses indicate that "the shallow meriodional overturning circulation in the tropical Pacific Ocean has rebounded since 1998, following 25 years of significantly weaker flow."  In fact, they say it "has recently rebounded to levels almost as high as in the 1970s."  Likewise, the areally-averaged sea surface temperature in the eastern and central equatorial Pacific Ocean concurrently dropped approximately 0.6°C to almost equal the low of the mid-1970s and to actually match the low of the previous regime in the mid-1950s.

What it means
With respect to tropical Pacific sea surface temperatures, McPhaden and Zhang conclude that the "precise magnitude of anthropogenic influences will be difficult to extract with confidence from the instrumental record given the rapidity with which observed warming trends can be reversed by natural variations," which is truly an understatement in view of the fact that it is currently no warmer in the eastern and central equatorial Pacific Ocean than it was a full half-century ago.

References
Chavez, F.P., Ryan, J., Lluch-Cota, S.E. and Niquen C., M.  2003.  From anchovies to sardines and back: multidecadal change in the Pacific Ocean.  Science 299: 217-221.

Peterson, W.T. and Schwing, F.B.  2003.  A new climate regime in the northeast Pacific Ocean.  Geophysical Research Letters 30: 10.1029/2003GL017528.