The Role of Ekman Ocean Heat Transport in the Northern Hemisphere Response to ENSO

The influence of oceanic Ekman heat transport (Q(ek)) on air-sea variability associated with ENSO teleconnections is examined via a pair of atmospheric general circulation model (AGCM) experiments. In the mixed layer model (MLM) experiment, observed sea surface temperatures (SSTs) for the years 1950-99 are specified over the tropical Pacific, while a grid of mixed layer models is coupled to the AGCM elsewhere over the global oceans. The experimental design was used in the Ekman transport/mixed layer model (EKM) experiments with the addition of Q(ek) in the mixed layer ocean temperature equation. The ENSO signal was evaluated using differences between composites of El Nino and La Nina events averaged over the 16 ensemble members in each experiment. In both experiments the Aleutian low deepend and the resulting surface heat fluxes cooled the central North Pacific and warmed the northeast Pacific during boreal winter in El Nino relative to La Nina events. Including Q(ek) amplified the ENSO-related SSTS by similar to 1/3 in the central and northeast Norht Pacific, producing anomalies comparable to those in nature. Differences between the ENSO-induced atmospheric circulation anomalies in the EKM and MLM experiments were not significant over the Noth Pacific. The sea level pressure (SLP) and SST response to ENSO over the Atlantic strongly projects on the North Atlantic Oscillation (NAO) and the SST tripole pattern in observations and both model experiments. The La Nina anomalies, which are stronger than during El Nino, include high pressure and positive SSTs in the central North Atlantic. Including Ekman transport enhanced the Atlantic SST anomalies, which is contrast to the Pacific, appeared to strengthen the overlying atmospheric circulation.