North Pacific Ocean Heat Content Shift In The Late 1980s

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The Ocean Heat Content (OHC) anomalies (0-700 meters) of the mid-to-high latitudes of the North Pacific Ocean shifted significantly in the late 1980s. The shift is visible in a graph of North Pacific OHC anomaly data from January 1955 to June 2009, Figure 1. The graph compares North Pacific OHC (24N-65N, 120E-110W) to arbitrarily scaled NINO3.4 SST anomalies and Sato Stratospheric Mean Optical Thickness data, with the latter datasets provided for the timing of ENSO events and explosive volcanic eruptions. Following an initial increase in the late 1950s, the North Pacific OHC data dropped for almost three decades, until the late 1980s. Then the North Pacific OHC anomalies rose significantly, with the majority occurring during the 1988/89 La Nina. Is the timing with the recharge and redistribution phase of that ENSO event a coincidence, or are the decadal changes in North Pacific OHC in synch with another variable?

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Figure 1

Figure 2 is a series of annual OHC anomaly maps from 1986 to 1991. It is clear that the shift in North Pacific OHC occurred east of Japan, in the western and central mid latitudes, an area known as the Kuroshio Extension.
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Figure 2

KUROSHIO EXTENSION OHC DATA
Figure 3 shows the region of the Northwest Pacific that I’ve used for Kuroshio Extension data in the remainder of the graphs. The coordinates are 30N-45N, 150E-160W.
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Figure 3

The OHC (0-700 meters) of the Kuroshio Extension is compared to that of the North Pacific in Figure 4. It is clear that the variability of the Kuroshio Extension OHC has a major impact on the OHC of the North Pacific.
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Figure 4

Figure 5 is a comparison graph of Kuroshio Extension OHC to arbitrarily scaled NINO3.4 SST anomalies and Sato Stratospheric Mean Optical Thickness data, again with the latter datasets provided for the timing of ENSO events and explosive volcanic eruptions. ENSO does not appear to be the primary driver of Kuroshio Extension OHC.
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Figure 5

THE NORTH PACIFIC INDEX EXHIBITS SIMILAR VARIATIONS

As defined by the University Corporation for Atmospheric Research (UCAR) Climate & Global Dynamics (CGD) webpage…
http://www.cgd.ucar.edu/cas/jhurrell/npindex.html
…the North Pacific (NP) Index is “the area-weighted sea level pressure over the region 30N-65N, 160E-140W, available since 1899.” It also exhibits variations that are similar to those of the Kuroshio Extension OHC. Figure 6 shows the North Pacific Index data smoothed with a 37-month running-average filter.
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Figure 6

Comparing the scaled North Pacific Index data with the OHC for the Kuroshio Extension, Figure 7, illustrates the agreement between the two datasets.
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Figure 7

As discussed in my post North Atlantic Ocean Heat Content (0-700 Meters) Is Governed By Natural Variables, Lozier et al (2008) “The Spatial Pattern and Mechanisms of Heat-Content Change in the North Atlantic” identified the North Atlantic Oscillation as the driver of decadal North Atlantic OHC variability. Link to Lozier et al (2008):
http://www.sciencemag.org/cgi/content/abstract/319/5864/800?rss=1

Could the sea level pressure of the North Pacific also be the primary driver of decadal North Pacific OHC variability?

CLOSING COMMENT

If anthropogenic greenhouse gases had a noticeable impact on the OHC of the North Pacific, one would expect of a gradual rise in OHC. To the contrary, North Pacific OHC data declines from the late 1950s until the late 1980s. There was then a sharp rise from the late 1980s to the early 1990s, appearing to be strongly influenced by the North Pacific Index, followed by a decade-long decline until the early 2000s.

SOURCES
The NODC OHC data and the HADISST data used for NINO3.4 SST anomalies are available through the KNMI Climate Explorer:http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere

Monthly North Pacific Index data from January 1899 to July 2009 is available through the UCAR CGD webpage:
http://www.cgd.ucar.edu/cas/jhurrell/indices.data.html#npmon

Sato Index data is available through GISS:
http://data.giss.nasa.gov/modelforce/strataer/
Specifically:
http://data.giss.nasa.gov/modelforce/strataer/tau_line.txt