Seasonal Variability of Mass in the Arctic Ocean Using GRACE and In Situ Bottom Pressure Measurements

The Ocean Bottom Pressure (OBP) measured by the Gravity Recovery and Climate Experiment (GRACE) was recently confirmed to be accurate for the Arctic Ocean Basin by Morison et al. (2007). They showed a high correlation between GRACE and in situ Arctic Bottom Pressure Recorder (ABPR) measurements near the North Pole, and showed that the multiyear trends in bottom pressure were consistent with the ocean circulation reverting to a pre-1990s anticyclonic pattern. Here we examine seasonal changes in bottom pressure. Both GRACE (2002-2007) and ABPR (2005-2007) pressure measurements at the North Pole show a seasonal peak-to-peak variability in the Arctic Ocean of about 6-10 cm of water equivalent with a maximum occurring during the summer and a minimum during the winter. In addition to the North Pole, annual cycles are also displayed by three years of in situ OBP measurements from the Beaufort Gyre, from south of the Aleutian Islands, and from the Fram Strait. The OBP time-series from the four locations (ABPR at the North Pole included) were treated equally, de-tided, and averaged to the same time resolution. The OBP time series from the Fram Strait, the North Pole and the Beaufort Gyre were all positively correlated at a 99% significance level, with correlation coefficients from 0.61 between North Pole and Beaufort Gyre to 0.8 between the Fram Strait and the Beaufort Gyre. The annual cycle, as determined by the first harmonic fitted to each time series, is nearly in phase between these three locations, indicating a mass maximum during the summer and a minimum during the winter. The OBP near the Aleutian Islands was not significantly correlated with any of the other OBP measurements in the Arctic Basin. Seasonal forcing by the atmosphere is investigated. Empirical Orthogonal Functions of sea level pressure, geostrophic winds and GRACE OBP have been used to analyze the response of the OBP to the seasonal atmospheric forcing and the spatial scale at which such relation occurs. The principal component time-series of the leading mode for the geostrophic winds (PCw1) reveals the annual cycle and explains 25% of the variance of the wind field. The leading EOF mode of the GRACE OBP (PCp1) in the Arctic Basin shows a strong annual cycle, which explains 36% of the total variance. Reference: Morison, J., J. Wahr, R. Kwok, and C. Peralta-Ferriz (2007), Recent trends in Arctic Ocean mass distribution revealed by GRACE, Geophys. Res. Lett., 34, L07602, doi:10.1029/2006GL029016.