Dynamics Of Sea Level Variation In The North Pacific Ocean, Kuroshio Region Of East Sea Of China And Kuroshio Extension

Sea level variation of the North Pacific Ocean during 1993 to 2006 is studied based on the AVISO altimeter data, and the most 5 important modes are given out. The contribution of steric effect, precipitation and evaporation to sea level variation is discussed. The sea level variation of the Kuroshio region of East Sea of China and Kuroshio Extension are also studied with a discussion of the contribution of steric effect, freshwater flux caused by precipitation and evaporation and flow intensity of Kuroshio. A two-layer ocean model is adopted that includes first-mode baroclinic Rossby wave dynamics and barotropic Sverdrup dynamics to investigate the contribution of wind-forcing to the sea level variation in the mid-latitude region of the North Pacific Ocean. The POP ocean model is applied in the simulation of global sea level variation to examine the dynamics of seasonal, inter-annual and long term sea level change.During 1993 to 2006, sea level rise occurs in the low and mid-latitude area of the western North Pacific Ocean basin while the low and mid-latitude area in the east of the basin and the high latitude area see sea level drop. The largest rise and drop rate occurs in the Kuroshio-Oyashio region. The average rate of the North Pacific sea level is 2.87mm/a. The seasonal sea level change is dominated by the annual cycle, with the highest and lowest sea level occurs in September and March. The annual sea level range increases in La Nina year and the sea level rise rate becomes smaller after the phase shift of Pacific Decadal Oscillation (PDO) in 1998/99. The Pacific Ocean has lower sea level during the positive phase of ENSO and PDO, and higher sea level during the negative phase of ENSO and PDO which is most obvious before and after the phase shift of 1998/99. The 5 most important Empirical Orthogonal Functions (EOF) can explain 48.3% of the sea level change in the Pacific Ocean with the first and the third EOFs being the seasonal modes , the second EOF being the ENSO mode and the fourth being a mixed mode of ENSO and PDO. The EOF decomposition shows one fourth of the sea level change is seasonal signal and one fourth is modulated by ENSO and PDO and the residual is higher or lower frequency signals.The steric sea level of the North Pacific Ocean is rising during 1993 to 2006 and the contribution of steric effect to sea level trend is 57%. The significant periods of the steric sea level is the same as the sea level in the North Pacific Ocean and the amplitudes of the oscillation on the significant periods are very close to each other. The 5 most important EOFs of steric sea level is similar to that of the sea level, which shows the steric effect can influence the sea level change in the North Pacific Ocean on both seasonal and inter-annual variability.During 1993 to 2006, precipitation in the western North Pacific Ocean is increasing which is making a contribution to the sea level rise in the area while the precipitation in the interior of the ocean in the mid-latitude and on the eastern boundary of the low latitude area is decreasing and the decreasing precipitation on the eastern boundary is contributing to the sea level drop in the area. The average precipitation rate of the North Pacific Ocean is 0.21mm/a which accounts for 7.4% of the sea level rate of the ocean. The seasonal variability of the precipitation is in favor of the seasonal sea level variability; the precipitation affects to the interannual sea level variation during the year with strong ENSO signals. The evaporation in the North Pacific Ocean during 1993 to 2006 is increasing at a rate of 0.09mm/a and it is in favor of the low sea level in 1998 and 2002. Contribution of the freshwater flux caused by precipitation and evaporation to the sea level trend is 4.2%.During 1993-2006, sea level of the Kuoshio region in the East Sea of China (ESC) is rising with a rate of 5.1mm/a and the rate on the right side of the Kuroshio is larger than the left side. The sea level of the Kuoshio region in the ESC shows prominent seasonal variability; the annual range is smaller during the positive phase of ENSO and PDO, and larger during the negative phase of them. The sea level changes in an opposite phase of PDO on the inter-annual timescale and the sea level shows large variation before and after the phase shift of ENSO and PDO. In 1995 the sea level drops dramatically and after 2002 the sea level rise becomes slower.The steric effect contributes about 81.3% to the seasonal variability of the sea level of the Kuoshio region in the ESC. Large difference can be found before 1996 between the sea level and steric sea level on the inter-annual timescale; low steric sea level after 2001 may be a reason for the decreasing of rising rate of the sea level. The steric sea level contributes 45.6% to the sea level trend. The precipitation contributes