1. Spatial Modes of Salinity and Temperature Comparison with PDO index
Decadal Changes in Upper Ocean Salinity in the North Pacific as Inferred from Argo Floats and Historical Observations
Li Ren, Stephen C. Riser, University of Washington
Decadal variability in upper ocean temperature in the North Pacific has been studied in detail in recent years, revealing phenomena such as Pacific Decadal Oscillation and the effects of ENSO on portions of the N. Pacific basin that are far removed from the eastern tropical Pacific. Yet the long-term variability of the large-scale, upper ocean salinity field has not been previously examined in much detail. Here we examine the upper ocean salinity filed in the N. Pacific from using data from the World Ocean Atlas and profiles from ARGO float collected since The data were grouped by 5-year intervals in order to optimize the resolution and stability of the salinity estimated. An EOF analysis of the resulting salinity (temperature) estimates was carried out on isopycnals. The results show an abrupt change in salinity (temperature) in the intermediate depths during 1970s as much as 0.1 PSU, perhaps coincident with other changes known to occur at that time. The float salinity data are consistent with the trends in salinity since the mid-1970s seen in the World Ocean Atlas.
EOF Analysis Results
Abstract
Spatial Modes of Salinity and Temperature
Percentage of variance in modes
First Modes
Percent(%)
First Mode
Second Mode
Isopycnal layers
Salinity
Temperature
26.5
27.81
22.14
16.58
17.68
26.6
27.06
20.89
17.18
17.75
26.7
26.42
21.26
17.32
16.87
26.8
26.73
22.52
17.37
16.15
26.9
26.11
22.83
18.08
15.50
Salinity
Temperature
Second Modes
Salinity
Temperature
On isopycnal surfaces 26.5 to 26.9 which contain the intermediate waters in the North Pacific, the variance explained by first mode is about 26.8% and the second mode is about 17.3%, totaling nearly half of the total variance. Based on North et al. (1982), the eigenvalues of these two modes are distinguishable (there is an eigenvalue spectrum), implying that they are significant
The first mode spatial pattern shows salty (warm) water in the central and eastern N. Pacific but fresh (cool) in the west and eastern equatorial Pacific. The second mode is the opposite of the first mode. These results are for the  =26.6 isopycnal layer; other nearby layers show similar results.
The First And Second Principal Component of salinity and temperature
Introduction
Comparison with PDO index
First PC
Second PC
945 Operational Floats,  November 4, 2003
The first principal component of both salinity and temperature (not show here) show an abrupt change in the 1970s on isopycnals 26.5, 26.6, 26.7, 26.8, and 26.9, which are the intermediate waters. Also, it appears there is a change late in the time series, in the year There appears to be a type of decadal oscillation in salinity and temperature in the intermediate water. Both spatial and time change of the first mode can be seen in the observations.
A primary focus of Argo is seasonal to decadal climate variability and predictability. Presently there are more than 900 floats Argo floats deployed over the world ocean, with more than 400 floats working in the Pacific ocean.
Comparing the first principal component (PC) with the PDO index (available from the site ftp.atmos.washington.edu, provided by Dr. N. Mantua), it is found there is a significant correlation between these two time series with the PDO leading PC1 by two to eleven ‘months’. Because of the methods used for reconstructing the data, it appears that the PDO signal leads the first PC by approximately one to five years.
Data Reconstruction
What does the North Pacific Appear Now?
The two figures on the left side panel are temperature and its anomaly compared with the WOA2001 climatology data on the isopyncal  = 26.6 in Sep. 2003; float positions are shown as black dots. The two figures on the right panel are similar but for salinity. There are presently approximately 300 Argo floats in the N. Pacific, so it is possible to begin to make useful maps. The water on this density surface is generally fresh and cold (compared to Levitus climatology) over most of the basin, except in the northwest pacific where the waters are saline and warm relative to climatology. Anomalies are approximately 1°C for temperature and 0.3 (PSS-78) for salinity.
 =26.6
Temperature
Salinity
We collected all the CTD and OSD data in the World Ocean Atlas from , and the profiles from Argo floats from 1999-present. These data were grouped by 5-year intervals in order to optimize the resolution and stability of the salinity estimates and to retain any decadal variability that might be present. It is known that there was a so-called regime shift during , so the data were divided as in groups as , , , , , , ,and We avoided mixing the data from 1976 and 1977.
Observations
Variations in salinity at  = 26.6 along 42°N clearly show a change in the region west of the 180° from saline to fresh in 1976; and fresh to saline in Results are opposite east of 180°.
Time
Conclusion: Based on Argo float data and historical archived observations, we find a marked change for salinity and temperature in 1970s in properties of the intermediate waters of the N. Pacific, and it seems possible that another marked change has been occurring since 2000.
Reference:
North, G.R.,T.L.Bell, R.F. Cahalan and F.J.Moeng (1982) Sampling errors in the Estimation of empirical orthogonal functions. Mon. Wea Rev.,110,
Variations in salinity at  = 26.6 along 220°E clearly show a change in the region north of the 20° N from fresh to saline in 1976 and saline to fresh in Results are opposite south of 20°N.
Time