Published in 2007 Cited 15 times Keywords: North Pacific, PDO-related air forcing, Eddy permitting model, Model heat/SST budget analysis

Introduction Regime shift in North Pacific Before 1977: AL weak, center SST high, east SST low After 1977: AL strong, center SST low, east SST high -> PDO: 1 st EOF of North Pacific SST -Previous studies showed PDO has its dynamical roots in the Tropics -But, midlatitude ocean processes also participate in the PDO Long-term SST variability processes: Heat flux, Reemergence, Interactions between atmosphere and ocean, Ocean currents variability => Study area is North Pacific, including near Equator region and Western Boundary region with PDO related forcing -Mesoscale eddy contribution to upper-ocean heat transport variability -Upper level heat content is a better predictor of air-sea heat flux than SST => Study both SST tendency and heat storage rate -Mesoscale eddy activity is a significant source of upper-ocean heat content => Eddy permitting high resolution (1/3 degree) model

Model description -Hallberg Isopycnal Model (HIM, Hallberg 1995) Primitive equation isopycnal model 25 layers (3 for ML, 2 for abyssal ocean) 1/3 degree resolution (5S ~ 65N, 110E ~ 80W) Thermodynamic sea ice model included SSS relaxation Bulk formula (Zeng et al. 1998) – da Silva monthly climatology + PDO-like anomaly fields (covariances between PDO index and climatology) Artificial (model) PDO index Spin-up 45 years (-1 PDO index state) 10 year model integration 1-4 negative PDO period 5-6 PDO transition period 7-10 positive PDO period

PDO change ((+) – (-)) * AL index indicates the strength of AL Showing strong positive PDO features ->

PDO change ((+) – (-)) Ocean loses more heats to air in positive PDO phase * Heat flux is in atmosphere-to-ocean

Heat budget equation SST (Mixed layer temperature, MLT) Upper layer water column (400 m) Diffusion term will be neglected ug + uag Units in degC/yr Units in W/m2

Budget difference: PDO (0) – PDO (-) -Clear PDO SST change (PDO+ : center SST ↓) Mostly come from Atmosphere (b) + contribution of ent. (c) and ageo. (e) -Kuroshio region Atm: cooling Geo/Ent: warming (against SST tendency) -North of 30N, atm and ageo dominate -South of 30N, vert dominates -Warming signal due mostly to vertical => thermocline deepening (Ekman pumping?) => No influence to SST (change occurred below MLD) -Difference in atmospheric term => Due to the MLD difference (Deep in Kuroshio)

Budget difference: PDO (+) – PDO (-) For SST tendency -SST tendency is nearly zero => SST change in PDO(+) is steady state => SST changes mostly in the transition period if PDO(-) SST change is in also steady state -Central Pacific region => increased by atm, compensated by ent/ageo -Kuroshio/Oyashio region => atm vs ent+geo - dipole change <= southward migration of Kuroshio separation point

For heat content change Budget difference: PDO (+) – PDO (-) -Still in adjustment (net change is not zero) -Kuroshio/Oyashio Extension region HCC is driven by vert. and geo. (equally) -WBC region geo. is more important In both KOE and WBC, opposite sign atm. (especially in WBC region) -Dipole structure along 40N -Central North Pacific (30~45N) ageo. cooling, atm. warming

Major points in budget difference analysis 1. Surface flux adjustment PDO(0) – PDO(-) Although the atmosphere cools the central Pacific during the model’s PDO transition period, once the transition is complete the MLT and heat content budgets both show enhanced warming by the atm. PDO(+) – PDO(-) SST tendency Ageo. and ent. causes SST cooling => surface flux becomes positively larger => differ from observational results by Yasuda and Hanawa (1997) (ocean-to-atmosphere flux is greater after 1976 shift in CN Pacific) => difference of dataset, overshooting of Kuroshio (Extension)…

Major points in budget difference analysis 2. Geostrophic advection and surface flux Shading: atm Contour: geo+vert There is a strong correspondence between the areas of geostrophic advection and atmospheric heat flux changes over the Kuroshio (+ KOE extension..). Geostrophic heat content advection changes are forcing the changes in atmospheric heat flux. 3. Difference in SST tendency and heat content change in atmospheric forcing PDO (+) – PDO(-) SST tendencyHeat content change A large atmospheric warming appears in the SST budget between 30 and 35N, which is not present in the heat flux field. See next page!

MLD effect PDO(+) – PDO(-) MLD changes can cause temperature budget changes without any associated heat flux changes, as the same heat flux acts on a greater or lesser amount of water. Surface flux term Change in A due to a change in h of Δh Changes in entrainment Seasonal cycle has a large impact upon MLD => Making Δh climatology for both positive and negative PDO phase. After that, ΔA and ΔE are calculated (average of all months) => See next page!

MLD effect Indeed, MLD changes have a significant impact on the SST tendency changes (Especially along the 30N in surface flux term). The large impact MLD on the SST tendency suggests that SST budgets are poor indicators of decadal heat flux changes (seasonal cycle is dominant in MLD changes).

Eddy fluxes Reynolds decomposition for eddy advective fluxes Any difference between the mean heat flux saved by the model code and the heat flux divergence calculated using the ubar and Tbar fields must be caused by eddy fluxes. F’ : eddy flux term Fbar: monthly mean advective heat budget calculated model interior code f : model advection scheme function ubar, Tbar : monthly mean u and T Using monthly averages resulted in a smaller eddy flux divergence than using seasonal averages but did not change the spatial patterns appreciably.

Eddy fluxes Eddy fluxes warm the ocean on the northern side of the boundary currents and cool it on the southern side (opposite to geostrophic part) Showing two different types of eddy response in the difference map between PDO(+) and PDO(-) - Southward shift in Oyashio Extension (~44N) - Doubles in flux magnitude in Kuroshio Extension (36~40N)

Summary Cooling primarily by atmospheric heat flux, during the transition from negative to positive PDO additional ent. and ageo. cooling in the SST tendency additional ageo. cooling in the heat content change Once the shift is completed, surface fluxes over the central Pacific begin to warm in response to continuing cooling by the ent. and ageo. SST budget is quickly adjusted to the new PDO regime, while heat content change still shows significant drift during the positive PDO Changes in geostrophic advection appear to control surface flux in the WBC region in both the SST tendency and heat content change Eddy field has a large effect on the heat budget near the WBC region as well as a smaller expression along the eastern equator