to the sea level change mainly on inter-annual timescale, and the precipitation rate accounts for 1.2% of the sea level rate. During 1995 when the sea level in the area drops dramatically, the precipitation weakens and the evaporation strengthens which is in favor of the sea level change; In the La Nina year of 1998 and 2002 when the area has high sea level, the precipitation strengthens while the evaporation weakens which may also contributes to the sea level change. During the strengthening period of the Kuroshio, the sea level on the left side usually drops while the sea level on the right side rises and the difference between them increases and the mean sea level in the Kuroshio region of ESC rises. The prominent low sea level occurs in 1995 can be explained by the dramatic decrease of the Kuroshio transport which is obviously not caused by steric effect.The sea level change south and north of the Kuroshio Extension(KE) is different. The sea level change on the north side of KE can be divided into three periods: the sea level drops during 1993-1998; in 1999 the sea level rises dramatically and then drops till 2003; after 2003 the sea level rises. The sea level on the north side of KE also shows prominent seasonal variability and the annual range is 18.6cm which accounts for about 60% of the sea level variation. The steric effect can explain 84% of the seasonal variability and 40% of the inter-annual variability of the sea level; the steric sea level trend contributes 70% to the sea level trend. The contribution of precipitation to sea level change is important in some years and the weakening of precipitation on the north side of KE contributes about 2-3% to the sea level trend. The contribution of evaporation is less than 1%. The sea level on the north side of KE drops as the flow strengthening and rises as the flow weakening.The sea level on the south side of the KE is also experienced three periods, the sea level drops during 1993-1996 and then rises during 1997-2006 and after 2003 the sea level drops. The sea level rate on the south side of KE is larger than north side. The seasonal variability caused an annual range of 22.6cm of sea level which accounts for about 50% of the sea level variability. The steric sea level contributes 88.5% to the seasonal sea level variability and one third to the inter-annual variability and trend of the sea level. The freshwater flux caused by precipitation and evaporation contributes little to sea level change. The sea level on the south side rises as the KE strengthening and drops as the KE weakening.The one and a half-layer ocean model shows the contribution of wind forcing to the sea level variation is different in different regions of the mid-latitude North Pacific Ocean. In the area south of the KE, the wind forced seasonal signal through barotropic Sverdrup transport is smaller than the inter-annual signals through boroclinic rossby wave; but in the area north of the KE, the seasonal variability is larger than the inter-annual variability caused by wind forcing. Considering the effect of steric sea level, the seasonal variability of the sea level south of the KE is mainly caused by steric effect while wind forcing is mainly causes the inter-annual and long term variability of the sea level. The seasonal and long term variability of the sea level north of the KE is mainly caused by steric effect and wind forcing is important in the inter-annual sea level variability.The anomalous sea level during 97/98 El Nino can be explained by the change of sea water temperature and transport caused by change of circulation. Controlling experiments show that wind dominates the 97/98 El Nino event; there is large difference in the current field and sea surface height between the El Nino event forced by real wind and climatology, while little difference can be found between the El Nino event forced by real heat flux and climatology. The sea level variation in the equatorial region can be well explained by wind forcing and the sea level variation in the mid and high latitude region is mainly caused by heat flux. The sea level variation during 97/98 El Nino caused by heat flux is about 20% and the residual is caused by wind.The simulated global mean steric sea level during 1993-2006 is rising at a rate of 0.74mm/a with most contribution coming from the upper 200m and the fast rising begins from 1998. The sea level rise of North Pacific Ocean caused by volume redistribution is 0.44mm/a; in 1997 the Pacific Ocean has a low sea level. The steric sea level of the North Pacific Ocean drops before 1998 and after 1998 begins to rise fast and this trend evolution is in agreement with the observation. The rising rate of the steric sea level of the North Pacific Ocean during 1993-2006 is 0.92mm/a and the contribution from the upper 200m is important. The sea level rise coming from water volume redistribution and steric effect can explain 47.4% of the sea level rise of the North Pacific Ocean.