Investors shouldn't expect the bumper gains returned by the equity market so far this year to continue, the CEO of the world's largest sovereign wealth fund told CNBC on Wednesday.

Nicolai Tangen, who leads Norges Bank Investment Management — which oversees Norway's $2.3 trillion oil fund — was speaking to CNBC's Ben Boulos after the fund posted a record first-half profit nearing $185 billion.

But the fund, overwhelmingly comprised of equities, saw a turbulent first six months of the year. NBIM's equity portfolio dropped 2.6% in the first quarter of the year before surging 15.98% in the subsequent three months, leading to a first-half return of 12.95%.

Asked what lies ahead for markets, Tangen told CNBC that he was surprised by how well markets and economies held up in the wake of the U.S.-Iran war and renewed inflationary pressure.

"If you went back two years and told me this is going to happen with the Hormuz strait, trade barriers, geopolitical tensions, and so on, I would never have thought that the market would be as resilient as it is," he said. "Companies are very good at managing under uncertainty and under changing operating conditions, and markets have been resilient, so it's difficult to say exactly how this is going to end up. But for sure, we should not be expecting the same kind of returns going forward as we've seen over the last six months."

However, he advised investors not to jump ship on their holdings during bouts of volatility.

"I would say the way to to make money is one, be very, very long term — don't change your strategy — and be well diversified," Tangen said. "I think that's a good philosophy, and then you know leave it to some professionals. It's more difficult than it looks to make money."

Much of the fund's first-half success came from a rally in semiconductor stocks, with its top performing holdings in the first half including Samsung, SK Hynix, TSMC, ASML, Intel and Nvidia.

"Chips, chips, chips, chips," Tangen had earlier told reporters at a news conference, while standing before a chart demonstrating the best-performing stocks in NBIM's portfolio.

During his interview with CNBC, Tangen conceded that the first half brought "very concentrated gains indeed" — but he noted that NBIM would not move to take profits or rebalance the fund.

"We are an index-near fund, so we typically will be invested in everything around the world," he explained. "We own 1.5% of all the companies in the world. In Europe, we own roughly 3% of all the [listed] companies, and so we typically are invested in everything, which is very good when you have periods like the first half of this year."

However, should there be a meaningful market downturn, Tangen warned the outlook could be bleak for the Norwegian sovereign wealth fund, which is the source of around 25% of Norway's fiscal budget.

"For sure, if there is a downturn in the markets we will lose money," he said. "We participate in the upturns and the downturns, and that's the way it's been for the last 30 years. And it's been a very, very good ride. I don't think we'll have a repeat of the last 30 years going forward; I think there'll be tougher times ahead. But I do think for a long-term large investor, you just need to be well spread out and invest for the very, very long term."

Global equity markets have been on a volatile ride this year, as headlines around AI capex, the Iran war, inflation and central banks have both weighed on and lifted investor sentiment. Wall Street's major averages are all up by more than 10% year-to-date, while Europe's Stoxx 600 has jumped more than 11% and South Korea's tech-heavy Kospi has surged more than 50%, despite multiple bouts of severe turbulence.