Analysis Of The Basic Features And Variability Of Heat Advection In The Tropical Pacific Ocean

Horizontal heat advection (HHA) plays an important role in the heat balance ofmixed layer in the tropical Pacific Ocean, and acts as an essential factor in theformation and variability of the tropical Pacific Ocean climate. Using the SimpleOcean Data Assimilation (SODA) Version2.02/4during1958–2007, this paperpresents a detailed analysis of HHA in the mixed-layer tropical Pacific Ocean,including climatological features, seasonal variations of zonal heat advection (ZHA)and its mechanisms, and interannual variations of HHA associated with ENSO.Especially, ZHA’s role in the heat budget of Ni o3.4region is estimated based on heatbudget equation, and its association with the seasonal displacement of easternboundary of the Western Pacific Warm Pool (WPWP) is discussed. Main results aresummarized as follows.1) Basic features of mixed layer HHA in the tropical Pacific Ocean. Inclimatology, the ZHA shows a zonal-band spatial pattern with its maximums near theequator and west boundary. Cold ZHA (about-50W/m2) appears between10°S and5°N in the mid-east Pacific Ocean, warm ZHA (about40W/m2) lies around5°N–10°N, and relatively weak cold ZHA about-20W/m2is in the region north of10°N. On the one hand, the spatial structure of ZHA is closely related to the zonalgradient of mixed layer temperature (this is why the ZHA inside the WPWP isrelatively small). On the other hand, the distribution of ZHA is associated withequatorial current system. In detail, the westward flowing South Equatorial Current(SEC) and North Equatorial Current (NEC) serve as significant cooling factors, whilethe eastward North Equatorial Countercurrent (NECC) warms the ocean.The distribution of meridional heat advection (MHA) seems to be lesscomplicated: cold the ocean in the mid-east Pacific near the equator while acts as aweak warm effect in the region north of10°N。The spatial pattern is mainly becausethe temperature decreases poleward while the meridional currents north of10°N arein the reversed direction relative to that south of10°N..2) Seasonal variability of mixed layer ZHA and its mechanisms in the Pacific Ocean. It is shown that the ZHA has significant seasonal variability. As a result ofin-phase seasonal variations of the SEC and NECC, cold ZHA in5°S–5°N variesout-of-phase with warm ZHA in5°–10°N on a seasonal time scale. The seasonalanomaly of ZHA features westward propagation, and large anomalous ZHA occupiesthe mid-east Pacific Ocean, especially in the Ni o3.4region.Investigation on the mechanism of ZHA’s seasonality in the Ni o3.4region isperformed and it is suggested that the seasonal variation of the zonal currents (i.e.,surface SEC and subsurface Equatorial Undercurrent) is the main reason, whichexplains about51.5%of the total ZHA’s total seasonality. Further study shows that theanomalous local wind stress is the direct reason, and the combination of equatorialRossby wave and Kelvin wave accounts for the westward propagation of ZHA’sseasonal signal.Heat budget analysis in the Ni o3.4region suggests that both the ZHA and seasurface heat flux are comparable important in the mixed layer heat balance there.Particularly, the anomalous ZHA from February to July is even more important thansurface heat flux. Furthermore, the connection between ZHA and EBWP is furtherexplored. A statistical relationship among the eastern boundary of WPWP, ZHA andsurface heat flux is established based on least-square fitting. The fitting equation isshown asLonE BWP Q0Qu, while ξ is214.2°E，α and β are weight coefficientsof sea surface heat flux (Q0) and ZHA (Qu), respectively as0.3°E m2W-1å'Œ0.4°Em2W-1. The equation fits the original series very well at99%confidence level.3）Interannual variability of HHA and its associations with traditional El Ni oevents. Results show that HHZ has prominent interannual variation associated withENSO cycle, and contributes to both the developing and decaying phases of El Ni oevents, based on correlation and regression analysis between the HHA and Ni o3.4index. Correlation between ZHA and Ni o3.4index presents a zonal-band structure,and is positive in NEC and SEC zones while negative in NECC region. MHAvariations show positive high correlations with Ni o3.4index in the mid-east PacificOcean, which suggests that the MHA plays an important role in heat transports and adjustment in ENSO cycles. Furthermore, the developing phase, mature phase anddecaying phase of traditional El Ni o have been composed in detecting HHA’s role indifferent periods of El Ni o events. During developing and mature phases, anomalousZHA warms the ocean near the equator of the middle Pacific and cools the ocean in5°–10°N. While during decaying phase, warm anomalous ZHA is weaken, andreplaced with cold anomalous ZHA on the equator. Anomalous MHA occurs in theeast Pacific Ocean, which serves as warm effects in developing and mature phaseswhile cold ones in decaying phases. It is worth noting that anomalous ZHAs mainlyoccurs in the middle Pacific Ocean while anomalous ZHAs occurs in the east PacificOcean,...