Abstract
Abstract. We examine the output of an ocean model forced by ECMWF winds to study the theoretical relationship between wind-induced changes in ocean bottom pressure in the North Pacific between 1992 until 2010 and ENSO. Our analysis indicates that while there are significant fluctuations correlated with some El Niño and La Niña events, the correlation is still relatively low. Moreover, the ENSO-correlated variability explains only 50 % of the non-seasonal, low-frequency variance. There are significant residual fluctuations in both wind-stress curl and ocean bottom pressure in the region with periods of 4-years and longer. One such fluctuation began in late 2002 and has been observed by the Gravity Recovery and Climate Experiment (GRACE). Even after accounting for possible ENSO-correlated variations, there is a significant trend in ocean bottom pressure in the region, equivalent to 0.7 ± 0.3 cm yr−1 of sea level from January 2003 until December 2008, which is confirmed with steric-corrected altimetry. Although this low-frequency fluctuation does not appear in the ocean model, we show that ECMWF winds have a significantly reduced trend that is inconsistent with satellite observations over the same time period, and so it appears that the difference is due to a forcing error in the model and not an intrinsic error.
Highlights
The area between 35◦ N to 50◦ N and 150◦ E to 180◦ E in the North Pacific has some of the highest variance of longwavelength ocean bottom pressure (OBP) in the world’s oceans
Since both Gravity Recovery and Climate Experiment (GRACE) and steric-corrected altimetry observe OBP changes over this period that are consistent with the trend in the satellite-derived wind stress curl, this suggests a significant error in the ECMWF winds in this region over this time period
After analyzing a long time-series of wind-stress curl from satellite observations, ocean bottom pressure from a model and GRACE, we conclude that while there is significant interannual variability in wind-stress curl that drives OBP variations in the sub-polar gyre, only part of it is related to El Nino/Southern Oscillation (ENSO) in the tropical Pacific, as previously pointed out by Song and Zlotnicki (2008)
Summary
The area between 35◦ N to 50◦ N and 150◦ E to 180◦ E in the North Pacific has some of the highest variance of longwavelength ocean bottom pressure (OBP) in the world’s oceans It is caused by mass transport into and out of the region due to changes in the wind-stress curl (WSC) over the sub-polar and sub-tropical gyres (e.g., Ponte, 1999). Song and Zlotnicki (2008) used a wind-forced model with realistic bathymetry and demonstrated significant interannual variations in OBP related to WSC in the region They concluded that there was a small, barely significant correlation between both WSC and OBP with an El Nino/Southern Oscillation (ENSO) climate index at a lag of 0 to 2 months. We will re-examine the ENSO-correlated variations in both OBP and WSC in the North Pacific, and
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